arXiv subjects

Paolo Cassata

Publications and source records attributed to Paolo Cassata.

36 records · Page 2Linked to original sources

arXiv2019

Panchromatic Study of the First Galaxies with Large ALMA Programs

Thanks to deep optical to near-IR imaging and spectroscopy, significant progress is made in characterizing the rest-frame UV to optical properties of galaxies in the early universe (z > 4). Surveys with Hubble, Spitzer, and ground-based facilities (Keck, Subaru, and VLT) provide spectroscopic and photometric redshifts, measurements of the spatial structure, stellar masses, and optical emission lines for large samples of galaxies. Recently, the Atacama Large (Sub) Millimeter Array (ALMA) has become a major player in pushing studies of high redshift galaxies to far-infrared wavelengths, hence making panchromatic surveys over many orders of frequencies possible. While past studies focused mostly on bright sub-millimeter galaxies, the sensitivity of ALMA now enables surveys like ALPINE, which focuses on measuring the gas and dust properties of a large sample of normal main-sequence galaxies at z > 4. Combining observations across different wavelengths into a single, panchromatic picture of galaxy formation and evolution is currently and in the future an important focus of the astronomical community.

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arXiv2018

Near-infrared emission lines in starburst galaxies at 0.5 < z < 0.9 : Discovery of a merger sequence of extreme obscurations

We obtained optical/near-IR rest-frame Magellan FIRE spectra (including Pa$β$ and Pa$γ$) of 25 starburst galaxies at 0.5<z<0.9, with average star formation rates (SFR) x7 above the Main Sequence (MS). We find that Paschen-to-Balmer line ratios saturate around a constant value corresponding to $A_{\rm V}\sim$2-3 mag, while line to IR luminosity ratios suggest a large range of more extreme obscurations and appear to be uncorrelated to the former. This behavior is not consistent with standard attenuation laws derived for local and distant galaxies, while being remarkably consistent with observations of starburst cores in which young stars and dust are homogeneously mixed. This model implies $A_{\rm V}=$2-30 mag attenuation to the center of starburst cores, with a median of ~9 mag (a factor of 4000). X-ray hardness ratios for 6 AGNs in our sample and column densities derived from observed dust masses and radio sizes independently confirm this level of attenuation. In these conditions observed optical/near-IR emission comes from surface regions, while inner starburst cores are invisible. We thus attribute the high [NII]/H$α$ ratios to widespread shocks from accretion, turbulence and dynamic disturbances rather than to AGNs. The large range of optical depths demonstrates that substantial diversity is present within the starburst population, possibly connected to different merger phases or progenitor properties. The majority of our targets are, in fact, morphologically classified as mergers. We argue that the extreme obscuration provides in itself smoking gun evidence of their merger origin, and a powerful tool for identifying mergers at even higher redshifts.

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arXiv2017

The Dust Attenuation of Star-forming Galaxies at $z\sim3$ and Beyond: New Insights from ALMA Observations

We present results on the dust attenuation of galaxies at redshift $\sim3-6$ by studying the relationship between the UV spectral slope ($β_{\rm UV}$) and the infrared excess (IRX; $L_{\rm IR}$/$L_{\rm UV}$) using ALMA far-infrared continuum observations. Our study is based on a sample of 67 massive, star-forming galaxies with a median mass of $M_{\ast}\sim 10^{10.7}\,M_{\rm \odot}$ spanning a redshift range $z=2.6-3.7$ (median $z=3.2$) that were observed with ALMA at $λ_{rest}=300\,{\rm μm}$. Both the individual ALMA detections (41 sources) and stacks including all galaxies show the IRX-$β_{\rm UV}$ relationship at $z\sim3$ is mostly consistent with that of local starburst galaxies on average. However, we find evidence for a large dispersion around the mean relationship by up to $\pm0.5$ dex. Nevertheless, the locally calibrated dust correction factors based on the IRX-$β_{\rm UV}$ relation are on average applicable to main-sequence $z\sim3$ galaxies. This does not appear to be the case at even higher redshifts, however. Using public ALMA observations of $z\sim4-6$ galaxies we find evidence for a significant evolution in the IRX-$β_{\rm UV}$ and the IRX-$M_{\ast}$ relations beyond $z\sim3$ toward lower IRX values. We discuss several caveats that could affect these results, including the assumed dust temperature. ALMA observations of larger $z>3$ galaxy samples will be required to confirm this intriguing redshift evolution.

