84 research outputs found
Spitzer view on the downsizing scenario of galaxy formation and the role of AGN
We present the latest results of the Spitzer Cosmological Surveys concerning the characterization of the evolution of galaxies in the last 12 Gyr (from z=4). We have analyzed the stellar mass function up to z=4 using a sample of more the 28,000 galaxies selected in the rest-frame near-infrared with Spitzer/IRAC. Our results confirm and quantify the “downsizing” scenario of galaxy formation. Based on the study of the specific SFRs of X-ray emitters, we discuss the role of AGN in the evolution of galaxies, arguing against the link between nuclear activity and the quenching of the star formation in massive galaxies at z<1.4
The galaxy major merger fraction to z ~ 1
Aims. The importance of disc-disc major mergers in galaxy evolution remains uncertain. We study the major merger fraction in a SPITZER/IRAC-selected catalogue in the GOODS-S field up to z ~ 1 for luminosity- and mass-limited samples. Methods. We select disc-disc merger remnants on the basis of morphological asymmetries/distortions, and address three main sources of systematic errors: (i) we explicitly apply morphological K-corrections; (ii) we measure asymmetries in galaxies artificially redshifted to z_d = 1.0 to deal with loss of morphological information with redshift; and (iii) we take into account the observational errors in z and A, which tend to overestimate the merger fraction, though use of maximum likelihood techniques. Results. We obtain morphological merger fractions (f_m^mph) below 0.06 up to z ~ 1. Parameterizing the merger fraction evolution with redshift as f_m^mph (z) = f_m^mph (0)(1 + z)^m, we find that m = 1.8 ± 0.5 for M(B)≤ -20 galaxies, while m = 5.4 ± 0.4 for M_* ≥ 10^10 M_⨀ galaxies. When we translate our merger fractions to merger rates (R_m^mph), their evolution, parameterized as R_m^mph (z) = R_m^mph (0)(1+ z)^n, is quite similar in both cases: n = 3.3 ± 0.8 for M(B) ≤ -20 galaxies, and n = 3.5 ± 0.4 for M_* ≥10^10 M_⨀ galaxies. Conclusions. Our results imply that only similar to 8% of today's M(star) ≥ 10^10 M_⨀ galaxies have undergone a disc-disc major merger since z ~ 1. In addition, ~ 21% of M_* ≥ 10(10) M_⨀ galaxies at z ~ 1 have undergone one of these mergers since z similar to 1.5. This suggests that disc-disc major mergers are not the dominant process in the evolution of M_* ≥ 10(10) M_⨀ galaxies since z 1, with only 0.2 disc-disc major mergers per galaxy, but may be an important process at z > 1, with ~ 1 merger per galaxy at 1 < z < 3
Dwarf galaxies show little ISM evolution from to : a spectroscopic study of metallicity, star formation, and electron density
We present gas-phase metallicity measurements for 583 emission line galaxies
at , including 388 dwarf galaxies with , and explore the dependence of the metallicity on the stellar mass and
star formation properties of the galaxies. Metallicities are determined through
the measurement of emission lines in very deep (7 hr exposure)
Keck/DEIMOS spectra taken primarily from the HALO7D survey. We measure
metallicity with three strong-line calibrations (O3H, R23, and O3O2) for
the overall sample, as well as with the faint [Ne III]3869 and [O
III]4363 emission lines for 112 and 17 galaxies where robust
detections were possible. We construct mass-metallicity relations (MZR) for
each calibration method, finding MZRs consistent with other strong-line results
at comparable redshift, as well as with galaxies. We quantify the
intrinsic scatter in the MZR as a function of mass, finding it increases with
lower stellar mass. We also measure a weak but significant correlation between
increased MZR scatter and higher specific star formation rate. We find a weak
influence of SFR in the fundamental metallicity relation as well, with an SFR
coefficient of . Finally, we use the flux ratios of the [O
II]3727,3729 doublet to calculate gas electron density in
1000 galaxies with as a function of
redshift. We measure low electron densities ( cm) for
galaxies, again consistent with conditions, but measure higher
densities ( cm) at . These results all suggest that
there is little evolution in star-forming interstellar medium conditions from
to , confirmed with a more complete sample of low-mass galaxies
than has previously been available in this redshift range.Comment: 22 pages, 10 figures, accepted to Ap
The Dwarf Galaxy Population at z ∼ 0.7: A Catalog of Emission Lines and Redshifts from Deep Keck Observations
We present a catalog of spectroscopically measured redshifts over
and emission line fluxes for 1440 galaxies. The majority (65\%) of the
galaxies come from the HALO7D survey, with the remainder from the DEEPwinds
program. This catalog includes redshifts for 646 dwarf galaxies with
. 810 catalog galaxies did not have previously
published spectroscopic redshifts, including 454 dwarf galaxies. HALO7D used
the DEIMOS spectrograph on the Keck II telescope to take very deep (up to 32
hours exposure, with a median of 7 hours) optical spectroscopy in the
COSMOS, EGS, GOODS-North, and GOODS-South CANDELS fields, and in some areas
outside CANDELS. We compare our redshift results to existing spectroscopic and
photometric redshifts in these fields, finding only a 1\% rate of discrepancy
with other spectroscopic redshifts. We measure a small increase in median
photometric redshift error (from 1.0\% to 1.3\%) and catastrophic outlier rate
(from 3.5\% to 8\%) with decreasing stellar mass. We obtained successful
redshift fits for 75\% of massive galaxies, and demonstrate a similar 70-75\%
successful redshift measurement rate in
galaxies, suggesting similar survey sensitivity in this low-mass range. We
describe the redshift, mass, and color-magnitude distributions of the catalog
galaxies, finding HALO7D galaxies representative of CANDELS galaxies up to
\textit{i}-band magnitudes of 25. The catalogs presented will enable studies of
star formation (SF), the mass-metallicity relation, SF-morphology relations,
and other properties of the dwarf galaxy population.Comment: 23 pages, 19 Figures, updated to version accepted by ApJ
What\u27s My Research? with Sarah Waltz and Guillermo Barro Calvo
Join us on March 2nd at noon, at the Digital Wall in the Library (1st floor) for a “What’s My Research?” presentation. Drs. Sarah Waltz, Music History, will talk about William Herschel, the musician who discovered Uranus, and Guillermo Barro Calvo, Physics, will speak about a new window into the Universe with the James Webb Space Telescope. A light lunch will be served
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