392 research outputs found
Spectral energy distributions of submm/radio bright gamma-ray burst host galaxies
We present optical to radio spectral energy distribution fitting of the host
galaxies of four long gamma-ray bursts: 980703, 000210, 000418 and 010222,
which were detected at submillimetre and/or radio wavelengths. We find that
only very young starburst galaxy models are consistent with the data having
both blue optical colors and a pronounced submm emission. For each host we are
able to construct a model consistent with the short- and long-wavelength parts
of the spectra. We find galaxy ages ranging from 0.09 to 2.0 Gyrs and star
formation rates ranging from 138 to 380 MSun/yr.Comment: 6 pages, 1 figure, Proceedings of the Conference "The Multicoloured
Landscape of Compact Objects and their Explosive Origins" (Cefalu, Sicily,
2006 June 11-24). Eds. L. Burderi et al. (New York: AIP), in press, for SED
templates, see http://archive.dark-cosmology.dk
Dust production scenarios in galaxies at z ~ 6-8.3
The mechanism of dust formation in galaxies at high redshift is still
unknown. Asymptotic giant branch (AGB) stars and explosions of supernovae (SNe)
are possible dust producers, and non-stellar processes may substantially
contribute to dust production, for example grain growth in the interstellar
medium (ISM). Our aim is to determine the contribution to dust production of
AGB stars and SNe in nine galaxies at z ~ 6-8.3, for which observations of dust
have been recently attempted. In order to determine the origin of the observed
dust we have determined dust yields per AGB star and SN required to explain the
total amounts of dust in these galaxies. We find that AGB stars were not able
to produce the amounts of dust observed in the galaxies in our sample. In order
to explain these dust masses, SNe would have to have maximum efficiency and not
destroy the dust which they formed. Therefore, the observed amounts of dust in
the galaxies in the early universe were formed either by efficient supernovae
or by a non-stellar mechanism, for instance the grain growth in the
interstellar medium.Comment: Accepted for publication in A&A, 5 pages, 1 figur
On the Distribution of Stellar Masses in Gamma-ray Burst Host Galaxies
We analyze Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M_*) based on the rest-frame K-band luminosities (L_K_(rest)) and constrain their star formation rates (SFRs; not corrected for dust extinction) based on the rest-frame UV continua. Further, we compute a mean M_*/ L_K_(rest) = 0.45 M_☉/L_☉. We find that the hosts are low M_*, star-forming systems. The median M_* in our sample ( = 10^(9.7) M_☉) is lower than that of "field" galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M_* is 10^7 M_☉ < M_* < 10^(11) M_☉, while the range spanned by the dust-uncorrected UV SFR is 10^(–2) M_☉ yr^(–1) < SFR < 10 M_☉ yr^(–1). There is no evidence for intrinsic evolution in the distribution of M_* with redshift. We show that extinction by dust must be present in at least 25% of the GRB hosts in our sample and suggest that this is a way to reconcile our finding of a relatively lower UV-based, specific SFR (φ ≡ SFR/M_*) with previous claims that GRBs have some of the highest φ values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on φ
The nature of GRB-selected submillimeter galaxies: hot and young
We present detailed fits of the spectral energy distributions (SEDs) of four
submillimeter (submm) galaxies selected by the presence of a gamma-ray burst
(GRB) event (GRBs 980703, 000210, 000418 and 010222). These faint ~3 mJy submm
emitters at redshift ~1 are characterized by an unusual combination of long-
and short-wavelength properties, namely enhanced submm and/or radio emission
combined with optical faintness and blue colors. We exclude an active galactic
nucleus as the source of long-wavelength emission. From the SED fits we
conclude that the four galaxies are young (ages <2 Gyr), highly starforming
(star formation rates ~150 MSun/yr), low-mass (stellar masses ~10^10 MSun) and
dusty (dust masses ~3x10^8 MSun). Their high dust temperatures (Td>45 K)
indicate that GRB host galaxies are hotter, younger, and less massive
counterparts to submm-selected galaxies detected so far. Future facilities like
Herschel, JCMT/SCUBA-2 and ALMA will test this hypothesis enabling measurement
of dust temperatures of fainter GRB-selected galaxies.Comment: 9 pages, 2 figures, submitted to ApJ, for SED templates, see
http://archive.dark-cosmology.dk
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
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
Star formation rates and stellar masses in z ~ 1 gamma ray burst hosts
We analyse 4.5, 8 and 24 um band Spitzer images of six gamma ray burst host
galaxies at redshifts close to 1. We constrain their star formation rates (SFR)
based on the entire available spectral energy distribution rather than the 24
um band only. Further, we estimate their stellar masses (M*) based on rest
frame K band luminosities. Our sample spans a wide range of galaxy properties:
derived SFRs range from less than 10 to a few hundred solar masses per year;
values of M* range from 10^9 to 10^10 Mo with a median of 5.6 x 10^9 Mo.
Comparing the specific star formation rate (PHI = SFR/M*) of our sample as a
function of M* to other representative types of galaxies (distant red galaxies,
Ly-alpha emitters, Lyman break galaxies, submillimeter galaxies and z ~ 2
galaxies from the Great Observatories Origins Deep Survey-North field), we find
that gamma ray burst hosts are among those with the highest PHI.Comment: 4 pages by emulateApJ, 1 table and 2 colour figures; published in ApJ
Letter
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
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
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