Dear Readers,
Welcome to the November 2 edition of the MIPS/IRAC GTO newsletter. This issue features six white papers and one letter. Jeremy Mould discusses how MIPS deep survey data can be used to discriminate between several prevailing theories for galaxy formation. Chad Engelbracht outlines a program of MIPS and IRS observations of starburst galaxies. Lee Hartmann discusses how IRAC can be used to study the evolution of protoplanetary disks through the planet forming stages. Phil Myers discusses a program to determine the spatial structure of nearby, young, embedded star clusters. Jocelyn Keene discusses possible observations of pre-stellar cores and embedded young stellar objects. Alice Quillen gives her perspective on ISO observations of cold dust in nearby galaxies and discusses how these studies could be expanded through SIRTF surveys. Finally, George Rieke presents an overview of the recent ISO conference in Paris, including a discussion of implications for SIRTF. Figures from Jeremy Mould's white paper are available at the MIPS web site.
The deadline for submissions to the next issue of the newsletter is Friday the 13th of November.
Doug Kelly, editor (dkelly@as.arizona.edu)
The reported cosmic infrared background (Puget et al. 1996; Hauser et al. 1998) implies that a large fraction of the radiation of early galaxies has been converted into the far infrared. The spectrum seems to indicate the presence of sources at large redshifts (3-10 in z).
Our knowledge of the early epochs of galaxies has increased rapidly in the last 5 years thanks to optical and near-UV observations with Keck and HST. Nevertheless, there is a broad range of theories consistent with these data that have very different signatures in the infrared.
These models and other luminosity evolution models (e.g. Lonsdale 1996) can be tested with MIPS with source counts orders of magnitude fainter than IRAS.
MIPS has an opportunity to test models of the history of galaxy formation by means of source counts, which Figure 1 shows, can extend four orders of magnitude fainter than IRAS.
In fact, classical radioastronomy P(D) techniques (see Oliver et al. 1997 for an example of the application of this technique to ISO galaxy counts) will allow us to test the predictions of models beyond the single source sensitivity limit shown in Figure 1.
MIPS has enough sensitivity to answer the question "can the cosmic background intensity around 100 microns be fully accounted for by discrete sources?" Presumably the answer is "yes" but the immediate follow-up questions are to measure their color and luminosity distribution. Beyond that, MIPS will open up the quest for the redshift distribution of the sources, which will lead smoothly into work with NGST.
Indeed, if nu*I_nu is close to the lower limit in Figure 2 of 5 nW m^-2 sr^-1, the background can be fully accounted for by 5 limiting MIPS 160 micron sources per MIPS beam. That is a reasonable supposition, but what will the data say? This trivial calculation reminds us that we will be operating at the confusion limit and that the P(D) approach will be the key.
The design brief for a deep MIPS survey should include sensitivity limits (depth), sampling statistics (area), and resolution. Candidate regions will include the Hubble Deep Fields (North and South) and the Lockman hole. At 450 microns and 850 microns SCUBA surveys of these regions (Barger et al. 1998; Hughes et al. 1998) will provide important complementary information at similar angular resolution. An interesting strategy adopted with SCUBA (Ivison et al. 1998) should be considered by the MIPS team for increasing the depth of faint source counts. That is to use microlensing amplification in rich clusters to see deeper into the Universe. Smail, Ivison, & Blain used the clusters Abell 370 and Cl2244-02 for this purpose. This advantage is bought at the cost of increased uncertainty in distant source luminosity arising from modelling the lens.
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Starburst galaxies are galaxies which are forming stars at a prodigious rate of up to several tens of solar masses per year. They typically have high luminosities (up to ~1e12 solar luminosities) that are dominated by warm FIR emission. A significant fraction of high-mass star formation in the local universe occurs in these galaxies (Heckman 1997), and their energetics are thus dominated by massive stars. Their optical and NIR spectra resemble those of HII regions, with the addition of a component of shocked gas from strong stellar winds and supernova remnants. The MIR spectra are characterized by strong PAH features and forbidden-line emission from low-ionization species. Starburst events are transient: gas depletion timescales in the nuclei of typical starbursts are on the order of 100 million years. Depending on the intensity, though, the starburst can dominate the luminous output of the galaxy for much longer times.
Massive starbursts, because of the large amounts of gas required to fuel them, are typically heavily obscured by dust. They show large measured values of extinction (e.g., Lutz et al. 1996, Engelbracht 1997), and most of the starburst luminosity is intercepted by dust and emerges as thermal reradiation in the FIR. Such systems are thus best studied in the infrared: the FIR spectral region contains the bulk of the luminosity, the MIR spectral region shows nebular emission which is driven by the massive-star component, and the NIR spectral region offers additional diagnostic emission lines as well as a direct measurement of the cool stellar population. In many of these systems, the optical and UV emission from the heart of the starburst is largely absorbed and observations at those wavelengths only sample the outer regions of star formation.
Much can be learned from evolutionary synthesis modeling of starburst galaxies, starting with Rieke et al. (1980), and continued by Rieke et al. (1993) and other authors (e.g., Leitherer & Heckman 1995, Engelbracht 1997). Our approach has been to use infrared observations, coupled with data at other wavelengths from the literature, to derive physical properties of the starburst population such as mass, luminosity, ionizing flux, CO index, etc. These then serve as constraints on the models, which allow exploration of parameters such as age, star formation history, and IMF. MIPS imaging will provide improved sensitivity and greatly improved spatial resolution over IRAS measurements of these galaxies, allowing much more accurate calculations of bolometric luminosity and some insight into the spatial distribution of the FIR emission (and hence the spatial evolution of the starburst) in these galaxies. IRS spectroscopy would provide invaluable temperature, density, and ionizing flux constraints, as well as allowing starburst/AGN discrimination as discussed below.
We propose a program of MIPS imaging and IRS spectroscopy of a sample of 20 nearby starburst galaxies. These galaxies are near enough that it should be possible to resolve individual star formation regions in some of them, and they are bright enough that observing them does not take much time. Plus, they have been well-studied so supporting observations at many other wavelengths are already available. In particular, Engelbracht (1997) presents NIR imaging and spectroscopy, plus some MIR spectroscopy, plus evolutionary synthesis modeling, of a sample of 20 northern-hemisphere starburst galaxies. Several of these galaxies are unsuitable for a MIPS program, and there are several excellent southern-hemisphere targets with NIR observations in the literature, so the size of the final sample will probably remain around 20.
The faintest galaxies in the sample are typically detected at a level of several Jy by IRAS, while the brightest approach several hundred Jy. We estimate that imaging of the whole sample in the three MIPS bands can be accomplished using less than 30 minutes of observing time. Similarly, IRS spectroscopy at high resolution from 10 to 40 microns could be obtained in less than 4 hours.