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arXiv2017

Morphology Dependence Of Stellar Age in Quenched Galaxies at Redshift ~ 1.2: Massive Compact Galaxies Are Older Than More Extended Ones

We report the detection of morphology dependent stellar age in massive quenched galaxies (QGs) at z~1.2. The sense of the dependence is that compact QGs are 0.5-2 Gyr older than normal-sized ones. The evidence comes from three different age indicators, Dn4000, Hdelta and fits to spectral synthesis models, applied to their stacked optical spectra. All age indicators consistently show that the stellar populations of compact QGs are older than their normally-sized counterparts. We detect weak [OII] emission in a fraction of QGs, and the strength of the line, when present, is similar between the two samples; however, compact galaxies exhibit significantly lower frequency of [OII] emission than normal ones. A fraction of both samples are individually detected in 7 Ms Chandra X-ray images (luminosities$\sim10^{40}$-$10^{41}$ erg/sec). 7 Ms stacks of non-detected galaxies show similarly low luminosities in the soft band only, consistent with a hot gas origin for the X-ray emission. While both [OII] emitters and non-emitters are also X-ray sources among normal galaxies, no compact galaxy with [OII] emission is an X-ray source, arguing against an AGN powering the line in compact galaxies. We interpret the [OII] properties as further evidence that compact galaxies are older and further along into the process of quenching star-formation and suppressing gas accretion. Finally, we argue that the older age of compact QGs is evidence of progenitor bias: compact QGs simply reflect the smaller sizes of galaxies at their earlier quenching epoch, with stellar density most likely having nothing directly to do with cessation of star-formation.

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arXiv2016

Hidden Starbursts and Active Galactic Nuclei at 0<z<4 from the Herschel-VVDS-CFHTLS-D1 Field: Inferences on Coevolution and Feedback

We investigate of the properties of $\sim$2000 Herschel/SPIRE-selected galaxies from $0 0.5$), the highest $L_{TIR}$ galaxies in a given redshift bin are unobserved by SPIRE at subsequently lower redshifts, a trend linked to downsizing. In conjunction with other results, this evidence is used to argue for prevalent AGN-driven quenching in starburst galaxies across cosmic time.

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arXiv2015

CANDELS Visual Classifications: Scheme, Data Release, and First Results

We have undertaken an ambitious program to visually classify all galaxies in the five CANDELS fields down to H<24.5 involving the dedicated efforts of 65 individual classifiers. Once completed, we expect to have detailed morphological classifications for over 50,000 galaxies up to z<4 over all the fields. Here, we present our detailed visual classification scheme, which was designed to cover a wide range of CANDELS science goals. This scheme includes the basic Hubble sequence types, but also includes a detailed look at mergers and interactions, the clumpiness of galaxies, $k$-corrections, and a variety of other structural properties. In this paper, we focus on the first field to be completed -- GOODS-S. The wide area coverage spanning the full field includes 7634 galaxies that have been classified by at least three different people. In the deep area of the field, 2534 galaxies have been classified by at least five different people at three different depths. With this paper, we release to the public all of the visual classifications in GOODS-S along with the GUI that we developed to classify galaxies. We find that the level of agreement among classifiers is good and depends on both the galaxy magnitude and the galaxy type, with disks showing the highest level of agreement and irregulars the lowest. A comparison of our classifications with the Sersic index and rest-frame colors shows a clear separation between disk and spheroid populations. Finally, we explore morphological k-corrections between the V-band and H-band observations and find that a small fraction (84 galaxies in total) are classified as being very different between these two bands. These galaxies typically have very clumpy and extended morphology or are very faint in the V-band.

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arXiv2015

Narrow band selected high redshift galaxy candidates contaminated by lower redshift O[III] ultrastrong emitter line galaxies

Context. Lyman Break Galaxies (LBG) and Narrow Band (NB) surveys have been successful at detecting large samples of high-redshift galaxies. Both methods are subject to contamination from low-redshift interlopers. Aims. In this paper, our aim is to investigate the nature of low-redshift interlopers in NB Lyman-$α$ emitters (LAE) searches. Methods. From previous HAWK-I NB imaging at z $\sim$7.7 we identify three objects that would have been selected as high-redshift LAEs had our optical data been one magnitude shallower (but still one to two magnitudes fainter than the near infrared data). We follow-up these objects in spectroscopy with XSHOOTER at the VLT. Results. Despite low quality data due to bad weather conditions, for each of the three objects we identify one, and only one emission line, in the spectra of the objects, that we identify as the O[III]5007A line. This result combined to spectral energy density fitting and tests based on line ratios of several populations of galaxies we infer that the 3 objects are ultrastrong line emitters at redshifts $\sim$1.1. Conclusions. From this work and the literature we remark that the O[III] line appears to be a common source of contamination in high-redshift LBG and LAE samples and we suggest that efforts be put to characterize with high accuracy the O[III] luminosity function out to redshift $\sim$3 or higher.

