arXiv subjects

Danilo Marchesini

Publications and source records attributed to Danilo Marchesini.

At least 181 records · Page 10Linked to original sources

arXiv2012

3D-HST Grism Spectroscopy of a Gravitationally Lensed, Low-metallicity Starburst Galaxy at z=1.847

We present Hubble Space Telescope (HST) imaging and spectroscopy of the gravitational lens SL2SJ02176-0513, a cusp arc at z=1.847. The UV continuum of the lensed galaxy is very blue, which is seemingly at odds with its redder optical colors. The 3D-HST WFC3/G141 near-infrared spectrum of the lens reveals the source of this discrepancy to be extremely strong [OIII]5007 and H-beta emission lines with rest-frame equivalent widths of 2000 +/- 100 and 520 +/- 40 Angstroms, respectively. The source has a stellar mass ~10^8 Msun, sSFR \sim 100/Gyr, and detection of [OIII]4363 yields a metallicity of 12 + log(O/H) = 7.5 +/- 0.2. We identify local blue compact dwarf analogs to SL2SJ02176-0513, which are among the most metal-poor galaxies in the SDSS. The local analogs resemble the lensed galaxy in many ways, including UV/optical SED, spatial morphology and emission line equivalent widths and ratios. Common to SL2SJ02176-0513 and its local counterparts is an upturn at mid-IR wavelengths likely arising from hot dust heated by starbursts. The emission lines of SL2SJ02176-0513 are spatially resolved owing to the combination of the lens and the high spatial resolution of HST. The lensed galaxy is composed of two clumps with combined size r_e \sim 300 pc, and we resolve significant differences in UV color and emission line equivalent width between them. Though it has characteristics occasionally attributed to active galactic nuclei, we conclude that SL2SJ02176-0513 is a low-metallicity star-bursting dwarf galaxy. Such galaxies will be found in significant numbers in the full 3D-HST grism survey.

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arXiv2012

Large-Scale Star Formation-Driven Outflows at 1<z<2 in the 3D-HST Survey

We present evidence of large-scale outflows from three low-mass (log(M/M_sun)~9.75) star-forming (SFR >4 M_sun/yr) galaxies observed at z=1.24, z=1.35 and z=1.75 in the 3D-HST Survey. Each of these galaxies is located within a projected physical distance of 60 kpc around the sight line to the quasar SDSS J123622.93+621526.6, which exhibits well-separated strong (W_r>0.8A) Mg II absorption systems matching precisely to the redshifts of the three galaxies. We derive the star formation surface densities from the H-alpha emission in the WFC3 G141 grism observations for the galaxies and find that in each case the star formation surface density well-exceeds 0.1 M_sun/yr/kpc^2, the typical threshold for starburst galaxies in the local Universe. From a small but complete parallel census of the 0.65 0.8A Mg II covering fraction of star-forming galaxies at 1 0.4A Mg II absorbing gas around star-forming galaxies may evolve from z~2 to the present, consistent with recent observations of an increasing collimation of star formation-driven outflows with time from z~3.

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arXiv2012

3D-HST: A wide-field grism spectroscopic survey with the Hubble Space Telescope

We present 3D-HST, a near-infrared spectroscopic Treasury program with the Hubble Space Telescope for studying the processes that shape galaxies in the distant Universe. 3D-HST provides rest-frame optical spectra for a sample of ~7000 galaxies at 1<z<3.5, the epoch when 60% of all star formation took place, the number density of quasars peaked, the first galaxies stopped forming stars, and the structural regularity that we see in galaxies today must have emerged. 3D-HST will cover 3/4 (625 sq.arcmin) of the CANDELS survey area with two orbits of primary WFC3/G141 grism coverage and two to four parallel orbits with the ACS/G800L grism. In the IR these exposure times yield a continuum signal-to-noise of ~5 per resolution element at H~23.1 and a 5sigma emission line sensitivity of 5x10-17 erg/s/cm2 for typical objects, improving by a factor of ~2 for compact sources in images with low sky background levels. The WFC3/G141 spectra provide continuous wavelength coverage from 1.1-1.6 um at a spatial resolution of ~0."13, which, combined with their depth, makes them a unique resource for studying galaxy evolution. We present the preliminary reduction and analysis of the grism observations, including emission line and redshift measurements from combined fits to the extracted grism spectra and photometry from ancillary multi-wavelength catalogs. The present analysis yields redshift estimates with a precision of sigma(z)=0.0034(1+z), or sigma(v)~1000 km/s. We illustrate how the generalized nature of the survey yields near-infrared spectra of remarkable quality for many different types of objects, including a quasar at z=4.7, quiescent galaxies at z~2, and the most distant T-type brown dwarf star known. The CANDELS and 3D-HST surveys combined will provide the definitive imaging and spectroscopic dataset for studies of the 1<z<3.5 Universe until the launch of the James Webb Space Telescope.

