774 research outputs found
Dusty Starbursts and the Growth of Cosmic Structure
Dusty starbursts were more numerous around z~1 than today and appear to be
responsible for the majority of cosmic star formation over the Hubble time. We
suggest that they represent a common phase within galaxies in general which is
triggered by the growth of cosmic structure. We discuss the origin of the
luminosity of luminous infrared galaxies at z~1. Are these galaxies dominated
by star formation or nuclear activity ? What is triggering their strong
activity ? Is it triggered by external interactions or did it happen naturally
within isolated galaxies ? We present HST-ACS high resolution optical images of
luminous infrared galaxies at z~0.7 showing the evolution of the morphology of
these galaxies as a function of infrared luminosity, or star formation rate,
and discuss the effect of the environment on their activity.Comment: 4 pages, 2 figures, to appear in Proceeding of the "Multi-Wavelength
Cosmology" Conference held in Mykonos, Greece, June 2003, ed.M. Plionis
(Kluwer
Optical Morphology Evolution of Infrared Luminous Galaxies in GOODS-N
We combine optical morphologies and photometry from HST, redshifts from Keck,
and mid-infrared luminosities from Spitzer for an optically selected sample
of~800 galaxies in GOODS-N to track morphology evolution of infrared luminous
galaxies (LIRGs) since redshift z=1. We find a 50% decline in the number of
LIRGs from z~1 to lower redshift, in agreement with previous studies. In
addition, there is evidence for a morphological evolution of the populations of
LIRGs. Above z=0.5, roughly half of all LIRGs are spiral, the
peculiar/irregular to spiral ratio is ~0.7, and both classes span a similar
range of L_{IR} and M_B. At low-z, spirals account for one-third of LIRGs, the
peculiar to spiral fraction rises to 1.3, and for a given M_B spirals tend to
have lower IR luminosity than peculiars. Only a few percent of LIRGs at any
redshift are red early-type galaxies. For blue galaxies (U-B < 0.2), M_B is
well correlated with log(L_{IR}) with an RMS scatter (about a bivariate linear
fit) of ~0.25 dex in IR luminosity. Among blue galaxies that are brighter than
M_B = -21, 75% are LIRGs, regardless of redshift. These results can be
explained by a scenario in which at high-z, most large spirals experience an
elevated star formation rate as LIRGs. Gas consumption results in a decline of
LIRGs, especially in spirals, to lower redshifts.Comment: 6 pages, 2 figures, accepted ApJ
Mid-infrared and optical spectroscopy of ultraluminous infrared galaxies: A comparison
New tools from Infrared Space Observatory (ISO) mid-infrared spectroscopy
have recently become available to determine the power sources of dust-obscured
ultraluminous infrared galaxies (ULIRGs). We compare ISO classifications -
starburst or active galactic nucleus (AGN) - with classifications from optical
spectroscopy, and with optical/near-infrared searches for hidden broad-line
regions. The agreement between mid-infrared and optical classification is
excellent if optical LINER spectra are assigned to the starburst group. The
starburst nature of ULIRG LINERs strongly supports the suggestion that LINER
spectra in infrared-selected galaxies, rather than being an expression of the
AGN phenomenon, are due to shocks that are probably related to galactic
superwinds. Differences between ISO and optical classification provide clues on
the evolution of ULIRGs and on the configuration of obscuring dust. We find few
ISO AGN with optical HII or LINER identification, suggesting that highly
obscured AGN exist but are not typical for the ULIRG phenomenon in general.
Rather, our results indicate that strong AGN activity, once triggered, quickly
breaks the obscuring screen at least in certain directions, thus becoming
detectable over a wide wavelength range.Comment: aastex, 1 eps figure. Accepted by ApJ (Letters
A Far-infrared Characterization of 24 μm Selected Galaxies at 0 < z < 2.5 using Stacking at 70 μm and 160 μm in the COSMOS Field
We present a study of the average properties of luminous infrared galaxies detected directly at 24 μm in the COSMOS field using a median stacking analysis at 70 μm and 160 μm. Over 35,000 sources spanning 0 ≤ z ≤ 3 and 0.06 mJy ≤ S_(24) ≤ 3.0 mJy are stacked, divided into bins of both photometric redshift and 24 μm flux. We find no correlation of S_(70)/S_(24) flux density ratio with S_(24), but find that galaxies with higher S_(24) have a lower S_(160)/S_(24) flux density ratio. These observed ratios suggest that 24 μm selected galaxies have warmer spectral energy distributions (SEDs) at higher mid-IR fluxes, and therefore have a possible higher fraction of active galactic nuclei. Comparisons of the average S_(70)/S_(24) and S_(160)/S_(24) colors with various empirical templates and theoretical models show that the galaxies detected at 24 μm are consistent with "normal" star-forming galaxies and warm mid-IR galaxies such as Mrk 231, but inconsistent with heavily obscured galaxies such as Arp 220. We perform a χ^2 analysis to determine best-fit galactic model SEDs and total IR luminosities for each of our bins. We compare our results to previous methods of estimating L IR and find that previous methods show considerable agreement over the full redshift range, except for the brightest S_(24) sources, where they overpredict the bolometric IR luminosity at high redshift, most likely due to their warmer dust SED. We present a table that can be used as a more accurate and robust method for estimating bolometric infrared luminosity from 24 μm flux densities
NIR Spectroscopy with the VLT of a sample of ISO selected Hubble Deep Field South Galaxies
A new population of faint galaxies characterized by an extremely high rate of
evolution with redshift up to z~1.5 has recently been discovered by ISO. These
sources are likely to contribute significantly to the cosmic far-IR
extragalactic background. We have carried out near-infrared VLT-ISAAC
spectroscopy of a sample of ISOCAM galaxies from the Hubble Deep Field South.
