Lyman-Alpha Escape from Low-Mass, Compact, High-Redshift Galaxies

We investigate the effects of stellar populations and sizes on Ly$\alpha$ escape in 27 spectroscopically confirmed and 35 photometric Lyman-Alpha Emitters (LAEs) at z $\approx$ 2.65 in seven fields of the Bo\"otes region of the NOAO Deep Wide-Field Survey. We use deep $HST$/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive ($M_{\star} \approx 10^{7} - 10^{9}~M_{\odot}$), younger (ages $\lesssim$ 1 Gyr), smaller ($r_{e} <$ 1 kpc), less dust-attenuated (E(B$-$V) $\le$ 0.26 mag), but have comparable star-formation-rates (SFRs $\approx 1 - 100~M_{\odot} {\rm yr^{-1}}$). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities (${\rm Z_{neb} \lesssim 0.2 Z_{\odot}}$) and/or high ionization parameters ($\log{(\rm U)} \gtrsim -2.4$). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Ly$\alpha$ escape, by computing star-formation-rate surface density, $\Sigma_{\rm SFR}$ and specific star-formation-rate surface density, $\Sigma_{\rm sSFR}$. For a given $\Sigma_{\rm SFR}$, the Ly$\alpha$ escape fraction is higher for LAEs with lower stellar masses. LAEs have higher $\Sigma_{\rm sSFR}$ on average compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Ly$\alpha$ photons. These results have important implications for the physics of Ly$\alpha$ radiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization.Comment: 36 pages, 15 figures; Accepted for publication in The Astronomical Journa

Reconstructing and Classifying SDSS DR16 Galaxy Spectra with Machine-Learning and Dimensionality Reduction Algorithms

Optical spectra of galaxies and quasars from large cosmological surveys are used to measure redshifts and infer distances. They are also rich with information on the intrinsic properties of these astronomical objects. However, their physical interpretation can be challenging due to the substantial number of degrees of freedom, various sources of noise, and degeneracies between physical parameters that cause similar spectral characteristics. To gain deeper insights into these degeneracies, we apply two unsupervised machine learning frameworks to a sample from the Sloan Digital Sky Survey data release 16 (SDSS DR16). The first framework is a Probabilistic Auto-Encoder (PAE), a two-stage deep learning framework consisting of a data compression stage from 1000 elements to 10 parameters and a density estimation stage. The second framework is a Uniform Manifold Approximation and Projection (UMAP), which we apply to both the uncompressed and compressed data. Exploring across regions on the compressed data UMAP, we construct sequences of stacked spectra which show a gradual transition from star-forming galaxies with narrow emission lines and blue spectra to passive galaxies with absorption lines and red spectra. Focusing on galaxies with broad emission lines produced by quasars, we find a sequence with varying levels of obscuration caused by cosmic dust. The experiments we present here inform future applications of neural networks and dimensionality reduction algorithms for large astronomical spectroscopic surveys.Comment: ASP Conference Series, Compendium of Undergraduate Research in Astronomy and Space Science (accepted), 24 pages, 14 figure

Dwarf Galaxies as Probes of Galaxy Evolution

Low-mass "dwarf" galaxies (stellar mass, M* <= 3 x 10^9 Msun), being ubiquitous in the universe, are excellent laboratories for testing critical components of galaxy evolution. The current and future photometric and spectroscopic surveys, including the Sloan Digital Sky Survey (SDSS), the Dark Energy Survey (DES), the Dark Energy Spectroscopic Instrument (DESI) survey, and the Legacy Survey of Space and Time (LSST) are facilitating the study of these galaxies in great detail. In this dissertation, I probe three different aspects of galaxy evolution using dwarf galaxies. Firstly, by conducting the widest (1.5 deg diameter field of view) and the deepest (down to g ~ 26.5 mag) survey of resolved stars in a Local Group dwarf galaxy, IC 1613, I found evidence of hierarchical accretion in the galaxy. This is an important first step in understanding structure formation in dwarf galaxies. Secondly, I demonstrated that the compact morphologies of Lyman-Alpha Emitters (LAEs), i.e., low-mass, high-redshift galaxies, are key to Ly-alpha escape, a process that is crucial for understanding the epoch of reionization. Finally, using an unprecedented spectroscopic sample of ~190,000 low-mass galaxies from the DESI survey, I identified >2,500 active galactic nuclei (AGNs) out to z ~ 0.45, more than tripling the existing census of optical dwarf AGN candidates. Compared to previous systematic searches, I have extended the discovery space of such candidates to lower galaxy masses and to higher redshifts. Additionally, I extended the black hole (BH) mass - galaxy stellar mass (M_BH - M*) scaling relation down to log (M*/Msun) ~ 8.4. This statistical sample of AGN in low-mass galaxies will have important implications for the study of galaxy-BH co-evolution at the low-mass end of the galaxy mass function. Furthermore, I created four publicly available Jupyter Notebooks that show how to access and analyze photometric and spectroscopic data for the scientific analysis of galaxies

