Search

We perform a ground-based near-infrared spectroscopic survey using the Keck/MOSFIRE spectrograph to target Ly$\alpha$ emission at $7.0$4$\sigma$ level in eight $z>7$ galaxies, which include additional members of the known $z\sim7.7$ Ly$\alpha$-emitter (LAE) cluster (Tilvi et al. 2020). With the addition of these newly-discovered $z\sim7.7$ LAEs, this is currently the largest measured LAE cluster at $z>7$. The unusually-high Ly$\alpha$ detection rate at $z\sim7.7$ in this field suggests significantly stronger Ly$\alpha$ emission from the clustered LAEs than from the rest of our targets. We estimate the ionized bubble sizes around these LAEs and conclude that the LAEs are clustered within an extended ionized structure created by overlapping ionized bubbles which allow the easier escape of Ly$\alpha$ from galaxies. It is remarkable that the brightest object in the cluster has the lowest measured redshift of the Ly$\alpha$ line, being placed in front of the other LAEs in the line-of-sight direction. This suggests that we are witnessing the enhanced IGM transmission of Ly$\alpha$ from galaxies on the rear side of an ionized area. This could be a consequence of Ly$\alpha$ radiative transfer: Ly$\alpha$ close to the central velocity is substantially scattered by the IGM while Ly$\alpha$ from the rear-side galaxies is significantly redshifted to where it has a clear path., Comment: Submitted to ApJ (17 pages, 8 figures, 4 tables)

The abundance of bright galaxies at z>8 can provide key constraints on models of galaxy formation and evolution, as the predicted abundance varies greatly when different physical prescriptions for gas cooling and star formation are implemented. We present the results of a search for bright z=9-10 galaxies selected from pure-parallel Hubble Space Telescope imaging programs. We include 132 fields observed as part of the Brightest of Reionizing Galaxies survey, the Hubble Infrared Pure Parallel Imaging Extragalactic Survey, and the WFC3 Infrared Spectroscopic Parallel survey. These observations cover a total of 620 sq. arcmin, about 70% of which is also covered with Spitzer Space Telescope infrared imaging. We identify thirteen candidate galaxies in the range 8.3

REQUIEM-2D (REsolving QUIEscent Magnified galaxies with 2D grism spectroscopy) is comprised of a sample of 8 massive ($\log M_*/M_\odot > 10.6$) strongly lensed quiescent galaxies at $z\sim2$. REQUIEM-2D combines the natural magnification from strong gravitational lensing with the high spatial-resolution grism spectroscopy of \emph{Hubble Space Telescope} through a spectrophotometric fit to study spatially resolved stellar populations. We show that quiescent galaxies in the REQUIEM-2D survey have diverse formation histories manifesting as a gradient in stellar ages, including examples of (1) a younger central region supporting outside-in formation, (2) flat age gradients that show evidence for both spatially-uniform early formation or inside-out quenching, and (3) regions at a fixed radial distance having different ages (such asymmetries cannot be recovered when averaging stellar population measurements azimuthally). The typical dust attenuation curve for the REQUIEM-2D galaxies is constrained to be steeper than Calzetti's law in the UV and generally consistent with $A_V<1$. Combined together and accounting for the different physical radial distances and formation time-scales, we find that the REQUIEM-2D galaxies that formed earlier in the universe exhibit slow and uniform growth in their inner core, whereas the galaxies that formed later have rapid inner growth in their inner core with younger ages relative to the outskirts. These results challenge the currently accepted paradigm of how massive quiescent galaxies form, where the earliest galaxies are thought to form most rapidly. Significantly larger samples close to the epoch of formation with similar data quality and higher spectral resolution are required to validate this finding., Comment: Submitted to ApJ

We present the $H_{160}$ morphological catalogs for the COSMOS-DASH survey, the largest area near-IR survey using HST-WFC3 to date. Utilizing the "Drift And SHift" observing technique for HST-WFC3 imaging, the COSMOS-DASH survey imaged approximately 0.5 deg$^2$ of the UltraVISTA deep stripes (0.7 deg$^2$ when combined with archival data). Global structural parameters are measured for 51,586 galaxies within COSMOS-DASH using GALFIT (excluding the CANDELS area) with detection using a deep multi-band HST image. We recover consistent results with those from the deeper 3D-HST morphological catalogs, finding that, in general, sizes and S\'ersic indices of typical galaxies are accurate to limiting magnitudes of $H_{160}<23$ and $H_{160}<22$ ABmag, respectively. In size-mass parameter space, galaxies in COSMOS-DASH demonstrate robust morphological measurements out to $z\sim2$ and down to $\log(M_\star/M_\odot)\sim9$. With the advantage of the larger area of COSMOS-DASH, we measure a flattening of the quiescent size-mass relation below $\log(M_\star/M_\odot)\sim10.5$ that persists out to $z\sim2$. We show that environment is not the primary driver of this flattening, at least out to $z=1.2$, whereas internal physical processes may instead govern the structural evolution., Comment: 27 pages, 17 figures. Accepted for publication in ApJ. For associated morphological catalog, see https://archive.stsci.edu/hlsp/cosmos-dash

