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Talia M.,University of Bologna | Mignoli M.,National institute for astrophysics | Cimatti A.,University of Bologna | Kurk J.,Max Planck Institute for Extraterrestrial Physics | And 12 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We use rest-frame UV spectroscopy to investigate the properties related to large-scale gas outflow, and both the dust extinction and star-formation rates (SFRs) of a sample of z ~ 2 star-forming galaxies from the Galaxy Mass Assembly ultradeep Spectroscopic Survey (GMASS). Methods. Dust extinction is estimated from the rest-frame UV continuum slope and used to obtain dust-corrected SFRs for the galaxies in the sample. A composite spectrum is created by averaging all the single spectra of our sample, and the equivalent widths and centroids of the absorption lines associated with the interstellar medium are measured. We then calculate the velocity offsets of these lines relative to the composite systemic velocity, which is obtained from photospheric stellar absorption lines and nebular emission lines. Finally, to investigate correlations between galaxy UV spectral characteristics and galaxy general properties, the sample is divided into two bins that are equally populated, according to the galaxy properties of stellar mass, color excess, and SFR. A composite spectrum for each group of galaxies is then created, and both the velocity offsets and the equivalent widths of the interstellar absorption lines are measured. Results. For the entire sample, we derive a mean value of the continuum slope β= -1.11 ± 0.44 (rms). For each galaxy, we calculate the dust extinction from the UV spectrum and then use this to correct the flux measured at 1500 (rest-frame), before converting the corrected UV flux into a SFR. We find that our galaxies have an average SFR of SFR= 52 ± 48 Mo yr -1 (rms) and that there is a positive correlation between SFR and stellar mass, in agreement with other works, the logarithmic slope of the relation being 1.10 ± 0.10. We discover that the low-ionization absorption lines associated with the interstellar medium measured in the composite spectrum, are blueshifted with respect to the rest frame of the system, which indicates that there is outflowing gas with typical velocities of about -100 kms -1. Finally, investigating the correlations between the galaxy UV spectral characteristics and general galaxy properties, we find a possible correlation between the equivalent width of the interstellar absorption lines and SFR, stellar mass, and color excess similar to that previously reported to hold at different redshifts. © ESO, 2012. Source


Rawlings J.I.,University College London | Seymour N.,University College London | Seymour N.,CSIRO | Page M.J.,University College London | And 17 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the mid-infrared (IR) spectra of seven of the most powerful radio-galaxies known to exist at 1.5 Source


Cassata P.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Guo Y.,University of Massachusetts Amherst | Renzini A.,National institute for astrophysics | And 10 more authors.
Astrophysical Journal | Year: 2011

We report on the evolution of the number density and size of early-type galaxies (ETGs) from z ∼ 2 to z ∼ 0. We select a sample of 563 massive (M > 1010 M ⊙), passively evolving (specific star formation rate <10-2 Gyr-1), and morphologically spheroidal galaxies at 0 < z < 2.5, using the panchromatic photometry and spectroscopic redshifts available in the Great Observatories Origins Deep Surveys fields. We combine Advanced Camera for Surveys and Wide Field Camera 3 Hubble Space Telescope images to study the morphology of our galaxies in their optical rest frame in the entire 0 < z < 2.5 range. We find that throughout the explored redshift range the passive galaxies selected with our criteria have weak morphological K-correction, with size being slightly smaller in the optical than in the UV rest frame (by ∼20% and ∼10% at z > 1.2 and z < 1.2, respectively). We measure a significant evolution of the mass-size relation of ETGs, with a fractional increment that is almost independent of the stellar mass. ETGs formed at z > 1 appear to be preferentially small, and the evolution of the mass-size relation at z < 1 is driven by both the continuous size growth of the compact galaxies and the appearance of new ETGs with large sizes. We also find that the number density of all passive ETGs increases rapidly, by a factor of five, from z ∼ 2 to z ∼ 1, and then more mildly by another factor of 1.5 from z ∼ 1 to z ∼ 0. We interpret these results as evidence that the bulk of the ETGs are formed at 1 < z < 3 through a mechanism that leaves very compact remnants. At z < 1 the compact ETGs grow gradually in size, becoming normal-size galaxies, and at the same time new ETGs with normal-large sizes are formed. © 2011. The American Astronomical Society. All rights reserved. Source


