arXiv subjects

M. Giavalisco

Publications and source records attributed to M. Giavalisco.

At least 145 records · Page 8Linked to original sources

arXiv2007

Large Structures and Galaxy Evolution in COSMOS at z < 1.1

We present the first identification of large-scale structures (LSS) at z $< 1.1$ in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3d space ($α,δ$,z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population -- recovering structures on all scales from 1 to 20\arcmin without {\it a priori} assumptions for the structure size or density profile. Our procedure makes {\bf no} {\it a priori} selection on galaxy spectral energy distribution (SED, for example the Red Sequence), enabling an unbiased investigation of environmental effects on galaxy evolution. The COSMOS photometric redshift catalog yields a sample of $1.5\times10^5$ galaxies with redshift accuracy, $Δz_{FWHM}/(1+z) \leq 0.1$ at z $< 1.1$ down to I$_{AB} \leq 25$ mag. Using this sample of galaxies, we identify 42 large-scale structures and clusters. abstract truncated for astroph 25 line limit -- see preprint

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arXiv2007

COSMOS : Hubble Space Telescope Observations

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$°) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution.

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arXiv2007

Lyman Alpha Emitters at Redshift 5.7 in the COSMOS Field

We present results from a narrow-band optical survey of a contiguous area of 1.95 deg^2, covered by the Cosmic Evolution Survey (COSMOS). Both optical narrow-band (lambda_c = 8150 AA and Delta_lambda = 120 AA) and broad-band (B, V, g', r', i', and z') imaging observations were performed with the Subaru prime-focus camera, Suprime-Cam on the Subaru Telescope. We provide the largest contiguous narrow-band survey, targetting Ly alpha emitters (LAEs) at z~5.7. We find a total of 119 LAE candidates at z~5.7. Over the wide-area covered by this survey, we find no strong evidence for large scale clustering of LAEs. We estimate a star formation rate (SFR) density of ~7*10^-4 M_sun yr^-1 Mpc^-3 for LAEs at z~5.7, and compare it with previous measurements.

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arXiv2007

The Spitzer Legacy Survey of the HST-ACS 2 sq. deg. COSMOS Field: survey strategy and first analysis

The Spitzer-COSMOS survey (S-COSMOS) is a Legacy program (Cycles 2+3) designed to carry out a uniform deep survey of the full 2 sq deg COSMOS field in all seven Spitzer bands (3.6, 4.5, 5.6, 8.0, 24.0, 70.0, 160.0 u). This paper describes the survey parameters, mapping strategy, data reduction procedures, achieved sensitivities to date, and the complete data set for future reference. We show that the observed infrared backgrounds in the S-COSMOS field are within 10% of the predicted background levels. The fluctuations in the background at 24u have been measured and do not show any significant contribution from cirrus, as expected. In addition, we report on the number of asteroid detections in the low galactic latitude COSMOS field. We use the Cycle 2 S-COSMOS data to determine preliminary number counts, and compare our results with those from previous Spitzer Legacy surveys (e.g. SWIRE, GOODS). The results from this "first analysis" confirm that the S-COSMOS survey will have sufficient sensitivity with IRAC to detect ~ L* disks and spheroids out to z ~ 3, and with MIPS to detect ultraluminous starbursts and AGN out to z ~3 at 24u and out to z ~1.5-2 at 70u and 160u.

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arXiv2006

A Potential Galaxy Threshing System in the Cosmos Field

We report on the discovery of a new potential galaxy threshing system in the COSMOS 2 square degree field using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru Telescope. This system consists of a giant elliptical galaxy with $M_V \approx -21.6$ and a tidally disrupted satellite galaxy with $M_V \approx -17.7$ at a photometric redshift of $z \approx 0.08$. This redshift is consistent with the spectroscopic redshift of 0.079 for the giant elliptical galaxy obtained from the Sloan Digital Sky Survey (SDSS) archive. The luminosity masses of the two galaxies are $3.7 \times 10^{12} \cal{M}_{\odot}$ and $3.1 \times 10^{9} \cal{M}_{\odot}$, respectively. The distance between the two galaxies is greater than 100 kpc. The two tidal tails emanating from the satellite galaxy extend over 150 kpc. This system would be the second well-defined galaxy threshing system found so far.

