arXiv subjects

Dawn K. Erb

Publications and source records attributed to Dawn K. Erb.

70 records · Page 4Linked to original sources

arXiv2007

Morphologies of Galaxies in and around a Protocluster at z=2.300

We present results from the first robust investigation of galaxy morphology as a function of environment at z>1.5. Our study is motivated by the fact that star-forming galaxies contained within a protocluster at z=2.3 in the HS1700+64 field have significantly older ages and larger stellar masses on average than those at similar redshifts but more typical environmental densities. In the analysis of HST/ACS images, we apply non-parametric statistics to characterize the rest-frame UV morphologies of a sample of 85 UV-selected star-forming galaxies at z=1.7-2.9, 22 of which are contained in the protocluster. The remaining 63 control-sample galaxies are not in the protocluster but have a similar mean redshift of ~2.3. We find no environmental dependence for the distributions of morphological properties. Combining the measured morphologies with the results of population synthesis modeling, we find only weak correlations, if any, between morphological and stellar population properties such as stellar mass, age, extinction and star-formation rate. Given the incomplete census of the protocluster galaxy population, and the lack of correlation between rest-frame UV morphology and star-formation history at z~2 within our sample, the absence of environmental trends in the distribution of morphological properties is not surprising. Additionally, using a larger sample of photometric candidates, we compare morphological distributions for 282 UV-selected and 43 near-IR-selected galaxies. While the difference in the degree of nebulosity between the two samples appears to be a byproduct of the fainter average rest-frame UV surface brightness of the near-IR-selected galaxies, we find that, among the lowest surface brightness galaxies, the near-IR-selected objects have significantly smaller angular sizes (abridged).

astro-ph↗

arXiv2006

The Physical Nature of Rest-UV Galaxy Morphology During the Peak Epoch of Galaxy Formation

Motivated by the irregular and little-understood morphologies of z ~ 2 - 3 galaxies, we use non-parametric coefficents to quantify the morphologies of 216 galaxies which have been spectroscopically confirmed to lie at redshifts z = 1.8 - 3.4 in the GOODS-N field. Using measurements of ultraviolet (UV) and optical spectral lines, multi-band photometric data, and stellar population models we statistically assess possible correlations between galaxy morphology and physical observables such as stellar mass, star formation rate, and the strength of galaxy-scale outflows. We find evidence that dustier galaxies have more nebulous UV morphologies and that larger, more luminous galaxies may drive stronger outflows, but otherwise conclude that UV morphology is either statistically decoupled from the majority of physical observables or determined by too complex a combination of physical processes to provide characterizations with predictive power. Given the absence of strong correlations between UV morphology and physical parameters such as star formation rates, we are therefore unable to support the hypothesis that morphologically irregular galaxies predominantly represent major galaxy mergers. Comparing galaxy samples, we find that IR-selected BzK galaxies and radio-selected submillimeter galaxies (SMGs) have UV morphologies similar to the optically selected sample, while distant red galaxies (DRGs) are more nebulous.

astro-ph↗

arXiv2006

A Spectroscopic Survey of Redshift 1.4<z<3.0 Galaxies in the Goods-North Field: Survey Description, Catalogs, and Properties

We present the results of a spectroscopic survey with the Keck I telescope of more than 280 star-forming galaxies and AGN at redshifts 1.4 1.4 in GOODS-N. Extensive multi-wavelength data allow us to investigate the stellar populations, stellar masses, bolometric luminosities (L_bol), and extinction of z~2 galaxies. Deep Chandra and Spitzer data indicate that the sample includes galaxies with a wide range in L_bol, from 10^10 L_sun to >10^12 L_sun, and covering 4 orders of magnitude in dust obscuration (L_bol/L_UV). The sample includes galaxies with a large dynamic range in evolutionary state, from very young galaxies (ages <50 Myr) with small stellar masses (M* ~ 10^9 M_sun) to evolved galaxies (ages >2 Gyr) with stellar masses comparable to the most massive galaxies at these redshifts (M* > 10^11 M_sun). Spitzer data indicate that the optical sample includes some fraction of the obscured AGN population at high redshifts: at least 3 of 11 AGN in the z>1.4 sample are undetected in the deep X-ray data but exhibit power-law SEDs longward of ~2 micron (rest-frame) indicative of obscured AGN. The results of our survey indicate that rest-frame UV selection and spectroscopy presently constitute the most time-wise efficient method of culling large samples of high redshift galaxies with a wide range in intrinsic properties, and the data presented here will add significantly to the multi-wavelength legacy of the GOODS survey. [Abridged]

astro-ph↗

arXiv2006

The Direct Detection of Lyman-Continuum Emission from Star-forming Galaxies at z~3