Guo Y.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Cassata P.,University of Massachusetts Amherst | Ferguson H.C.,US Space Telescope Science Institute | And 16 more authors.
Astrophysical Journal | Year: 2012

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 ≲ z ≲ 3.5 by using rest-frame UV-optical (V - J versus J - L) colors. The criteria are thoroughly tested with theoretical stellar population synthesis models and real galaxies with spectroscopic redshifts to evaluate their efficiency and contamination. We apply the well-tested VJL criteria to the HST/WFC3 Early Release Science field and study the physical properties of selected galaxies. The redshift distribution of selected SFGs peaks at z ∼ 2.7, slightly lower than that of Lyman break galaxies at z ∼ 3. Comparing the observed mid-infrared fluxes of selected galaxies with the prediction of pure stellar emission, we find that our VJL method is effective at selecting massive dusty SFGs that are missed by the Lyman break technique. About half of the star formation in massive (Mstar > 1010 M⊙) galaxies at 2.3 ≲ z ≲ 3.5 is contributed by dusty (extinction E(B - V) > 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 six 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 these types of galaxies began to form their stars at z ≳ 5. We measure the integrated stellar mass density (ISMD) of PEGs at z ∼ 2.5 and set constraints on it at z > 3. We find that the ISMD grows by at least about a factor of 10 in 1Gyr at 3 < z <5 and by another factor of 10 in the next 3.5Gyr (1 < z < 3). © 2012 The American Astronomical Society. All rights reserved. Source


Guo Y.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Cassata P.,University of Massachusetts Amherst | Ferguson H.C.,US Space Telescope Science Institute | And 9 more authors.
Astrophysical Journal | Year: 2011

We report the detection of color gradients in six massive (stellar mass (M star) > 1010 M ) and passively evolving (specific star formation rate <10-11 yr-1) galaxies at redshift 1.3 < z < 2.5 identified in the Hubble Ultra Deep Field using ultra-deep Hubble Space Telescope (HST) Advanced Camera for Surveys and WFC3/IR images. After carefully matching the different point-spread functions, we obtain color maps and multi-band optical/near-IR photometry (BVizYJH) in concentric annuli, from the smallest resolved radial distance (1.7kpc) 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 (U- V, U- B, and B- V) than the outer parts. The slopes of the color gradient have no obvious dependence on the redshift and on the stellar mass of the galaxies. They do mildly depend, however, on the overall dust obscuration (E(B - V)) 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-type ones. The gradient of a single parameter (age, extinction, or metallicity) cannot fully explain the observed color gradients. Fitting the spatially resolved HST seven-band photometry to stellar population synthesis models, we find that, regardless of assumptions on the metallicity gradient, the redder inner regions of the galaxies have slightly higher dust obscuration than the bluer outer regions, implying that dust partly contributes to the observed color gradients, although the magnitude depends on the assumed extinction law. Due to the age-metallicity degeneracy, the derived age gradient depends on the assumptions for the metallicity gradient. We discuss the implications of a number of assumptions for metallicity gradients on the formation and evolution of these galaxies. We find that the evolution of the mass-size relationship from z 2 to the present cannot be driven by in situ extended star formation, which implies that accretion or merger is mostly responsible for the growth of their stellar mass and size. The lack of a correlation between the strength of the color gradient and the stellar mass argues against the metallicity gradient predicted by the monolithic-collapse scenario, which would require significant major mergers to evolve into the one observed at the present. © 2011. The American Astronomical Society. All rights reserved.. Source

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