Mariska Kriek

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 NOAO/Yale NEWFIRM Medium Band Survey. The sample is selected at a constant number density of n = 2 ? 10?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 Mn (z) = 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 of galaxies with masses near Mn (z), 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 kpc < r < 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 re (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 buildup from z = 2 to z = 0. A direct consequence of these results is that massive galaxies do not evolve in a self-similar way: their structural profiles change as a function of redshift, complicating analyses which (often implicitly) assume self-similarity. 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.

Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z ~ 2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse

Using deep near-infrared spectroscopy, Kriek et al. found that ∼45% of massive galaxies at have evolved z ∼ 2.3 stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)–assisted Keck/adaptive optics (AO). Considering that their median stellar mass is , the galaxies are remarkably small, with 11 1.7 # 10 M, a median effective radius kpc. Galaxies of similar mass in the nearby universe have sizes of ≈5 kpc and r p 0.9 e average stellar densities that are 2 orders of magnitude lower than the galaxies. These results extend earlier z ∼ 2.3 work at and confirm previous studies at that lacked spectroscopic redshifts and imaging of sufficient z ∼ 1.5 z 1 2 resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ∼10% of the population of K-selected quiescent galaxies at , effectively ruling out simple monolithic z ∼ 2.3 models for their formation. The galaxies must evolve significantly after , through dry mergers or other z ∼ 2.3 processes, consistent with predictions from hierarchical models. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation

THE RELATION BETWEEN COMPACT, QUIESCENT HIGH-REDSHIFT GALAXIES AND MASSIVE NEARBY ELLIPTICAL GALAXIES: EVIDENCE FOR HIERARCHICAL, INSIDE-OUT GROWTH

Recent studies have shown that massive quiescent galaxies at high redshift are much more compact than present-day galaxies of the same mass. Here we compare the radial stellar density profiles and the number density of a sample of massive galaxies at z ~ 2.3 to nearby massive elliptical galaxies. We confirm that the average stellar densities of the z ~ 2.3 galaxies within the effective radius, ρ(<re ), are two orders of magnitude higher than those of local elliptical galaxies of the same stellar mass. However, we also find that the densities measured within a constant physical radius of 1 kpc, ρ(<1 kpc), are higher by a factor of 2-3 only. This suggests that inside-out growth scenarios are plausible, in which the compact high-redshift galaxies make up the centers of normal nearby ellipticals. The compact galaxies are common at high redshift, which enables us to further constrain their evolution by requiring that the number density of their descendants does not exceed constraints imposed by the z = 0 galaxy mass function. We infer that size growth must be efficient, with (r 1+2/r 1) ~ (M 1+2/M 1)2. A simple model where compact galaxies with masses ~1011 M ☉ primarily grow through minor mergers produces descendants with the approximate sizes, stellar densities, and number density of elliptical galaxies with masses 2-3 × 1011 M ☉ in the local universe. We note that this model also predicts evolution in the M BH – σ relation, such that the progenitors of elliptical galaxies have lower black hole masses at fixed velocity dispersion. The main observational uncertainty is the conversion from light to mass; measurements of kinematics are needed to calibrate the masses and stellar densities of the high-redshift galaxies.

3D-HST WFC3-selected Photometric Catalogs in the Five CANDELS/3D-HST Fields: Photometry, Photometric Redshifts and Stellar Masses

