Andrew Newman

I am broadly interested in galaxy evolution, dark matter, and instrument development. Please find a summary of my research below.

Mapping the z~2.5 Cosmic Web with Lyman-alpha Tomography

When and how did galaxy's large-scale environments begin to shape their evolution? A key barrier to answering this question is the difficulty of mapping large-scale structures and galaxy environments in the critical "Cosmic Noon" era around z~2. I am leading the Lyman-alpha Tomography IMACS Survey (LATIS), a roughly 60-night survey at Magellan, which will produce the largest Mpc-resolution 3D map of the z~2.5 universe by measuring H I absorption in a network of faint background galaxies. We will detect, map, and weigh statistical samples of over- and underdense structures, ranging from large voids to massive protoclusters, in a novel way that is independent of their galaxy populations. LATIS is conducted in well-observed patches of sky, including COSMOS, enabling us to link the detailed properties of galaxies with their (physical) Mpc-scale environments.

Survey Paper: Newman et al (2020)

First science results: Newman et al (2022), Nature

Below: partial map of the COSMOS field (representing 1/4 of the LATIS survey volume) adapted from Newman et al (2020)

The Formation and Growth of Massive Galaxies

I am interested in all phases of the lives of massive galaxies. How did they form stars rapidly in the early universe, and why did they stop? When and how did galaxies transform into ellipticals? How do "dead" galaxies continue to grow once star formation has ended? I investigate these questions primarily using optical/near-infrared spectroscopy on large telescopes along with space-based imaging.

Galaxies' sizes are perhaps their most fundamental parameter after mass. I tracked the size growth of quiescent galaxies over the last 10 Gyr and related these galaxies' growth to the rate of consumption of their satellite populations (Newman et al 2012). Connecting galaxies' structures to their star formation histories, derived from deep spectra, allowed myself and collaborators to distinguish growth that occurred before quenching (from star formation) versus afterwards (from merging; Belli, Newman & Ellis 2015). We found that z~1.5 quiescent galaxies followed a wide range of formation histories likely linked to distinct quenching processes (Belli, Newman & Ellis 2019).

Stellar dynamics plays a key role in tracking galaxies' structural evolution. I and collaborators undertook a Keck spectroscopic survey that yielded the largest library of velocity dispersions of z > 1.3 quiescent galaxies (Newman et al 2010; Belli, Newman et al 2014a, 2014b, 2017). We used these data to investigate how "dead" galaxies continued to grow. I discovered 4 rare examples of early quiescent galaxies that are gravitationally lensed (Newman et al 2018a). The magnification enabled the first resolved spectroscopic studies of these galaxies. Surprisingly, we found that all of these sources remained rapidly rotating, disk-dominated galaxies after quenching, even though they are destined to become giant non-rotating ellipticals (Newman et al 2015, 2018b). This result challenges the traditional view that the quenching of star formation and transformation into an elliptical occurred in a single event.

The first early quiescent galaxy in which rotation was observed, a triply-imaged source at z=2.6 (Newman et al 2015)

Ultra-high S/N spectrum of a lensed z=2 quiescent galaxy (Jafariyazani et al 2020) using Keck/MOSFIRE (top) and Magellan/FIRE (bottom)

Stellar chemical abundances provide an important fossil record of massive galaxies' early star formation. In Jafariyazani, Newman et al (2020), we used an extremely high-quality spectrum of a spectacular lensed source to measure the first detailed (7 element) stellar abundance pattern of a z=2 quiescent galaxy. Seen just ~1 Gyr after quenching, we found the galaxy's abundances to be surprisingly different from local massive galaxies. This implies that the abundance patterns seen in the centers of today's ellipticals may not reflect these galaxies' formation conditions---the key assumption of the "archaeological" program---but instead have been significantly polluted by stars formed in other galaxies, which have merged and produced the chemical evolution.

Environment may play a key role in modulating galaxies' star formation and growth histories. I investigated galaxy growth in a distant galaxy cluster (z=1.8) using Hubble Space Telescope grism spectroscopy (Newman et al 2014). I found that this cluster harbors a remarkably quiescent galaxy population, but that the ages and structures of the "dead" galaxies are similar to those in the field.

The initial mass function of stars seems to vary in nearby early-type galaxies based on evidence from stellar dynamics, gravitational lensing, and detailed stellar population synthesis models. I performed a critical comparison of these methods in the 3 closest gravitational lenses (Newman et al 2017) as a fundamental test of their consistency. This work is discussed extensively in a recent Annual Review (Smith 2020).

The z=1.8 cluster JKCS 041 (Newman et al 2014)

Dark Matter in Galaxies, Groups, and Clusters

I seek to learn about the nature of dark matter by observing the structures of dark matter halos, on scales ranging from massive galaxy clusters to dwarf galaxies. In Newman et al (2009, 2011, 2013a, 2013b), I measured the dark and baryonic mass profiles of seven relaxed galaxy clusters on scales ranging from ~3 kpc to ~3 Mpc. The wide range of scales was achieved by combining measures of weak lensing, strong lensing, and stellar dynamics (within the central galaxy). The main goal of this work was to measure the slope of the dark matter density profile on small scales and to compare it to the "universal" shape that characterizes collisionless cold dark matter. Unexpectedly, I found that the total density profile (dominated by dark matter plus stars) follows this theoretical expectation quite closely down to small scales, whereas the dark matter alone in fact traces a shallower profile within ~30 kpc. Extending this methodology to lower-mass group-scale lenses, Newman et al (2015) found consistency with canonical dark matter profiles. This supports the idea that assembly histories dominated by collisionless mergers (e.g., brightest cluster galaxies) are physically linked to the removal of dark matter from the interior. Intriguingly, however, self-interacting dark matter models can also explain this trend in dark matter profiles.

I am also interested in the dark matter profiles of much lower-mass galaxies. In Relatores, Newman et al (2019a, 2019b) we measured the dark matter density profiles of disk galaxies with masses around log M = 9. We found that these galaxies show a wide range of density profiles. On average, they are shallower than a canonical CDM profile, but much steeper than the "cored" profiles reported in still lower-mass dwarfs. Supernova feedback is the favored explanation for creating these "cores". State-of-the-art simulations of galaxy formation that produce appropriate cores in low-mass dwarfs fail to match the steeper slopes in our more massive sample, implying that this process is not yet fully understood.

MIRMOS: The Magellan Infrared Multiobject Spectrograph

I am a project scientist for MIRMOS, a powerful near-infrared multi-object spectrograph and IFU currently under development for the Magellan telescopes. Please see the project webpage for more information.

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