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arXiv2006

Photometric Redshifts of Galaxies in COSMOS

We measure photometric redshifts and spectral types for galaxies in the COSMOS survey. We use template fitting technique combined with luminosity function priors and with the option to simultaneously estimate dust extinction (i.e. E(B-V)) for each galaxy.Our estimated redshifts are accurate to i<25 and z~1.2. Using simulations with sampling and noise characteristics similar to those in COSMOS, the accuracy and reliability is estimated for the photometric redshifts as a function of the magnitude limits of the sample, S/N ratios and the number of bands used. From the simulations we find that the ratio of derived 95% confidence interval in the redshift probability distribution to the estimated photometric redshift (D95) can be used to identify and exclude the catastrophic failures in the photometric redshift estimates. We compare the derived redshifts with high-reliability spectroscopic redshifts for a sample of 868 normal galaxies with z < 1.2 from zCOSMOS. Considering different scenarios, depending on using prior, no prior and/or extinction, we compare the photometric and spectroscopic redshifts for this sample. This corresponds to an rms scatter of 0.031, with a small number of outliers (<2.5%). We also find good agreement (rms=0.10) between photometric and spectroscopic redshifts for Type II AGNs. We compare results from our photometric redshift procedure with three other independent codes and find them in excellent agreement. We show preliminary results, based on photometric redshifts for the entire COSMOS sample (to i < 25 mag.).

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arXiv2006

The Cosmic Evolution Survey (COSMOS) -- Overview

The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) with large-scale structure (LSS) over the redshift range z $> 0.5 $ to 6. The survey includes multi-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 $\sq$°area, including HST imaging. Given the very high sensitivity and resolution of these datasets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS datasets, and summarize the science goals.

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arXiv2006

The Cosmic Evolution Survey (COSMOS): Subaru Observations of the HST COSMOS Field

We present deep optical imaging observations of 2 square degree area, covered by the Cosmic Evolution Survey (COSMOS), made by the prime-focus Camera (Supreme-Cam) on the 8.2m Subaru Telescope. Observations were done in six broad-band [B (4459.7 AA), g' (4723.1 AA), V (5483.8 AA), r' (6213.0 AA), i' (7640.8 AA), z' (8855.0 AA)], and one narrow-band (NB816) filters. A total of 10^6 galaxies were detected to i'~26.5 mag. These data, combined with observations at u* and K-band are used to construct the photometric catalogs for the COSMOS and to measure their photometric redshifts, multi-band spectral energy distributions, stellar masses and identification of high redshift candidates. This catalog provides multi-waveband data for scientific analysis of the COSMOS survey.

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arXiv2006

Lyman Break Galaxies in the VLT/FORS2 spectroscopic campaign in the GOODS-S field

We present initial results from our ongoing campaign of spectroscopic identifications of Lyman--break galaxies (LBGs) at z ~ 4, 5 and 6 with FORS2 at the ESO VLT. The sample of LBGs is selected from the HST/ACS images in the GOODS southern field (CDF--S), and here we discuss the spectroscopic properties of 103 LBGs that we have cofirmed in the redshift interval 3.5 < z < 6.3, of which 34 at z>5 and 27 between 5.5 and 6.3. As for LBGs at lower redshifts, we find that LBGs with LyA in emission are characterized by weaker intestellar absorption lines and bluer UV continua than galaxies whose LyA is observed in absorption. No correlation between the LyA properties and the apparent elongation of the UV morphology of the galaxies is observed, suggesting that geometrical effects are not the cause of the spectral differences. A trend of increasing LyA equivalent width with decreasing luminosity is also observed. We also present new measures of the UV luinosity function of LBGs.

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arXiv2006

The Morphological Diversities Among Star-forming Galaxies at High Redshifts in the Great Observatories Origins Deep Survey (GOODS)

We have used the HST/ACS images to identify 4700 Lyman break galaxies (LBGs) in GOODS. We present the results from a parametric analysis of the 2-D surface brightness profiles, for 1333 LBGs at z > 2.5 with rest-frame UV(1600 Angstrom) AB magnitude < -20.5. Based on the Sersic index, n, which measures the profile shape, we find that about 40% of LBGs at z=3 have light profiles close to exponential, and only 30% have the high concentrations seen for spheroids. About 30% of LBGs appear to have multiple cores or disturbed morphologies suggestive of close pairs or on-going mergers. The fraction of spheroid-like (n > 2.5) LBGs decreases by about 15% from z = 5 to 3. A comparison of LBGs with the starburst galaxies at z = 1.2, shows that the fraction of spheroid-like profiles is about 20% higher among LBGs. The ellipticity distribution for LBGs exhibits a pronounced skew towards high ellipticities (> 0.5), which cannot be explained by morphologies similar to the local disks and spheroids viewed at random orientations. The peak of the distribution evolves toward lower ellipticities, from 0.7 at z = 4 to 0.5 at z = 3. At z = 1.2 the distribution is relatively flat as seen among the present-day galaxies. The dominance of elongated morphologies among LBGs suggests that in a significant fraction of them we may be witnessing star-formation in clumps along gas-rich filaments, or the earliest gas-rich bars that encompass essentially the entire visible galaxy. Similar features are found to be ubiquitous in hydrodynamical simulations in which galaxy formation at high redshifts occurs in filamentary inflows of dynamically cold gas within the dark matter halos, and involves gas- rich mergers.