We present a detailed stellar population analysis of 11 bright ($H<26.6$) galaxies at $z=9-11$ (three spectroscopically confirmed) to constrain the chemical enrichment and growth of stellar mass of early galaxies. We use the flexible Bayesian spectral energy distribution (SED) fitting code Prospector with a range of star-formation histories (SFHs), a flexible dust attenuation law and a self-consistent modeling of emission lines. This approach allows us to assess how different priors affect our results, and how well we can break degeneracies between dust attenuation, stellar ages, metallicity and emission lines using data which probe only the rest-frame ultraviolet to optical wavelengths. We measure a median observed ultraviolet spectral slope $\beta=-1.87_{-0.43}^{+0.35}$ for relatively massive star-forming galaxies ($9<\log(M_{\star}/M_{\odot})<10$), consistent with no change from $z=4$ to $z=9-10$ at these stellar masses, implying rapid enrichment. Our SED-fitting results are consistent with a star-forming main sequence with sub-linear slope ($0.7\pm0.2$) and specific star-formation rates of $3-10~\mathrm{Gyr}^{-1}$. However, the stellar ages and SFHs are less well constrained. Using different SFH priors, we cannot distinguish between median mass-weighted ages of $\sim50-150$ Myr, which corresponds to 50\% formation redshifts of $z_{50}\sim10-12$ at $z\sim9$ and is of the order of the dynamical timescales of these systems. Importantly, the models with different SFH priors are able to fit the data equally well. We conclude that the current observational data cannot tightly constrain the mass-buildup timescales of these $z=9-11$ galaxies, with our results consistent with SFHs implying both a shallow and steep increase of the cosmic SFR density with time at $z>10$., Comment: 27 pages + Appendix; accepted by ApJ

We present the results from a new search for candidate galaxies at z ~ 8.5-11 discovered over the 850 arcmin^2 area probed by the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). We use a photometric redshift selection including both Hubble and Spitzer Space Telescope photometry to robustly identify galaxies in this epoch at F160W < 26.6. We use a detailed vetting procedure, including screening for persistence, stellar contamination, inclusion of ground-based imaging, and followup space-based imaging to build a robust sample of 11 candidate galaxies, three presented here for the first time. The inclusion of Spitzer/IRAC photometry in the selection process reduces contamination, and yields more robust redshift estimates than Hubble alone. We constrain the evolution of the rest-frame ultraviolet luminosity function via a new method of calculating the observed number densities without choosing a prior magnitude bin size. We find that the abundance at our brightest probed luminosities (M_UV=-22.3) is consistent with predictions from simulations which assume that galaxies in this epoch have gas depletion times at least as short as those in nearby starburst galaxies. Due to large Poisson and cosmic variance uncertainties we cannot conclusively rule out either a smooth evolution of the luminosity function continued from z=4-8, or an accelerate decline at z > 8. We calculate that the presence of seven galaxies in a single field (EGS) is an outlier at the 2-sigma significance level, implying the discovery of a significant overdensity. These scenarios will be imminently testable to high confidence within the first year of observations of the James Webb Space Telescope., Comment: 47 figures, 25 pages, 10 tables, Accepted to the Astrophysical Journal. Photometric catalogs available via email request to lead author

Ly$\alpha$ emission from galaxies can be utilized to characterize the ionization state in the intergalactic medium (IGM). We report our search for Ly$\alpha$ emission at $z>7$ using a comprehensive Keck/MOSFIRE near-infrared spectroscopic dataset, as part of the Texas Spectroscopic Search for Ly$\alpha$ Emission at the End of Reionization Survey. We analyze data from 10 nights of MOSFIRE observations which together target 72 high-$z$ candidate galaxies in the GOODS-N field, all with deep exposure times of 4.5-19 hr. Utilizing an improved automated emission-line search, we report 10 Ly$\alpha$ emission lines detected ($>$4$\sigma$) at $z>7$, significantly increasing the spectroscopically confirmed sample. Our sample includes large equivalent-width (EW) Ly$\alpha$ emitters ($>$50\r{A}), and additional tentative Ly$\alpha$ emission lines detected at 3 - 4$\sigma$ from five additional galaxies. We constrain the Ly$\alpha$ EW distribution at $z\sim7.6$, finding a significant drop from $z\lesssim6$, suggesting an increasing fraction of neutral hydrogen (HI) in the IGM in this epoch. We estimate the Ly$\alpha$ transmission through the IGM ($=$EW$_{z\sim\text{7.6}}$/EW$_{z\sim\text{2-6}}$), and infer an IGM HI fraction ($X_{\text{HI}}$) of $49^{+19}_{-19}\%$ at $z\sim7.6$, which is lower in modest tension ($>$1$\sigma$) with recent measurements at $z \sim$ 7.6. The spatial distribution of the detected Ly$\alpha$ emitters implies the presence of a potential highly ionized region at $z\sim7.55$ which hosts four Ly$\alpha$ emitters within a $\sim$ 40 cMpc spatial separation. The prominence of this ionized region in our dataset could explain our lower inferred value of $X_{\text{HI}}$, though our analysis is also sensitive to the chosen reference Ly$\alpha$ EW distribution values and reionization models., Comment: Published 2020 November 27 in ApJ. Changed to match the published version (HI fraction & Figure 12 updated and other minor changes). 27 pages, 15 figures, 3 tables