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arXiv2014

The interstellar medium and feedback in the progenitors of the compact passive galaxies at z~2

Quenched galaxies at z>2 are nearly all very compact relative to z~0, suggesting a physical connection between high stellar density and efficient, rapid cessation of star-formation. We present restframe UV spectra of Lyman-break galaxies (LBGs) at z~3 selected to be candidate progenitors of quenched galaxies at z~2 based on their compact restframe optical sizes and high surface density of star-formation. We compare their UV properties to those of more extended LBGs of similar mass and star formation rate (non-candidates). We find that candidate progenitors have faster ISM gas velocities and higher equivalent widths of interstellar absorption lines, implying larger velocity spread among absorbing clouds. Candidates deviate from the relationship between equivalent widths of Lyman-alpha and interstellar absorption lines in that their Lyman-alpha emission remains strong despite high interstellar absorption, possibly indicating that the neutral HI fraction is patchy such that Lyman-alpha photons can escape. We detect stronger CIV P-Cygni features (emission and absorption) and HeII emission in candidates, indicative of larger populations of metal rich Wolf-Rayet stars compared to non-candidates. The faster bulk motions, broader spread of gas velocity, and Lyman-alpha properties of candidates are consistent with their ISM being subject to more energetic feedback than non-candidates. Together with their larger metallicity (implying more evolved star-formation activity) this leads us to propose, if speculatively, that they are likely to quench sooner than non-candidates, supporting the validity of selection criteria used to identify them as progenitors of z~2 passive galaxies. We propose that massive, compact galaxies undergo more rapid growth of stellar mass content, perhaps because the gas accretion mechanisms are different, and quench sooner than normally-sized LBGs at these early epochs.

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arXiv2014

Lyman-alpha Forest Tomography from Background Galaxies: The First Megaparsec-Resolution Large-Scale Structure Map at z>2

We present the first observations of foreground Lyman-$α$ forest absorption from high-redshift galaxies, targeting 24 star-forming galaxies (SFGs) with $z\sim 2.3-2.8$ within a $5' \times 15'$ region of the COSMOS field. The transverse sightline separation is $\sim 2\,h^{-1}\mathrm{Mpc}$ comoving, allowing us to create a tomographic reconstruction of the 3D Ly$α$ forest absorption field over the redshift range $2.20\leq z\leq 2.45$. The resulting map covers $6\,h^{-1}\mathrm{Mpc} \times 14\,h^{-1}\mathrm{Mpc}$ in the transverse plane and $230\,h^{-1}\mathrm{Mpc}$ along the line-of-sight with a spatial resolution of $\approx 3.5\,h^{-1}\mathrm{Mpc}$, and is the first high-fidelity map of large-scale structure on $\sim\mathrm{Mpc}$ scales at $z>2$. Our map reveals significant structures with $\gtrsim 10\,h^{-1}\mathrm{Mpc}$ extent, including several spanning the entire transverse breadth, providing qualitative evidence for the filamentary structures predicted to exist in the high-redshift cosmic web. Simulated reconstructions with the same sightline sampling, spectral resolution, and signal-to-noise ratio recover the salient structures present in the underlying 3D absorption fields. Using data from other surveys, we identified 18 galaxies with known redshifts coeval with our map volume enabling a direct comparison to our tomographic map. This shows that galaxies preferentially occupy high-density regions, in qualitative agreement with the same comparison applied to simulations. Our results establishes the feasibility of the CLAMATO survey, which aims to obtain Ly$α$ forest spectra for $\sim 1000$ SFGs over $\sim 1 \,\mathrm{deg}^2$ of the COSMOS field, in order to map out IGM large-scale structure at $\langle z \rangle \sim 2.3$ over a large volume $(100\,h^{-1}\mathrm{Mpc})^3$.

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arXiv2013

The Progenitors of the Compact Early-Type Galaxies at High-Redshift

We use GOODS and CANDELS images to identify progenitors of massive (log M > 10 Msun) compact "early-type" galaxies (ETGs) at z~1.6. Since merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z~3 based on their mass, SFR and central stellar density and find that these account for a large fraction of, and possibly all, compact ETGs at z~1.6. We find that the average far-UV SED of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration, and consistent with more evolved (aging) star-formation. This is in line with other evidence that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended "halos" surrounding the compact "core", both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas-rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the HST images, with their stellar mass assembling in-situ, and that they have not experienced any major merging until the epoch of observations at z~1.6.