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arXiv2006

The Great Observatories Origins Deep Survey VLT/FORS2 Spectroscopy in the GOODS-South Field: Part II

We present the second campaign of the ESO/GOODS program of spectroscopy of faint galaxies in the GOODS-South field. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams and comparing the results with public data. 807 spectra of 652 individua targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 501 redshift determinations. The typical redshift uncertainty is estimated to be sigma_z ~ 0.001. Galaxies have been selected adopting three different color criteria and using the photometric redshifts.The resulting redshift distribution typically spans two redshift domains: from z=0.5 to 2 and z=3.5 to 6.2. In particular, 94 B435-,V606-,i775 "dropout" Lyman break galaxies have been observed, yielding redshifts for 65 objects in the interval 3.4<z<6.2. Three sources have been serendipitously discovered in the redshift interval 4.8<z<5.5. Together with the previous release, 930 sources have now been observed and 724 redshift determinations have been carried out. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/. Large scale structures are clearly detected at z=0.666, 0.734, 1.096, 1.221, 1.300, and 1.614. A sample of 34 sources with tilted [OII]3727 emission has been identified, 32 of them in the redshift range 0.9<z<1.5.

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arXiv2006

Ultraviolet-to-Far-Infrared Properties of Local Star-Forming Galaxies

We present the results of a multiwavelength study of nearby galaxies, aimed at understanding the relation between the ultraviolet and far-infrared emission in star-forming galaxies. The dataset comprises new ultraviolet (from HST/STIS), ground-based Halpha, and radio continuum observations, together with archival infrared data (from IRAS and ISO). The local galaxies are used as benchmarks for comparison of the infrared-to-ultraviolet properties with two populations of high-redshift galaxies: the sub-millimeter star-forming galaxies detected by SCUBA and the ultraviolet-selected Lyman Break galaxies. In addition, the long wavelength baseline covered by the present data enables us to compare the star formation rates (SFRs) derived from the observed ultraviolet, Halpha, infrared, and radio luminosities, and to gauge the impact of dust opacity in the local galaxies. We also derive a new calibration for the non-thermal part of the radio SFR estimator, based on the comparison of 1.4 GHz measurements with a new estimator of the bolometric luminosity of the star forming regions. We find that more actively star forming galaxies show higher dust opacities, in line with previous results. We find that the local star-forming galaxies have lower F(205um)/F(UV) ratio, by two-three orders of magnitude than the submillimeter-selected galaxies, and may have similar or somewhat higher F(205um)/F(UV) than Lyman Break Galaxies. The F(205um)/F(UV) ratio of the local galaxy population may be influenced by the cool dust emission in the far-infrared heated by non-ionizing stellar populations, which may be reduced or absent in the LBGs.

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arXiv2006

Multiwavelength star formation indicators: Observations

We present a compilation of multiwavelength data on different star formation indicators for a sample of nearby star forming galaxies. Here we discuss the observations, reductions and measurements of ultraviolet images obtained with STIS, on board the Hubble Space Telescope, ground-based Halpha, and VLA 8.46 GHz radio images. These observations are complemented with infrared fluxes, as well as large aperture optical radio and ultraviolet data from the literature. This database will be used in a forthcoming paper to compare star formation rates at different wavebands. We also present spectral energy distributions (SEDs) for those galaxies with at least one far-infrared measurements from ISO, longward of 100 um. These SEDs are divided in two groups, those which are dominated by the far-infrared emission, and those where the contribution from the far-infrared and optical emission is comparable. These SEDs are useful tools to study the properties of high redshift galaxies.

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arXiv2005

Spitzer Observations of Massive Red Galaxies at High Redshift

We investigate the properties of massive galaxies at z=1-3.5 using HST observations, ground-based near-IR imaging, and Spitzer Space Telescope observations at 3-24 micron. We identify 153 distant red galaxies (DRGs) with J-K > 2.3 mag (Vega) in the southern GOODS field. This sample is approximately complete in stellar mass for passively evolving galaxies above 10^11 solar masses and z < 3. The galaxies identified by this selection are roughly split between objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation, and galaxies whose light is dominated by heavily reddened starbursts. Very few of the galaxies (< 10%) have no indication of current star formation. Using SFR estimates that include the reradiated IR emission, the DRGs at z=1.5-3 with stellar masses > 10^11 solar masses have specific SFRs (SFRs per unit stellar mass) ranging from 0.2 to 10 Gyr^-1, with a mean value of ~2.4 Gyr^-1. The DRGs with stellar masses > 10^11 solar masses and 1.5 < z < 3 have integrated specific SFRs greater the global value over all galaxies. In contrast, we find that galaxies at z = 0.3-0.75 with these stellar masses have integrated specific SFRs less than the global value, and more than an order of magnitude lower than that for massive DRGs at z = 1.5-3. 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. [Abridged]