In addition to supplying constraints on the stellar population, the MIR spectra provide a powerful discriminant between AGN and starburst activity. Limited access to the MIR spectral region from the ground, on a very bright galaxy, allowed Engelbracht et al. (1998) to show that the LINER characteristics of NGC 253 arise from an evolving starburst and not an active nucleus, an issue that was not possible to resolve from optical spectra. Similar techniques have allowed Genzel et al. (1998) to show that the dominant power source in most ultraluminous IRAS galaxies (ULIRG) is star formation, with only a small contribution from an active nucleus. We suspect that many members of the class of LINER galaxies discussed by Engelbracht et al. (1998) (these systems are less luminous than the ULIRGs) will prove to be evolved starbursts, an issue that should be trivial to resolve with the IRS spectra.
Approximately half of all the young stars in molecular clouds with ages of 1 Myr or less show bright infrared emission in the 2-100 micron wavelength range attributable to optically-thick, dusty circumstellar disks. The optically-thick disk emission disappears at least by ages of 30 Myr, and probably much earlier, though the requisite studies have not yet been undertaken because of limited sensitivity and a lack of suitable samples to study. The explanation for this evolution of disk emission is probably the coagulation of dust grains into larger bodies, leading to the agglomeration of planetesimals, and ultimately to planets which can sweep up much of the remaining gas and dust. Indeed, the Vega-like systems, which exhibit relatively modest amounts of emission from optically-thin dust, typically show inner holes in their disks, as expected in the coagulation scenario since grain size evolution is more rapid at smaller radial distances (Weidenschilling & Cuzzi 1993). The inner holes in the disks of the young A stars HR 4796A and Beta Pic are about 50 AU in size (Pantin et al. 1997; Jayawardhana et al. 1998; Koerner et al. 1998), suggestively comparable to the size of the Sun's planetary system.
SIRTF provides the sensitivity to study the evolution of protoplanetary disks through the agglomeration (and planet-forming) stage(s) for statistically significant numbers of stars. This capability could provide the first real observational constraints on how protoplanetary disk evolution is affected by stellar properties, binary companions, etc. It may even be possible to recognize systems in the process of forming giant planets as gaps are cleared in disk dust. Accomplishing these goals will depend upon observing young stars in the appropriate age range.
Many nearby star-forming regions are known with ages 0.5-2 Myr. Several of these regions are likely to be studied by SIRTF if for no other reason than to characterize the stellar populations present; one proposal for such a study is appended below. There are a few nearby open clusters with ages of 30 Myr and up, some of which (e.g., Pleiades) will probably be surveyed for brown dwarfs, but the timescale for the disappearance of disk dust emission is probably substantially shorter than this. HR 4796A, which has an estimated age of 8 +/- 3 Myr from its M-star companion (Stauffer et al. 1995), has an optically thin, low-mass debris disk (Jayawardhana et al. 1998; Koerner et al. 1998), and revised estimates for the similar Beta Pic system suggest an age that is not much larger, 20-100 Myr (Heap et al. 1998).
The real problem lies in identifying appropriate samples of stars with ages in the critical 3-10 Myr period. Unfortunately, many of the young clusters originally thought to have these ages probably are much younger, due to mass-dependent age determinations. The classic case is NGC 2264, which originally was estimated to have an age of about 5 Myr, based on the main-sequence turn-on among A stars (Walker 1956; see also Sung et al. 1997). However, the recent study by Makidon et al. (1997) clearly shows that most of the low-mass M stars have ages of about 1 Myr, and their optical emission properties appear to be similar to those of other 1 Myr old T Tauri stars.
This mass-dependent age trend turns up over and over again in cluster studies. For example, Herbig (1998) recently found that the mean age of the low-mass stars in IC 348 is about 1 Myr, but that the A- and F-type stars have much larger ages, roughly 3-6 Myr. The clearest and most disturbing demonstration of this effect is in the Orion Nebula Cluster, which is so compact (and so filled with gas) that it must be quite young. Hillenbrand (1997) found that most of the stars below about 0.8 Msun have apparent ages <1 Myr, with a small age spread, as might be expected. However, the estimated ages of 1 Msun stars are ~ 1 Myr, with a spread between 3 and 0.5 Myr; the 1.5-3 Msun stars have ages between 1 and 10 Myr; and all the higher mass stars have ages < 1 Myr. Hillenbrand suggested that this trend of ages is not real and is due to a "birthline" effect (Palla & Stahler 1992); that is, the A-F stars appear too old because they started out closer to the main sequence than assumed in current evolutionary tracks.
In summary, there needs to be a careful reevaluation of young cluster ages before identifying the samples of stars needed to follow disk evolution through the crucial 3-10 Myr age range. Simply using age estimates from the literature will not suffice; older estimates are likely to be biased toward more massive stars (i.e., A stars) and thus result in relatively large ages. Clusters need to be compared mass range by mass range to avoid this bias; otherwise, it is quite possible to develop a program to study disk evolution in which the stars all have roughly the same ages. (Note that several young clusters appear to have some age spread among the low-mass stars and so may contain a few stars in the desired age range. However, one must be aware that observational errors, both random and systematic, certainly increase the apparent age spread beyond its true value, and in any event it is difficult to construct a statistically significant sample of such older stars.)
My colleagues (Erik Gullbring, Cesar Briceno, Nuria Calvet) and I are currently engaged in a project to identify relatively young clusters of stars with the appropriate ages. We have a total of 20 candidate clusters and have photometry for at least one good target. These clusters are typically at distances comparable to NGC 2264 (~ 1 Kpc), and the sensitivity of SIRTF is sufficient to detect dusty disks around these objects. The compact nature of these populous clusters should produce reasonable samples of stars in a modest number of fields of view. The problem is, at this time we still don't know which of these clusters are the best targets. We have one likely candidate in hand and should be able to confirm this by mid-1999; the other cluster should be selected by the end of 1999.
We propose a survey of infrared disk emission of 5 young open clusters at wavelengths from 3 to 70 microns. The goals of this project are to study the evolution of protoplanetary disks through the expected times of planet formation, surveying large populations of stars through a substantial mass range to understand the range of disk properties. Special emphasis will be placed on detecting the existence of inner disk holes caused by dust coagulation and on tracing the evolution of such holes.
One candidate cluster will be NGC 2264, which is relatively well studied and has approximately 400 known members that can be surveyed. The two older clusters would be selected from the results of our ongoing survey. We also suggest surveying the nearby Cha region, since the stars there appear to be slightly older (1.5 - 2 Myr) than NGC 2264, Taurus, etc. Finally we suggest observing the compact, relatively nearby IC348 group. These last two groups might be contained within the star formation region surveys being proposed separately.
We focus on NGC 2264 initially because it is of comparable distance (800 pc), richness, and angular size to the old cluster candidates. Membership is well-established for NGC 2264 from combining studies of proper motions, spectra, and photometry (Makidon et al. 1998). We propose to take 36 fields of 5' x 5' with IRAC and MIPS. This area covers most of the cluster stars (~ 200), providing a good statistical sample. We assume that we can sample all four bands simultaneously with IRAC and the 24 and 70 micron bands with MIPS.
