177 research outputs found
The mid-IR Luminosity Function at z<0.3 from 5MUSES: Understanding the Star-formation/AGN Balance from a Spectroscopic View
We present rest-frame 15 and 24 um luminosity functions and the corresponding
star-forming luminosity functions at z<0.3 derived from the 5MUSES sample.
Spectroscopic redshifts have been obtained for ~98% of the objects and the
median redshift is ~0.12. The 5-35 um IRS spectra allow us to estimate
accurately the luminosities and build the luminosity functions. Using a
combination of starburst and quasar templates, we quantify the star-formation
and AGN contributions in the mid-IR SED. We then compute the star-formation
luminosity functions at 15 um and 24 um, and compare with the total 15 um and
24 um luminosity functions. When we remove the contribution of AGN, the bright
end of the luminosity function exhibits a strong decline, consistent with the
exponential cutoff of a Schechter function. Integrating the differential
luminosity function, we find that the fractional contribution by star formation
to the energy density is 58% at 15 um and 78% at 24 um, while it goes up to
~86% when we extrapolate our mid-IR results to the total IR luminosity density.
We confirm that the active galactic nuclei play more important roles
energetically at high luminosities. Finally, we compare our results with work
at z~0.7 and confirm that evolution on both luminosity and density is required
to explain the difference in the LFs at different redshifts.Comment: 31 pages, 12 figures, accepted for Ap
The Spatial Extent of (U)LIRGs in the mid-Infrared I: The Continuum Emission
We present an analysis of the extended mid-infrared (MIR) emission of the
Great Observatories All-Sky LIRG Survey (GOALS) sample based on 5-15um low
resolution spectra obtained with the IRS on Spitzer. We calculate the fraction
of extended emission as a function of wavelength for the galaxies in the
sample, FEE_lambda. We can identify 3 general types of FEE_lambda: one where it
is constant, one where features due to emission lines and PAHs appear more
extended than the continuum, and a third which is characteristic of sources
with deep silicate absorption at 9.7um. More than 30% of the galaxies have a
median FEE_lambda larger than 0.5 implying that at least half of their MIR
emission is extended. Luminous Infrared Galaxies (LIRGs) display a wide range
of FEE in their warm dust continuum (0<=FEE_13.2um<=0.85). The large values of
FEE_13.2um that we find in many LIRGs suggest that their extended MIR continuum
emission originates in scales up to 10kpc. The mean size of the LIRG cores at
13.2um is 2.6kpc. However, once the LIR of the systems reaches the threshold of
~10^11.8Lsun, all sources become clearly more compact, with FEE_13.2um<=0.2,
and their cores are unresolved. Our estimated upper limit for the core size of
ULIRGs is less than 1.5kpc. The analysis indicates that the compactness of
systems with LIR>~10^11.25Lsun strongly increases in those classified as
mergers in their final stage of interaction. The FEE_13.2um is also related to
the contribution of an active galactic nucleus (AGN) to the MIR. Galaxies which
are more AGN-dominated are less extended, independently of their LIR. We
finally find that the extent of the MIR continuum emission is correlated with
the far-IR IRAS log(f_60um/f_100um) color. This enables us to place a lower
limit to the area in a galaxy from where the cold dust emission may originate,
a prediction which can be tested soon with the Herschel Space Telescope.Comment: 18 pages, 8 figures, accepted for publication in Ap
Far-Ultraviolet and Far-Infrared Bivariate Luminosity Function of Galaxies: Complex Relation between Stellar and Dust Emission
Far-ultraviolet (FUV) and far-infrared (FIR) luminosity functions (LFs) of
galaxies show a strong evolution from to , but the FIR LF
evolves much stronger than the FUV one. The FUV is dominantly radiated from
newly formed short-lived OB stars, while the FIR is emitted by dust grains
heated by the FUV radiation field. It is known that dust is always associated
with star formation activity. Thus, both FUV and FIR are tightly related to the
star formation in galaxies, but in a very complicated manner. In order to
disentangle the relation between FUV and FIR emissions, we estimate the UV-IR
bivariate LF (BLF) of galaxies with {\sl GALEX} and {\sl AKARI} All-Sky Survey
datasets. Recently we invented a new mathematical method to construct the BLF
with given marginals and prescribed correlation coefficient. This method makes
use of a tool from mathematical statistics, so called "copula". The copula
enables us to construct a bivariate distribution function from given marginal
distributions with prescribed correlation and/or dependence structure. With
this new formulation and FUV and FIR univariate LFs, we analyze various FUV and
FIR data with {\sl GALEX}, {\sl Spitzer}, and {\sl AKARI} to estimate the UV-IR
BLF. The obtained BLFs naturally explain the nonlinear complicated relation