We present the results of rest-frame ultraviolet spectroscopic observations of a sample of 14 z~3 star-forming galaxies in the SSA22a field. These spectra are characterized by unprecedented depth in the Lyman-continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with two of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 AA to that in the Lyman-continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_1500/f_900,corr(C49)=4.5 and f_1500/f_900,corr(D3)=2.9. The average emergent flux-density ratio in our sample is =22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum in Steidel et al. (2001). If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_nu(900)~2.6x10^{-22} erg/s/cm^2/Hz/sr, comparable to the contribution of optically-selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z~3, inferred from the HI Ly-alpha forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or non-detection of Lyman-continuum emission from galaxies are not well-determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z~3 and beyond (abridged).

astro-ph↗

arXiv2006

H-alpha Observations of a Large Sample of Galaxies at z~2: Implications for Star Formation in High Redshift Galaxies

Using H-alpha spectra of 114 rest-frame UV-selected galaxies at z~2, we compare inferred star formation rates (SFRs) with those determined from the UV continuum luminosity. After correcting for extinction using standard techniques based on the UV continuum slope, we find excellent agreement between the indicators, with = 31 Msun/yr and = 29 Msun/yr. The agreement between the indicators suggests that the UV luminosity is attenuated by an typical factor of ~4.5 (with a range from no attenuation to a factor of ~100 for the most obscured object in the sample), in good agreement with estimates of obscuration from X-ray, radio and mid-IR data. The H-alpha luminosity is attenuated by a factor of ~1.7 on average, and the maximum H-alpha attenuation is a factor of ~5. In agreement with X-ray and mid-IR studies, we find that the SFR increases with increasing stellar mass and at brighter K magnitudes, to ~ 60 Msun/yr for galaxies with K<20; the correlation between K magnitude and SFR is much stronger than the correlation between stellar mass and SFR. All galaxies in the sample have SFRs per unit area Sigma_SFR in the range observed in local starbursts. We compare the instantaneous SFRs and the past average SFRs as inferred from the ages and stellar masses, finding that for most of the sample, the current SFR is an adequate representation of the past average. There is some evidence that the most massive galaxies (M_star >10^11 Msun) have had higher SFRs in the past.

astro-ph↗

arXiv2006

The Stellar, Gas and Dynamical Masses of Star-Forming Galaxies at z~2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z~2. By combining the H-alpha spectra with photometric measurements from observed 0.3--8 micron, we assess the relationships between kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The H-alpha line widths give a mean dynamical mass M_dyn=6.9 +/- 0.6 x 10^10 Msun within a typical radius of ~6 kpc, after excluding AGN. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has M_dyn >> M_star. These objects are best fit by young stellar populations and tend to have high H-alpha equivalent widths, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 sigma significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation, and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of three for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars. [Abridged]

astro-ph↗

arXiv2006

Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations

Using very deep Spitzer/MIPS 24 micron observations, we present an analysis of the bolometric luminosities (L[bol]) and UV extinction properties of more than 200 spectroscopically identified, optically selected (UGR) z~2 galaxies in the GOODS-N field. The large spectroscopic sample is supplemented with near-IR selected (BzK/DRG) galaxies and submm sources at similar redshifts in the same field, providing a representative collection of relatively massive (M*>1e10 Msun) galaxies at high redshifts. We focus on the redshift range 1.5-2.6, where MIPS is sensitive to the strength of the mid-IR PAH features in the galaxy spectra (rest-frame 5-8.5 micron). We demonstrate, using stacked X-ray data and a subset of galaxies with H-alpha measurements, that L(5-8.5) provides a reliable estimate of L(IR) for most star forming galaxies at z~2. The range of L(IR) in the samples considered extends from ~1e10 to >1e12 Lsun, with a mean of 2e11 Lsun. Using 24 micron observations to infer dust extinction in high redshift galaxies, we find that, as in the local universe, the obscuration (L[IR]/L[1600]) is strongly dependent on L(bol), and ranges in value from <1 to \~1000. However, the obscuration is ~10 times smaller at a given L(bol) at z~2 than at z=0. We show that the values of L(IR) and obscuration inferred from the UV spectral slope beta generally agree well with the values inferred from L(5-8.5) for L(bol)<1e12 Lsun. For galaxies with L(bol)>1e12 Lsun, it is common for UV-based estimates to underpredict L(IR) by a factor of ~10-100. Using the specific SFR as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z~2, from those which have just begun to form stars to those which have already accumulated most of their stellar mass and are about to become, or already are, passively-evolving. [Abridged]