We study the stellar populations and assembly of the nearby spiral galaxy NGC 2903's bulge, bar, and outer disc using the VIRUS-P Exploration of Nearby Galaxies IFS survey. We observe NGC 2903 with a spatial resolution of 185 pc using the Mitchell Spectrograph's 4.25 arcsec fibres at the 2.7 Harlan J. Smith telescope. Bulge-bar-disc decomposition on the 2MASS Ks-band image of NGC 2903 shows that it has ~6%, 6%, and 88%, of its stellar mass in the bulge, bar, and outer disc, respectively, and its bulge has a low Sersic index of ~0.27, suggestive of a disky bulge. We perform stellar population synthesis and find that the outer disc has 46% of its mass in stars >5 Gyr, 48% in stars between 1 and 5 Gyr, and <10% in younger stars. Its stellar bar has 65% of its mass in ages 1-5 Gyr and has metallicities similar to the outer disc, suggestive of the evolutionary picture where the bar forms from disc material. Its bulge is mainly composed of old high-metallicity stars though it also has a small fraction of young stars. We find enhanced metallicity in the spiral arms and central region, tracing areas of high star formation as seen in the Halpha map. These results are consistent with the idea that galaxies of low bulge-to-total mass ratio and low bulge Sersic index like NGC 2903 has not had a recent major merger event, but has instead grown mostly through minor mergers and secular processes., Comment: 14 pages, 9 figures, 2 tables, accepted in MNRAS, added reference

We present Keck/MOSFIRE $H$-band spectroscopy targeting C$\,$III] $\lambda$1907,1909 in a $z=7.5056$ galaxy previously identified via Ly$\alpha$ emission. We detect strong line emission at $1.621\pm0.002\,\mu$m with a line flux of ($2.63\pm0.52$)$\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$. We tentatively identify this line as [C$\,$III] $\lambda$1907, but we are unable to detect C$\,$III] $\lambda$1909 owing to sky emission at the expected location. This gives a galaxy systemic redshift, $z_{sys}=7.5032\pm0.0003$, with a velocity offset to Ly$\alpha$ of $\Delta$v$_{Ly\alpha}$ = $88\pm27$ km s$^{-1}$. The ratio of combined C$\,$III]/Ly$\alpha$ is 0.30-0.45, one of the highest values measured for any $z>2$ galaxy. We do not detect Si$\,$III] $\lambda\lambda$1883, 1892, and place an upper limit on Si$\,$III]/C$\,$III] $<$ 0.35 ($2\sigma$). Comparing our results to photoionization models, the C$\,$III] equivalent width (W$_{\text{CIII]}} = 16.23\pm2.32\,$\AA), low Si$\,$III]/C$\,$III] ratio, and high implied [O$\,$III] equivalent width (from the $Spitzer$/IRAC [3.6]$-$[4.5]$\simeq$0.8 mag color) require sub-Solar metallicities ($Z\simeq0.1-0.2 Z_{\odot}$) and a high ionization parameter, log$\,$U $\gtrsim -1.5$. These results favor models that produce higher ionization, such as the BPASS models for the photospheres of high-mass stars, and that include both binary stellar populations and/or an IMF that extends to 300 M$_{\odot}$. The combined C$\,$III] equivalent width and [3.6]$-$[4.5] color are more consistent with ionization from young stars than AGN, however we cannot rule out ionization from a combination of an AGN and young stars. We make predictions for $James~Webb~Space~Telescope$ spectroscopy using these different models, which will ultimately test the nature of the ionizing radiation in this source., Comment: 19 pages, 12 figures, accepted for publication in ApJ

