388 research outputs found
GOODS-Herschel: a population of 24 μm dropout sources at z<2
Using extremely deep PACS 100- and 160 μm Herschel data from the GOODS-Herschel program, we identify 21 infrared bright galaxies previously missed in the deepest 24 μm surveys performed by Spitzer/MIPS. These MIPS dropouts are predominantly found in two redshift bins, centred at z ~ 0.4 and ~1.3. Their S_(100)/S_(24) flux density ratios are similar to those of local (ultra-) luminous infrared galaxies (LIRGs and ULIRGs), whose silicate absorption features at 18 μm (at z ~ 0.4) and 9.7 μm (at z ~ 1.3) are shifted into the 24 μm MIPS band at these redshifts. The high-z sub-sample consists of 11 infrared luminous sources, accounting for ~2% of the whole GOODS-Herschel sample and putting strong upper limits on the fraction of LIRGs/ULIRGs at 1.0 43 colour cut selects galaxies with a redshift distribution similar to that of the MIPS dropouts and when combined with a second colour cut, S_(16)/S_8 > 4, isolates sources at 1.0 < z < 1.7. We show that these sources have elevated specific star formation rates (sSFR) compared to main sequence galaxies at these redshifts and are likely to be compact starbursts with moderate/strong 9.7 μm silicate absorption features in their mid-IR spectra. Herschel data reveal that their infrared luminosities extrapolated from the 24 μm flux density are underestimated, on average, by a factor of ~3. These silicate break galaxies account for 16% (8%) of the ULIRG (LIRG) population in the GOODS fields, indicating a lower limit in their space density of 2.0 × 10^(-5) Mpc^(-3). Finally, we provide estimates of the fraction of z < 2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250- and 350 μm sensitivity limits, and conclude that previous predictions of a population of silicate break galaxies missed by the major 24 μm extragalactic surveys have beenoverestimated
A mid-IR study of Hickson Compact Groups II. Multi-wavelength analysis of the complete GALEX-Spitzer Sample
We present a comprehensive study on the impact of the environment of compact
galaxy groups on the evolution of their members using a multi-wavelength
analysis, from the UV to the infrared, for a sample of 32 Hickson compact
groups (HCGs) containing 135 galaxies. Fitting the SEDs of all galaxies with
the state-of-the-art model of da Cunha (2008) we can accurately calculate their
mass, SFR, and extinction, as well as estimate their infrared luminosity and
dust content. We compare our findings with samples of field galaxies,
early-stage interacting pairs, and cluster galaxies with similar data. We find
that classifying the groups as dynamically "old" or "young", depending on
whether or not at least one quarter of their members are early-type systems, is
physical and consistent with past classifications of HCGs based on their atomic
gas content. [...ABRIDGED...] We also examine their SF properties, UV-optical
and mid-IR colors, and we conclude that all the evidence point to an
evolutionary scenario in which the effects of the group environment and the
properties of the galaxy members are not instantaneous. Early on, the influence
of close companions to group galaxies is similar to the one of galaxy pairs in
the field. However, as the time progresses, the effects of tidal torques and
minor merging, shape the morphology and star formation history of the group
galaxies, leading to an increase of the fraction of early-type members and a
rapid built up of the stellar mass in the remaining late-type galaxies.Comment: Accepted for publication in A&A. Figure resolution degraded for arXiv
limits, full resolution paper available at
http://www.physics.uoc.gr/~bitsakis/paperII_bitsakis.pd
On the far-infrared metallicity diagnostics: applications to high-redshift galaxies
In an earlier paper we modeled the far-infrared emission from a star-forming
galaxy using the photoionisation code CLOUDY and presented metallicity
sensitive diagnostics based on far-infrared fine structure line ratios. Here,
we focus on the applicability of the [OIII]88/[NII]122 microns line ratio as a
gas phase metallicity indicator in high redshift submillimetre luminous
galaxies. The [OIII]88/[NII]122 microns ratio is strongly dependent on the
ionization parameter (which is related to the total number of ionizing photons)
as well as the gas electron density. We demonstrate how the ratio of 88/$122
continuum flux measurements can provide a reasonable estimate of the ionization
parameter while the availability of the [NII]205 microns line can constrain the
electron density. Using the [OIII]88/[NII]122 microns line ratios from a sample
of nearby normal and star-forming galaxies we measure their gas phase
metallicities and find that their mass metallicity relation is consistent with
the one derived using optical emission lines. Using new, previously
unpublished, Herschel spectroscopic observations of key far-infrared fine
structure lines of the z~3 galaxy HLSW-01 and additional published measurements
of far-infrared fine structure lines of high-z submillimetre luminous galaxies
we derive gas phase metallicities using their [OIII]88/[NII]122 microns line
ratio. We find that the metallicities of these z~3 submm luminous galaxies are
consistent with solar metallicities and that they appear to follow the
mass-metallicity relation expected for z~3 systems.Comment: 10 pages, 7 figures, MNRAS in pres
Disentangling the AGN and Star-Formation connection using XMM-Newton
There is growing evidence supporting the coeval growth of galaxies and their
resident SMBH. Most studies also claim a correlation between the activity of
the SMBH and the star-formation of the host galaxy. It is unclear, though,
whether this correlation extends to all redz and LX. In this work, we use data
from the X-ATLAS and XMM-XXL North fields and compile the largest X-ray sample