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arXiv2012

The Evolution of the Rest-frame V-band Luminosity Function from z=4: A Constant Faint-end Slope over the Last 12 Gyr of Cosmic History

We present the rest-frame V-band luminosity function (LF) of galaxies at 0.4<z<4.0, measured from a near-infrared selected sample constructed from the NMBS, the FIRES, the FIREWORKS, and the ultra-deep NICMOS and WFC3 observations in the HDFN, HUDF, and GOODS-CDFS, all having high-quality optical to mid-infrared data. This unique sample combines data from surveys with a large range of depths and areas in a self-consistent way, allowing us to (1) minimize the uncertainties due to cosmic variance; and (2) simultaneously constrain the bright and faint ends with unprecedented accuracy over the targeted redshift range, probing the LF down to 0.1 L* at z~3.9. We find that (1) the faint end is fairly flat and with a constant slope from z=4, with alpha= -1.27 +/- 0.05; (2) the characteristic magnitude has dimmed by 1.3 mag from z~3.7 to z=0.1; (3) the characteristic density has increased by a factor of ~8 from z~3.7 to z=0.1, with 50% of this increase from z~4 to z~1.8; and (4) the luminosity density peaks at z~1-1.5, increasing by a factor of ~4 from z=4.0 to z~1-1.5, and subsequently decreasing by a factor of ~1.5 by z=0.1. We find no evidence for a steepening of the faint-end slope with redshift out to z=4, in contrast with previous observational claims and theoretical predictions. The constant faint-end slope suggests that the efficiency of stellar feedback may evolve with redshift. Alternative interpretations are discussed, such as different masses of the halos hosting faint galaxies at low and high redshifts and/or environmental effects.

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arXiv2011

First Results from the 3D-HST Survey: The Striking Diversity of Massive Galaxies at z>1

We present first results from the 3D-HST program, a near-IR spectroscopic survey performed with the Wide Field Camera 3 on the Hubble Space Telescope. We have used 3D-HST spectra to measure redshifts and Halpha equivalent widths for a stellar mass-limited sample of 34 galaxies at 1 10^11 M(sun) in the COSMOS, GOODS, and AEGIS fields. We find that a substantial fraction of massive galaxies at this epoch are forming stars at a high rate: the fraction of galaxies with Halpha equivalent widths >10 A is 59%, compared to 10% among SDSS galaxies of similar masses at z=0.1. Galaxies with weak Halpha emission show absorption lines typical of 2-4 Gyr old stellar populations. The structural parameters of the galaxies, derived from the associated WFC3 F140W imaging data, correlate with the presence of Halpha: quiescent galaxies are compact with high Sersic index and high inferred velocity dispersion, whereas star-forming galaxies are typically large two-armed spiral galaxies, with low Sersic index. Some of these star forming galaxies might be progenitors of the most massive S0 and Sa galaxies. Our results challenge the idea that galaxies at fixed mass form a homogeneous population with small scatter in their properties. Instead we find that massive galaxies form a highly diverse population at z>1, in marked contrast to the local Universe.

