About me

I am a Professor of Extragalactic Astronomy at Leiden Observatory. My research group studies how galaxies, including our own Milky Way, came into existence. In order to unravel their formation histories, we examine masses, sizes, ages, stellar birth rates, stellar motions, chemical compositions, black hole signatures, and many more properties of galaxies over cosmic time. I started at Leiden Observatory in the summer of 2021. Previously, I was a Professor (Assistant, Associate, and Full) in the Astronomy Department at UC Berkeley (2012-2022), a Clay Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics (2010-2011), and an H. N. Russell Postdoctoral Fellow at Princeton University (2007-2010). I obtained my Ph.D. at Leiden Observatory in 2007, working with Professors Pieter van Dokkum at Yale and Marijn Franx at Leiden.

Group

Current group members:
  • Aliza Beverage, UCB PhD student (2025)
  • Brian Lorenz, UCB PhD student (2025)
  • Yipeng Lyu, Leiden MSc student (2022)
  • Shravya Shenoy, Leiden MSc student (2022)
  • Yilun Ma, UCB BA student (2022)
Former PhD students:
  • Katherine (Wren) Suess (UCB 2021), "The Growth & Transformation of Galaxies Across Cosmic Time"
    Stanford-Santa Cruz Cosmology Fellow / UC Chancellor's Postdoctoral Fellow
  • Sedona Price (UCB 2017), "Galaxies in the Young Universe: Structures, Masses, and Composition of Star-Forming Galaxies at z ∼ 1.5 − 3"
    Postdoctoral Scholar at MPE
  • Francesca Fornasini (UCB 2016, w/ J. Tomsick), "The Faint, the Poor, and the Steady: studies of low-luminosity, metal-poor, and non-pulsating populations of high-mass X-ray binaries"
    Assistant Professor at Stonehill College
  • Jesse van de Sande (Leiden 2014, w/ M. Franx), "The Dawn of the Red and Dead: Stellar Kinematics of Massive Quiescent Galaxies out to z=2"
    ARC DECRA Fellow at the University of Sydney
Former Fellows/Associates:
  • Guillermo Barro (UCB Postdoctoral Fellow), Assistant Professor at the University of the Pacific
  • Ryan Trainor (Miller Fellow), Assistant Professor at Franklin & Marshall College
Other former group members: Oscar Chavez (UCB BA); Justin Clarke (UCB BA, w/ R. Feldmann); Michael Gordon (Princeton BA, w/ J. Greene); Therese Jones (UCB MA); Jamie Lin (UCB BA); Meng Luo (UCB BA); Alejandro (AJ) Olvera (UCB BA); Imad Pasha (UCB BA); Willem de Pous (visiting PhD student); Dyas Utomo (UCB MA); Michael Yano (UCB BA); Kevin Yu (UCB BA); Tom Zick (UCB MA)

Research Interests

My research interests center on the formation and evolution of galaxies across cosmic time. I am pursuing several avenues, which range from detailed studies to understand the physical processes in galaxies, to large photometric and spectroscopic surveys to examine how galaxies evolve over time, to improving the tools and techniques used to study galaxies. Below are external links to my major research programs and publications, as well as research highlights of my group.

New pathways to studying galaxy growth

In the past decade, large and deep photometric and spectroscopic surveys have significantly advanced our understanding of galaxy growth, from the most active time in the universe (z=2−3) to the present day. My group has worked on several new approaches to push this field forward. First, we used half-mass (instead of half-light) radii combined with a clustering-based classification method, to understand how galaxies grow during the star-forming and quiescent phase. This work is featured in four papers by former Berkeley graduate student Wren Suess (Suess, Kriek, Price, & Barro 2019A, 2019b, 2020, 2021). Second, we presented a large comprehensive study of the kinematic evolution of star-forming galaxies over cosmic time (z~1.4-3.8). This work used a new approach based on randomly oriented slit observations from the MOSDEF survey and a comprehensive forward modeling technique based on HST imaging, and is featured in two papers by former Berkeley graduate student Sedona Price (Price et al. 2016, 2020). Over the coming years we will apply this new approach to distant quiescent galaxies in our Heavy Metal survey and JWST/NIRSpec program, to further unravel their kinematic evolution.

