397 research outputs found
Determining the stellar masses of submillimetre galaxies: the critical importance of star formation histories
Submillimetre (submm) galaxies are among the most rapidly star-forming and
most massive high-redshift galaxies; thus, their properties provide important
constraints on galaxy evolution models. However, there is still a debate about
their stellar masses and their nature in the context of the general galaxy
population. To test the reliability of their stellar mass determinations, we
used a sample of simulated submm galaxies for which we derived stellar masses
via spectral energy distribution (SED) modelling (with Grasil, Magphys, Hyperz
and LePhare) adopting various star formation histories (SFHs). We found that
the assumption of SFHs with two independent components leads to the most
accurate stellar masses. Exponentially declining SFHs (tau) lead to lower
masses (albeit still consistent with the true values), while the assumption of
single-burst SFHs results in a significant mass underestimation. Thus, we
conclude that studies based on the higher masses inferred from fitting the SEDs
of real submm galaxies with double SFHs are most likely to be correct, implying
that submm galaxies lie on the high-mass end of the main sequence of
star-forming galaxies. This conclusion appears robust to assumptions of whether
or not submm galaxies are driven by major mergers, since the suite of simulated
galaxies modelled here contains examples of both merging and isolated galaxies.
We identified discrepancies between the true and inferred stellar ages (rather
than the dust attenuation) as the primary determinant of the success/failure of
the mass recovery. Regardless of the choice of SFH, the SED-derived stellar
masses exhibit a factor of ~2 scatter around the true value; this scatter is an
inherent limitation of the SED modelling due to simplified assumptions.
Finally, we found that the contribution of active galactic nuclei does not have
any significant impact on the derived stellar masses.Comment: Accepted to A&A. 11 pages, 9 figures, 1 table. V2 main changes: 1)
discussion of the stellar age as the main parameter influencing the success
of an SED model (Fig. 4, 5, 7); 2) discussion of the age-dust degeneracy (Fig
9); 3) the comparison of real and simulated submm galaxies (Fig 1
Dust grain growth in the interstellar medium of 5<z<6.5 quasars
We investigate whether stellar dust sources i.e. asymptotic giant branch
(AGB) stars and supernovae (SNe) can account for dust detected in 5<z<6.5
quasars (QSOs). We calculate the required dust yields per AGB star and per SN
using the dust masses of QSOs inferred from their millimeter emission and
stellar masses approximated as the difference between the dynamical and the H_2
gas masses of these objects. We find that AGB stars are not efficient enough to
form dust in the majority of the z>5 QSOs, whereas SNe may be able to account
for dust in some QSOs. However, they require very high dust yields even for a
top-heavy initial mass function. This suggests additional non-stellar dust
formation mechanism e.g. significant dust grain growth in the interstellar
medium of at least three out of nine z>5 QSOs. SNe (but not AGB stars) may
deliver enough heavy elements to fuel this growth.Comment: A&A, accepted. 5 pages, 2 figures, 2 table
A nearby GRB host prototype for z~7 Lyman-break galaxies: Spitzer-IRS and X-shooter spectroscopy of the host galaxy of GRB031203
Gamma-ray burst (GRB) host galaxies have been studied extensively in optical
photometry and spectroscopy. Here we present the first mid-infrared spectrum of
a GRB host, HG031203. It is one of the nearest GRB hosts at z=0.1055, allowing
both low and high-resolution spectroscopy with Spitzer-IRS. Medium resolution
UV-to-K-band spectroscopy with the X-shooter spectrograph on the VLT is also
presented, along with Spitzer IRAC and MIPS photometry, as well as radio and
sub-mm observations. These data allow us to construct a UV-to-radio spectral
energy distribution with almost complete spectroscopic coverage from 0.3-35
micron of a GRB host galaxy for the first time, potentially valuable as a
template for future model comparisons. The IRS spectra show strong,
high-ionisation fine structure line emission indicative of a hard radiation
field in the galaxy, suggestive of strong ongoing star-formation and a very