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arXiv2011

The Evolution of Early-type Galaxies Selected by Their Spatial Clustering

Aims: We present a new method that uses luminosity or stellar mass functions combined with clustering measurements to select samples of galaxies at different redshifts likely to follow a progenitor-to-descendant relationship. As the method uses clustering information, we refer to galaxy samples selected this way as clustering-selected samples. We apply this method to infer the number of mergers during the evolution of MUSYC early-type galaxies (ETGs) from z~1 to the present-day. Methods: The method consists in using clustering information to infer the typical dark-matter halo mass of the hosts of the selected progenitor galaxies. Using LambdaCDM predictions, it is then possible to follow these haloes to a later time where the sample of descendants will be that with the clustering of these descendant haloes. Results: This technique shows that ETGs at a given redshift evolve into brighter galaxies at lower redshifts (considering rest-frame, passively evolved optical luminosities). This indicates that the stellar mass of these galaxies increases with time and that, in principle, a stellar mass selection at different redshifts does not provide samples of galaxies in a progenitor-descendant relationship. Conclusions: The comparison between high redshift ETGs and their likely descendants at z=0 points to a higher number density for the progenitors by a factor 5.5+-4.0, implying the need for mergers to decrease their number density by today. Because the luminosity densities of progenitors and descendants are consistent, our results show no need for significant star-formation in ETGs since z=1, which indicates that the needed mergers are dry, i.e. gas free.

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arXiv2011

The NEWFIRM Medium-band Survey: Photometric Catalogs, Redshifts and the Bimodal Color Distribution of Galaxies out to z~3

We present deep near-infrared (NIR) medium-bandwidth photometry over the wavelength range 1-1.8 microns in the All-wavelength Extended Groth strip International Survey (AEGIS) and Cosmic Evolution Survey (COSMOS) fields. The observations were carried out as part of the NEWFIRM Medium-Band Survey (NMBS), an NOAO survey program on the Mayall 4m telescope on Kitt Peak using the NOAO Extremely Wide-Field Infrared Imager (NEWFIRM). In this paper, we describe the full details of the observations, data reduction and photometry for the survey. We also present a public K-selected photometric catalog, along with accurate photometric redshifts. The redshifts are computed with 37 (20) filters in the COSMOS (AEGIS) fields, combining the NIR medium-bandwidth data with existing ultraviolet (UV; Galaxy Evolution Explorer), visible and NIR (Canada-France-Hawaii Telescope and Subaru) and mid-IR (Spitzer/IRAC) imaging. We find excellent agreement with publicly available spectroscopic redshifts, with sigma_z/(1+z)~1-2% for ~4000 galaxies at z=0-3. The NMBS catalogs contain ~13,000 galaxies at z>1.5 with accurate photometric redshifts and rest-frame colors. Due to the increased spectral resolution obtained with the five NIR medium-band filters, the median 68% confidence intervals of the photometric redshifts of both quiescent and star-forming galaxies are a factor of ~2 times smaller when comparing catalogs with medium-band NIR photometry to NIR broadband photometry. We show evidence for a clear bimodal color distribution between quiescent and star-forming galaxies that persists to z~3, a higher redshift than has been probed so far.

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arXiv2011

The number density and mass density of star-forming and quiescent galaxies at 0.4 < z < 2.2

We study the build-up of the bimodal galaxy population using the NEWFIRM Medium-Band Survey, which provides excellent redshifts and well-sampled spectral energy distributions of ~27,000 galaxies with K<22.8 at 0.4 < z < 2.2. We first show that star-forming galaxies and quiescent galaxies can be robustly separated with a two-color criterion over this entire redshift range. We then study the evolution of the number density and mass density of quiescent and star-forming galaxies, extending the results of the COMBO-17, DEEP2, and other surveys to z=2.2. The mass density of quiescent galaxies with M > 3 10^11 solar masses increases by a factor of ~10 from z=2 to the present day, whereas the mass density in star-forming galaxies is flat or decreases over the same time period. Modest mass growth by a factor of 2 of individual quiescent galaxies can explain roughly half of the strong density evolution at M>10^11 solar masses, due to the steepness of the exponential tail of the mass function. The rest of the density evolution of massive, quiescent galaxies is likely due to transformation (e.g. quenching) of the massive star-forming population, a conclusion which is consistent with the density evolution we observe for the star-forming galaxies themselves, which is flat or decreasing with cosmic time. Modest mass growth does not explain the evolution of less massive quiescent galaxies (~10^10.5 solar masses), which show a similarly steep increase in their number densities. The less massive quiescent galaxies are therefore continuously formed by transforming galaxies from the star-forming population