astro-ph↗

arXiv2006

The Mass-Metallicity Relation at z~2

We use a sample of 87 rest-frame UV-selected star-forming galaxies with mean spectroscopic redshift z=2.26 to study the correlation between metallicity and stellar mass at high redshift. Using stellar masses determined from SED fitting to 0.3-8 micron photometry, we divide the sample into six bins in stellar mass, and construct six composite H-alpha+[NII] spectra from all of the objects in each bin. We estimate the mean oxygen abundance in each bin from the [NII]/H-alpha ratio, and find a monotonic increase in metallicity with increasing stellar mass, from 12+log(O/H) < 8.2 for galaxies with = 2.7e9 Msun to 12+log(O/H) = 8.6 for galaxies with = 1e11 Msun. We use the empirical relation between star formation rate density and gas density to estimate the gas fractions of the galaxies, finding an increase in gas fraction with decreasing stellar mass. These gas fractions combined with the observed metallicities allow the estimation of the effective yield y_eff as a function of stellar mass; in constrast to observations in the local universe which show a decrease in y_eff with decreasing baryonic mass, we find a slight increase. Such a variation of metallicity with gas fraction is best fit by a model with supersolar yield and an outflow rate ~4 times higher than the star formation rate. We conclude that the mass-metallicity relation at high redshift is driven by the increase in metallicity as the gas fraction decreases through star formation, and is likely modulated by metal loss from strong outflows in galaxies of all masses. There is no evidence for preferential loss of metals from low mass galaxies as has been suggested in the local universe. [Abridged]

astro-ph↗

arXiv2005

Predictions and Strategies for Integral-Field Spectroscopy of High-Redshift Galaxies

We investigate the ability of infrared integral-field spectrographs to map the velocity fields of high redshift galaxies, presenting a formalism which may be applied to any telescope and imaging spectrograph system. We discuss the 5-sigma limiting line fluxes which current integral-field spectrographs will reach, and extend this discussion to consider future large aperture telescopes with cryogenically cooled adaptive reimaging optics. In particular, we simulate observations of spectral line emission from star-forming regions at redshifts z = 0.5 to 2.5 using a variety of spatial sampling scales and give predictions for the signal-to-noise ratio expected as a function of redshift. Using values characteristic of the W.M. Keck II telescope and the new OH-Suppressing InfraRed Imaging Spectrograph (OSIRIS) we calculate integral-field signal-to-noise ratio maps for a sample of U_nGR color-selected star-forming galaxies at redshift z ~ 2 - 2.6 and demonstrate that OSIRIS will be able to reconstruct the two-dimensional projected velocity fields of these galaxies on scales of 100 mas (~ 1 kpc at redshift z ~ 2). With signal-to-noise ratios per spatial sample up to ~ 20, OSIRIS will in some cases be able to distinguish between merger activity and ordered disk rotation. Structures on scales smaller than 1 kpc may be detected by OSIRIS for particularly bright sources, and will be easy targets for future 30m class telescopes.

astro-ph↗

arXiv2005

A Census of Optical and Near-Infrared Selected Star-Forming and Passively Evolving Galaxies at Redshift Z~2