young stellar population. The selection of HG031203 via the presence of a GRB
suggests that it might be a useful analogue of very young star-forming galaxies
in the early universe, and hints that local BCDs may be used as more reliable
analogues of star-formation in the early universe than typical local
starbursts. We look at the current debate on the ages of the dominant stellar
populations in z~7 and z~8 galaxies in this context. The nebular line emission
is so strong in HG031203, that at z~7, it can reproduce the spectral energy
distributions of z-band dropout galaxies with elevated IRAC 3.6 and 4.5 micron
fluxes without the need to invoke a 4000A break.Comment: Published in ApJ. 9 pages, 6 figures, emulateapj styl
New light on gamma-ray burst host galaxies with Herschel
Until recently, dust emission has been detected in very few host galaxies of
gamma-ray bursts (GRBHs). With Herschel, we have now observed 17 GRBHs up to
redshift z~3 and detected seven of them at infrared (IR) wavelengths. This
relatively high detection rate (41%) may be due to the composition of our
sample which at a median redshift of 1.1 is dominated by the hosts of dark
GRBs. Although the numbers are small, statistics suggest that dark GRBs are
more likely to be detected in the IR than their optically-bright counterparts.
Combining our IR data with optical, near-infrared, and radio data from our own
datasets and from the literature, we have constructed spectral energy
distributions (SEDs) which span up to 6 orders of magnitude in wavelength. By
fitting the SEDs, we have obtained stellar masses, dust masses, star-formation
rate (SFR), and extinctions for our sample galaxies. We find that GRBHs are
galaxies that tend to have a high specfic SFR (sSFR), and like other
star-forming galaxies, their ratios of dust-to-stellar mass are well correlated
with sSFR. We incorporate our Herschel sample into a larger compilation of
GRBHs, and compare this combined sample to SFR-weighted median stellar masses
of the widest, deepest galaxy survey to date. This is done in order to
establish whether or not GRBs can be used as an unbiased tracer of cosmic
comoving SFR density (SFRD) in the universe. In contrast with previous results,
this comparison shows that GRBHs are medium-sized galaxies with relatively high
sSFRs; stellar masses and sSFRs of GRBHs as a function of redshift are similar
to what is expected for star-forming galaxy populations at similar redshifts.
We conclude that there is no strong evidence that GRBs are biased tracers of
SFRD; thus they should be able to reliably probe the SFRD to early epochs.Comment: 18 pages, 9 figures, accepted for publication in A&A. Revised to
include Fig. 6, mistakenly omitted in origina
Modelling high resolution ALMA observations of strongly lensed highly star forming galaxies detected by <i>Herschel</i>
We have modelled ∼ 0.1 arcsec resolution ALMA imaging of six strong gravitationally lensed galaxies detected by the Herschel Space Observatory. Our modelling recovers mass properties of the lensing galaxies and, by determining magnification factors, intrinsic properties of the lensed sub-millimetre sources. We find that the lensed galaxies all have high ratios of star formation rate to dust mass, consistent with or higher than the mean ratio for high redshift sub-millimetre galaxies and low redshift ultra-luminous infra-red galaxies. Source reconstruction reveals that most galaxies exhibit disturbed morphologies. Both the cleaned image plane data and the directly observed interferometric visibilities have been modelled, enabling comparison of both approaches. In the majority of cases, the recovered lens models are consistent between methods, all six having mass density profiles that are close to isothermal. However, one system with poor signal to noise shows mildly significant differences
Herschel-ATLAS/GAMA: A difference between star formation rates in strong-line and weak-line radio galaxies