between FUV and FIR emission from star-forming galaxies. Though the faint-end
of the BLF was not well constrained for high- samples, the estimated linear
correlation coefficient was found to be very high, and is remarkably
stable with redshifts (from 0.95 at to 0.85 at ). This implies
the evolution of the UV-IR BLF is mainly due to the different evolution of the
univariate LFs, and may not be controlled by the dependence structure.Comment: 10 pages, 7 figures, Earth, Planets and Space, in pres
The evolution of the dust and gas content in galaxies
We use deep Herschel observations taken with both PACS and SPIRE imaging cameras to estimate the dust mass of a sample of galaxies extracted from the GOODS-S, GOODS-N and the COSMOS fields. We divide the redshift–stellar mass (M star )–star formation rate (SFR) parameter space into small bins and investigate average properties over this grid. In the first part of the work we investigate the scaling relations between dust mass, stellar mass and SFR out to z = 2.5. No clear evolution of the dust mass with redshift is observed at a given SFR and stellar mass. We find a tight correlation between the SFR and the dust mass, which, under reasonable assumptions, is likely a consequence of the Schmidt-Kennicutt (S-K) relation. The previously observed correlation between the stellar content and the dust content flattens or sometimes disappears when considering galaxies with the same SFR. Our finding suggests that most of the correlation between dust mass and stellar mass obtained by previous studies is likely a consequence of the correlation between the dust mass and the SFR combined with the main sequence, i.e., the tight relation observed between the stellar mass and the SFR and followed by the majority of star-forming galaxies. We then investigate the gas content as inferred from dust mass measurements. We convert the dust mass into gas mass by assuming that the dust-to-gas ratio scales linearly with the gas metallicity (as supported by many observations). For normal star-forming galaxies (on the main sequence) the inferred relation between the SFR and the gas mass (integrated S-K relation) broadly agrees with the results of previous studies based on CO measurements, despite the completely different approaches. We observe that all galaxies in the sample follow, within uncertainties, the same S-K relation. However, when investigated in redshift intervals, the S-K relation shows a moderate, but significant redshift evolution. The bulk of the galaxy population at z ∼ 2 converts gas into stars with an efficiency (star formation efficiency, SFE = SFR/M gas , equal to the inverse of the depletion time) about 5 times higher than at z ∼ 0. However, it is not clear what fraction of such variation of the SFE is due to an intrinsic redshift evolution and what fraction is simply a consequence of high-z galaxies having, on average, higher SFR, combined with thesuper-linear slope of the S-K relation (whileother studies finda linear slope). We confirm that the gas fraction (f gas = M gas /(M gas + M star )) decreases with stellar mass and increases with the SFR. We observe no evolution with redshift once M star and SFR are fixed. We explain these trends by introducing a universal relation between gas fraction, stellar mass and SFR that does not evolve with redshift, at least out to z ∼ 2.5. Galaxies move across this relation as their gas content evolves across the cosmic epochs. We use the 3D fundamental f gas –M star –SFR relation, along with the evolution of the main sequence with redshift, to estimate the evolution of the gas fraction in the average population of galaxies as a function of redshift and as a function of stellar mass: we find that M star > ∼ 10 11 M ? galaxies show the strongest evolution at z > ∼ 1.3 and a flatter trend at lower redshift, while f gas decreases more regularly over the entire redshift range probed in M star < ∼ 10 11 Mo galaxies, in agreement with a downsizing scenario
Deep observations of CO line emission from star-forming galaxies in a cluster candidate at z=1.5
We report results from a deep Jansky Very Large Array (JVLA) search for CO
1-0 line emission from galaxies in a candidate galaxy cluster at z~1.55 in the
COSMOS field. We target 4 galaxies with optical spectroscopic redshifts in the
range z=1.47-1.59. Two of these 4 galaxies, ID51613 and ID51813, are nominally
detected in CO line emission at the 3-4 sigma level. We find CO luminosities of
2.4x10^10 K km/s pc^2 and 1.3x10^10 K km/s pc^2, respectively. Taking advantage
from the clustering and 2-GHz bandwidth of the JVLA, we perform a search for
emission lines in the proximity of optical sources within the field of view of
our observations. We limit our search to galaxies with K<23.5 (AB) and
z_phot=1.2-1.8. We find 2 bright optical galaxies to be associated with
significant emission line peaks (>4 sigma) in the data cube, which we identify
with the CO line emission. To test the reliability of the line peaks found, we
performed a parallel search for line peaks using a Bayesian inference method.