Using the extensive multi-wavelength data in the GOODS-North field, we construct and draw comparisons between samples of optical and near-IR selected star-forming and passively evolving galaxies at redshifts 1.4 2.3 galaxies (Distant Red Galaxies; DRGs) are very similar as a function of K, with K<20 galaxies having ~120 Msun/yr, a factor of two to three higher than those with K>20.5. The absence of X-ray emission from the reddest DRGs and BzK galaxies with z-K>3 indicates they must have declining star formation histories to explain their red colors and low SFRs. While the M/L ratio of passively-evolving galaxies may be larger on average, the Spitzer/IRAC data indicate that their inferred stellar masses do not exceed the range spanned by optically selected galaxies, suggesting that the disparity in current SFR may not indicate a fundamental difference between optical and near-IR selected massive galaxies (M* > 10^11 Msun). We consider the contribution of UGR, BzK, DRG, and submillimeter galaxies (SMGs) to the SFRD at z~2, taking into account sample overlap. The total SFRD in the interval 1.4 80% could be selected by the UGR, BzK, and/or DRG criteria.

astro-ph↗

arXiv2005

Spectroscopic Identification of a Proto-Cluster at z=2.300: Environmental Dependence of Galaxy Properties at High Redshift

We have discovered a highly significant over-density of galaxies at z=2.300+/-0.015 in the course of a redshift survey designed to select star-forming galaxies in the redshift range z=2.3+/-0.4 in the field of the bright z=2.72 QSO HS1700+643. The structure has a redshift-space galaxy over-density of delta_g,z ~= 7 and an estimated matter over-density in real space of delta_m ~= 1.8, indicating that it will virialize by z~0 with a mass scale of ~= 1.4x10^15 M_sun, that of a rich galaxy cluster. Detailed modeling of the spectral energy distribution -- from the rest-far-UV to the rest-near-IR -- of the 72 spectroscopically confirmed galaxies in this field for which we have obtained K_s and Spitzer/IRAC photometry, allows for a first direct comparison of galaxy properties as a function of large-scale environment at high redshift. We find that galaxies in the proto-cluster environment have mean stellar masses and inferred ages that are ~2 times larger (at z=2.30) than identically UV-selected galaxies outside of the structure, and show that this is consistent with simple theoretical expectations for the acceleration of structure formation in a region that is over-dense on large scales by the observed amount. The proto-cluster environment contains a significant number of galaxies that already appear old, with large stellar masses (>10^11 M_sun), by z=2.3.

astro-ph↗

arXiv2005

UV to Mid-IR Observations of Star-forming Galaxies at z~2: Stellar Masses and Stellar Populations

We present the broad-band UV through mid-infrared spectral energy distributions (SEDs) of a sample of 72 spectroscopically-confirmed star-forming galaxies at z=2.30+/-0.3. Located in a 72 arcmin-squared field centered on the bright background QSO, HS1700+643, these galaxies were pre-selected to lie at z~2 based solely on their rest-frame UV colors and luminosities, and should be representative of UV-selected samples at high redshift. In addition to deep ground-based photometry spanning from 0.35-2.15 microns, we make use of Spitzer/IRAC data, which probes the rest-frame near-IR at z~2. The range of stellar populations present in the sample is investigated with simple, single-component stellar population synthesis models. Emphasizing stellar mass estimates, which are less subject to systematic uncertainties than other parameters, we find =10.32+/-0.51 for the sample. Allowing for the possibility of episodic star formation, we find that typical galaxies in our sample could contain up to three times more stellar mass in an old underlying burst than what was inferred from single-component modeling. In contrast, mass estimates for the most massive galaxies in the sample (M*>10^11 Msun) are fairly insensitive to the method used to model the stellar population. Galaxies in this massive tail, which are also the oldest objects in the sample, could plausibly evolve into the passive galaxies discovered at z~1.7 with near-IR selection techniques. In the general framework of hierarchical galaxy formation and mergers, which implies episodic star-formation histories, galaxies at high redshift may pass in and out of UV-selected and near-IR color-selected samples as they evolve from phases of active star formation to quiescence and back again (Abridged).

astro-ph↗

arXiv2005

Strong Spatial Clustering of UV-selected Galaxies with Magnitude Ks<20.5 and Redshift Z~2