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arXiv2010

The Spectral Energy Distribution of Post-Starburst Galaxies in the NEWFIRM Medium-Band Survey: A Low Contribution from TP-AGB Stars

Stellar population synthesis (SPS) models are a key ingredient of many galaxy evolution studies. Unfortunately, the models are still poorly calibrated for certain stellar evolution stages. Of particular concern is the treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase, as different implementations lead to systematic differences in derived galaxy properties. Post-starburst galaxies are a promising calibration sample, as TP-AGB stars are thought to be most prominently visible during this phase. Here, we use post-starburst galaxies in the NEWFIRM medium-band survey to assess different SPS models. The available photometry allows the selection of a homogeneous and well-defined sample of 62 post-starburst galaxies at 0.7<z<2.0, from which we construct a well-sampled composite spectral energy distribution (SED) over the range 1200-40 000 Angstrom. The SED is well-fit by the Bruzual & Charlot SPS models, while the Maraston models do not reproduce the rest-frame optical and near-infrared parts of the SED simultaneously. When the fitting is restricted to lambda < 6000 Angstrom, the Maraston models overpredict the near-infrared luminosity, implying that these models give too much weight to TP-AGB stars. Using the flexible SPS models by Conroy et al, and assuming solar metallicity, we find that the contribution of TP-AGB stars to the integrated SED is a factor of ~3 lower than predicted by the latest Padova TP-AGB models. Whether this is due to lower bolometric luminosities, shorter lifetimes, and/or heavy dust obscuration of TP-AGB stars remains to be addressed. Altogether, our data demand a low contribution from TP-AGB stars to the SED of post-starburst galaxies.

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arXiv2010

Galaxy clustering in the NEWFIRM Medium Band Survey: the relationship between stellar mass and dark matter halo mass at 1 < z < 2

We present an analysis of the clustering of galaxies as a function of their stellar mass at 1 < z < 2 using data from the NEWFIRM Medium Band Survey (NMBS). The precise photometric redshifts and stellar masses that the NMBS produces allows us to define a series of mass limited samples of galaxies more massive than 0.7, 1 and 3x10^10 Msun in redshift intervals centered on z = 1.1, 1.5 and 1.9 respectively. In each redshift interval we show that there exists a strong dependence of clustering strength on the stellar mass limit of the sample, with more massive galaxies showing a higher clustering amplitude on all scales. We further interpret our clustering measurements in the LCDM cosmological context using the halo model of galaxy clustering. We show that the typical halo mass of central and satellite galaxies increases with stellar mass, whereas the satellite fraction decreases with stellar mass, qualitatively the same as is seen at z < 1. We see little evidence of any redshift dependence in the stellar mass-to-halo mass relationship over our narrow redshift range. However, when we compare with similar measurements at z~0, we see clear evidence for a change in this relation. If we assume a universal baryon fraction, the ratio of stellar mass to halo mass reveals the fraction of baryons that have been converted to stars. We see that the peak in this star formation efficiency for central galaxies shifts to higher halo masses at higher redshift, moving from ~7x10^11 Msun at z~0 to ~3x10^12 Msun at z~1.5, revealing evidence of `halo downsizing'. Finally we show that for highly biased galaxy populations at z > 1 there may be a discrepancy between the measured space density and clustering and that predicted by the halo model. This could imply that there is a problem with one or more ingredients of the halo model at these redshifts, for instance the halo bias relation or the halo profile.