Monte Carlo simulations show that such associations are statistically
significant, with probabilities of chance association of 3.5% and 10.7% for ID
51207 and ID 51380, respectively. Modeling of their optical/IR SEDs indicates
that the CO detected galaxies and candidates have stellar masses and SFRs in
the range (0.3-1.1)x10^11 M_sun and 60-160 M_sun/yr, with SFEs comparable to
that found in other star-forming galaxies at similar redshifts. By comparing
the space density of CO emitters derived from our observations with the space
density derived from previous CO detections at z~1.5, and with semi-analytic
predictions for the CO luminosity function, we suggest that the latter tend to
underestimate the number of CO galaxies detected at high-redshift. Finally, we
argue about the benefits of future blind CO searches in clustered fields with
upcoming submm/radio facilities.Comment: Accepted for publication in MNRAS. Abstract has been slightly
shortened compared to original pdf versio
ACCESS III: The Nature of Star Formation in the Shapley Supercluster
We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX
ultraviolet imaging of the Shapley supercluster at z=0.048. Combining this with
spectra of 814 supercluster members and 1.4GHz radio continuum maps, this
represents the largest complete census of star-formation (both obscured and
unobscured) in local cluster galaxies to date, reaching SFRs~0.02Msun/yr. We
take advantage of this comprehensive panchromatic dataset to perform a detailed
analysis of the nature of star formation in cluster galaxies, using several
quite independent diagnostics of the quantity and intensity of star formation
to develop a coherent view of the types of star formation within cluster
galaxies. We observe a robust bimodality in the infrared (f_24/f_K) galaxy
colours, which we are able to identify as another manifestation of the broad
split into star-forming spiral and passive elliptical galaxy populations seen
in UV-optical surveys. This diagnostic also allows the identification of
galaxies in the process of having their star formation quenched as the infrared
analogue to the UV "green valley" population. The bulk of supercluster galaxies
on the star-forming sequence have specific-SFRs consistent with local field
specific-SFR-M* relations, and form a tight FIR-radio correlation confirming
that their FIR emission is due to star formation. We show that 85% of the
global SFR is quiescent star formation within spiral disks, as manifest by the
observed sequence in the IRX-beta relation being significantly offset from the
starburst relation of Kong et al. (2004), while their FIR-radio colours
indicate dust heated by low-intensity star formation. Just 15% of the global
SFR is due to nuclear starbursts. The vast majority of star formation seen in
cluster galaxies comes from normal infalling spirals who have yet to be
affected by the cluster environment.Comment: 17 pages, 9 figures. Accepted for publication in MNRA
The physical scale of the far-infrared emission in the most luminous submillimetre galaxies II: evidence for merger-driven star formation
We present high-resolution 345 GHz interferometric observations of two
extreme luminous (L_{IR}>10^{13} L_sun), submillimetre-selected galaxies (SMGs)
in the COSMOS field with the Submillimeter Array (SMA). Both targets were
previously detected as unresolved point-sources by the SMA in its compact
configuration, also at 345 GHz. These new data, which provide a factor of ~3
improvement in resolution, allow us to measure the physical scale of the
far-infrared in the submillimetre directly. The visibility functions of both
targets show significant evidence for structure on 0.5-1 arcsec scales, which
at z=1.5 translates into a physical scale of 5-8 kpc. Our results are
consistent with the angular and physical scales of two comparably luminous
objects with high-resolution SMA followup, as well as radio continuum and CO
sizes. These relatively compact sizes (<5-10 kpc) argue strongly for