We obtained deep 8.5'x8.5' near-infrared images within four high-redshift survey fields, measured the Ks magnitudes of 300 optically selected galaxies with spectroscopic redshift 1.8 20.5. We found at greater than 95% confidence that the brighter galaxies cluster more strongly. The best-fit correlation lengths for the bright and faint samples are 10+-3 and 4+-0.8 comoving Mpc/h, respectively (1sigma), although the unusual density of bright QSOs in one of our survey fields may imply that the result is not representative of the universe as a whole. Neglecting this possibility, the correlation length for the optically selected sample with Ks<20.5 agrees well with that reported for comparably bright near-IR-selected samples. The differences in correlation length between optically selected and near-IR-selected samples have been presented as evidence that the two techniques find orthogonal populations of high-redshift galaxies. Our results favor a more nuanced view.

astro-ph↗

arXiv2004

The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field

We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the GOODS-N field. The galaxies were selected primarily because they appear to have elongated morphologies, and slits were aligned with the major axes (as determined from the rest-frame UV emission) of 11 of the 13. If the galaxies are elongated because they are highly inclined, alignment of the slit and major axis should maximize the observed velocity and reveal velocity shear, if present. In spite of this alignment, we see spatially resolved velocity shear in only two galaxies. We show that the seeing makes a large difference in the observed velocity spread of a tilted emission line, and use this information to place limits on the velocity spread of the ionized gas of the galaxies in the sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the velocity shear (half of the velocity range of a tilted emission line) that would be observed under our best seeing conditions of ~0.5". When combined with previous work, our data also indicate that aligning the slit along the major axis does not increase the probability of observing a tilted emission line. We then focus on the one-dimensional velocity dispersion σ, which is much less affected by the seeing, and see that the elongated subsample exhibits a significantly lower velocity dispersion than galaxies selected at random from our total H-alpha sample, not higher as one might have expected. We also see some evidence that the elongated galaxies are less reddened than those randomly selected using only UV colors. Both of these results are counter to what would be expected if the elongated galaxies were highly inclined disks. It is at least as likely that the galaxies' elongated morphologies are due to merging subunits.

astro-ph↗

arXiv2004

Optical Selection of Galaxies at Redshifts 1<z<3

Few galaxies have been found between the redshift ranges z < ~1 probed by magnitude-limited surveys and z > ~3 probed by Lyman-break surveys. Comparison of galaxy samples at lower and higher redshift suggests that large numbers of stars were born and the Hubble sequence began to take shape at the intermediate redshifts 1<z<3, but observational challenges have prevented us from observing the process in much detail. We present simple and efficient strategies that can be used to find large numbers of galaxies throughout this important but unexplored redshift range. All the strategies are based on selecting galaxies for spectroscopy on the basis of their colors in ground-based images taken through a small number of optical filters: GRi for redshifts 0.85<z<1.15, GRz for 1<z<1.5, and UGR for 1.4<z<2.1 and 1.9<z<2.7. The performance of our strategies is quantified empirically through spectroscopy of more than 2000 galaxies at 1<z<3.5. We estimate that more than half of the UV-luminosity density at 1<z<3 is produced by galaxies that satisfy our color-selection criteria. Our methodology is described in detail, allowing readers to devise analogous selection criteria for other optical filter sets.

astro-ph↗

arXiv2003

H-alpha Spectroscopy of Galaxies at z>2: Kinematics and Star Formation

We present near-infrared spectroscopy of H-alpha emission lines in a sample of 16 star-forming galaxies at redshifts 2.0 4e10 M_sun. One-dimensional velocity dispersions for the 16 galaxies range from ~50 to ~260 km/s, and in cases where we have both virial masses implied by the velocity dispersions and dynamical masses derived from the spatially extended emission lines, they are in rough agreement. We compare our kinematic results to similar measurements made at z~3, and find that both the observed rotational velocities and velocity dispersions tend to be larger at z~2 than at z~3. We find a mean SFR_H-alpha of 16 M_sun/yr and an average SFR_H-alpha/SFR_UV ratio of 2.4, without correcting for extinction. We see moderate evidence for an inverse correlation between the UV continuum luminosity and the ratio SFR_H-alpha/SFR_UV, such as might be observed if the UV-faint galaxies suffered greater extinction. We discuss the effects of dust and star formation history on the SFRs, and conclude that extinction is the most likely explanation for the discrepancy between the two SFRs.

astro-ph↗