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arXiv2010

Well-Sampled Far-Infrared Spectral Energy Distributions of z ~ 2 Galaxies: Evidence for Scaled up Cool Galaxies

We present an analysis of the far-infrared (FIR) spectral energy distributions (SEDs) of two massive K-selected galaxies at z = 2.122 and z = 2.024 detected at 24um, 70um, 160um by Spitzer, 250um, 350um, 500um by BLAST, and 870um by APEX. The large wavelength range of these observations and the availability of spectroscopic redshifts allow us to unambiguously identify the peak of the redshifted thermal emission from dust at ~ 300um. The SEDs of both galaxies are reasonably well fit by synthetic templates of local galaxies with L_IR ~ 10^{11} L_{sun} -- 10^{12} L_{sun} yet both galaxies have L_{IR} ~ 10^{13} L_{sun}. This suggests that these galaxies are not high redshift analogues of the Hyper-LIRGs/ULIRGs used in local templates, but are instead "scaled up" versions of local ULIRGs/LIRGs. For both galaxies the star formation rate determined from the best-fit FIR SEDs (SFR(L_{IR})) agrees with the SFR determined from the dust corrected H-alpha luminosity (SFR(H-alpha)) to within a factor of ~ 2; however, when the SFR of these galaxies is estimated using only the observed 24um flux and the standard luminosity-dependent template method (SFR(24um)), it systematically overestimates the SFR by as much as a factor of 6. A larger sample of 24 K-selected galaxies at z ~ 2.3 drawn from the Kriek et al. (2008) GNIRS sample shows the same trend between SFR(24um) and SFR(H-alpha). Using that sample we show that SFR(24um) and SFR(H-alpha) are in better agreement when SFR(24um) is estimated using the log average of local templates rather than selecting a single luminosity-dependent template, because this incorporates lower luminosity templates. The better agreement between SFRs from lower luminosity templates suggests that the FIR SEDs of the BLAST-detected galaxies may be typical for massive galaxies at z ~ 2, and that the majority are scaled up versions of lower luminosity local galaxies.

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arXiv2010

z~7 galaxy candidates from NICMOS observations over the HDF South and the CDF-S and HDF-N GOODS fields

We use ~88 arcmin**2 of deep (>~26.5 mag at 5 sigma) NICMOS data over the two GOODS fields and the HDF South to conduct a search for bright z>~7 galaxy candidates. This search takes advantage of an efficient preselection over 58 arcmin**2 of NICMOS H-band data where only plausible z>~7 candidates are followed up with NICMOS J-band observations. ~248 arcmin**2 of deep ground-based near-infrared data (>~25.5 mag, 5 sigma) is also considered in the search. In total, we report 15 z-dropout candidates over this area -- 7 of which are new to these search fields. Two possible z~9 J-dropout candidates are also found, but seem unlikely to correspond to z~9 galaxies. The present z~9 search is used to set upper limits on the prevalence of such sources. Rigorous testing is undertaken to establish the level of contamination of our selections by photometric scatter, low mass stars, supernovae (SNe), and spurious sources. The estimated contamination rate of our z~7 selection is ~24%. Through careful simulations, the effective volume available to our z>~7 selections is estimated and used to establish constraints on the volume density of luminous (L*(z=3), or -21 mag) galaxies from these searches. We find that the volume density of luminous star-forming galaxies at z~7 is 13_{-5}^{+8}x lower than at z~4 and >25x lower (1 sigma) at z~9 than at z~4. This is the most stringent constraint yet available on the volume density of >~L* galaxies at z~9. The present wide-area, multi-field search limits cosmic variance to <20%. The evolution we find at the bright end of the UV LF is similar to that found from recent Subaru Suprime-Cam, HAWK-I or ERS WFC3/IR searches. The present paper also includes a complete summary of our final z~7 z-dropout sample (18 candidates) identified from all NICMOS observations to date (over the two GOODS fields, the HUDF, galaxy clusters).

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arXiv2010

The Most Massive Galaxies at 3.0<z<4.0 in the NEWFIRM Medium-Band Survey: Properties and Improved Constraints on the Stellar Mass Function

