2,759 research outputs found
Role of Galaxy Mergers in Cosmic Star Formation History
We present a morphology study of intermediate-redshift (0.2<z<1.2) luminous
infrared galaxies (LIRGs) and general field galaxies in the GOODS fields using
a revised asymmetry measurement method optimized for deep fields. By taking
careful account of the importance of the underlying sky-background structures,
our new method does not suffer from systematic bias and offers small
uncertainties. By redshifting local LIRGs and low-redshift GOODS galaxies to
different higher redshifts, we have found that the redshift dependence of the
galaxy asymmetry due to surface-brightness dimming is a function of the
asymmetry itself, with larger corrections for more asymmetric objects. By
applying redshift-, IR-luminosity- and optical-brightness-dependent asymmetry
corrections, we have found that intermediate-redshift LIRGs generally show
highly asymmetric morphologies, with implied merger fractions ~50% up to z=1.2,
although they are slightly more symmetric than local LIRGs. For general field
galaxies, we find an almost constant relatively high merger fraction (20-30%).
The B-band LFs of galaxy mergers are derived at different redshifts up to z=1.2
and confirm the weak evolution of the merger fraction after breaking the
luminosity-density degeneracy. The IR luminosity functions (LFs) of galaxy
mergers are also derived, indicating a larger merger fraction at higher IR
luminosity. The integral of the merger IR LFs indicates a dramatic evolution of
the merger-induced IR energy density [(1+z)^(5-6)}], and that galaxy mergers
start to dominate the cosmic IR energy density at z>~1.Comment: Accepted for publication in ApJ, 25 pages, 23 figures (2 colors). The
high-resolution pdf is at
http://cztsy.as.arizona.edu/~yong/Research/SHI_MERGER.pd
Star Formation in a Stellar Mass Selected Sample of Galaxies to z=3 from the GOODS NICMOS Survey (GNS)
We present a study of the star-forming properties of a stellar mass-selected
sample of galaxies in the GOODS NICMOS Survey (GNS), based on deep Hubble Space
Telescope imaging of the GOODS North and South fields. Using a stellar mass
selected sample, combined with HST/ACS and Spitzer data to measure both UV and
infrared derived star formation rates (SFR), we investigate the star forming
properties of a complete sample of ~1300 galaxies down to log M*=9.5 at
redshifts 1.5<z<3. Eight percent of the sample is made up of massive galaxies
with M*>10^11 Msun. We derive optical colours, dust extinctions, and
ultraviolet and infrared SFR to determine how the star formation rate changes
as a function of both stellar mass and time. Our results show that SFR
increases at higher stellar mass such that massive galaxies nearly double their
stellar mass from star formation alone over the redshift range studied, but the
average value of SFR for a given stellar mass remains constant over this 2 Gyr
period. Furthermore, we find no strong evolution in the SFR for our sample as a
function of mass over our redshift range of interest, in particular we do not
find a decline in the SFR among massive galaxies, as is seen at z < 1. The most
massive galaxies in our sample (log M*>11) have high average SFRs with values,
SFR(UV,corr) = 103+/-75 Msun/yr, yet exhibit red rest-frame (U-B) colours at
all redshifts. We conclude that the majority of these red high-redshift massive
galaxies are red due to dust extinction. We find that A(2800) increases with
stellar mass, and show that between 45% and 85% of massive galaxies harbour
dusty star formation. These results show that even just a few Gyr after the
first galaxies appear, there are strong relations between the global physical
properties of galaxies, driven by stellar mass or another underlying feature of
galaxies strongly related to the stellar mass.Comment: 18 pages, 10 figures, accepted for publication in MNRA
The evolution of the X-ray luminosity functions of unabsorbed and absorbed AGNs out to z~5
We present new measurements of the evolution of the X-ray luminosity
functions (XLFs) of unabsorbed and absorbed Active Galactic Nuclei (AGNs) out
to z~5. We construct samples containing 2957 sources detected at hard (2-7 keV)
X-ray energies and 4351 sources detected at soft (0.5-2 keV) energies from a
compilation of Chandra surveys supplemented by wide-area surveys from ASCA} and
ROSAT. We consider the hard and soft X-ray samples separately and find that the
XLF based on either (initially neglecting absorption effects) is best described
by a new flexible model parametrization where the break luminosity,
normalization and faint-end slope all evolve with redshift. We then incorporate
absorption effects, separately modelling the evolution of the XLFs of
unabsorbed () and absorbed ()
AGNs, seeking a model that can reconcile both the hard- and soft-band samples.
We find that the absorbed AGN XLF has a lower break luminosity, a higher
normalization, and a steeper faint-end slope than the unabsorbed AGN XLF out to
z~2. Hence, absorbed AGNs dominate at low luminosities, with the absorbed
fraction falling rapidly as luminosity increases. Both XLFs undergo strong
luminosity evolution which shifts the transition in the absorbed fraction to
higher luminosities at higher redshifts. The evolution in the shape of the
total XLF is primarily driven by the changing mix of unabsorbed and absorbed
populations.Comment: 36 pages, 20 figures, 11 tables. A casual reader is directed to
figures 7, 8, 9 and 20. Updated to version accepted for publication in MNRA
Testing Diagnostics of Nuclear Activity and Star Formation in Galaxies at z>1
We present some of the first science data with the new Keck/MOSFIRE
instrument to test the effectiveness of different AGN/SF diagnostics at z~1.5.