With 500s exposures IRAC can detect any dust excess above photospheric emission at lambda <~ 8 microns for stars down to the hydrogen burning limit. With this exposure time, the short-wavelength channels will detect objects well below the hydrogen-burning mass limit. The detectability of disks depends upon the disk optical depth, inner disk hole size, and whether there are large grains in the inner disk hole. A sample disk model provided by Nuria Calvet indicates that for a typical low-mass star of 10 Myr age, inner disk holes of 1 AU result in no detectable disk emission in the IRAC range. However, if the dust has not completely disappeared but has just begun agglomerating into larger grains, a noticeable effect may be observed. The specific model indicates that detectable excesses can be observed in the IRAC bands for coagulation into dust of sizes < 100 microns. Thus, IRAC may be able to probe the first stages of accumulation.
For an optically-thick disk beyond 1 AU, the predicted fluxes for a typical 10 Myr old low-mass star are around 0.8 mJy at wavelengths of 24 microns and 70 microns. 500s exposures with MIPS would result in a 15 sigma detection at 24 microns and a 2.7 sigma detection at 70 microns. The corresponding disk around a solar-mass star would be about a factor of 3 brighter and thus would be easily detected.
Dust emission from more tenuous, evolved disks should be concentrated at longer wavelengths. Scaling from the 8 +/- 3 Myr-old A0 star HR 4796A, at 1 Kpc the corresponding fluxes would be ~ 10 and 20 mJy at 24 microns and 70 microns, respectively, and so would be very easy to detect in a 500s exposure. Scaling to a 5 sigma detection at 24 microns in 500s (250 microJy), we could detect a dust disk of similar temperature and relative luminosity around a star of 0.5 Lsun, corresponding to a star of 0.9 Msun at an age of 10 Myr. For the same temperature distribution, hole sizes must be smaller for lower-luminosity stars; making appropriate scaling, dust around the 0.9 Msun star would have the same temperature (~ 100 K) for a hole of 10 times smaller, or 5 AU (Jupiter's orbit). Thus, MIPS can usefully constrain the development of disk evolution in the giant planet region for the more luminous stars in these clusters (solar mass and greater).
The following table assumes 5x5 arcmin fields for both IRAC and MIPS.
________________________________________________________________________ TABLE 1 Exposure times per cluster ------------------------------------------------------------------------ Instrument Bands Exposure time Obs. time Fields Total ------------------------------------------------------------------------ IRAC 6um and 8um 500s 625s 36 2.3 x 10^4 sec MIPS 24um and 70um 460s 750s 36 2.7 x 10^4 sec ________________________________________________________________________
This comes to a total of 14 hours of observatory time per cluster; 3 clusters then amounts to 42 hours of observatory time.
To cover Cha we need to take many more fields with shorter exposures. This region is about 5 times closer than the older clusters, so the exposure times can be shorter. Using a MIPS minimum integration time of 77 seconds means about 10 times less exposure per field; for IRAC, the exposures can be 20 times shorter. To cover Cha I and Cha II (Schwartz 1991), we require approximately 4 square degrees, or about 570 fields. This would suggest about 4.4 x 10^4 sec of MIPS exposure and about 1.8 x 10^4 sec IRAC exposure time, for an additional 17 hr.
Finally, we consider surveying the region of IC 348 that has Halpha emission stars. A field of approximately 15' square should suffice, or 9 fields. For five times shorter exposures than for the distant clusters (300 pc distance), this adds less than 1 hour of time so is relatively efficient. The IRAC/MIPS total exposure time is then about 60 hr.
To use IRS we need to select individual targets for observation. However, the possibility of studying the evolution of the silicate emission feature should be considered. Exposure times of 320s should get us at least 3 sigma detections of stellar photospheres for stars of one solar mass and greater in these clusters; surveying 20 objects spread through spectral types A-G should take another 1.5 x 10^4 sec of observatory time, or another 4 hr/cluster, for a (three instrument) total of 80 hours for the five clusters/groups.
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We plan to use IRAC to measure the spatial distribution of the youngest stars in ~ 50 groups and clusters within 1 kpc and to measure the degree of clustering in 7 of the nearest star-forming complexes. In this contribution we describe the background motivation for this work, our observational goals, and our plan of observations.
In the last few years, it has become clear that the relatively isolated young stars in Taurus and similar complexes cannot be considered typical of most stars and the circumstances of their formation. Infrared observations of young clusters (Lada & Lada 1991), young stellar groups (Chen & Tokunaga 1994; Hodapp 1994), and pre-main sequence binaries (Mathieu 1994) have shown that star formation is a process whose typical outcome is more than one star. Most pre-main sequence stars in Taurus and Ophiuchus are in binary systems, as is also seen in the field star population (Ghez, Neugebauer, & Matthews 1993). The number of young stars in groups is at least comparable to the number of stars in isolation, and may be significantly greater, according to a near-infrared imaging study of 164 young stellar objects with molecular outflows (Hodapp 1994). The largest nearby young clusters, such as in Orion and M17, have ~10 massive stars accompanied by ~1000 low-mass stars. The distribution of stellar fluxes at 2 microns in such young clusters is generally consistent with that expected from the initial mass function (IMF) for field stars (Lada & Lada 1995). This consistency suggests that such clusters can provide most of the stars that are counted to estimate the IMF.
As a result, it is also clear that the typical star cannot be considered to arise solely from the initial conditions derived from quiescent, thermally-dominated, low-mass cores, as in the "standard model" of isolated star formation (Shu, Adams & Lizano 1987). This is so even though such cores are evidently the birth sites of many young low-mass stars (Benson & Myers 1989; Beichman et al 1986). The physical properties of dense cores in cluster-forming regions are different from those of low-mass cores in Taurus, Lupus, and Chamaeleon, which tend to make relatively isolated low-mass stars and binaries. "Massive cores" are larger, denser, and more turbulent than low-mass cores, even when they do not contain any embedded young stars (Jijina, Myers & Adams 98). Their molecular spectral line widths are dominated by nonthermal motions (Harju, Walmsley, & Wouterlout 1993; Caselli & Myers 1995; Walmsley 1995), due probably to MHD turbulence and waves (Arons & Max 1975; Myers & Goodman 1988; Myers & Khersonsky 1995). Thus we may safely neglect turbulence in modelling the formation of some low-mass stars, but not in modelling the formation of the typical low-mass star.
Despite our understanding of these differences (cf. Pringle 1989, Whitworth et al 1996), there has been relatively little progress in understanding the formation of stars in clusters. There are two main suggestions for the coeval formation of multiple stars in groups: (1) the precursors are essentially a cluster of Jeans-mass condensations which are made gravitationally unstable by compression (Larson 1985; Mouschovias 1991; Clarke 1997; Myers 1998), and (2) the precursors are a cluster of small "seed fragments" which orbit in their cluster-forming cores until they gain enough mass to collapse (Bonnell et al 1997). These models match to some degree the stellar masses seen in clusters but do not directly account for the number and density of stars in clusters or for their complex spatial structure.