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arXiv2012

GMASS ultradeep spectroscopy of galaxies at z~2 - VII. Sample selection and spectroscopy

The populations of both quiescent and actively star-forming galaxies at 1 1.4, called Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). To determine redshifts and physical properties, such as metallicity, dust content, dynamical masses, and star formation history, we performed ultra-deep spectroscopy with the red-sensitive optical spectrograph FORS2 at the VLT. Our sample consists of objects, within the CDFS/GOODS area, detected at 4.5 micron, to be sensitive to stellar mass rather than star formation intensity. The spectroscopic targets were selected with a photometric redshift constraint (z>1.4) and magnitude constraints (B(AB)<26, I(AB)<26.5), which should ensure that these are faint, distant, and fairly massive galaxies. We present the sample selection, survey design, observations, data reduction, and spectroscopic redshifts. Up to 30 hours of spectroscopy of 174 spectroscopic targets and 70 additional objects enabled us to determine 210 redshifts, of which 145 are at z>1.4. From the redshifts and photometry, we deduce that the BzK selection criteria are efficient (82%) and suffer low contamination (11%). Several papers based on the GMASS survey show its value for studies of galaxy formation and evolution. We publicly release the redshifts and reduced spectra. In combination with existing and on-going additional observations in CDFS/GOODS, this data set provides a legacy for future studies of distant galaxies.

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arXiv2012

Multi-Wavelength View of Kiloparsec-Scale Clumps in Star-Forming Galaxies at z~2

This paper studies the properties of kiloparsec-scale clumps in star-forming galaxies at z~2 through multi-wavelength broad band photometry. A sample of 40 clumps is identified through auto-detection and visual inspection from 10 galaxies with 1.5<z<2.5 in the Hubble Ultra Deep Field, where deep and high-resolution HST/WFC3 and ACS images enable us to resolve structures of z~2 galaxies down to kpc scale in the rest-frame UV and optical bands as well as to detect clumps toward the faint end. The physical properties of clumps are measured through fitting spatially resolved seven-band (BVizYJH) spectral energy distribution to models. On average, the clumps are blue and have similar median rest-frame UV--optical color as the diffuse components of their host galaxies, but the clumps have large scatter in their colors. Although the star formation rate (SFR)--stellar mass relation of galaxies is dominated by the diffuse components, clumps emerge as regions with enhanced specific SFRs, contributing individually ~10% and together ~50% of the SFR of the host galaxies. However, the contributions of clumps to the rest-frame UV/optical luminosity and stellar mass are smaller, typically a few percent individually and ~20% together. On average, clumps are younger by 0.2 dex and denser by a factor of 8 than diffuse components. Clump properties have obvious radial variations in the sense that central clumps are redder, older, more extincted, denser, and less active on forming stars than outskirts clumps. Our results are broadly consistent with a widely held view that clumps are formed through gravitational instability in gas-rich turbulent disks and would eventually migrate toward galactic centers and coalesce into bulges. Roughly 40% of the galaxies in our sample contain a massive clump that could be identified as a proto-bulge, which seems qualitatively consistent with such a bulge-formation scenario.

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arXiv2012

Rest-frame UV--Optically Selected Galaxies at 2.3<z<3.5: Searching for Dusty Star-forming and Passively-Evolving Galaxies

A new set of color selection criteria (VJL) analogous with the BzK method is designed to select both star-forming galaxies (SFGs) and passively-evolving galaxies (PEGs) at 2.3 10^{10}M_{Sun}) galaxies at 2.3 0.4) SFGs, which however, only account for ~20% of the number density of massive SFGs. We also use the mid-infrared fluxes to clean our PEG sample, and find that galaxy size can be used as a secondary criterion to effectively eliminate the contamination of dusty SFGs. The redshift distribution of the cleaned PEG sample peaks at z~2.5. We find 6 PEG candidates at z>3 and discuss possible methods to distinguish them from dusty contamination. We conclude that at least part of our candidates are real PEGs at z~3, implying that this type of galaxies began to form their stars at z>5. We measure the integrated stellar mass density of PEGs at z~2.5 and set constraints on it at z>3. We find that the integrated stellar mass density grows by at least about factor of 10 in 1 Gyr at 3<z<5 and by another factor of 10 in next 3.5 Gyr (1<z<3).