Realizing the utility of Ly$\alpha$ emission to trace the evolution of the intergalactic medium (IGM) during the epoch of reionization requires deep spectroscopy across the boundary of optical and near-infrared (NIR) spectrographs at $z\sim7.2$ when Ly$\alpha$ emission is at $\sim$1$\mu$m. Our Texas Spectroscopic Search for Ly$\alpha$ Emission at the End of Reionization includes 18 nights of deep spectroscopic observations using the Keck DEIMOS (optical) and MOSFIRE (NIR) spectrographs. Within this dataset we observe Ly$\alpha$ emission from 183 photometric-redshift selected galaxies at $z =$ 5.5 - 8.3 from the Cosmic Assembly Near infrared Deep Extragalactic Legacy Survey (CANDELS). Our overlapping MOSFIRE observations, over 84 galaxies total, provide the deepest NIR spectroscopic data yet obtained for Ly$\alpha$ from galaxies $z > 7$, with $>16$ hr integration time for four observed galaxies. Here we analyze these four targets, and we report the discovery of a new $z = 7.60$ Ly$\alpha$ detection as well as provide an updated observation of the previously confirmed $z=7.51$ Ly$\alpha$ emission from Finkelstein et al. (2013) with a $\sim$3$\times$ longer exposure time. Our analysis of these Ly$\alpha$ emission line profiles reveal a significant asymmetric shape. The two detected Ly$\alpha$ emission lines from bright sources ($M_{\text{UV}}<-20.25$) could imply that these bright galaxies inhabit ionized bubbles in a partially neutral IGM, although deeper exposures may yet reveal Ly$\alpha$ emission in the fainter sources., Comment: 10 pages, 2 figures, 3 tables, Submitted to ApJ

The distribution of Ly$\alpha$ emission is an presently accessible method for studying the state of the intergalactic medium (IGM) into the reionization era. We carried out deep spectroscopic observations in order to search for Ly$\alpha$ emission from galaxies with photometric redshifts $z$ = 5.5 - 8.3 selected from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). Utilizing data from the Keck/DEIMOS spectrograph, we explore a wavelength coverage of Ly$\alpha$ emission at $z$ ~ 5 - 7 with four nights of spectroscopic observations for 118 galaxies, detecting five emission lines with ~ 5$\sigma$ significance: three in the GOODS-N and two in the GOODS-S field. We constrain the equivalent width (EW) distribution of Ly$\alpha$ emission by comparing the number of detected objects with the expected number constructed from detailed simulations of mock emission lines that account for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The Ly$\alpha$ EW distribution is well described by an exponential form, $\text{dN/dEW}\propto \text{exp(-EW/}W_0)$, characterized by the $e$-folding scale ($W_0$) of ~ 60 - 100$\AA$ at 0.3 < $z$ < 6. By contrast, our measure of the Ly$\alpha$ EW distribution at 6.0 < $z$ < 7.0 rejects a Ly$\alpha$ EW distribution with $W_0$ > 36.4$\AA$ (125.3$\AA$) at 1$\sigma$ (2$\sigma$) significance. This provides additional evidence that the EW distribution of Ly$\alpha$ declines at $z$ > 6, suggesting an increasing fraction of neutral hydrogen in the IGM at that epoch., Comment: 16 pages, 12 figures, 3 tables, Submitted to ApJ

We perform the first spatially-resolved stellar population study of galaxies in the early universe (z = 3.5 - 6.5), utilizing the Hubble Space Telescope Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) imaging dataset over the GOODS-S field. We select a sample of 418 bright and extended galaxies at z = 3.5 - 6.5 from a parent sample of ~ 8000 photometric-redshift selected galaxies from Finkelstein et al. (2015). We first examine galaxies at 3.5< z < 4.0 using additional deep K-band survey data from the HAWK-I UDS and GOODS Survey (HUGS) which covers the 4000A break at these redshifts. We measure the stellar mass, star formation rate, and dust extinction for galaxy inner and outer regions via spatially-resolved spectral energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By comparing specific star formation rates (sSFRs) between inner and outer parts of the galaxies we find that the majority of galaxies with the high central mass densities show evidence for a preferentially lower sSFR in their centers than in their outer regions, indicative of reduced sSFRs in their central regions. We also study galaxies at z ~ 5 and 6 (here limited to high spatial resolution in the rest-frame ultraviolet only), finding that they show sSFRs which are generally independent of radial distance from the center of the galaxies. This indicates that stars are formed uniformly at all radii in massive galaxies at z ~ 5 - 6, contrary to massive galaxies at z < 4., Comment: Accepted to ApJ, 20 pages, 15 figures