A major limitation on progress in this area is the lack of observations with systematic sensitivity and coverage from region to region. Although we know that star formation is "clustered" in Orion and "isolated" in Taurus, we have very little quantitative understanding of the relative variation of clustering within a cluster, within a cluster-forming complex, or from complex to complex. Thus it is difficult to relate clustered star formation to the known variations in structure and properties of the parent star-forming gas from region to region. In this proposal we seek to use the IRAC and MIPS cameras to help observe, quantify and analyze clustering in the nearest star-forming regions.
Until recently, there has been little attention paid to the spatial stucture of clusters as a set of clues to understand cluster formation, even though inspection of cluster images reveals a wealth of information, including multiple centers of clustering and clustering on a range of size scales. Thanks to the near-infrared imaging observations in the last decade, there is now a wealth of observational data that tell us the location and richness of nearby embedded clusters. Some of the best examples are the studies by Hodapp (1994), Chen & Tokunaga (1994), and Testi et al (1997). We have recently made a compilation of near-infrared surveys of star-forming regions within 1 kpc, which lists 129 entries from 46 papers, with median number of projected stars in the cluster image > 100 stars (Christopher et al 1998).
These regions have been observed at J, H, or K, with a variety of instruments, under varying conditions of seeing. Therefore the presently available observations of these regions suffer from nonuniform sensitivity, incompleteness, and uncertain backgrounds. Furthermore these studies are not always sensitive to the very youngest and most extincted members, which are systematically redder than most of the stars visible at J, H, or K. These factors limit the utility of the observations, in their present form, for a systematic study of cluster structure, and argue for observation at the longer wavelengths covered by IRAC and MIPS. Nonetheless our compilation provides an excellent source list for a systematic imaging study of cluster spatial structure, and it is on this basis that we plan to image 50 infrared clusters within 1 kpc, with IRAC and MIPS on SIRTF.
The IRAC observations will be sensitive enough to reach 5-sigma sensitivity to the flux density corresponding to luminosity 0.01 Lsun at typical source distance, integrating for 60 s at 8 microns, 20 s at 6 microns, and allowing 20 s overhead. For each of the 50 targets we will make a 3 x 3 mosaic, to image a 15 x 15 arcmin field, giving substantial on-source and background coverage. We assume equal time between the IRAC and MIPS surveys, for a total of about 25 hours time.
We plan to analyze the spatial structure of the imaged clusters using surface density imaging algorithms similar to those described by Gladwin et al (1998) and developed independently by Christopher et al (1998). These provide contour maps of local surface density of stars with much greater sensitivity than is provided by simply counting the number of stars in a grid cell of fixed size. We will use these to systematically measure the density structure of young clusters, in relation to their total size, age, and incidence of very young members. We will identify those clusters whose surface density maps have single maxima, multiple maxima, and the typical sizes and separations of these features. These structural aspects of young clusters will provide strong constraints on models of cluster formation.
An important benefit of this survey, and a strong reason to use both IRAC and MIPS, is the detailed spectral energy distribution of the YSOs resolved by both instruments. This distribution will help to identify the contributions from circumstellar disk and envelope from each source and will help to deduce the spatial structure of each cluster as a function of its apparent age. Thus it should be possible to deduce whether the youngest stars in the clusters have systematically formed in a well-defined portion of their cluster, or whether they have a more extended distribution.
The foregoing study selects known clusters but does not address the question of how concentrated each cluster is, in relation to the stellar content in its surrounding molecular cloud complex. At present, our picture of clustering is somewhat bimodal in that star-forming regions are considered "clustered" or "isolated", whereas in reality there is a wide range of spatial concentrations of young stars within a complex. The Oph cloud core has a rich cluster, but the Oph streamers are sites of isolated star formation (e.g. Wilking 1991). The young stars in Taurus are themselves organized into groups, although their space densities are too low to qualify as clusters by most definitions (Gomez et al 1993). With the regional studies by ISO (Nordh et al 1998) and with the forthcoming wider-field studies by the WIRE satellite, we are building up a much more complete census of the stellar content of the nearest star-forming regions at mid- infrared wavelengths. Our group (P. Myers, PI) has a program of WIRE observations to image all of the molecular gas within 350 pc that is visible to WIRE at 12 and 25 microns wavelength, with ~ 100 times the sensitivity of IRAS, to catalog the stellar content and structure of the nearest regions. We expect to build on this list with our IRAC program by taking advantage of IRAC's sensitivity to mid-infrared emission and its much finer angular resolution (~1 arcsec) than that of WIRE (~ 20 arcsec). We expect that in many instances we will find multiple IRAC sources associated with single WIRE sources, much as groups and clusters of near-infrared sources are frequently associated with single IRAS sources. These studies of spatial concentration in extended star forming complexes are essential for comparison of regions of strong and weak clustering with the turbulence and density structure of molecular cloud maps of the same regions.
To study this spatial concentration of stars in regions which do and do not contain clusters, we plan a second study of clustering, targeting not the nearest known clusters, but the nearest star forming regions, some of which are known to have clusters. Accordingly we plan to image the following complexes, in units of 30 x 30 arcmin fields, to match our corresponding WIRE fields of view:
Complex Number of 30 x 30 arcmin fields Cluster
Taurus 3 no
Ophiuchus 3 yes
Corona Australis 1 yes
Chamaeleon I 1 yes
Lupus I 1 no
Serpens 1 yes
Perseus 3 yes
With equal time for the IRAC and MIPS components, as in the cluster imaging section, we estimate 26 hours observing time.
The total time for the two projects is thus 25 + 26 = 51 hours, divided equally between IRAC and MIPS time.
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Whitworth, A. P., Bhattal, A. S., Francis, N., and Watkins, S. J. 1996, MNRAS,
283, 1061
Wilking, B. A. 1991, in Low Mass Star Formation in Southern Molecular Clouds,
ed. B. Reipurth (Garching: ESO Scientific Report No. 11), 159
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Here are some rough notes that I threw together after my return from the ISO conference in Paris. While I was there I noticed that, although there were reported to be lots of ISO measurements of pre-stellar cores, there seemed to me to be no measurements of their FIR emission in the Wien part of the spectrum, i.e. lambda < 100 microns. In fact, one abstract states that none of them was detected at 90 microns! In addition, I believe that there have been no systematic maps made of the temperatures of pre-stellar or proto-stellar cores. Generally they have only been observed in at most two wavelength bands. This has implications for deriving masses and/or densities from FIR and SMM measurements. Since most of these observations have not been published, and the abstracts are not very detailed, I am retreating to the distant past to discuss some proposed observations in terms of my graduate student research.
The first survey of the thermal dust emission from nine Bok Globules (now known as pre-stellar cores) was conducted by Keene (1981). These observations were made laboriously and with great difficulty utilizing the facilities then available - i.e., the KAO and the IRTF. At that time I showed that the intensity of FIR emission from these objects was, in general, comparable to the heating from the Interstellar Radiation Field (ISRF).
Since that time, of course, there has been an enormous advance in the facilities available. Further investigations by Keene et al. (1983), still using the KAO, and by Beichman et al. (1986), using IRAS, have shown that at least three of these nine cores contain proto-stars or young stellar objects (YSOs). IRAS, of course, has enormously simplified the task of selecting pre-stellar cores (Class -I sources) from those containing YSOs.