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arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey - The Hubble Space Telescope Observations, Imaging Data Products and Mosaics

This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at $z\sim1.5-8$, and to study Type Ia SNe beyond $z>1.5$. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers \sim125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of \sim800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

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arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8--1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z>1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10^9 M_\odot to z \approx 2, reaching the knee of the ultraviolet luminosity function (UVLF) of galaxies to z \approx 8. The survey covers approximately 800 arcmin^2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H=27.7 mag) covers \sim 125 arcmin^2 within GOODS-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (EGS, COSMOS, and UDS) and covers the full area to a 5σ point-source limit of H \gtrsim 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

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arXiv2011

Discovery Of Cold, Pristine Gas Possibly Accreting Onto An Overdensity Of Star-Forming Galaxies At Redshift z ~ 1.6

We report the discovery of large amounts of cold (T ~ 10^4 K), chemically young gas in an overdensity of galaxies at redshift z ~ 1.6 in the Great Observatories Origins Deep Survey southern field (GOODS-S). The gas is identified thanks to the ultra-strong Mg II absorption features it imprints in the rest-frame UV spectra of galaxies in the background of the overdensity. There is no evidence that the optically-thick gas is part of any massive galaxy (i.e. M_star > 4x10^9 M_sun), but rather is associated with the overdensity; less massive and fainter galaxies (25.5 < z_850 < 27.5 mag) have too large an impact parameter to be causing ultra-strong absorption systems, based on our knowledge of such systems. The lack of corresponding Fe II absorption features, not detected even in co-added spectra, suggests that the gas is chemically more pristine than the ISM and outflows of star-forming galaxies at similar redshift, including those in the overdensity itself, and comparable to the most metal-poor stars in the Milky Way halo. A crude estimate of the projected covering factor of the high-column density gas (N_H >~ 10^20 cm-2) based on the observed fraction of galaxies with ultra-strong absorbers is C_F ~ 0.04. A broad, continuum absorption profile extending to the red of the interstellar Mg II absorption line by <~ 2000 km/s is possibly detected in two independent co-added spectra of galaxies of the overdensity, consistent with a large-scale infall motion of the gas onto the overdensity and its galaxies. Overall, these findings provides the first tentative evidence of accretion of cold, chemically young gas onto galaxies at high redshift, possibly feeding their star formation activity. The fact that the galaxies are members of a large structure, as opposed to field galaxies, might play a significant role in our ability to detect the accreting gas.

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arXiv2011

Color and Stellar Population Gradients in Passively Evolving Galaxies at z~2 from HST/WFC3 Deep Imaging in the Hubble Ultra Deep Field

We report the detection of color gradients in six massive (stellar mass > 10^{10} M_{sun}) and passively evolving (specific SFR < 10^{-11}/yr) galaxies at redshift 1.3<z<2.5 identified in the HUDF using HST ACS and WFC3/IR images. After matching different PSFs, we obtain color maps and multi-band optical/near-IR photometry (BVizYJH) in concentric annuli, from the smallest resolved radial (~1.7 kpc) up to several times the H-band effective radius. We find that the inner regions of these galaxies have redder rest-frame UV--optical colors than the outer parts. The slopes of the color gradients mildly depend on the overall dust obscuration and rest-frame (U-V) color, with more obscured or redder galaxies having steeper color gradients. The z~2 color gradients are also steeper than those of local early-types. The gradient of a single parameter (age, extinction or metallicity) cannot fully explain the observed color gradients. Fitting spatially resolved HST seven-band photometry to stellar population synthesis models, we find that, regardless of assumptions for metallicity gradient, the redder inner regions of the galaxies have slightly higher dust obscuration than the bluer outer regions, although the magnitude depends on the assumed extinction law. The derived age gradient depends on the assumptions for metallicity gradient. We discuss the implications of a number of assumptions for metallicity gradient on the formation and evolution of these galaxies. We find that the evolution of the mass--size relationship from z~2 to z~0 cannot be driven by in--situ extended star formation, implying that accretion or merger is mostly responsible for the evolution. The lack of a correlation between color gradient and stellar mass argues against the metallicity gradient predicted by the monolithic collapse, which would require significant major mergers to evolve into the one observed at z~0. (Abridged)

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arXiv2008

A spectroscopic study of a z=1.6 galaxy overdensity with GMASS

The Galaxy Mass Assembly ultra-deep Spectroscopic Survey samples a part of the CDFS to unprecedented depth. The resulting distribution of 150 z>1.4 redshifts reveals a significant peak at z=1.6, part of a larger overdensity found at this redshift. The 42 spectroscopic members of this structure, called Cl 0332-2742, form an overdensity in redshift of a factor 11+/-3 and have a velocity dispersion of 450 km/s. We derive a total mass for Cl 0332-2742 of ~7x10^14 Msol. The colours of its early-type galaxies are consistent with a theoretical red sequence of galaxies with stars formed at z=3.0. In addition, there are more massive, passive and older, but less star forming galaxies in CL 0332-2742 than in the field. We conclude that this structure is a cluster under assembly at z=1.6.

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