[Abridged] We use the NEWFIRM Medium-Band Survey (NMBS) to characterize the properties of a mass-complete sample of 14 galaxies at 3.0 2.5x10^11 Msun, and to derive more accurate measurements of the high-mass end of the stellar mass function (SMF) of galaxies at z=3.5, with significantly reduced contributions from photometric redshift errors and cosmic variance to the total error budget of the SMF. The typical very massive galaxy at z=3.5 is red and faint in the observer's optical, with median r=26.1, and rest-frame U-V=1.6. About 60% of the sample have optical colors satisfying either the U- or the B-dropout color criteria, although ~50% of these galaxies have r>25.5. About 30% of the sample has SFRs from SED modeling consistent with zero. However, >80% of the sample is detected at 24 micron, with total infrared luminosities in the range (0.5-4.0)x10^13 Lsun. This implies the presence of either dust-enshrouded starburst activity (with SFRs of 600-4300 Msun/yr) and/or highly-obscured active galactic nuclei (AGN). The contribution of galaxies with M_star>2.5x10^11 Msun to the total stellar mass budget at z=3.5 is ~8%. We find an evolution by a factor of 2-7 and 3-22 from z~5 and z~6, respectively, to z=3.5. The previously found disagreement at the high-mass end between observed and model-predicted SMFs is now significant at the 3sigma level. However, systematic uncertainties dominate the total error budget, with errors up to a factor of ~8 in the densities, bringing the observed SMF in marginal agreement with the predicted SMF. Additional systematic uncertainties on the high-mass end could be introduced by either 1) the intense star-formation and/or the very common AGN activities as inferred from the MIPS 24 micron detections, and/or 2) contamination by a significant population of massive, old, and dusty galaxies at z~2.6.

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arXiv2009

The Growth of Massive Galaxies Since z=2

We study the growth of massive galaxies from z=2 to the present using data from the NEWFIRM Medium Band Survey. The sample is selected at a constant number density of n=2x10^-4 Mpc^-3, so that galaxies at different epochs can be compared in a meaningful way. We show that the stellar mass of galaxies at this number density has increased by a factor of ~2 since z=2, following the relation log(M)=11.45-0.15z. In order to determine at what physical radii this mass growth occurred we construct very deep stacked rest-frame R-band images at redshifts z=0.6, 1.1, 1.6, and 2.0. These image stacks of typically 70-80 galaxies enable us to characterize the stellar distribution to surface brightness limits of ~28.5 mag/arcsec^2. We find that massive galaxies gradually built up their outer regions over the past 10 Gyr. The mass within a radius of r=5 kpc is nearly constant with redshift whereas the mass at 5-75 kpc has increased by a factor of ~4 since z=2. Parameterizing the surface brightness profiles we find that the effective radius and Sersic n parameter evolve as r_e~(1+z)^-1.3 and n~(1+z)^-1.0 respectively. The data demonstrate that massive galaxies have grown mostly inside-out, assembling their extended stellar halos around compact, dense cores with possibly exponential radial density distributions. Comparing the observed mass evolution to the average star formation rates of the galaxies we find that the growth is likely dominated by mergers, as in-situ star formation can only account for ~20% of the mass build-up from z=2 to z=0. The main uncertainties in this study are possible redshift-dependent systematic errors in the total stellar masses and the conversion from light-weighted to mass-weighted radial profiles.

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arXiv2009

How Massive are Massive Compact Galaxies?

Using a sample of nine massive compact galaxies at z ~ 2.3 with rest-frame optical spectroscopy and comprehensive U through 8um photometry we investigate how assumptions in SED modeling change the stellar mass estimates of these galaxies, and how this affects our interpretation of their size evolution. The SEDs are fit to Tau-models with a range of metallicities, dust laws, as well as different stellar population synthesis codes. These models indicate masses equal to, or slightly smaller than our default masses. The maximum difference is 0.16 dex for each parameter considered, and only 0.18 dex for the most extreme combination of parameters. Two-component populations with a maximally old stellar population superposed with a young component provide reasonable fits to these SEDs using the models of Bruzual & Charlot (2003); however, using models with updated treatment of TP-AGB stars the fits are poorer. The two-component models predict masses that are 0.08 to 0.22 dex larger than the Tau-models. We also test the effect of a bottom-light IMF and find that it would reduce the masses of these galaxies by 0.3 dex. Considering the range of allowable masses from the Tau-models, two-component fits, and IMF, we conclude that on average these galaxies lie below the mass-size relation of galaxies in the local universe by a factor of 3-9, depending on the SED models used.