The rest-frame R-band spectral properties of the ISO population resembles that
of powerful dust-enshrouded active starburst galaxies.Comment: 8 pages, 3 figures, to appear in "ISO Surveys of a Dusty Universe",
eds. D. Lemke, M. Stickel, K. Wilke, typos correcte
NGC6240: extended CO structures and their association with shocked gas
We present deep CO observations of NGC6240 performed with the IRAM Plateau de
Bure Interferometer (PdBI). NGC6240 is the prototypical example of a major
galaxy merger in progress, caught at an early stage, with an extended,
strongly-disturbed butterfly-like morphology and the presence of a heavily
obscured active nucleus in the core of each progenitor galaxy. The CO line
shows a skewed profile with very broad and asymmetric wings detected out to
velocities of -600 km/s and +800 km/s with respect to the systemic velocity.
The PdBI maps reveal the existence of two prominent structures of blueshifted
CO emission. One extends eastward, i.e. approximately perpendicular to the line
connecting the galactic nuclei, over scales of ~7 kpc and shows velocities up
to -400 km/s. The other extends southwestward out to ~7 kpc from the nuclear
region, and has a velocity of -100 km/s with respect to the systemic one.
Interestingly, redshifted emission with velocities 400 to 800 km/s is detected
around the two nuclei, extending in the east-west direction, and partly
overlapping with the eastern blue-shifted structure, although tracing a more
compact region of size ~1.7 kpc. The overlap between the southwestern CO blob
and the dust lanes seen in HST images, which are interpreted as tidal tails,
indicates that the molecular gas is deeply affected by galaxy interactions. The
eastern blueshifted CO emission is co-spatial with an Halpha filament that is
associated with strong H2 and soft X-ray emission. The analysis of Chandra
X-ray data provides strong evidence for shocked gas at the position of the
Halpha emission. Its association with outflowing molecular gas supports a
scenario where the molecular gas is compressed into a shock wave that
propagates eastward from the nuclei. If this is an outflow, the AGN are likely
the driving force.Comment: Accepted for publication in A&
A Submillimeter and Radio Survey of Gamma-Ray Burst Host Galaxies: A Glimpse into the Future of Star Formation Studies
We present the first comprehensive search for submillimeter and radio
emission from the host galaxies of twenty well-localized gamma-ray bursts
(GRBs). With the exception of a single source, all observations were undertaken
months to years after the GRB explosions to ensure negligible contamination
from the afterglows. We detect the host galaxy of GRB 000418 in both the sub-mm
and radio, and the host galaxy of GRB 000210 only in the sub-mm. These
observations, in conjunction with the previous detections of the host galaxies
of GRB 980703 and GRB 010222, indicate that about 20% of GRB host galaxies are
ultra-luminous and have star formation rates of about 500 M_sun/yr. As an
ensemble, the non-detected hosts have a star formation rate of about 100
M_sun/yr (5-sigma) based on their radio emission. The detected and ensemble
star formation rates exceed the optical values by an order of magnitude,
indicating significant dust obscuration. In the same vein, the ratio of
bolometric dust luminosity to UV luminosity for the hosts detected in the
sub-mm and radio ranges from 20-800, and follows the known trend of increasing
obscuration with increasing bolometric luminosity. We also show that, both as a
sample and individually, the GRB host galaxies have bluer R-K colors as
compared with galaxies selected in the sub-mm in the same redshift range. This
possibly indicates that the stellar populations in the GRB hosts are on average
younger, supporting the massive stellar progenitor scenario for GRBs, but it is
also possible that GRB hosts are on average less dusty.Comment: Submitted to ApJ; 36 pages, 3 tables, 7 figures; updated reference
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