up to date, to investigate how X-ray AGN affect the star-formation of their
host galaxies in a wide redz and luminosity baseline, i.e., 0.03<z<3 and log LX
(2-10 keV)= (41-45.5) ergs-1. Our sample consists of 3336 AGN. 1872 X-ray
objects have spectroscopic redz. For the remaining sources we calculate
photometric redz using TPZ, a machine-learning algorithm. We estimate stellar
masses (M*) and Star Formation Rates (SFRs) by applying SED fitting through the
CIGALE code, using optical, near-IR and mid-IR photometry. 608 of our sources
also have far-IR photometry (Herschel). We use these sources to calibrate the
SFR calculations of the rest of our X-ray sample. Our results show a
correlation between the LX and the SFR, at all redz and LX spanned by our
sample. We also find a dependence of the specific SFR (sSFR) on redz, while
there are indications that LX enhances the sSFR even at low redz. We then
disentangle the effects of M* and redz on the SFR and study again its
dependence on the LX. Towards this end, we use the Schreiber et al. formula to
estimate the SFR of main sequence galaxies that have the same stellar mass and
redshift as our X-ray AGN. Our analysis reveals that the AGN enhances the
star-formation of its host galaxy, when the galaxy lies below the main sequence
and quenches the star-formation of the galaxy it lives in, when the host lies
above the main sequence. Therefore, the effect of AGN on the SFR of the host
galaxy, depends on the location of the galaxy relative to the main sequence.Comment: A&A Accepted 10 pages, 8 figures, 2 table
Dust and gas in star forming galaxies at z~3 - extending galaxy uniformity to 11.5 billion years
We present millimetre dust emission measurements of two Lyman Break Galaxies at z~3 and construct for the first time fully sampled infrared spectral energy distributions (SEDs), from mid-IR to the Rayleigh-Jeans tail, of individually detected, unlensed, UV-selected, main sequence (MS) galaxies at z=3. The SED modelling of the two sources confirms previous findings, based on stacked ensembles, of an increasing mean radiation field with redshift, consistent with a rapidly decreasing gas metallicity in z > 2 galaxies. Complementing our study with CO[3-2] emission line observations, we measure the molecular gas mass (M_H2) reservoir of the systems using three independent approaches: 1) CO line observations, 2) the dust to gas mass ratio vs metallicity relation and 3) a single band, dust emission flux on the Rayleigh-Jeans side of the SED. All techniques return consistent M_H2 estimates within a factor of ~2 or less, yielding gas depletion time-scales (tau_dep ~ 0.35 Gyrs) and gas-to-stellar mass ratios (M_H2/M* ~ 0.5-1) for our z~3 massive MS galaxies. The overall properties of our galaxies are consistent with trends and relations established at lower redshifts, extending the apparent uniformity of star-forming galaxies over the last 11.5 billion years
The large molecular gas fraction of post-starburst galaxies at z > 1
Post-starburst galaxies are sources that had the last major episode of star
formation about 1 Gyr before the epoch of the observations and are on their way
to quiescence. It is important to study such galaxies at redshift z > 1, during
their main quenching phase, and estimate their molecular gas content to
constrain the processes responsible for the cessation of star formation. We
present CO(3-2) ALMA observations of two massive (Mstar ~ 5 x 10^10 Msun)
post-starburst galaxies at z > 1. We measure their molecular gas fraction to be
f_H2 = M_H2/Mstar ~ 8% - 16%, consistent with z < 1 post-starburst galaxies
from the literature. The star formation efficiency of our targets is ~ 10x
lower than that of star-forming galaxies at similar redshift, and they are
outliers of the f_H2 - specific star formation rate (sSFR) relation of
star-forming galaxies, as they have larger f_H2 than expected given their sSFR.
The gas fraction of post-starbursts from our sample and the literature
correlates with the Dn4000 spectral index, a proxy of the stellar population
age. This suggests that their gas content decreases after the last major burst
of star formation. Finally, one of our targets is undergoing a major merger
phase with two highly star-forming companions. This hints at a picture where a
perturber event (e.g., major merger) quenches star formation without completely
removing the molecular gas.Comment: Accepted for publication in MNRA
HerMES: spectral energy distributions of submillimeter galaxies at z > 4.
We present a study of the infrared properties for a sample of seven spectroscopically confirmed submillimetre galaxies (SMGs) at z > 4.0. By combining ground-based near-infrared, Spitzer IRAC and MIPS, Herschel SPIRE, and ground-based submillimetre / millimeter photometry, we construct their spectral energy distributions (SEDs) and a composite model to fit the SEDs. The model includes a stellar emission component at λ rest 50μm. Six objects in the sample are detected at 250 and 350μm. The dust temperatures for the sources in this sample are in the range of 40–80 K, and their L FIR ∼ 10 13 Lo qualifies them as hyper-luminous infrared galaxies. The mean FIR-radio index for this sample is around (q) = 2.2 indicating no radio excess in their radio emission. Most sources in the sample have 24μmdetections corresponding to a rest-frame 4.5μm luminosity of Log 10 (L 4.5 /L ? )=11 ∼ 11.5. Their L 4.5 /L FIR ratios are very similar to those of starburst-dominated SMGs at z ∼ 2. The L CO − L FIR relation for this sample is consistent with that determined for local ULIRGs and SMGs at z ∼ 2. We conclude that SMGs at z > 4 are hotter and more luminous in the FIR but otherwise very similar to those at z ∼ 2. None of these sources show any sign of the strong QSO phase being triggered
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