A New Infrared Criterion for Selecting Active Galactic Nuclei to Lower Luminosities

We present a spectroscopic and photometric analysis of a sample of 416,288 galaxies from the Sloan Digital Sky Survey (SDSS) matched to mid-infrared (mid-IR) data from the Wide-Field Infrared Survey Explorer (WISE). By using a new spectroscopic fitting package, GELATO (Galaxy/AGN Emission Line Analysis TOol), we are able to retrieve emission line fluxes and uncertainties for SDSS spectra and robustly determine the presence of broad lines and outflowing components, enabling us to investigate WISE color space as a function of optical spectroscopic properties. In addition, we pursue SED template fitting to assess the relative AGN contribution and nuclear obscuration to compare to existing mid-IR selection criteria with WISE. We present a selection criterion in mid-IR color space to select Active Galactic Nuclei (AGNs) with a $\sim$80% accuracy and a completeness of $\sim$16%. This is the first mid-IR color selection defined by solely using the distribution of Type I and Type II optical spectroscopic AGNs in WISE mid-IR color space. Our selection is an improvement of $\sim$50% in the completeness of targeting spectroscopic AGNs with WISE down to an SDSS $r<17.77$ mag. In addition, our new criterion targets a less luminous population of AGNs, with on average lower [O III] luminosities by $\sim$30% ($>0.1$ dex) compared to typical WISE color-color selections. With upcoming large photometric surveys without corresponding spectroscopy, our method presents a way to select larger populations of AGNs at lower AGN luminosities and higher nuclear obscuration levels than traditional mid-IR color selections.Comment: 31 pages, 22 figures. Accepted for publication in The Astronomical Journa

Tidal Destruction in a Low-mass Galaxy Environment: The Discovery of Tidal Tails around DDO 44

We report the discovery of a 1° (~50 kpc) long stellar tidal stream emanating from the dwarf galaxy DDO 44, a likely satellite of Local Volume galaxy NGC 2403 located ~70 kpc in projection from its companion. NGC 2403 is a roughly Large Magellanic Cloud (LMC) stellar-mass galaxy 3 Mpc away, residing at the outer limits of the M81 group. We are mapping a large region around NGC 2403 as part of our Magellanic Analogs\u27 Dwarf Companions and Stellar Halos survey, reaching point-source depths (90% completeness) of (g, i) = (26.5, 26.2). Density maps of old, metal-poor RGB stars reveal tidal streams extending on two sides of DDO 44, with the streams directed toward NGC 2403. We estimate total luminosities of the original DDO 44 system (dwarf and streams combined) to be M i,tot = −13.4 and M g,tot = −12.6, with ~25%–30% of the luminosity in the streams. Analogs of ~LMC-mass hosts with massive tidally disrupting satellites are rare in the Illustris simulations, especially at large separations such as that of DDO 44. The few analogs that are present in the models suggest that even low-mass hosts can efficiently quench their massive satellites

Hyper Wide Field Imaging of the Local Group Dwarf Irregular Galaxy IC 1613: An Extended Component of Metal-poor Stars

