We present a joint analysis of the rest-frame ultraviolet (UV) luminosity functions of continuumselected star-forming galaxies and galaxies dominated by active galactic nuclei (AGNs) at z ∼ 4. These 3,740 z ∼ 4 galaxies are selected from broad-band imaging in nine photometric bands over 18 deg 2 in the Spitzer/HETDEX Exploratory Large Area Survey (SHELA) field. The large area and moderate depth of our survey provide a unique view of the intersection between the bright end of the galaxy UV luminosity function (M AB < −22) and the faint end of the AGN UV luminosity function. We do not separate AGN-dominated galaxies from star-formation-dominated galaxies, but rather fit both luminosity functions simultaneously. These functions are best fit with a double power-law (DPL) for both the galaxy and AGN components, where the galaxy bright-end slope has a power-law index of −3.80 ± 0.10, and the corresponding AGN faint-end slope is α AGN = −1.49 +0.30 −0.21 . We cannot rule out a Schechter-like exponential decline for the galaxy UV luminosity function, and in this scenario the AGN luminosity function has a steeper faint-end slope of −2.08 +0.18 −0.11 . Comparison of our galaxy luminosity function results with a representative cosmological model of galaxy formation suggests that the molecular gas depletion time must be shorter, implying that star formation is more efficient in bright galaxies at z = 4 than at the present day. If the galaxy luminosity function does indeed have a power-law shape at the bright end, the implied ionizing emissivity from AGNs is not inconsistent with previous observations. However, if the underlying galaxy distribution is Schechter, it implies a significantly higher ionizing emissivity from AGNs at this epoch.

We present the ugriz-band Dark Energy Camera (DECam) plus 3.6 and 4.5 µm IRAC catalogs for the Spitzer /HETDEX Exploratory Large-Area (SHELA) survey. SHELA covers ∼ 24 deg 2 of the Sloan Digital Sky Survey (SDSS) "Stripe 82" region, with seven bandpasses spanning a wavelength range of 0.35 to 4.5 µm. SHELA falls within the footprint of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), which will provide spectroscopic redshifts for ∼ 200,000 Lyα emitters at 1.9 < z < 3.5 and also for ∼ 200,000 [OII] emitters at z < 0.5. SHELA's deep, wide-area multiwavelength images combined with HETDEX's spectroscopic information, will facilitate many extragalactic studies, including measuring the evolution of galaxy stellar mass, halo mass, and environment from 1.5 < z < 3.5. Here we present riz-band selected ugriz-band DECam catalogs that reach a 5σ depth of ∼ 24.5 AB mag (for point sources with an aperture that encloses 70% of the total flux) and cover 17.5 deg 2 of the overall SHELA field. We validate our DECam catalog by comparison to the DECam Legacy Survey (DECaLS) DR5 and the Dark Energy Survey (DES) DR1. We perform IRAC forced photometry with The Tractor image modeling code to measure 3.6 and 4.5 µm fluxes for all objects within our DECam catalog. We demonstrate the utility of our catalog by computing galaxy number counts and estimating photometric redshifts. Our photometric redshifts recover the available z = 0.33 SDSS spectroscopic redshifts with a 1σ scatter in ∆z/(1 + z) of 0.04.