The 3D-HST and CANDELS programs have provided WFC3 and ACS spectroscopy and photometry over ≈900 arcmin2 in five fields: AEGIS, COSMOS, GOODS-North, GOODS-South, and the UKIDSS UDS field. All these fields have a wealth of publicly available imaging data sets in addition to the Hubble Space Telescope (HST) data, which makes it possible to construct the spectral energy distributions (SEDs) of objects over a wide wavelength range. In this paper we describe a photometric analysis of the CANDELS and 3D-HST HST imaging and the ancillary imaging data at wavelengths 0.3-8 μm. Objects were selected in the WFC3 near-IR bands, and their SEDs were determined by carefully taking the effects of the point-spread function in each observation into account. A total of 147 distinct imaging data sets were used in the analysis. The photometry is made available in the form of six catalogs: one for each field, as well as a master catalog containing all objects in the entire survey. We also provide derived data products: photometric redshifts, determined with the EAZY code, and stellar population parameters determined with the FAST code. We make all the imaging data that were used in the analysis available, including our reductions of the WFC3 imaging in all five fields. 3D-HST is a spectroscopic survey with the WFC3 and ACS grisms, and the photometric catalogs presented here constitute a necessary first step in the analysis of these grism data. All the data presented in this paper are available through the 3D-HST Web site (http://3dhst.research.yale.edu).

The Stellar Mass Density and Specific Star Formation Rate of the Universe at z ~ 7

We use a robust sample of 11 z ~ 7 galaxies (z 850 dropouts) to estimate the stellar mass density (SMD) of the universe when it was only ~750?Myr old. We combine the very deep optical to near-infrared photometry from the Hubble Space Telescope Advanced Camera for Surveys and NICMOS cameras with mid-infrared Spitzer Infrared Array Camera (IRAC) imaging available through the GOODS program. After carefully removing the flux from contaminating foreground sources, we have obtained reliable photometry in the 3.6??m and 4.5??m IRAC channels. The spectral shapes of these sources, including their rest-frame optical colors, strongly support their being at z ~ 7 with a mean photometric redshift of z = 7.2 ? 0.5. We use Bruzual & Charlot synthetic stellar population models to constrain their stellar masses and star formation histories. We find stellar masses that range over (0.1-12) ? 109 M ? and average ages from 20?Myr to 425?Myr with a mean of ~300?Myr, suggesting that in some of these galaxies most of the stars were formed at z > 8 (and probably at z 10). The best fits to the observed SEDs are consistent with little or no dust extinction, in agreement with recent results at z ~ 4-8. The star formation rates (SFRs) are in the range from 5 to 20 M ? yr-1. From this sample, we measure an SMD of 6.6+5.4 ?3.3 ? 105 M ? Mpc-3 to a limit of M UV,AB L*) and does not include the dominant contribution of the fainter galaxies. Strikingly, we find that the specific SFR is constant from z ~ 7 to z ~ 2 but drops substantially at more recent times.

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

We present deep near-IR (NIR) medium-bandwidth photometry over the wavelength range 1-1.8 μm in the All-wavelength Extended Groth strip International Survey (AEGIS) and Cosmic Evolution Survey (COSMOS) fields. The observations were carried out using the NOAO Extremely Wide-Field Infrared Imager (NEWFIRM) on the Mayall 4 m Telescope on Kitt Peak as part of the NEWFIRM Medium-Band Survey (NMBS), an NOAO survey program. 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 UV (Galaxy Evolution Explorer), visible and NIR (Canada-France-Hawaii Telescope and Subaru Telescope), and mid-IR (Spitzer/IRAC) imaging. We find excellent agreement with publicly available spectroscopic redshifts, with σ 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 about two 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.

A high stellar velocity dispersion for a compact massive galaxy at redshift z = 2.186

Recent studies have found that the oldest and most luminous galaxies in the early Universe are surprisingly compact, having stellar masses similar to present-day elliptical galaxies but much smaller sizes. This finding has attracted considerable attention, as it suggests that massive galaxies have grown in size by a factor of about five over the past ten billion years (10 Gyr). A key test of these results is a determination of the stellar kinematics of one of the compact galaxies: if the sizes of these objects are as extreme as has been claimed, their stars are expected to have much higher velocities than those in present-day galaxies of the same mass. Here we report a measurement of the stellar velocity dispersion of a massive compact galaxy at redshift z = 2.186, corresponding to a look-back time of 10.7 Gyr. The velocity dispersion is very high at  km s-1, consistent with the mass and compactness of the galaxy inferred from photometric data. This would indicate significant recent structural and dynamical evolution of massive galaxies over the past 10 Gyr. The uncertainty in the dispersion was determined from simulations that include the effects of noise and template mismatch. However, we cannot exclude the possibility that some subtle systematic effect may have influenced the analysis, given the low signal-to-noise ratio of our spectrum.