We present a study of the excitation conditions and metallicity of ionized gas (Z_gas) in eight nearby barred and unbarred spiral galaxies from the VIRUS-P Exploration of Nearby Galaxies (VENGA) survey, which provides high spatial sampling and resolution (median ~ 387 pc), large coverage from the bulge to outer disc, broad wavelength range (3600-6800 Ang.), and medium spectral resolution (~ 120 km s^-1 at 5000 Ang.). Our results are: (1) We present high resolution gas excitation maps to differentiate between regions with excitation typical of Seyfert, LINER, or recent star formation. We find LINER-type excitation at large distances (3-10 kpc) from the centre, and associate this excitation with diffuse ionized gas (DIG). (2) After excluding spaxels dominated by Seyfert, LINER, or DIG, we produce maps with the best spatial resolution and sampling to date of the ionization parameter q, star formation rate, and Z_gas using common strong line diagnostics. We find that isolated barred and unbarred spirals exhibit similarly shallow Z_gas profiles from the inner kpc out to large radii (7-10 kpc or 0.5-1.0 R_25). This implies that if profiles had steeper gradients at earlier epochs, then the present-day bar is not the primary driver flattening gradients over time. This result contradicts earlier claims, but agrees with recent IFU studies. (3) The Z_gas gradients in our z ~ 0 massive spirals are markedly shallower, by ~ 0.2 dex kpc^-1, than published gradients for lensed lower mass galaxies at z ~ 1.5-2.0. Cosmologically-motivated hydrodynamical simulations best match this inferred evolution, but the match is sensitive to adopted stellar feedback prescriptions., Comment: 48 pages, 16 figures, 6 tables. Accepted to MNRAS

We report the results from some of the deepest Keck/Multi-Object Spectrometer For Infra-Red Exploration data yet obtained for candidate $z \gtrsim 7$ galaxies. Our data show one significant line detection with 6.5$\sigma$ significance in our combined 10 hr of integration which is independently detected on more than one night, thus ruling out the possibility that the detection is spurious. The asymmetric line profile and non-detection in the optical bands strongly imply that the detected line is Ly$\alpha$ emission from a galaxy at $z$(Ly$\alpha)=7.6637 \pm 0.0011$, making it the fourth spectroscopically confirmed galaxy via Ly$\alpha$ at $z>7.5$. This galaxy is bright in the rest-frame ultraviolet (UV; $M_{\rm UV} \sim -21.2$) with a moderately blue UV slope ($\beta=-2.2^{+0.3}_{-0.2}$), and exhibits a rest-frame Ly$\alpha$ equivalent width of EW(Ly$\alpha$) $\sim 15.6^{+5.9}_{-3.6}$ \AA. The non-detection of the 11 other $z \sim$ 7-8 galaxies in our long 10 hr integration, reaching a median 5$\sigma$ sensitivity of 28 \AA\ in the rest-frame EW(Ly$\alpha$), implies a 1.3$\sigma$ deviation from the null hypothesis of a non-evolving distribution in the rest-frame EW(Ly$\alpha$) between $36.5$, which may signal the evolving neutral fraction in the intergalactic medium at the end of the reionization epoch, although our weak evidence suggests the need for a larger statistical sample to allow for a more robust conclusion., Comment: 10 pages, 4 figures, ApJ, in press

We present galaxy stellar mass functions (GSMFs) at $z=$ 4-8 from a rest-frame ultraviolet (UV) selected sample of $\sim$4500 galaxies, found via photometric redshifts over an area of $\sim$280 arcmin$^2$ in the CANDELS/GOODS fields and the Hubble Ultra Deep Field. The deepest Spitzer/IRAC data yet-to-date and the relatively large volume allow us to place a better constraint at both the low- and high-mass ends of the GSMFs compared to previous space-based studies from pre-CANDELS observations. Supplemented by a stacking analysis, we find a linear correlation between the rest-frame UV absolute magnitude at 1500 \AA\ ($M_{\rm UV}$) and logarithmic stellar mass ($\log M_*$) that holds for galaxies with $\log(M_*/M_{\odot}) \lesssim 10$. We use simulations to validate our method of measuring the slope of the $\log M_*$-$M_{\rm UV}$ relation, finding that the bias is minimized with a hybrid technique combining photometry of individual bright galaxies with stacked photometry for faint galaxies. The resultant measured slopes do not significantly evolve over $z=$ 4-8, while the normalization of the trend exhibits a weak evolution toward lower masses at higher redshift. We combine the $\log M_*$-$M_{\rm UV}$ distribution with observed rest-frame UV luminosity functions at each redshift to derive the GSMFs, finding that the low-mass-end slope becomes steeper with increasing redshift from $\alpha=-1.55^{+0.08}_{-0.07}$ at $z=4$ to $\alpha=-2.25^{+0.72}_{-0.35}$ at $z=8$. The inferred stellar mass density, when integrated over $M_*=10^8$-$10^{13} M_{\odot}$, increases by a factor of $10^{+30}_{-2}$ between $z=7$ and $z=4$ and is in good agreement with the time integral of the cosmic star formation rate density., Comment: 27 pages, 17 figures, ApJ, in press

