Using ALMA to discover the obscured high-redshift Universe

Using the Atacama Large Millimeter Array (ALMA) I serendipitiously discovered an infrared-dark "ALMA-only" galaxy that is so dust obscured, it was only detectable through its dust emission with deep ALMA observations. Our survey was small, indicating these galaxies could be relatively common, comprising a yet-undiscovered population of massive star-forming galaxies at z > 5. Given how small our survey was, this population is likely more abundant than the rare and extreme sub-millimeter galaxies, which are probably just the tip of the iceberg. This population could explain the origin of the the highest redshift quiescent galaxies ever discovered, at 3 < z < 4, and indicate that galaxies can grow extremely rapidly in the early Universe. You can learn more about this missing population, and our infrared-dark galaxy in this publication: Williams et al., 2019, ApJ Vol. 884
This paper also got some press coverage, you can read more in articles in CNN , Forbes , Discover , Newsweek, Smithsonian, and Boston Globe

JWST Imaging and Spectroscopy

I am part of the JWST NIRCam/NIRSpec joint GTO team, and in JWST Cycle 1 we will make some of the deepest observations of the early Universe. Our planned survey, called the JWST Advanced Deep Extragalactic Survey (JADES), will probe down to ~29-30 (AB magnitude) from 1-5 microns using NIRCam, and will obtain deep spectroscopy for galaxies down to ~27 ABmag with NIRSpec. JADES will be the deepest near-infrared imaging and spectroscopy ever achieved. Our survey is ambitious, and new JWST instruments that offer unprecedented sensitivity and unique observing modes will require careful testing, simulation, and analysis of systematics.

To design the best possible survey and analysis tools, our team developed a novel phenomenological model that reproduces observed distributions of galaxy properties out to redshift~10, enabling JWST predictions based on real observations. Our model is the foundation of the JAdes extraGalactic Ultra-deep Artifical Realizations (JAGUAR), a software and data package that includes mock extragalactic catalogs that feature both photometry and spectra (Python code to produce your own JWST realizations is coming soon). JAGUAR is a critical component of the JADES data challenges that will prepare us for this groundbreaking survey. The banner photo features simulated JWST images of our deep survey using JAGUAR and the NIRCam simulation tool developed by Christopher Willmer. The simulated JWST images are also available for download so you can explore whats in store once JWST launches! Details of the phenomenological model that we developed for JAGUAR, as well as some science predictions for JADES, can be found in this publication: Williams et al., 2018, ApJS Vol. 236

The formation of massive and quenched galaxies

One of the major open questions in galaxy evolution is why massive galaxies quench star-formation. This process affects the most massive galaxies very early in the Universe, and gives rise to the well-known color-morphology bimodality among galaxies known as the Hubble Sequence. A surprising finding from studying early quenched galaxies with Hubble was that the first quenched galaxies are remarkably compact compared to their local counterparts. These compact, massive and quenched early galaxies challenge our theoretical models of galaxy formation and pose two key questions that are the focus of my research interests: First, how do such massive, compact galaxies form? Second, why do massive and compact galaxies quench star-formation?

Understanding the detailed structure and stellar populations of quenched galaxies at high-redshift is critical to reconstructing their history. My work investigating the relationship between stellar populations and compactness of recently quenched galaxies showed that the densest galaxies are also the oldest, favoring evolutionary models where denser galaxies formed and evolved earlier in the Universe's history than larger galaxies, and disfavoring models where quenching is triggered by violent transformational events such as gas-rich mergers. (Williams et al, 2017) However, why star-formation stops in massive galaxies remains an open question. My work has also identified hot gas in and around early quenched galaxies, 10 billion years ago, which may prevent new gas from accreting onto galaxies. Further investigation is needed to understand the source of the hot gas, and any impact it may have on truncating star-formation.