Spectroscopic Identification of Massive Galaxies at z ~ 2.3 with Strongly Suppressed Star Formation*

We present first results of a spectroscopic survey targeting K-selected galaxies at z = 2.0-2.7 using the Gemini near-infrared spectrograph (GNIRS). We obtained near-infrared spectra with a wavelength coverage of 1.0-2.5 μm for 26 K-bright galaxies (K < 19.7) selected from the Multi-wavelength Survey by Yale-Chile (MUSYC) using photometric redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies using rest-frame optical emission lines or the redshifted Balmer/4000 A break. Twenty galaxies have spectroscopic redshifts in the range 2.0 < z < 2.7, for which bright emission lines like Hα and [O III] fall in atmospheric windows. Surprisingly, we detected no emission lines for nine of these 20 galaxies. The median 2 σ upper limit on the rest-frame equivalent width of Hα for these nine galaxies is ~10 A. The stellar continuum emission of these same nine galaxies is best fitted by evolved stellar population models. The best-fit star formation rate (SFR) is zero for five out of nine galaxies and is consistent with zero within 1 σ for the remaining four. Thus, both the Hα measurements and the independent stellar continuum modeling imply that 45% of our K-selected galaxies are not forming stars intensely. This high fraction of galaxies without detected line emission and low SFRs may imply that the suppression of star formation in massive galaxies occurs at higher redshift than is predicted by current cold dark matter (CDM) galaxy formation models. However, obscured star formation may have been missed, and deep mid-infrared imaging is needed to clarify this situation.

Star Formation Rates and Stellar Masses of z = 7–8 Galaxies from IRAC Observations of the WFC3/IR Early Release Science and the HUDF Fields

We investigate the Spitzer/IRAC properties of 36 z ~ 7 z 850-dropout galaxies and three z ~ 8 Y 098 galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the spectral energy distributions to derive mean redshifts, stellar masses, and ages. The z ~ 7 galaxies are best characterized by substantial ages (>100 Myr) and M/LV 0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from ? ~ ?2.0 to ? ~ ?3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1L* z = 3), where low metallicity and smooth star formation histories (SFHs) fail to match the blue far-UV and moderately red H ? [3.6] color. Such colors may require episodic SFHs with short periods of activity and quiescence (on-off cycles) and/or a contribution from emission lines. The stellar mass of our sample of z ~ 7 star-forming galaxies correlates with star formation rate (SFR) according to log M* = 8.70(?0.09) + 1.06(?0.10)log SFR, implying that star formation may have commenced at z > 10. No galaxies are found with SFRs much higher or lower than the past averaged SFR suggesting that the typical star formation timescales are probably a substantial fraction of the Hubble time. We report the first IRAC detection of Y 098-dropout galaxies at z ~ 8. The average rest-frame U ? V 0.3 (AB) of the three galaxies are similar to faint z ~ 7 galaxies, implying similar M/L. The stellar mass density to M UV,AB < ?18 is ?*(z = 8) = 1.8+0.7 ?1.0 ? 106 M ? Mpc?3, following log ?*(z) = 10.6(?0.6) ? 4.4(?0.7) log(1 + z) [M ? Mpc?3] over 3 < z < 8.

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 1 there may be a discrepancy between the space density and clustering predicted by the halo model and the measured clustering and space density. This could imply that there is a problem with one or more ingredient of the halo model at these redshifts, for instance, the halo bias relation may not yet be precisely calibrated at high halo masses or galaxies may not be distributed within halos following a Navarro-Frenk-White profile.