In addition, pioneering work on finding the cold dust component of pre-stellar cores with IRAS data has been done by Laureijs, Clark, & Prusti (1991) and more recently with ISO data by Laureijs et al. (1996). In this latter work, Laureijs et al have finally, unambiguously seen the FIR limb brightening of dark clouds heated only by the ISRF that was predicted long ago by Spencer & Leung (1978) and Leung, O'Brien, & Dubisch (1998) from radiative transfer calculations.
This points out a problem with the work that has been done on pre-stellar cores with JCMT and ISO. Observations at SMM wavelengths at JCMT (Ward-Thompson et al 1994) and in the FIR with ISO as reported at the meeting by P. André indicate that cores containing Class 0 YSOs have steep interior density gradients but that Class -I sources tend to have flat density profile cores. The ISOPHOT survey reported by P. André was made at 90, 170, and 200 microns of pre-stellar (Class -I) and proto-stellar (Class 0) cores. Oddly, the cores in this survey were not detected at 90 microns (which makes me think that they must have been embedded within a much larger cloud, such as rho Oph). Unfortunately, at SMM wavelengths or with FIR detections at only two wavelengths you can't get a very accurate temperature measurement. Class 0 sources should have local temperature maxima at the source locations while the Class -I sources should have local temperature minima at their centers. Thus, even if they both had the same density profile, the observed FIR profiles of Class -I sources would be much flatter because of the difference in central temperatures.
I propose that we use the SED mode and 160 micron super-resolution to map the temperature profiles of a sample of Class 0 and Class -I sources. Combined with long-wavelength continuum observations from the ground at 350 microns to 1.3 mm wavelength from the CSO, JCMT, and/or IRAM these temperature profiles will enable us to assess much more accurately the density profiles of the cores and thus the process of evolution from one stage to the next.
Most of the ISO results have not yet been published so I am taking the predicted flux densities from my own work (Keene 1981, Keene et al 1983).
B133 is the first Class -I source to have its SED measured in the FIR, and it is about 50 Jy in a 2.2' beam at the peak of its emission, ~250 microns (Keene 1981). At the longest wavelength accessible from SIRTF (160 microns), it is about 40 Jy. If the surface brightness is uniform (as it would be for a flat density core) the brightness within the 45" MIPS beam would be about 5 Jy, or about 2 Jy per pixel. This will obviously be an easy detection, since the 1 sigma sensitivity in 20 sec on source time is expected to be about 5 mJy. We can make a high S/N map at 160 microns with just one exposure per pixel. However, to compare optimally with the SED mode observations we should do a super-resolution (SR) observation at 160 microns in case we actually see any sharp features - as we would expect to in Class 0 sources. This will entail approximately 40 integrations to cover a field of view of about 5 sq arcmin and take about 30 minutes of time.
I did not detect B133 at wavelengths shorter than 190 microns, but an extrapolation of a 13 K blackbody multiplied with an emission efficiency proportional to nu^2 (which fits the long-wavelength spectrum) passes through about 10 Jy at 100 microns. The IRAS PSC gives an upper limit of 14 Jy at 100 microns and a measurement of 0.9 Jy at 60 microns. If we assume that the sensitivity of the SED mode is approximately 20 times the photometry sensitivity at 70 microns, then it will be approximately 22 mJy in 20 sec on-source time. So the S/N will be about 36 at 60 microns and 450 at 100 microns in 20 sec on-source time in SED mode. We will need to map the source so will need to take integrations at 32 positions at least. Probably 64 would be more satisfactory. So the total SED mode map would take approximately 45 minutes.
B335 is the first Class 0 to have had its SED measured in the FIR. In a large beam (1.7') its flux density is only slightly larger than that of B133 (Keene 1981), however, with a small beam it becomes obvious that there is a small source at the center (Keene et al 1983). Measurements in a 90" beam at 180 microns showed that the source flux is approximately 80 Jy. If this is a point source then there will be at least 10 Jy in the central pixel. If it is an extended source there could be as little as 2.5 Jy per pixel. In any case, we will have to use very short integrations since we will be near saturation on the detectors. Three sec exposures on source would give us approximately 15 mJy sensitivity at 160 microns. The sensitivity would presumably be worse if we have to use high-dynamic-range mode. Even so, the S/N will be very high with only about 1/3 the integration time as for a Class -I source, i.e., a total of about 10 minutes to map a 5' square region.
The measured flux at 60 microns is 7 Jy in a 33" beam and at 110 microns is 35 Jy in a 42" beam (Keene et al 1983). Given the same integration time as for the SED mode map on a Class -I source, the S/N will be very high. Again, it might be better to use short integrations (3 sec) and thus have a total observation time of 15 minutes.
In summary, to map an isolated Class -I core of size <5' it will take only of order 1 - 1.5 hours, and to map a Class 0 source it will take about 0.5 hours. In addition, we may consider using the 25 micron array and IRAC to get information on any embedded YSOs.
A catalog of isolated globules exists (Clemens & Barvainis 1988), and there should be no difficulty in finding plenty of candidates for observations.
André, P. 1998, in Observing the Universe with ISO
Beichman, C. et al. 1986, ApJ, 307, 337
Clemens, D., & Barvainis, R. 1988, ApJS, 68, 257
Keene, J. 1981, ApJ, 245, 115
Keene, J. et al. 1983, ApJ, 247, L43
Laureijs, R. C., Clark, F. O., & Prusti, T. 1991, ApJ, 372, 185
Laureijs, R.J. et al 1996, A&A, 315, L317
Leung, C. M., O'Brien, E. V., & Dubisch, R. 1989, 337, 293
Spencer, R. G. & Leung, C. M. 1978, ApJ, 222, 140
Ward-Thompson, D., Scott, P.F., Hills, R.E., and André, P. 1994,
MNRAS, 268, 276
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Following the excellent review on cold dust given by R. Laureijs at the ISO conference last week, I will try put in perspective what results ISO has on cold dust in galaxies. MIPS in combination with a ground based mm study and existing HI observations should make some progress on the distribution and state of cold gas and dust in nearby galaxies.
The cold gas content is one of the fundamental parameters of a galaxy because it consists of a reservoir for future star formation. One way to estimate this cold gas mass is by observing the optically thin emission from dust at 0.1 - 1 mm or 100 - 1000 microns. The detected flux and the total dust mass are directly proportional, depending only on the distance to the galaxy, the absorption at the wavelength observed, and the Planck function. Because dust emissivity is strongly dependent on temperature, moderate changes in temperature result in large changes in the energy. An estimate of the dust mass therefore requires an accurate measurement of the dust temperature. To study the coldest material (T<30K), deep images at a range of wavelengths across the region 60-1000 microns are required.