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arXiv2005

Evidence for a Massive Post-Starburst Galaxy at z ~ 6.5

We present results from a search for high-redshift J--band ``dropout'' galaxies in the portion of the GOODS southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF).Using observations at optical, near-infrared and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the ESO-VLT, we search for very massive galaxies at high redshifts and find one particularly remarkable candidate. Its spectral energy distribution is consistent with a galaxy at z ~ 6.5 and a stellar mass of 6x10e11 M(sun) (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646A Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ~ 2.5 are possible but provide significantly poorer fits. The object is detected with Spitzer at 24 micron. This emission originats from an obscured active nucleus or star formation. We present optical and near-infrared spectroscopy which has, thus far, failed to detect any spectral features. This helps limit the solution in which the galaxy is a starburst or active galaxy at z ~ 2.5. If the high-redshift interpretation is correct, this object would be an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in a way which a very large mass of stars formed within a remarkably short period of time, at very high redshift.

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arXiv2005

Direct evidence for an early reionization of the Universe?

We examine the possible reionization of the intergalactic medium (IGM) by the source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep {\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al (2005) have identified HUDF-JD2 as a massive ($\sim6\times10^{11}M_\odot$) post-starburst galaxy at redshift z$\gtrsim6.5$. We find that HUDF-JD2 may be capable of reionizing its surrounding region of the Universe, starting the process at a redshift as high as z$\approx 15 \pm5$.

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arXiv2005

The Rest-Frame FUV Morphologies of Star-Forming Galaxies at z ~ 1.5 and z ~ 4

We apply a new approach to quantifying galaxy morphology and identifying galaxy mergers to the rest-frame far-ultraviolet images of 82 z ~ 4 Lyman break galaxies (LBGs) and 55 1.2 < z < 1.8 emission-line galaxies in the GOODS and Ultra Deep Fields. We compare the distributions of the Gini coefficient (G), second-order moment of the brightest 20% of galaxy light (M20), and concentration (C) for high-redshift and low-redshift galaxies with average signal to noise per pixel > 2.5 and Petrosian radii > 0.3 arcsec. Ten of the 82 LBGs have M20 >= -1.1 and possess bright double or multiple nuclei, implying a major-merger fraction of star-forming galaxies ~ 10-25% at M_{FUV} < -20, depending on our incompleteness corrections. Galaxies with bulge-like morphologies (G >= 0.55, M20 < -1.6) make up ~ 30% of the z ~ 4 LBG sample, while the remaining ~ 50% have G and M20 values higher than expected for smooth bulges and disks and may be star-forming disks, minor-mergers or post-mergers. The star-forming z ~ 1.5 galaxy sample has a morphological distribution which is similar to the UDF z ~ 4 LBGs, with an identical fraction of major-merger candidates but fewer spheroids. The observed morphological distributions are roughly consistent with current hierarchical model predictions for the major-merger rates and minor-merger induced starbursts at z ~ 1.5 and ~4. We also examine the rest-frame FUV-NUV and FUV-B colors as a function of morphology and find no strong correlations at either epoch.

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arXiv2005

The population of BzK selected ULIRGs at z~2

We investigate the multi-wavelength emission of BzK selected star forming galaxies at z~2 in the Great Observatories Origins Deep Survey (GOODS) North region. Most (82%) of the sources are individually detected at 24um in the Spitzer MIPS imaging, and one fourth (26%) in the VLA radio data. Significant detections of the individually undetected objects are obtained through stacking in the radio, submm and X-ray domains. The typical star forming galaxy with stellar mass ~10^{11}Mo at z=2 is an Ultra-luminous Infrared Galaxy (ULIRG), with L_IR ~ 1-2x10^{12}Lo and star formation rate SFR 200-300Mo/yr, implying a comoving density of ULIRGs at z=2 at least 3 orders of magnitude above the local one. SFRs derived from the reddening corrected UV luminosities agree well, on average, with the longer wavelength estimates. The high 24um detection rate suggests a relatively large duty cycle for the BzK star forming phase, consistently with the available independent measurements of the space density of passively evolving galaxies at z>1.4. If the IMF at z=2 is similar to the local one, and in particular is not a top-heavy IMF, this suggests that a substantial fraction of the high mass tail (>10^{11}Mo) of the galaxy stellar mass function was completed by z~1.4.

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