Chemical footprints

Chemical compositions of galaxies enable a unique view into their chemical enrichment, star formation and assembly histories. While metallicities of star-forming galaxies have been studied out to z~4 using bright emission lines, this work is probitively challenging for distant quiescent galaxies for which we rely on faint absorption lines shifted to near-IR wavelengths. Nonetheless, my group has significantly advanced this field over the past few years. To obtain a full evolutionary senses, we are using the public SDSS and LEGA-C surveys at lower redshifts (Beverage et al. 2021) and Keck Observatory (MOSFIRE & LRIS) at higher redshifts (Kriek et al. 2016, 2019). By studying the stellar ages and elemental abundances in relation to other galaxy poperties, such as mass and size, we are learning (i) when, how fast, and how efficient quiescent galaxies formed their star and enriched their gas (ii) why they stopped forming stars, and (iii) how they evolved after becoming quiescent. Over the coming years we will use the ultra-deep spectra from the Heavy Metal survey and from our JWST/NIRSpec program to fully exploit the use of metals for understanding the formation histories of quiescent galaxies.

Galaxy transformations

We have known for more than a century that galaxies come in two flavors: spiral galaxies with high stellar birth rates and more massive elliptical galaxies with quiescent stellar populations. Nonetheless, the processes by which star-forming disks transform into quiescent ellipticals are poorly understood. My group is tackling this problem using several approaches. First, we use massive post-starburst galaxies at z=0.5-1.0 as laboratories for understanding this transformation (SQuIGGLE; Suess et al. submitted) and showed that galaxies with recently suppressed star formation can still host large molecular gas reservoirs (Suess et al. 2017 ). Second, we studied the molecular gas and kinematic properties of compact star-forming galaxies, catching the phase just before the star formation is quenched (Barro et al. 2016; 2017). Finally, we unravelled the structural transformation during the transitional phase by combining stacked spectra from the MOSDEF survey or composite spectral energy distributions with structural properties of galaxies (Yano et al. 2014; Zick et al. 2018, Suess et al. 2021). Over the coming years, the Heavy Metal survey, our JWST/NIRSpec program, and new ALMA observations will further advance this field.

Toward more accurate measurements and model ingredients

Whereas studies of the distant galaxy population have grown tremendously in both size and depth over the past years, scientific progress has been limited by severe systematic uncertainties. Deriving physical properties from galaxy observations strongly relies on stellar population synthesis and dust models, as well as empirical calibrations. Such models and calibrations rely on many assumptions regarding stellar evolution, the initial mass function, stellar binary populations, chemical abundance patterns, dust geometry, etc. Over the past decade, my group has focused on improving our understanding of many of these ingredients, ranging from the thermally-pulsing AGB phase, the dust attenuation law, star formation rate indicators, to (X-ray)-binary populations (e.g., Kriek et al. 2010, 2013; Price et al. 2014; Utomo et al. 2014; van de Sande et al. 2015; Fornasini et al. 2019). Furthermore, using cosmological simulations we have assessed how well our current analysis methods can measure the sizes and masses of distant galaxies (Price et al. 2018).

Downloads

Teaching

  • Introduction to Research (Astro 290), UC Berkeley graduate class (2020)
  • Introduction to Research (Astro 198), UC Berkeley undergraduate class (2020)
  • Introduction to Astrophysics, part II (Astro 7B), UC Berkeley undergraduate class (2019-2021)
  • Introduction to Astrophysics, part I (Astro 7A), UC Berkeley undergraduate class (2013-2016)
  • Galaxies (Astro 218), UC Berkeley graduate class (2012-2019)
  • Math, Prison Teaching Initiative (2008-2010)

Acknowledgement

The research of my group has been funded by many agencies, including the NSF (AAG AST-1313171 and AST-1909942), NASA (ADAP NNX14AR86G and NNX16AF54G), STScI (GO-11135, GO-12177, AR-12847, GO-15436, AR-13907, and AR-16141), Chandra (AR6-17011X), NRAO (SOS 360180, 71847, and 72128), the Hellman Fellows fund, and the UCO mini-grant program. Furthermore, graduate students A. Beverage, F. Fornasini, T. Jones, B. Lorenz, S. Price and K. Suess were all supported by fellowships from the NSF-GRFP.