GOODS-S Field Research Articles

Star formation history of galaxies fromz= 0 toz= 0.7

Aims. We investigate whether the mean star formation activity of star-forming galaxies from to in the GOODS-S field can be reproduced by simple evolution models of these systems. In this case, such models might be used as first-order references for studies at higher z to decipher when and to what extent a secular evolution is sufficient to explain the star formation history in galaxies.

A multi-wavelength study ofz= 3.15 Lyman-$\mathsf{\alpha}$ emitters in the GOODS South Field

Context: Ly-alpha-emitters have proven to be excellent probes of faint, star-forming galaxies in the high redshift universe. However, although the sample of known emitters is increasingly growing, their nature (e.g. stellar masses, ages, metallicities, star-formation rates) is still poorly constrained. Aims: We aim to study the nature of Ly-alpha-emitters, to find the properties of a typical Ly-alpha-emitting galaxy and to compare these properties with the properties of other galaxies at similar redshift, in particular Lyman-break galaxies. Methods: We have performed narrow-band imaging at the VLT, focused on Ly-alpha at redshift z ~ 3.15, in the GOODS-S field. We have identified a sample of Ly-alpha-emitting candidates, and we have studied their Spectral Energy Distributions (SEDs). Results: We find that the emitters are best fit by an SED with low metallicity (Z/Z_sun = 0.005), low dust extinction (A_V ~ 0.32) and medium stellar masses of approximately 10^9 M_sun. The age is not very well constrained. One object out of 24 appears to be a high redshift Ly-alpha-emitting dusty starburst galaxy. We find filamentary structure as traced by the Ly-alpha-emitters at the 4 sigma level. The rest-frame UV SED of these galaxies is very similar to that of Lyman Break Galaxies (LBGs) and comply with the selection criteria for U-band drop-outs, except they are intrinsically fainter than the current limit for LBGs. Conclusion: Ly-alpha-emitters are excellent probes of galaxies in the distant universe, and represent a class of star-forming, dust and AGN free, medium mass objects.

Comparing dynamical and photometric-stellar masses of early-type galaxies atz ~ 1

Aims. The purpose of this study is to explore the relationship between galaxy stellar masses, based on multiwavelength photometry spectral template fitting and dynamical masses based on published velocity dispersion measurements, for a sample of 48 early-type galaxies at z ∼ I with HST/ACS morphological information. Methods. We determine photometric-stellar masses and perform a quantitative morphological analysis of cluster and field galaxies at redshift 0.6 < z < 1.2, using ground- and space-based multiwavelength data available on the GOODS-S field and on the field around the X-ray luminous cluster RDCS 1252.9-2927 at z = 1.24. We use multi-band photometry over 0.4-8 μm from HST/ACS, VLT/ISAAC and Spiher/IRAC to estimate photometric-stellar masses using Composite Stellar Population (CSP) templates computed with PEGASE.2 (Fioc & Rocca-Volmerange 1997) models. We compare stellar masses with those obtained using CSPs built with Bruzual & Charlot (2003, MNRAS, 344, 1000; BC03) and Maraston (2005, MNRAS, 362, 799; M05) models. We then compare photometric-stellar mass and dynamical mass estimates as a function of morphological parameters obtained from HST/ACS imaging. Results. Based on our sample, which spans the mass range log M phot ≃ [10, 11.5], we find that 1) PEGASE.2, BC03, M05 yield consistent photometric-stellar masses for early-type galaxies at z ∼ I with a small scatter (0.15 dex rms); 2) adopting a Kroupa IMF, photometric-stellar masses match dynamical mass estimates for early-type galaxies with an average offset of 0.27 dex; 3) assuming a constant IMF, increasing dark matter fraction with the increasing galaxy mass can explain the observed trend; 4) we observe that early-type galaxies with significant disk components (Sa/Sab) or with signs of dynamical interaction tend to have the largest deviation from a one-to-one M dyn vs. M phot relation.

SpitzerObservations of Massive, Red Galaxies at High Redshift

We study massive galaxies at z ~ 1-3.5 using HST optical imaging, ground-based near-IR imaging, and Spitzer observations at 3-24 μm. From Ks-selected galaxies in the 130 arcmin2 GOODS-S field, we identify 153 distant red galaxies (DRGs) with (J - Ks)Vega ≥ 2.3. This sample is approximately complete in stellar mass for passively evolving galaxies above 1011 M☉ and z ≤ 3. Roughly half of the DRGs are objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation (at zmed ~ 2.5), and the others are galaxies whose light is dominated by heavily reddened (A1600 4-6 mag) starbursts (at zmed ~ 1.7). Very few DRGs (10%) have no indication of current star formation. DRGs at z ~ 1.5-3 with stellar masses ≥1011 M☉ have specific star formation rates (SFRs per unit mass) including the reradiated far-IR emission that range from 0.2 to 10 Gyr-1. Based on the X-ray luminosities and rest-frame near-IR colors, roughly one-quarter of the DRGs contain AGNs, implying that the growth of supermassive black holes coincides with the formation of massive galaxies. At 1.5 ≤ z ≤ 3, the DRGs with M ≥ 1011 M☉ have an integrated specific SFR comparable to the global value of all galaxies. In contrast, galaxies at z ~ 0.3-0.75 with M ≥ 1011 M☉ have an integrated specific SFR less than the global value and more than an order of magnitude lower than that for massive DRGs. At z 1, lower mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z ~ 1.5. Further mass assembly in these galaxies takes place with low specific SFRs.