Recent observations have shown that the characteristic luminosity of the rest-frame ultraviolet (UV) luminosity function does not significantly evolve at 4 < z < 7 and is approximately M*_UV ~ -21. We investigate this apparent non-evolution by examining a sample of 178 bright, M_UV < -21 galaxies at z=4 to 7, analyzing their stellar populations and host halo masses. Including deep Spitzer/IRAC imaging to constrain the rest-frame optical light, we find that M*_UV galaxies at z=4-7 have similar stellar masses of log(M/Msol)=9.6-9.9 and are thus relatively massive for these high redshifts. However, bright galaxies at z=4-7 are less massive and have younger inferred ages than similarly bright galaxies at z=2-3, even though the two populations have similar star formation rates and levels of dust attenuation. We match the abundances of these bright z=4-7 galaxies to halo mass functions from the Bolshoi Lambda-CDM simulation to estimate the halo masses. We find that the typical halo masses in ~M*_UV galaxies decrease from log(M_h/Msol)=11.9 at z=4 to log(M_h/Msol)=11.4 at z=7. Thus, although we are studying galaxies at a similar mass across multiple redshifts, these galaxies live in lower mass halos at higher redshift. The stellar baryon fraction in units of the cosmic mean Omega_b/Omega_m rises from 5.1% at z=4 to 11.7% at z=7; this evolution is significant at the ~3-sigma level. This rise does not agree with simple expectations of how galaxies grow, and implies that some effect, perhaps a diminishing efficiency of feedback, is allowing a higher fraction of available baryons to be converted into stars at high redshifts., Comment: Accepted to ApJ. 15 pages, 5 figures, 6 tables

We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity function at z=4-8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Year 1 Hubble Frontier Field deep parallel observations. These surveys provides an effective volume of 0.6-1.2 x 10^6 Mpc^3 over this epoch, allowing us to perform a robust search for faint (M_UV=-18) and bright (M_UV < -21) galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 galaxies at 3.51000 galaxies at z~6-8. We measure the luminosity function using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our results agree with previous studies, yet we find a higher abundance of UV-bright galaxies at z>6, with M* ~ -21 at z>5, different than that inferred based on previous trends at lower redshift. At z=8, a single power-law provides an equally good fit to the UV luminosity function, while at z=6 and 7, an exponential cutoff at the bright-end is moderately preferred. We compare to semi-analytical models, and find that the lack of evolution in M* is consistent with models where the impact of dust attenuation on the bright-end of the luminosity function decreases at higher redshift. We measure the evolution of the cosmic star-formation rate density, correcting for dust attenuation, and find that it declines as (1+z)^(-4.3 +/- 0.5) at z>4, consistent with observations at z>9. Our observations are consistent with a reionization history that starts at z>10, completes at z>6, and reaches a midpoint (x_HII = 0.5) at 6.7

We present the results from a VLT/SINFONI and Keck/NIRSPEC near-infrared spectroscopic survey of 16 Lyman-alpha emitters (LAEs) at $z$ = 2.1 - 2.5 in the COSMOS and GOODS-N fields discovered from the HETDEX Pilot Survey. We detect rest-frame optical nebular lines (H$\alpha$ and/or [OIII]$\lambda$5007) for 10 of the LAEs and measure physical properties, including the star formation rate (SFR), gas-phase metallicity, gas-mass fraction, and Ly$\alpha$ velocity offset. We find that LAEs may lie below the mass-metallicity relation for continuum-selected star-forming galaxies at the same redshift. The LAEs all show velocity shifts of Ly$\alpha$ relative to the systemic redshift ranging between +85 and +296 km s$^{-1}$ with a mean of +180 km s$^{-1}$. This value is smaller than measured for continuum-selected star-forming galaxies at similar redshifts. The Ly$\alpha$ velocity offsets show a moderate correlation with the measured star formation rate (2.5$\sigma$), but no significant correlations are seen with the SFR surface density, specific SFR, stellar mass, or dynamical mass ($\lesssim$ 1.5$\sigma$). Exploring the role of dust, kinematics of the interstellar medium (ISM), and geometry on the escape of Ly$\alpha$ photons, we find no signature of selective quenching of resonantly scattered Ly$\alpha$ photons. However, we also find no evidence that a clumpy ISM is enhancing the Ly$\alpha$ equivalent width. Our results suggest that the low metallicity in LAEs may be responsible for yielding an environment with a low neutral hydrogen column density as well as less dust, easing the escape of Ly$\alpha$ photons over that in continuum-selected star-forming galaxies., Comment: 26 pages, accepted for publication in ApJ

The observed deficit of strongly Lyman-alpha emitting galaxies at z>6.5 is attributed to either increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this, we have performed very deep near-IR spectroscopy of z>7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lyman-alpha fraction at z~8 (combined photometric redshift peak at z=7.7) using two methods. First, we derived NLy{\alpha}/Ntot directly using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al. 2012) that compares the z>7 galaxy spectra to models of the Lyman-alpha equivalent width (WLy{\alpha}) distribution at z~6. We explored two simple evolutionary scenarios: smooth evolution where Lyman-alpha is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity), and patchy evolution where Lyman-alpha is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature we find that at z~8 the Lyman-alpha fraction has dropped by a factor >3(84% confidence interval) using both the smooth and patchy scenarios compared to the z~6 values. Furthermore, we find a tentative evidence that the data favor the patchy scenario over smooth (with "positive" Bayesian evidence), extending trends observed at z~7 to higher redshift. If this decrease is a result of reionization as predicted by theory, then our data imply the volume averaged neutral hydrogen fraction in the IGM to be >0.3 suggesting that the reionization of the universe is in progress at z~8., Comment: Published in ApJ