merger-driven starbursts, rather than extended gas-rich disks, as the preferred
channel for forming SMGs. For the most luminous objects, the derived sizes may
also have important physical consequences; under a series of simplifying
assumptions, we find that these two objects in particular are forming stars
close to or at the Eddington limit for a starburst.Comment: 9 pages, 3 Figures, submitted to MNRA
A physical model for the origin of the diffuse cosmic infrared background
We present a physical model for origin of the cosmic diffuse infrared
background (CDIRB). By utilizing the observed stellar mass function and its
evolution as input to a semi-empirical model of galaxy formation, we isolate
the physics driving diffuse IR emission. The model includes contributions from
three primary sources of IR emission: steady-state star formation owing to
isolated disk galaxies, interaction-driven bursts of star formation owing to
close encounters and mergers, and obscured active galactic nuclei (AGN). We
find that most of the CDIRB is produced by equal contributions from objects at
z=0.5-1 and z>1, as suggested by recent observations. Of those sources, the
vast majority of the emission originates in systems with low to moderate IR
luminosities (L_{IR}<10^{12} $L_sun); the most luminous objects contribute
significant flux only at high-redshifts (z>2). All star formation in ongoing
mergers accounts for <10% of the total at all wavelengths and redshifts, while
emission directly attributable to the interaction-driven burst itself accounts
for <5%. We furthermore find that obscured AGN contribute <1-2% of the CDIRB at
all wavelengths and redshifts, with a strong upper limit of less than 4% of the
total emission. Finally, since electron-positron pair production interactions
with the CDIRB represent the primary source of opacity to very high energy
(VHE: E_\gamma > 1 TeV) \gamma-rays, the model provides predictions for the
optical depth of the Universe to the most energetic photons. We find that these
predictions agree with observations of high-energy cutoffs at TeV energies in
nearby blazars, and suggest that while the Universe is extremely optically
thick at >10 TeV, the next generation of VHE \gamma-ray telescopes can
reasonably expect detections from out to 50-150 Mpc.Comment: 14 pages, 13 figures, submitted to MNRA
Invaders in hot water: a simple decontamination method to prevent the accidental spread of aquatic invasive non-native species.
Watersports equipment can act as a vector for the introduction and spread of invasive non native species (INNS) in freshwater environments. To support advice given to recreational water users under the UK Government’s Check Clean Dry biosecurity campaign and ensure its effectiveness at killing a range of aquatic INNS, we conducted a survival experiment on seven INNS which pose a high risk to UK freshwaters. The efficacy of exposure to hot water (45 °C, 15 min) was tested as a method by which waters users could ‘clean’ their equipment and was compared to drying and a control group (no treatment). Hot water had caused 99 % mortality across all species 1 h after treatment and was more effective than drying at all time points (1 h: χ2 = 117.24, p < 0.001; 1 day χ2 = 95.68, p < 0.001; 8 days χ2 = 12.16, p < 0.001 and 16 days χ2 = 7.58, p < 0.001). Drying caused significantly higher mortality than the control (no action) from day 4 (χ2 = 8.49, p < 0.01) onwards. In the absence of hot water or drying, 6/7 of these species survived for 16 days, highlighting the importance of good biosecurity practice to reduce the risk of accidental spread. In an additional experiment the minimum lethal temperature and exposure time in hot water to cause 100 % mortality in American signal crayfish (Pacifastacus leniusculus), was determined to be 5 min at 40 °C. Hot water provides a simple, rapid and effective method to clean equipment. We recommend that it is advocated in future biosecurity awareness campaigns
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