We explore the buildup of quiescent galaxies using a sample of 28,469 massive (M⋆ ≥ 1011M⊙) galaxies at redshifts 1.5 &lt; z &lt; 3.0, drawn from a 17.5 deg2 area (0.33 Gpc3 comoving volume at these redshifts). This allows for a robust study of the quiescent fraction as a function of mass at 1.5 &lt; z &lt; 3.0 with a sample ∼40 times larger at log(M⋆/$\rm M_{\odot })\ge 11.5$ than previous studies. We derive the quiescent fraction using three methods: specific star-formation rate, distance from the main sequence, and UVJ color-color selection. All three methods give similar values at 1.5 &lt; z &lt; 2.0, however the results differ by up to a factor of two at 2.0 &lt; z &lt; 3.0. At redshifts 1.5 &lt; z &lt; 3.0 the quiescent fraction increases as a function of stellar mass. By z = 2, only 3.3 Gyr after the Big Bang, the universe has quenched ∼25% of M⋆ = 1011M⊙ galaxies and ∼45% of M⋆ = 1012M⊙ galaxies. We discuss physical mechanisms across a range of epochs and environments that could explain our results. We compare our results with predictions from hydrodynamical simulations SIMBA and IllustrisTNG and semi-analytic models (SAMs) SAG, SAGE, and Galacticus. The quiescent fraction from IllustrisTNG is higher than our empirical result by a factor of 2 − 5, while those from SIMBA and the three SAMs are lower by a factor of 1.5 − 10 at 1.5 &lt; z &lt; 3.0.

We investigate the relation between AGN and star formation (SF) activity at 0.5 &lt; z &lt; 3 by analyzing 898 galaxies with X-ray luminous AGN (LX &gt; 1044 erg s−1) and a large comparison sample of ∼320, 000 galaxies without X-ray luminous AGN. Our samples are selected from a large (11.8 deg2) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume (∼0.3 Gpc3) at 0.5 &lt; z &lt; 3 minimizes the effects of cosmic variance and captures a large number of massive galaxies (∼30, 000 galaxies with M* &gt; 1011 M⊙) and X-ray luminous AGN. While many galaxy studies discard AGN hosts, we fit the SED of galaxies with and without X-ray luminous AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. We find that without this inclusion, stellar masses and star formation rates (SFRs) in AGN host galaxies can be overestimated, on average, by factors of up to ∼5 and ∼10, respectively. The average SFR of galaxies with X-ray luminous AGN is higher by a factor of ∼3 to 10 compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high SFRs and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority ($&gt; 95 \%$) of galaxies with X-ray luminous AGN at z = 0.5 − 3 do not show quenched SF: this suggests that if AGN feedback quenches SF, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity.

We study the evolution in the number density of the highest mass galaxies over 0.4 < z < 1.5 (covering 9 Gyr). We use the Spitzer/HETDEX Exploratory Large-Area (SHELA) Survey, which covers 17.5 deg 2 with eight photometric bands spanning 0.3-4.5 µm within the SDSS Stripe 82 field. This size produces the lowest counting uncertainties and cosmic variance yet for massive galaxies at z ∼ 1.0. We study the stellar mass function (SMF) for galaxies with log(M * /M ) > 10.3 using a forward-modeling method that fully accounts for statistical and systematic uncertainties on stellar mass. From z=0.4 to 1.5 the massive end of the SMF shows minimal evolution in its shape: the characteristic mass (M * ) evolves by less than 0.1 dex (±0.05 dex); the number density of galaxies with log M * /M > 11 stays roughly constant at log(n/Mpc −3 ) −3.4 (±0.05), then declines to log n/Mpc −3 =−3.7 (±0.05) at z=1.5. We discuss the uncertainties in the SMF, which are dominated by assumptions in the star formation history and details of stellar population synthesis models for stellar mass estimations. For quiescent galaxies, the data are consistent with no (or slight) evolution ( 0.1 dex) in the characteristic mass nor number density from z ∼ 1.5 to the present. This implies that any mass growth (presumably through "dry' mergers) of the quiescent massive galaxy population must balance the rate of mass losses from late-stage stellar evolution and the formation of quenching galaxies from the star-forming population. We provide a limit on this mass growth from z = 1.0 to 0.4 of ∆M * /M * ≤ 45% (i.e., 0.16 dex) for quiescent galaxies more massive than 10 11 M .