Dust in equilibrium with the interstellar radiation field in the Milky Way achieves a temperature of ~ 18 K. However dense clouds could have lower temperatures if they are clumpy and self shielded. Changes in the chemical composition and shape of the dust grains (Fogel, M. E. and Leung C. M., 1998, ApJ, 501, 175) can also result in a lower temperature equilibrium state. The spatial distribution of the emission from the cold material provides a valuable clue as to the mass and temperature distribution. Far infrared mapping with higher angular resolution observations than possible with IRAS, ISO or COBE is required to assess this distribution. MIPS imaging at 70 and 160 microns in comparison with longer wavelength mm maps should both address the spatial distribution at the same time as the temperature distribution. This should then allow us to determine accurately the cold dust mass as well as its physical state.
Recent observations by ISOPHOT at 175/200 microns of galaxies are memorable (probably because they were shown too many times at the ISO conference). Haas et al. (1998, A&A, 338, L33) presented a 175 micron image of M31 which has identical morphology to the HI atomic hydrogen map of Brinks, E. & Shane, W.W. (1984, A&AS, 55, 179). I can't resist suggesting that Brinks's interpretation in terms of a clumpy flaring disk in the geometry of a warp (also based on a study of the HI velocity channel maps) might be more profound than Haas's 10 and 14kpc double rings. The ISOPHOT SED study of Krugel, E., Seibenmorgen, R., Zota, V., & Chini, R. (1998, A&A, 331, L9) of 3 normal galaxies suggests that cold dust masses are large (a few times 10^8 Msol) and a few times larger than previously estimated with IRAS. A better measurement of the spectral energy distribution is really needed to confirm this.
Observation of the spatial distribution in a few more galaxies (e.g. M51 and M83 as well as M31) in comparison to HI maps would determine if a relatively massive cold dusty component is commonly associated with the extended HI distribution typically found in the outer parts of galaxies. Since the outer parts of galaxies are reduced in metallicity, these observations might provide interesting constraints on metallicity evolution in galaxies. Mapping of the density and temperature distribution of this cold dust component should determine the importance of self shielding. It may be that the decreasing flux of the interstellar radiation field as a function of distance from the center of the galaxy naturally results in a decrease in dust temperature with radius.
In M31, Haas et al (1998) measure at 175microns the outer most `ring'
is 0.1 MJy/sr, which converts to 0.5 mJy per 15"x15" pixel.
M31 covers 50' or so, so this would be a good project for a quick scan
(e.g. 20s moderate scale scan map mode). We expect that large scale
scan maps of some nearby galaxies (e.g. M31, M51, M83, M101) would
lend themselves to a variety of other studies. For example (sticking
to the topic of cold dust), dark cloud cores detected could be compared
to those in our galaxy (e.g. Laureijs et al 1991), which can reach
temperatures as low as 12-15 K in the absense of any central star
or core, and might or might not be sites of future star formation.
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The papers presented at this conference were rather mixed in quality, many being little more than progress reports on data reductions and interpretation. However, there are a number of results of substantial interest in planning the MIPS/IRAC GTO program. Unavoidably, I emphasize the MIPS aspects of the program below, but with help from other attendees at the meeting I hope we can expand this description into a broader one.
ISO has done very well in surveying at 7 and 15 microns, and at 175 microns. The 7 micron survey is heavily contaminated by stars and is probably not very useful by itself for cosmological studies. Rowan-Robinson described a large project -- ELAIS -- that is being conducted by 68 investigators based on surveys as follows:
wavelength 15um 90um 175um 850um 21cm instrument ISO ISO ISO SCUBA VLA detect. Limit 3mJy 50 75 8 0.1-0.4 sq. deg. 11 12 3 0.02 8.2
Followup is planned with AXAF, lots of groundbased telescopes, and radio telescopes in Australia for southern hemisphere coverage. Puget and Kawara described a number of 175 micron surveys, many of which appear to be included in the above compilation and have similar detection limits. Kawara also discussed a 15um survey with a "limit" of .02mJy, but the limit definition was not specified and I had the impression it was not 5-sigma as for the other limits I have quoted. Elbaz discussed other deep surveys at 15 microns, with detection limits and areas on the sky as follows:
"SHALLOW" 0.8mJy 0.41 sq deg "DEEP" 0.5mJy 0.3 sq deg "HDF/ultradeep" 0.1mJy 0.013 sq deg
If MIPS works as well as we hope, it would reach a detection limit of about 0.2mJy in 1 sq degree in 72 hours (plus overheads) at 24 microns. Given the typical spectral energy distributions of galaxies in deep surveys, this survey would be somewhat deeper than the deepest ISO surveys at 15um (over two orders of magnitude greater area). In the same 72 hours, MIPS should reach a detection limit of about 0.7mJy at 70 microns and a confusion limited detection level of about 15 mJy at 160 microns over a square degree. The IRAC survey bands were not covered in any coherent way with ISO, so deep IRAC surveys have a totally clear playing field.
From these comparisons, it is clear that MIPS can improve on these surveys, but also that the objectives for the MIPS surveys have to be ambitious if they are to have scientific impact. One big advantage for MIPS is the ability to cover a significant area to great depth. The deep ISO 15um surveys cover the equivalent of about an 8x8 arcmin area, at most (i.e., about two MIPS fields of view), and the 90um surveys do not reach very interesting detection limits. Thus, the surveys proposed for MIPS under the quasar SED program white paper remain a unique contribution -- only MIPS has the ability to cover enough sky to make identifications of significant numbers of faint xray sources and characterize their SEDs.
For the study of galaxy evolution, I propose that we set a goal of fully resolving the infrared background. At 160um, where we know what the total background is from COBE, it appears this goal is within reach. The ISO 175um deep surveys appear to resolve about 10% of the background into sources (although there are still some calibration uncertainties that could affect this number). At these wavelengths, confusion is the sensitivity limit, not photon noise. Nominally, our beam should be 2 times smaller in area than the ISO one because of the large telescope aperture. In addition we oversample the Airy disk, whereas they undersampled it, and the net beam size for us with optimal source extraction could be as much as 10 times smaller. Thus, we can expect to probe much deeper into the confusion. Their source counts at 100mJy are about 60/square degree, so one square degree is about the minimum area we need to survey to get good statistics (we would detect sources roughly inversely proportional to beam area, so we might find of order 500 in a square degree).
The accompanying MIPS survey of one degree at 24um would only reach slightly deeper than the available ISO ones of smaller areas, assuming we dedicate 72 hours to the area (determined by requirements for 160um). It would still have a significant advantage. The average redshift of the objects found at 15um is z ~ 0.7. However, at z = 0.9, the rest frame wavelength of their band is at the 7.7um PAH feature -- that is, the powerful infrared continuum emission of starburst galaxies is redshifted out of their detection range just at the redshifts where they stop seeing galaxies! Thus, a 24um survey would presumably have many new galaxies up to z ~ 1.5, where it would have the same problem as the ISO 15um surveys: starburst SEDs will be redshifted out of our spectral band. From the ISO data, it was argued that most star formation out to z = 0.7 was hidden in dust. Characterizing the rate of star formation to z = 1.5 is of substantial interest, since the same situation is likely to hold and z = 1.5 is close to the peak of star formation as deduced by other means. However, the comparison with the ISO results also suggests that we should consider a smaller area, very deep 24um survey if we are to be sure to resolve the infrared background. Confusion is not really a strong issue at this wavelength, and we can integrate as long as we can afford. ISO shows that about two MIPS FOVs is enough to yield very interesting results, and I would suggest we do something like one ultradeep field on each of HDF north and south. If we devoted 10 hours to each of these fields, we should achieve 5-sigma detection limits of about 0.05mJy. Allowing for the typical SEDs of starburst galaxies, this level is 5-10 times deeper than the deepest ISO surveys, so it probes a significant new region of phase space.