We have constructed a sample of radio-loud objects with optical spectroscopy from the Galaxy and Mass Assembly (GAMA) project over the Herschel Astrophysical Terahertz Large Area Survey (Herschel-ATLAS) Phase 1 fields. Classifying the radio sources in terms of their optical spectra, we find that strong-emission-line sources ('high-excitation radio galaxies') have, on average, a factor of ~4 higher 250-μm Herschel luminosity than weak-line ('lowexcitation') radio galaxies and are also more luminous than magnitude-matched radio-quiet galaxies at the same redshift. Using all five H-ATLAS bands, we show that this difference in luminosity between the emission-line classes arises mostly from a difference in the average dust temperature; strong-emission-line sources tend to have comparable dust masses to, but higher dust temperatures than, radio galaxies with weak emission lines. We interpret this as showing that radio galaxies with strong nuclear emission lines are much more likely to be associated with star formation in their host galaxy, although there is certainly not a one-to-one relationship between star formation and strong-line active galactic nuclei (AGN) activity. The strong-line sources are estimated to have star formation rates at least a factor of 3-4 higher than those in the weak-line objects. Our conclusion is consistent with earlier work, generally carried out using much smaller samples, and reinforces the general picture of high-excitation radio galaxies as being located in lower-mass, less evolved host galaxies than their low-excitation counterparts.Peer reviewe
The properties of (sub)millimetre-selected galaxies as revealed by CANDELS HST WFC3/IR imaging in GOODS-South
We have exploited the HST CANDELS WFC3/IR imaging to study the properties of
(sub-)mm galaxies in GOODS-South. After using the deep radio and Spitzer
imaging to identify galaxy counterparts for the (sub-)mm sources, we have used
the new CANDELS data in two ways. First, we have derived improved photometric
redshifts and stellar masses, confirming that the (sub-)mm galaxies are massive
(=2.2x10^11 M_solar) galaxies at z=1-3. Second, we have exploited the depth
and resolution of the WFC3/IR imaging to determine the sizes and morphologies
of the galaxies at rest-frame optical wavelengths, fitting two-dimensional
axi-symmetric Sersic models. Crucially, the WFC3/IR H-band imaging enables
modelling of the mass-dominant galaxy, rather than the blue high-surface
brightness features which often dominate optical (rest-frame UV) images of
(sub-)mm galaxies, and can confuse visual morphological classification. As a
result of this analysis we find that >95% of the rest-frame optical light in
almost all of the (sub-)mm galaxies is well-described by either a single
exponential disk, or a multiple-component system in which the dominant
constituent is disk-like. We demonstrate that this conclusion is consistent
with the results of high-quality ground-based K-band imaging, and explain why.
The massive disk galaxies which host luminous (sub-)mm emission are reasonably
extended (r_e=4 kpc), consistent with the sizes of other massive star-forming
disks at z~2. In many cases we find evidence of blue clumps within the sources,
with the mass-dominant disk becoming more significant at longer wavelengths.
Finally, only a minority of the sources show evidence for a major galaxy-galaxy
interaction. Taken together, these results support the view that most (sub-)mm
galaxies at z~2 are simply the most extreme examples of normal star-forming
galaxies at that era.Comment: 30 pages, 9 figure
A quiescent galaxy at the position of the long GRB 050219A
Long-duration gamma-ray bursts (LGRBs) are produced by the collapse of very
massive stars. Due to the short lifetime of their progenitors, LGRBs pinpoint
star-forming galaxies. We present here a multi-band search for the host galaxy
of the long dark GRB 050219A within the enhanced Swift/XRT error circle. We
used spectroscopic observations acquired with VLT/X-shooter to determine the
redshift and star-formation rate of the putative host galaxy. We compared the
results with the optical/IR spectral energy distribution obtained with
different facilities. Surprisingly, the host galaxy is a old and quiescent
early-type galaxy at z = 0.211 characterised by an unprecedentedly low specific
star-formation rate. It is the first LGRB host to be also an early-type
post-starburst galaxy. This is further evidence that GRBs can explode in all
kind of galaxies, with the only requirement being an episode of star-formation.Comment: 11 pages, 10 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
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