We present the high-mass end of the galaxy stellar mass function using the largest sample to date (5,352) of star-forming galaxies with M > 10 11 M at cosmic noon, 1.5 < z < 3.5. This sample is uniformly selected across 17.2 deg 2 (∼0.44 Gpc 3 comoving volume from 1.5 < z < 3.5), mitigating the effects of cosmic variance and encompassing a wide range of environments. This area, a factor of 10 larger than previous studies, provides robust statistics at the high-mass end. Using multi-wavelength data in the Spitzer /HETDEX Exploratory Large Area (SHELA) footprint we find that the SHELA footprint star-forming galaxy stellar mass function is steeply declining at the high-mass end probing values as high as ∼10 −4 Mpc 3 /dex and as low as ∼5×10 −8 Mpc 3 /dex across a stellar mass range of log(M /M ) ∼ 11 -12. We compare our empirical star-forming galaxy stellar mass function at the high mass end to three types of numerical models: hydrodynamical models from IllustrisTNG, abundance matching from the UniverseMachine, and three different semi-analytic models (SAMs; SAG, SAGE, GALACTICUS). At redshifts 1.5 < z < 3.5 we find that results from Illus-trisTNG and abundance matching models agree within a factor of ∼2 to 10, however the three SAMs strongly underestimate (up to a factor of 1,000) the number density of massive galaxies. We discuss the implications of these results for our understanding of galaxy evolution.

We present the results of a deep K s -band (2.1 μm) imaging survey of the Spitzer/HETDEX Exploratory Large-Area (SHELA) field using the NEWFIRM near-infrared camera on the KPNO Mayall 4 m telescope. This NEWFIRM HETDEX Survey reaches a 5σ depth of 22.4 AB mag (2″-diameter apertures corrected to total), is ∼50% and 90% complete at K ∼ 22.65 and K ∼ 22.15, respectively, and covers 22 deg2 of the 24 deg2 SHELA Spitzer/IRAC footprint (within “Stripe 82” of the Sloan Digital Sky Survey). We present a K s -band-selected catalog that includes deep ugriz imaging from the Dark Energy Camera and 3.6 and 4.5 μm imaging from Spitzer/IRAC, with forced photometry of 1.7 million sources across 17.5 deg2. The large area and moderate depth of this catalog enable the study of the most massive galaxies at high redshift and minimize uncertainties associated with counting statistics and cosmic variance. As a demonstration, we derive stellar masses (M *) and star formation rates (SFRs) for candidate galaxies at 3 ≲ z ≲ 5 and select a conservative sample of nine candidate massive (M * > 1011 M ⊙) quiescent galaxies, which have measured SFRs significantly below the main sequence at this redshift. Five are ultramassive with M * > 1012, though uncertainties in IRAC blending, gravitational lensing, or active galactic nucleus emission could result in true masses that are lower. Simulations predict that these galaxies should be extremely rare; thus, we conclude by discussing what physical processes in models could be altered to allow the formation of such massive quiescent galaxies at such early times.

We present the main sequence for all galaxies and star-forming galaxies for a sample of 28,469 massive (M⋆ ≥ 1011M⊙) galaxies at cosmic noon (1.5 &lt; z &lt; 3.0), uniformly selected from a 17.5 deg2 area (0.33 Gpc3 comoving volume at these redshifts). Our large sample allows for a novel approach to investigating the galaxy main sequence that has not been accessible to previous studies. We measure the main sequence in small mass bins in the SFR-M⋆ plane without assuming a functional form for the main sequence. With a large sample of galaxies in each mass bin, we isolate star-forming galaxies by locating the transition between the star-forming and green valley populations in the SFR-M⋆ plane. This approach eliminates the need for arbitrarily defined fixed cutoffs when isolating the star-forming galaxy population, which often biases measurements of the scatter around the star-forming galaxy main sequence. We find that the main sequence for all galaxies becomes increasingly flat towards present day at the high-mass end, while the star-forming galaxy main sequence does not. We attribute this difference to the increasing fraction of the collective green valley and quiescent galaxy population from z = 3.0 to z = 1.5. Additionally, we measure the total scatter around the star-forming galaxy main sequence and find that it is ∼0.5 − 1.0 dex with little evolution as a function of mass or redshift. We discuss the implications that these results have for pinpointing the physical processes driving massive galaxy evolution.