The 70um part of the one degree survey would already be close to the confusion limit and it is not obvious that deeper surveying is worthwhile. The same argument applies at 160um.
To see if we can demonstrate that we have resolved the full infrared background, we need to evaluate what observations can be obtained in total power mode. Our problem is that the background at 24 and 70um will be dominated by the zodiacal light, and at some level we will have the same problem that the COBE team had in subtracting it off. At another level, though, we have the advantage that we can subtract off the resolved galaxies and stars, and any residual background will have a very different spectral energy distribution than the zodiacal light. I propose we evaluate using the MIPS SED mode in total power (this is not a sanctioned operating mode, so we would need to think about how to implement it). Together with the imaging modes, we would get a good measure of the spectrum of the background. We also need to consider total power measurements at selected fields (low cirrus) at different zodiacal latitudes. We need to evaluate whether such a program has a reasonable chance of separating zodiacal and cosmic backgrounds.
With continuing work on the ISOPHOT reduction software, it is now possible to extract measurements to significantly fainter levels than the IRAS survey, i.e., to a real one standard deviation level of 10 mJy at 90 microns. As a result, the debris disk search programs are leading to new detections. Extensive programs of this type were presented by Becklin et al., Dominik, and Robberto.
The first of these programs, a U.S. key project, targeted three samples: 1.) 39 F and G type stars that show IRAS 60um excesses, which were measured at other FIR wavelengths; 2.) measurements in nearby open clusters to study the evolution of debris disks (clusters observed are alpha Per, Coma Berenices, Hyades, Pleiades, and U Maj, covering roughly 50 to 700 Myrs in age); and 3.) 41 nearby T Tau stars with IRAS detections at 60um, observed at other bands to obtain more information on their SEDS (as in objective 1.). The results were presented in terms of average disk luminosity, which strongly emphasizes the most luminous members of the samples and hence constitutes a significant selection effect that might influence the results (but with ISOPHOT's inability to measure faint debris disks, there is little choice). The average luminosity of the disks fades by a modest factor for the first few 10s of million years, then stabilizes out to the oldest members of the sample.
C. Dominik described a volume limited sample, stars with distance < 25 pc. They had excluded O and B stars because of the possibility of free-free excesses, and M stars because of the poor atmospheric modeling. They also excluded variables, close binaries, and all stars with expected photospheric emission at 60um < 50 mJy. The sample has a total of 91 members. Excesses were found (> 2 standard deviation significance at 60um) in 19 cases, with possible excesses in 32 more. Thus, 20-50% of the sample show excess emission in the FIR. They obtained age estimates by various means for nearly all of the sample. They claim that the average brightness of the excesses faded with a timescale of about 400 Myr, but the statistics were too poor for this result to be given much weight.
Lemke also mentioned a small ISOPHOT program that led to detections of 1 excess (beta Uma) among 9 A stars in the relatively young UMa association,
M. Robberto reported on the Beckwith/Meyer et al. study of debris disks around YSOs. They used ISOPHOT photometry in Chameleon, IC2692, alpha Per, Pleiades, and NGC 7092 to study the disk decay with age. There appears to be a steady drop in disk emission with age, with virtually no detections for > 10MYr (a result that is not readily reconciled with the debris disk incidence in the other programs). A total of about 80 sources were observed, all with ISOPHOT.
Olofsson and Prusti described the results of surveys of nearby star forming regions at 7 and 15 microns, conceptually similar to a possible IRAC/MIPS survey. Prusti showed that the sources divided into two well-differentiated classes in the color magnitude diagram involving these colors; the 15 micron band is long enough in wavelength to single out excess sources clearly compared with heavily obscured sources with no excess or only weak excesses. He demonstrated that the ISO imaging could identify many new members of embedded clusters by virtue of their excesses, often doubling the known membership. The emphasis in these works was to determine the IMF, using the sample of sources with strong excesses and calibrating the source luminosity on the basis of 15 micron excess luminosity and a comparison with IRAS. It was shown that the sensitivity limits in nearby star forming regions (e.g., Chamaeleon) reach the bottom of the stellar range, 0.1 M_sun. There are many problems with this particular approach, and I believe the goals of this program have already been reached, or nearly reached, from the ground in the near infrared - nonetheless, the data are of interest as a benchmark for our plans.
An interesting series of papers discussed low resolution spectroscopy of debris disks. Waelkens presented a review. Spectra were possible only of the brightest systems -- Ae/Be stars, beta Pic, etc. However, they show many mineralogical features. A useful series lies from 16 to 40 microns and is associated with silicates. A pair of features at 33 and 34 microns can be used for mineralogy: the former is bright in pyroxenes and the latter in olivines (Molster, Kemper, Cami, cited by Waters). In beta Pic, the structure associated with crystalline silicates in the 10um region is confirmed (Lagage). The conclusion from the ISO studies is that the material in the debris disks has mineralogy closely resembling that of comets, rather than the more amorphous silicates (with smooth features without the crystalline bumps and wiggles) found in the ISM. This resemblance is primarily based on ISO observations of Comet Hale Bopp (Malfait et al. A&A 1998).
In addition to the silicate features, Waelkens discussed a number of Ae/Be stars that show prominent water ice features. There is a relatively sharp feature near 40um, falling between the spectral ranges covered by SIRTF, and the broad 63um bump covered by the MIPS SED mode is quite dramatic in, for example, HD 100546, HD179218, and HD142527. The latter spectrum also shows a broad feature from 80 to 150um attributed to hydrous silicate, that we would have trouble characterizing with SIRTF. Other similar stars were roughly similar in properties, but the S/N quickly got inadequate for detailed comparisons beyond a half dozen or so examples.
Waelkens' conclusion was that there is interesting evidence for variations in the spectral behavior of debris disks, but that they do not appear to be correlated with age alone. He also indicated that the youngest objects (e.g., Class I) appear not to have crystalline silicate features (and generally weak features of any kind).
A number of young debris disks have detected molecular hydrogen at 17 and 28 microns (GG Tau, HD 163296 for example, Thi et al.). The H_2 can be used to constrain the temperature structure of the gaseous portion of the disks (discussed in a review by van Dishoeck).