We use broadband photometry extending from the rest-frame UV to the near-IR to fit the individual spectral energy distributions (SEDs) of 63 bright (L(Ly-alpha) > 10^43 ergs/s) Ly-alpha emitting galaxies (LAEs) in the redshift range 1.9 < z < 3.6. We find that these LAEs are quite heterogeneous, with stellar masses that span over three orders of magnitude, from 7.5 < log M < 10.5. Moreover, although most LAEs have small amounts of extinction, some high-mass objects have stellar reddenings as large as E(B-V) ~0.4. Interestingly, in dusty objects the optical depths for Ly-alpha and the UV continuum are always similar, indicating that Ly-alpha photons are not undergoing many scatters before escaping their galaxy. In contrast, the ratio of optical depths in low-reddening systems can vary widely, illustrating the diverse nature of the systems. Finally, we show that in the star formation rate (SFR)-log mass diagram, our LAEs fall above the "main-sequence" defined by z ~ 3 continuum selected star-forming galaxies. In this respect, they are similar to sub-mm-selected galaxies, although most LAEs have much lower mass., Comment: Accepted to the ApJ

We present new results on the spectrally resolved Lyman-alpha (LyA) emission of three LyA emitting field galaxies at z~2.4 with high LyA equivalent width (>100 Angstroms) and LyA luminosity (~10^43 erg/s). At 120 km/s (FWHM) spectral resolution, the prominent double-peaked LyA profile straddles the systemic velocity, where the velocity zero-point is determined from spectroscopy of the galaxies' rest-frame optical nebular emission lines. The average velocity offset from systemic of the stronger redshifted emission component for our sample is 176 km/s while the average total separation between the redshifted and main blueshifted emission components is 380 km/s. These measurements are a factor of ~2 smaller than for UV continuum-selected galaxies that show LyA in emission with lower LyA equivalent width. We compare our LyA spectra to the predicted line profiles of a spherical "expanding shell" LyA radiative transfer grid that models large-scale galaxy outflows. Specifically blueward of the systemic velocity where two galaxies show a weak, highly blueshifted (by ~1000 km/s) tertiary emission peak, the model line profiles are a relatively poor representation of the observed spectra. Since the neutral gas column density has a dominant influence over the shape of the LyA line profile, we caution against equating the observed LyA velocity offset with a physical outflow velocity, especially at lower spectral resolution where the unresolved LyA velocity offset is a convoluted function of several degenerate parameters. Referring to rest-frame ultraviolet and optical Hubble Space Telescope imaging, we find that galaxy-galaxy interactions may play an important role in inducing a starburst that results in copious LyA emission, as well as perturbing the gas distribution and velocity field which have strong influence over the LyA emission line profile., Comment: 20 pages, 8 figures, 3 tables; Accepted for publication in The Astrophysical Journal; Updated v2 fixes incorrectly processed LaTeX symbols

We present the survey design, data reduction, and spectral fitting pipeline for the VIRUS-P Exploration of Nearby Galaxies (VENGA). VENGA is an integral field spectroscopic survey, which maps the disks of 30 nearby spiral galaxies. Targets span a wide range in Hubble type, star formation activity, morphology, and inclination. The VENGA data-cubes have 5.6'' FWHM spatial resolution, ~5A FWHM spectral resolution, sample the 3600A-6800A range, and cover large areas typically sampling galaxies out to ~0.7 R_25. These data-cubes can be used to produce 2D maps of the star formation rate, dust extinction, electron density, stellar population parameters, the kinematics and chemical abundances of both stars and ionized gas, and other physical quantities derived from the fitting of the stellar spectrum and the measurement of nebular emission lines. To exemplify our methods and the quality of the data, we present the VENGA data-cube on the face-on Sc galaxy NGC 628 (a.k.a. M 74). The VENGA observations of NGC 628 are described, as well as the construction of the data-cube, our spectral fitting method, and the fitting of the stellar and ionized gas velocity fields. We also propose a new method to measure the inclination of nearly face-on systems based on the matching of the stellar and gas rotation curves using asymmetric drift corrections. VENGA will measure relevant physical parameters across different environments within these galaxies, allowing a series of studies on star formation, structure assembly, stellar populations, chemical evolution, galactic feedback, nuclear activity, and the properties of the interstellar medium in massive disk galaxies., Comment: Accepted for publication in AJ, 25 pages, 18 figures, 6 tables