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arXiv2009

Introducing the Photometric Maximum Likelihood Method: Galaxy Luminosity Functions at z<1.2 in MUSYC-ECDFS

We present a new maximum likelihood method for the calculation of galaxy luminosity functions from multi-band photometric surveys without spectroscopic data. The method evaluates the likelihood of a trial luminosity function by directly comparing the predicted distribution of fluxes in a multi-dimensional photometric space to the observations, and thus does not require the intermediate step of calculating photometric redshifts. We apply this algorithm to ~27,000 galaxies with m_R<=25 in the MUSYC-ECDFS field, with a focus on recovering the luminosity function of field galaxies at z<1.2. Our deepest LFs reach M_r=-14 and show that the field galaxy LF deviates from a Schechter function, exhibiting a steep upturn at intermediate magnitudes that is due to galaxies of late spectral types.

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arXiv2009

An ultra-deep near-infrared spectrum of a compact quiescent galaxy at z=2.2

Several recent studies have shown that about half of the massive galaxies at z~2 are in a quiescent phase. Moreover, these galaxies are commonly found to be ultra-compact with half-light radii of ~1 kpc. We have obtained a ~29 hr spectrum of a typical quiescent, ultra-dense galaxy at z=2.1865 with the Gemini Near-Infrared Spectrograph. The spectrum exhibits a strong optical break and several absorption features, which have not previously been detected in z>2 quiescent galaxies. Comparison of the spectral energy distribution with stellar population synthesis models implies a low star formation rate (SFR) of 1-3 Msol/yr, an age of 1.3-2.2 Gyr, and a stellar mass of ~2x10^11 Msol. We detect several faint emission lines, with emission-line ratios of [NII]/Halpha, [SII]/Halpha and [OII]/[OIII] typical of low-ionization nuclear emission-line regions. Thus, neither the stellar continuum nor the nebular emission implies active star formation. The current SFR is <1% of the past average SFR. If this galaxy is representative of compact quiescent galaxies beyond z=2, it implies that quenching of star formation is extremely efficient and also indicates that low luminosity active galactic nuclei (AGNs) could be common in these objects. Nuclear emission is a potential concern for the size measurement. However, we show that the AGN contributes <8% to the rest-frame optical emission. A possible post-starburst population may affect size measurements more strongly; although a 0.5 Gyr old stellar population can make up <10% of the total stellar mass, it could account for up to ~40% of the optical light. Nevertheless, this spectrum shows that this compact galaxy is dominated by an evolved stellar population.

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arXiv2009

The Evolution of the Stellar Mass Function of Galaxies from z=4.0 and the First Comprehensive Analysis of its Uncertainties: Evidence for Mass-dependent Evolution

[Abridged] We present the evolution of the stellar mass function (SMF) of galaxies from z=4.0 to z=1.3 measured from a sample constructed from the deep NIR MUSYC, the FIRES, and the GOODS-CDFS surveys, all having very high-quality optical to mid-infrared data. This sample, unique in that it combines data from surveys with a large range of depths and areas in a self-consistent way, allowed us to 1) minimize the uncertainty due to cosmic variance and empirically quantify its contribution to the total error budget; 2) simultaneously probe the high-mass end and the low-mass end (down to ~0.05 times the characteristic stellar mass) of the SMF with good statistics; and 3) empirically derive the redshift-dependent completeness limits in stellar mass. We provide, for the first time, a comprehensive analysis of random and systematic uncertainties affecting the derived SMFs. We find that the mass density evolves by a factor of ~17(+7,-10) since z=4.0, mostly driven by a change in the normalization Phi*. If only random errors are taken into account, we find evidence for mass-dependent evolution, with the low-mass end evolving more rapidly than the high-mass end. However, we show that this result is no longer robust when systematic uncertainties due to the SED-modeling assumptions are taken into account. Taking our results at face value, we find that they are in conflict with semi-analytic models of galaxy formation. The models predict SMFs that are in general too steep, with too many low-mass galaxies and too few high-mass galaxies. The discrepancy at the high-mass end is susceptible to uncertainties in the models and the data, but the discrepancy at the low-mass end may be more difficult to explain.

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