My impressions from this material are as follows. We need to develop samples carefully if we are to get all the information possible on the debris disk phenomenon. For example, we could select an A star sample. These stars have the virtues that they are luminous enough that we can measure their SEDs very well even out to the distances of the nearest star forming regions, that they are hot enough to reveal other aspects of disks through, for example, CO emission at 2.3um, that they do not have free-free excesses that could be confused with disks, and that they live long enough to reveal debris disk evolution effects. Another sample might be the nearest solar-like (G&K) stars to search for debris systems down to the level of the Kuiper belt, a unique opportunity with SIRTF. We might also construct a sample comparing stars with and without evidence for planetary systems judged from Doppler recoil measurements. In studying these systems, we should be sure to include low resolution spectroscopy to characterize their mineralogy.
It is still difficult to get an overall impression of the return from Quasar SED studies with ISO. This type of program depends on good performance in the 30-150um ISOPHOT bands, which is where the largest problems persist in data reductions. There were many quasar programs with ISO (Wilkes et al., US key program, an ISOPHOT key program, Chini et al. radio loud quasars, Bemmel and Barthel, 4 matched pairs of radio loud quasars and radio galaxies to test unification). Nonetheless, it is not clear whether ISO will have an adequate sensitivity gain over IRAS to reach qualitatively new conclusions, although the availability of a band longer than 100um is definitely helpful in tying the SEDs to groundbased measurements in the submm and from there into the radio regime.
B. Wilkes reviewed this work, stating three major conclusions: 1.) the accretion torus around radio loud quasar AGNs is optically thick to beyond 200um; 2.) the far infrared and submm continuum is strongly dominated by warm dust in radio quiet quasars, dominated by nonthermal emission in core dominated quasars and particularly in flat spectrum ones, and is a mixture of the two in lobe dominated quasars; and 3.) a number of core dominated quasars show variability compared with the IRAS data. None of these conclusions is completely new.
Having said that only modest progress has been made in this area, what is left to do? As pointed out in 1.), a deep survey for xray bright AGNs is an interesting program. Radio galaxy detections are poorly represented in both IRAS and ISO samples - they are sufficiently fainter than quasars to fall below the achieved limits as a group. Variability studies are useful even of bright quasars, connecting IRAS to ISO to SIRTF. Specific types of quasars, such as BALQSOs and optically quiet objects are not well represented, or not represented at all in the ISO far infrared data because the sensitivity reaches only to the brightest objects. Much of the program in our current white paper therefore appears still to be highly suitable.
One program that will need careful thinking-through is the testing of the unification of radio loud quasars and radio galaxies. The ISO results confirm the conclusions from IRAS that the radio galaxies are less luminous than the quasars (van Bemmel and Barthel, Haas et al. 1998, ApJL, 503, L109). The suggestion that the difference could arise from a significant beamed nonthermal component in the RLQSOs is not supported by the ISO measurements of SEDs extending to the 200um region, which show a clearly defined thermal SED bump in the far infrared. Thus, the test originally devised for the unification appears to contradict it. However, as is often the case with unification models, once this result has been demonstrated, the model is modified - in this case to argue that the accretion disks must be optically thick through 200um so that they emit asymmetrically (van Bemmel and Barthel). A test of this argument requires a carefully selected sample that includes the "missing link" between radio galaxy and core dominated quasar, e.g., broad line radio galaxies, for which the disk must be approximately face on according to the proposed unification. A robust prediction of the unification is that these transitional active galaxies should have far infrared properties similar to the core dominated quasars.
If the accretion disks are truly optically thick in the far infrared, it suggests another class of experiment. It is pretty well demonstrated both through VLA data and a comparison of groundbased small aperture mid infrared measurements with IRAS ones that ULIRGs produce as a class significant luminosity from outside their nuclei in the far infrared. If quasars are a following evolutionary state to these objects, we expect that the properties of the quasar host galaxies should have specific properties, including a residual strong far infrared output. Thus, measurements of the radio galaxies should show a sensible progression from the star formation state in their ULIRG predecessors.
There was relatively little new information on galaxy clusters. Only a few clusters have been studied at all at high redshift, and the ones that have are only observed with ISOCAM. It was shown that the infrared bright galaxies (15um) tend to lie around the periphery of the cluster for a few cases - this behavior is similar to the behavior of nearby clusters studied previously in the radio, Halpha, and with IRAS. Metcalfe described an ambitious program. A small number of clusters known to have gravitational potentials favorable for lensing were imaged very deeply, and followup groundbased spectra show that many of the detected galaxies are background (z up to ~ 1). It is argued that on average the galaxies should be lensed to increase their apparent brightness by factors of 2-3, so the data were used statistically to try to constrain infrared galaxy counts out to z ~ 1. The results are consistent with those obtained from direct deep fields. Although a clever way to increase the effective telescope aperture, there was considerable skepticism about the ability to interpret the results quantitatively. It will be interesting to watch how these observations and their interpretation play out.
In addition to Coma (see white paper on cluster observations), a number of clusters have been observed with ISOPHOT to look for intergalaxy heated dust. The results are not yet reduced (Stickel, private communication). Given the relatively poor performance of the ISOPHOT bands from 60-140 microns, I suspect that this issue will not be settled definitively without SIRTF observations that fully map the clusters down to faint limits. In this way, the contributions of the individual galaxies can be measured rather than deduced indirectly as is being done to interpret the ISOPHOT work.
Helou reviewed the ISO results on normal galaxies. The overall far infrared luminosity is roughly independent of morphological type. However, early type galaxies have strong nuclear emission, and if that component is removed the portion of luminosity in the FIR shows an increasing trend with later type as expected from optical studies underlying the type classification.
The ISO low resolution spectra show that PAH emission is virtually universal among infrared galaxies. Vigroux showed this with a color-color plot; however, dwarf galaxies stand out as potentially having very weak PAH bands. This behavior is confirmed by ISOCAM CVF spectra of the very low metallicity dwarf SBS 0335-052, 2.5% solar metallicity, which shows very strong infrared continuum but no PAH at all (Thuan, Sauvage, and Madden). Madden et al. reported similar data on the low, but not so low as SBS 0335, metallicity galaxies IIZw40, NGC 1569, and NGC 1140. IIZw40 is similar to SBS 0335, NGC 1140 has PAHs at "normal" strength, and NGC 1569 is intermediate. I do not know of ISO measurements of these galaxies in the far infrared, although some are available from the IRAS data. These results indicate the importance of understanding this type of galaxy to help interpret deep surveys, where many of the detected galaxies are expected also to be low in metals.
As a reward(?) for anyone who has read this far, we discuss a report by Valentijn on NGC 891. Measurements of the 17 and 28 micron H_2 lines along the disk of this edge-on galaxy indicate that the 17um line has a plausible mapping into other indicators of interstellar gas, but the 28um line appears to have much greater extent. This result could indicate a very cold component of molecular hydrogen (the 28um line comes from the lowest lying level), which could account for a very large mass - potentially enough to account for a significant portion of the dark matter! Higher signal to noise spectra and a better sampling of the plane of the galaxy are required to confirm the claim.
Another outstanding ISO result is the detailed study of far
infrared fine structure lines, using high resolution in LWS.
Since these measurements cannot be extended or even repeated with
SIRTF, we will not try to summarize them.
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