MOSFIRE spectra were obtained in three H-band multi-slit masks in the GOODS-S
field, resulting in two hour exposures of 36 emission-line galaxies. We compare
X-ray data with the traditional emission-line ratio diagnostics and the
alternative mass-excitation and color-excitation diagrams, combining new
MOSFIRE infrared data with previous HST/WFC3 infrared spectra (from the 3D-HST
survey) and multiwavelength photometry. We demonstrate that a high [OIII]/Hb
ratio is insufficient as an AGN indicator at z>1. For the four X-ray detected
galaxies, the classic diagnostics ([OIII]/Hb vs. [NII]/Ha and [SII]/Ha) remain
consistent with X-ray AGN/SF classification. The X-ray data also suggest that
"composite" galaxies (with intermediate AGN/SF classification) host bona-fide
AGNs. Nearly 2/3 of the z~1.5 emission-line galaxies have nuclear activity
detected by either X-rays or the classic diagnostics. Compared to the X-ray and
line ratio classifications, the mass-excitation method remains effective at
z>1, but we show that the color-excitation method requires a new calibration to
successfully identify AGNs at these redshifts.Comment: 7 pages, 4 figures. Accepted to ApJ Letter
Near-Infrared and Star-forming properties of Local Luminous Infrared Galaxies
We use HST NICMOS continuum and Pa-alpha observations to study the
near-infrared and star-formation properties of a representative sample of 30
local (d ~ 35-75Mpc) luminous infrared galaxies (LIRGs, infrared 8-1000um
luminosities of L_IR=11-11.9[Lsun]). The data provide spatial resolutions of
25-50pc and cover the central ~3.3-7.1kpc regions of these galaxies. About half
of the LIRGs show compact (~1-2kpc) Pa-alpha emission with a high surface
brightness in the form of nuclear emission, rings, and mini-spirals. The rest
of the sample show Pa-alpha emission along the disk and the spiral arms
extending over scales of 3-7kpc and larger. About half of the sample contains
HII regions with H-alpha luminosities significantly higher than those observed
in normal galaxies. There is a linear empirical relationship between the mid-IR
24um and hydrogen recombination (extinction-corrected Pa-alpha) luminosity for
these LIRGs, and the HII regions in the central part of M51. This relation
holds over more than four decades in luminosity suggesting that the mid-IR
emission is a good tracer of the star formation rate (SFR). Analogous to the
widely used relation between the SFR and total IR luminosity of Kennicutt
(1998), we derive an empirical calibration of the SFR in terms of the
monochromatic 24um luminosity that can be used for luminous, dusty galaxies.Comment: Accepted for publication in ApJ. Contact first author for high
qualitity version of figure
Breaking the Curve with CANDELS: A Bayesian Approach to Reveal the Non-Universality of the Dust-Attenuation Law at High Redshift
Dust attenuation affects nearly all observational aspects of galaxy
evolution, yet very little is known about the form of the dust-attenuation law
in the distant Universe. Here, we model the spectral energy distributions
(SEDs) of galaxies at z = 1.5--3 from CANDELS with rest-frame UV to near-IR
imaging under different assumptions about the dust law, and compare the amount
of inferred attenuated light with the observed infrared (IR) luminosities. Some
individual galaxies show strong Bayesian evidence in preference of one dust law
over another, and this preference agrees with their observed location on the
plane of infrared excess (IRX, ) and UV slope
(). We generalize the shape of the dust law with an empirical model,
where
is the dust law of Calzetti et al. (2000), and show that there
exists a correlation between the color excess and tilt with
+ . Galaxies with high
color excess have a shallower, starburst-like law, and those with low color
excess have a steeper, SMC-like law. Surprisingly, the galaxies in our sample
show no correlation between the shape of the dust law and stellar mass,
star-formation rate, or . The change in the dust law with color excess
is consistent with a model where attenuation is caused by by scattering, a
mixed star-dust geometry, and/or trends with stellar population age,
metallicity, and dust grain size. This rest-frame UV-to-near-IR method shows
potential to constrain the dust law at even higher () redshifts.Comment: 20 pages, 18 figures, resubmitted to Ap
Towards an understanding of the rapid decline of the cosmic star formation rate
We present a first analysis of deep 24 micron observations with the Spitzer
Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice,
0.65<z<0.75. We combine the infrared data with redshifts, rest-frame
luminosities, and colors from COMBO-17, and with morphologies from Hubble Space
Telescope images collected by the GEMS and GOODS projects. To characterize the
decline in star-formation rate (SFR) since z~0.7, we estimate the total thermal
infrared (IR) luminosities, SFRs, and stellar masses for the galaxies in this
sample. At z~0.7, nearly 40% of intermediate and high-mass galaxies (with
stellar masses >2x10^10 solar masses) are undergoing a period of intense star
formation above their past-averaged SFR. In contrast, less than 1% of
equally-massive galaxies in the local universe have similarly intense star
formation activity. Morphologically-undisturbed galaxies dominate the total
infrared luminosity density and SFR density: at z~0.7, more than half of the
intensely star-forming galaxies have spiral morphologies, whereas less than
\~30% are strongly interacting. Thus, a decline in major-merger rate is not the
underlying cause of the rapid decline in cosmic SFR since z~0.7. Physical
properties that do not strongly affect galaxy morphology - for example, gas
consumption and weak interactions with small satellite galaxies - appear to be
responsible.Comment: To appear in the Astrophysical Journal 1 June 2005. 14 pages with 8
embedded figure
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