Stellar halos offer fossil evidence for hierarchical structure formation. Since halo assembly is predicted to be scale-free, stellar halos around low-mass galaxies constrain properties such as star formation in the accreted subhalos and the formation of dwarf galaxies. However, few observational searches for stellar halos in dwarfs exist. Here we present gi photometry of resolved stars in isolated Local Group dwarf irregular galaxy IC 1613 (M sstarf ~ 108 M ⊙). These Subaru/Hyper Suprime-Cam observations are the widest and deepest of IC 1613 to date. We measure surface density profiles of young main-sequence, intermediate to old red giant branch, and ancient horizontal branch stars outside of 12\u27 (~2.6 kpc; 2.5 half-light radii) from the IC 1613 center. All of the populations extend to ~24\u27 (~5.2 kpc; 5 half-light radii), with the older populations best fit by a broken exponential in these outer regions. Comparison with earlier studies sensitive to IC 1613\u27s inner regions shows that the density of old stellar populations steepens substantially with distance from the center; we trace the g-band effective surface brightness to an extremely faint limit of ~33.7 mag arcsec−2. Conversely, the distribution of younger stars follows a single, shallow exponential profile in the outer regions, demonstrating different formation channels for the younger and older components of IC 1613. The outermost, intermediate-age and old stars have properties consistent with those expected for accreted stellar halos, though future observational and theoretical work is needed to definitively distinguish this scenario from other possibilities

The Extremely Luminous Quasar Survey in the Sloan Digital Sky Survey Footprint. II. The North Galactic Cap Sample

We present the North Galactic Cap sample of the Extremely Luminous Quasar Survey (ELQS-N), which targets quasars with M-1450 70% over z similar to 3.0-5.0 at m(i) less than or similar to 17.5, limited toward fainter magnitudes by the depth of the Two Micron All Sky Survey. The presented quasar catalog consists of a total of 270 objects, of which 39 are newly identified in this work with spectroscopy obtained at the Vatican Advanced Technology Telescope and the MMT 6.5 m telescope. In addition to the high completeness, which allowed us to discover new quasars in the already well-surveyed SDSS North Galactic Cap, the efficiency of our selection is relatively high at similar to 79%. Using 120 objects of this quasar sample we are able to extend the previously measured optical quasar luminosity function (QLF) by one magnitude toward the bright end at 2.8 <= z <= 4.5. A first analysis of the QLF suggests a relatively steep bright-end slope of beta approximate to -4 for this sample. This result contrasts with previous results in the same redshift range, which find a much flatter slope around beta similar to -2.5, but agrees with recent measurements of the bright-end slope at lower and higher redshifts. Our results constrain the bright-end slope at z = 2.8-4.5 to beta < -2.94 with a 99% confidence.U.S. NSF [AST 15-15115]; NASA ADAP grant [NNX17AF28G]; National Aeronautics and Space Administration; National Science Foundation; Alfred P. Sloan Foundation; U.S. Department of Energy Office of Science; Center for High-Performance Computing at the University of Utah; Brazilian Participation Group; Carnegie Institution for Science; Carnegie Mellon University; Chilean Participation Group; French Participation Group; Harvard-Smithsonian Center for Astrophysics; Instituto de Astrofsica de Canarias; Johns Hopkins University; Kavli Institute for the Physics and Mathematics of the Universe (IPMU)/University of Tokyo; Lawrence Berkeley National Laboratory; Leibniz Institut fur Astrophysik Potsdam (AIP); Max-Planck-Institut fur Astronomie (MPIA Heidelberg); Max-Planck-Institut fur Astrophysik (MPA Garching); Max-Planck-Institut fur Extraterrestrische Physik (MPE); National Astronomical Observatories of China; New Mexico State University; New York University; University of Notre Dame; Observatrio Nacional/MCTI; Ohio State University; Pennsylvania State University; Shanghai Astronomical Observatory; United Kingdom Participation Group; Universidad Nacional Autnoma de Mexico; University of Arizona; University of Colorado Boulder; University of Oxford; University of Portsmouth; University of Utah; University of Virginia; University of Washington; University of Wisconsin; Vanderbilt University; Yale UniversityThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]