Compact star-forming galaxies

One of the best opportunities to study quenching is to catch star-forming galaxies "in the act." A major challenge with this method is correctly identifying galaxies likely to quench. My work shows that compact star-forming galaxies with high surface densities of star-formation are the likely predecessors of compact quenched galaxies, enabling them to be targeted to study quenching in detail. This work also showed these compact galaxies likely did not form by gas-rich mergers: their morphologies are inconsistent with the theoretical expectation for merger remnants. More likely, they formed very early in the Universe when galaxies were typically smaller (Williams et al, 2014). This "early formation" evolutionary scenario for compact galaxies has since been identified in cosmological simulations (Wellons et al. 2015).

Compact star-forming galaxies have been followed up by myself and collaborators in order to look for signatures of quenching mechanisms such as stellar or AGN feedback. These works indicate compact galaxies experience stronger feedback than ordinary galaxies, possibly from the high density of stars ( Williams et al, 2015), or AGN (Kocevski et al, 2017). Using the VLA and more recently ALMA, my collaborators and I have measured short depletion times of molecular gas in compact star-forming galaxies, further evidence that they are likely about to quench star-formation (Spilker et al, 2016).

Submillimeter Galaxies

With extremely high star-formation rates, it is possible that sub-mm galaxies live in the most massive dark-matter halos in the Universe. If true, they could be the early signposts of galaxy cluster formation and evolve into the most massive quenched galaxies in the Universe. Their high dust obscuration makes it difficult to study their stars directly, so we must observe them with submillimeter telescopes, which have low resolution and map the sky very slowly. I worked with the Atacama Submillimeter Telescope Experiment (ASTE) to produce the (circa 2011) largest submillimeter map of an extragalactic field to date, detecting hundreds of submillimeter galaxies and enabling one of the first measurements of their clustering, which directly probes the masses of their dark matter halos. My work shows that submillimeter galaxies do not cluster as strongly as expected for the most massive halos, contrary to the idea that they will become the most massive elliptical galaxies in the Universe. This work also indicates that future ~50-meter-class single dish telescopes, such as the Large Millimeter Telescope, will be able to robustly measure the halo masses of submillimeter galaxies, so we can better understand their fate. Williams et al., 2011, ApJ Vol. 733

Past Research

    In collaboration with the Sea Ice group at the Geophysical Institute of the University of Alaska (lead by Dr. Hajo Eicken) I helped to develop a real-time sea ice montioring system in, also known as sea ice observatories, on the Arctic coast of Alaska. These monitoring systems are important because warming due to climate change occurs more rapidly in Arctic regions and results in the coastal, grounded (landfast) sea ice becoming less predictable. The landfast ice is a critical part of the coastal ecosystem and the subsistence resources for Native Alaskans, and unstable ice conditions put hunters and fishers in danger. The ice observatory helps assess the coastal ice conditions in real-time. Details of the ice observatory can be found in Druckenmiller et al, 2009. See real-time radar and images of current ice conditions off the Arctic coast of Alaska at the following links:
    Barrow Sea Ice Observatory
    Wales Sea Ice Observatory

    For my masters thesis at University of Alaska with Dr. Bernard Coakley, I studied the tectonic history of the Arctic Ocean, one of the most enigmatic and poorly understood ocean basins on the planet due to its remoteness and ice cover. I used gravity and bathymetry data collected from submarine transets under the Arctic sea ice to better understand how the Arctic basin formed.
    Arctic Ocean Bathmetry
    Scientific Ice EXpeditions

    As an undergrad, I was a Research Experience for Undergraduates (REU) researcher in planetary geophysics at the University of Alaska. I used numerical calculations with Dr. John Chappelow to study the latitudinal extent of the carbon dioxide ice cap on Mars, and how this has changed over Mars's history due to changes in its orbit (in particular the obliquity).
    UAF Planetary Science

    As an undergraduate at Johns Hopkins working with Drs. David Golimowski and Todd Henry, I made imaging and spectroscopic observations at Apache Point Observatory as part of the nearby and low-mass stars group. I worked to spectroscopically characterize L and T type dwarfs and candidate brown dwarfs. More info on the Research Consortium on Nearby Stars and our findings about dwarf stars can be found here: Knapp et al., 2004.