We measure the radial profile of the 12CO(1-0) to H_2 conversion factor (Xco) in NGC 628. The H\alpha emission from the VENGA integral field spectroscopy is used to map the star formation rate surface density (\Sigma_{SFR}). We estimate the molecular gas surface density (\Sigma_{H2}) from \Sigma_{SFR} by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure Xco. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (H\alpha vs. far-UV plus 24\mu m), and CO maps from different telescopes (single-dish and interferometers). The observed Xco profile is robust against these systematics, drops by a factor of 2 from R~7 kpc to the center of the galaxy, and is well fit by a gradient \Delta log(Xco)=0.06\pm0.02 dex kpc^-1. We study how changes in Xco follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in Xco can be explained by a combination of decreasing metallicity, and decreasing \Sigma_{H2} with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of Xco in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single Xco value when large changes in gas surface density or metallicity are present., Comment: 10 pages, 7 figures, accepted for publication in The Astrophysical Journal

We report the discovery of 40 bright quasars and active galactic nuclei (AGNs) at low Galactic latitude (b<20deg). The low Galactic latitude region has been considered a place to avoid when searching for extragalactic sources, because of the high Galactic extinction, as well as a large number of stars contaminating the sample selection. Bright quasars (R<~17) suffer more from such difficulties because they look like bright stars, which are numerous at low b, yet their surface number density is very low. In order to find quasars in this region of the sky less explored for extragalactic sources, we have started a survey of low Galactic latitude bright quasars as a part of the Seoul National University Quasar Survey in Optical (SNUQSO). Quasar candidates have been selected from radio and near-infrared (NIR) data. Out of 88 targets, we identify 29 bright quasars/AGNs around the antigalactic center, and 11 bright quasars/AGNs in the outskirts of the Galactic center, from two observing runs in 2006 at the Bohyunsan Optical Astronomical Observatory (BOAO) in Korea. Our finding demonstrates that quasars/AGNs can be discovered effectively, even at low Galactic latitude, using multiwavelength data., Comment: 16 pages, 3 figures

We perform the first spatially-resolved stellar population study of galaxies in the early universe z equals 3.5 -6.5, utilizing the Hubble Space Telescope Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) imaging dataset over the GOODS-S field. We select a sample of 418 bright and extended galaxies at z less than or approximately equal to 3.5-6.5 from a parent sample of approximately 8000 photometric-redshift selected galaxies from Finkelstein et al. We first examine galaxies at 3.5 less than or equal to z less than or approximately equal to 4.0 using additional deep K-band survey data from the HAWK-I UDS and GOODS Survey (HUGS) which covers the 4000 Angstrom break at these redshifts. We measure the stellar mass, star formation rate, and dust extinction for galaxy inner and outer regions via spatially-resolved spectral energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By comparing specific star formation rates (sSFRs) between inner and outer parts of the galaxies we find that the majority of galaxies with the high central mass densities show evidence for a preferentially lower sSFR in their centers than in their outer regions, indicative of reduced sSFRs in their central regions. We also study galaxies at z approximately equal to 5 and 6 (here limited to high spatial resolution in the rest-frame ultraviolet only), finding that they show sSFRs which are generally independent of radial distance from the center of the galaxies. This indicates that stars are formed uniformly at all radii in massive galaxies at z approximately equal to 5-6, contrary tomassive galaxies at z. less than approximately equal to 4.

We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity function at z = 4 to 8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1- 2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 ×10(exp 6) Mpc(exp 3) over this epoch, allowing us to perform a robust search for bright (M(sub UV) less than −21) and faint (M(sub UV) = −18) galaxies. We select galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 galaxies at 3.5 less than z less than 8.5, with more than 1000 galaxies at z of approximately 6 - 8. We measure both a stepwise luminosity function for galaxies in our redshift samples, as well as a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright galaxies at z of greater than or equal to 6. Our bestfit value of the characteristic magnitude M* is consistent with −21 at z of greater than or equal to 5, different than that inferred based on previous trends at lower redshift. At z = 8, a single power-law provides an equally good fit to the UV luminosity function, while at z = 6 and 7, an exponential cutoff at the bright-end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in M* is consistent with models where the impact of dust attenuation on the bright-end of the luminosity function decreases at higher redshift, though a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to M(sub UV) = −17, correcting for dust attenuation, and find that the SFR density declines proportionally to (1 + z)((exp −4.3)(+/-)(0.5)) at z greater than 4, consistent with observations at z greater than or equal to 9. Our observed luminosity functions are consistent with a reionization history that starts at redshift of approximately greater than 10, completes at z greater than 6, and reaches a midpoint (x(sub HII) = 0.5) at 6.7 less than z less than 9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, though consistent with recent estimates of bright z similar to 10 galaxies.