The Evolution of the Mass Function Split by Morphology up to Redshift 1 in the FORS Deep and the GOODS-S Fields

We study the evolution of the stellar mass density for the separate families of bulge-dominated and disk-dominated galaxies over the redshift range . We derive quantitative morphology for a statistically significant 0.25 ≤ z ≤ 1.15 galaxy sample of 1645 objects selected from the FORS Deep Field and the GOODS-S field. We find that the morphological mix evolves monotonically with time: the higher the redshift, the more disk systems dominate the total mass content. At , massive objects ( ) host about half of the mass contained in objects 10 z ∼ 1 M ≥ 7 # 10 M ∗ , of similar mass in the local universe. The contribution from early- and late-type galaxies to the mass budget at is nearly equal. We show that in situ star formation is not sufficient to explain the changing mass budget. z ∼ 1 Moreover, we find that the star formation rate per unit stellar mass of massive galaxies increases with redshift only for the intermediate and early morphological types while it stays nearly constant for late-type objects. This suggests that merging and/or frequent accretion of small-mass objects has a key role in the shaping of the Hubble sequence as we observe it now and also in decreasing the star formation activity of the bulge-dominated descendants of massive disk galaxies. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: fundamental parameters — galaxies: luminosity function, mass function — surveys

Specific Star Formation Rates to Redshift 5 from the FORS Deep Field and the GOODS-S Field

We explore the build-up of stellar mass in galaxies over a wide redshift range 0.4 < z < 5.0 by studying the evolution of the specific star formation rate (SSFR), defined as the star formation rate per unit stellar mass, as a function of stellar mass and age. Our work is based on a combined sample of ~ 9000 galaxies from the FORS Deep Field and the GOODS-S field, providing high statistical accuracy and relative insensitivity against cosmic variance. As at lower redshifts, we find that lower-mass galaxies show higher SSFRs than higher mass galaxies, although highly obscured galaxies remain undetected in our sample. Furthermore, the highest mass galaxies contain the oldest stellar populations at all redshifts, in principle agreement with the existence of evolved, massive galaxies at 1 < z < 3. It is remarkable, however, that this trend continues to very high redshifts of z ~ 4. We also show that with increasing redshift the SSFR for massive galaxies increases by a factor of ~ 10, reaching the era of their formation at z ~ 2 and beyond. These findings can be interpreted as evidence for an early epoch of star formation in the most massive galaxies, and ongoing star-formation activity in lower mass galaxies.

Merging Galaxies in GOODS-S: First Extragalactic Results from Keck Laser Adaptive Optics

The Center for Adaptive Optics Treasury Survey aims to combine deep Hubble Space Telescope (HST) images in the optical with deep Keck adaptive optics (AO) data in the near-infrared (NIR) to study distant galaxies, active galactic nuclei, and supernovae. We recently achieved an important new milestone by securing the first Keck laser guide star AO image of faint galaxies. Six galaxies with redshifts ranging from 0.3 to 1.0 were targeted in one pointing in the GOODS-S field. Two are Chandra Deep Field-South sources, XID-56 and XID-536, with complex morphologies suggestive of recent merger activity. Substructures seen in the NIR AO image (FWHM ~01), including multiple tight knots in XID-56 and a double nucleus in XID-536, are confirmed in the optical HST images. These structures are unresolved in the best seeing-limited (~05 FWHM) NIR images. Stellar population synthesis models of the substructures indicate that XID-56 is a gas-rich merger with a recent burst of star formation and significant amounts of dust. XID-536 appears to be a merger of two evolved stellar populations.

Hubble Space TelescopeImaging and Keck Spectroscopy ofz≈ 6i‐Band Dropout Galaxies in the Advanced Camera for Surveys GOODS Fields

We measure the surface density of i'-band dropout galaxies at z ~ 6 through wide-field Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging and ultradeep Keck DEIMOS spectroscopy. Using deep HST ACS SDSS i' (F775W) and SDSS z' (F850LP) imaging from the Great Observatories Origins Deep Survey-North (GOODS-N; 200 arcmin2), we identify nine i'-drops satisfying (i'-z')AB > 1.5 to a depth of zimg1.gif = 25.6 (corresponding to Limg2.gif at z ~ 3). We use HK' imaging data to improve the fidelity of our sample, discriminating against lower redshift red galaxies and cool Galactic stars. Three i'-drops are consistent with M/L/T dwarf stars. We present ultradeep Keck DEIMOS spectroscopy of 10 objects from our combined GOODS-N and GOODS-S i'-drop sample. We detect Lyα emission at z = 5.83 from one object in the GOODS-S field, which lies only 8' (i.e., 3 himg3.gif Mpc) away from a previously confirmed z = 5.78 object. One possible Lyα emitter at z = 6.24 is found in the GOODS-N field (although identification of this spatially offset emission line is ambiguous). Using the rest-frame UV continuum from our six candidate z ~ 6 galaxies from the GOODS-N field, we determine a lower limit to the unobscured volume-averaged global star formation rate at z ~ 6 of (5.4 ± 2.2) × 10-4 h70 M☉ yr-1 Mpc-3. We find that the cosmic star formation density in Lyman break galaxies (LBGs) with unobscured star formation rates greater than 15 M☉ yr-1 falls by a factor of 8 between z ~ 3 and z ~ 6. Hence, the luminosity function of LBGs must evolve in this redshift interval: a constant integrated star formation density at z > 3 requires a much steeper faint-end slope, or a brighter characteristic luminosity. This result is in agreement with our previous measurement from the GOODS-S field, indicating that cosmic variance is not a dominant source of uncertainty.