147 research outputs found
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
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
The galaxies in the field of the nearby GRB980425/SN1998bw
We present spectroscopic observations of ESO 184-G82, the host galaxy of
GRB980425/SN1998bw, and six galaxies in its field. A host redshift of
z=0.0087+/-0.0006 is derived, consistent with that measured by Tinney et al.
(1998). Redshifts are obtained for the six surrounding galaxies observed. Three
of these galaxies lie within 11 Mpc of each other, confirming the suggestion
that some of these galaxies form a group. However, all of the field galaxies
observed lie at significantly greater distances than ESO 184-G82 and are
therefore not associated with it. The host galaxy of GRB980425/SN1998bw thus
appears to be an isolated dwarf galaxy and interactions with other galaxies do
not seem to be responsible for its star formation.Comment: 5 pages, 2 figures, accepted for publication in A&
A High Signal-to-Noise Ratio Composite Spectrum of Gamma-ray Burst Afterglows
We present a composite spectrum of 60 long duration gamma-ray burst (GRB)
afterglows with redshifts in the range 0.35<z<6.7 observed with low resolution
optical spectra. The composite spectrum covers the wavelength range 700-6600 A
in the rest frame and has a mean signal-to-noise ratio of 150 per 1 A pixel and
reaches a maximum of ~300 in the range 2500-3500 A. Equivalent widths are
measured from metal absorption lines from the Lya line to ~5200 A, and
associated metal and hydrogen lines are identified between the Lyman break and
Lya line. The average transmission within the Lyman forest is consistent with
that found along quasar lines of sight. We find a temporal variation in fine
structure lines when dividing the sample into bursts observed within 2 hours
from their trigger and those observed later. Other lines in the predominantly
neutral gas show variations too, but this is most likely a random effect caused
by weighting of individual strong absorption lines and which mimics a temporal
variation. Bursts characterized with high or low prompt GRB energy release
produce afterglows with similar absorption line strengths, and likewise for
bursts with bright or faint optical afterglows. Bursts defined as dark from
their optical to X-ray spectral index have stronger absorption lines relative
to the optically bright bursts. The composite spectrum has strong CaII and MgII
absorption lines as commonly found in dusty galaxies, however, we find no
evidence for dust or a significant molecular content based on the non-detection
of diffuse interstellar bands. Compared to starburst galaxy spectra, the GRB
composite has much stronger fine structure lines, while metal absorption lines
are weaker.Comment: Accepted for publication in ApJ, 24 page
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
Constraints on the parameters of the Left Right Mirror Model
We study some phenomenological constraints on the parameters of a left right
model with mirror fermions (LRMM) that solves the strong CP problem. In
particular, we evaluate the contribution of mirror neutrinos to the invisible Z
decay width (\Gamma_Z^{inv}), and we find that the present experimental value
on \Gamma_Z^{inv}, can be used to place an upper bound on the Z-Z' mixing angle
that is consistent with limits obtained previously from other low-energy
observables. In this model the charged fermions that correspond to the standard
model (SM) mix with its mirror counterparts. This mixing, simultaneously with
the Z-Z' one, leads to modifications of the \Gamma(Z --> f \bar{f}) decay
width. By comparing with LEP data, we obtain bounds on the standard-mirror
lepton mixing angles. We also find that the bottom quark mixing parameters can
be chosen to fit the experimental values of R_b, and the resulting values for
the Z-Z' mixing angle do not agree with previous bounds. However, this
disagreement disappears if one takes the more recent ALEPH data.Comment: 7 pages, 2 figures, REVTe
Outshining the quasars at reionisation: The X-ray spectrum and lightcurve of the redshift 6.29 Gamma-Ray Burst GRB050904
Gamma-ray burst (GRB) 050904 is the most distant X-ray source known, at
z=6.295, comparable to the farthest AGN and galaxies. Its X-ray flux decays,
but not as a power-law; it is dominated by large variability from a few minutes
to at least half a day. The spectra soften from a power-law with photon index
Gamma=1.2 to 1.9, and are well-fit by an absorbed power-law with possible
evidence of large intrinsic absorption. There is no evidence for discrete
features, in spite of the high signal-to-noise ratio. In the days after the
burst, GRB 050904 was by far the brightest known X-ray source at z>4. In the
first minutes after the burst, the flux was >10^{-9} erg cm^-2 s^-1 in the
0.2-10keV band, corresponding to an apparent luminosity >10^5 times larger than
the brightest AGN at these distances. More photons were acquired in a few
minutes with Swift-XRT than XMM-Newton and Chandra obtained in ~300 ks of
pointed observations of z>5 AGN. This observation is a clear demonstration of
concept for efficient X-ray studies of the high-z IGM with large area,
high-resolution X-ray detectors, and shows that early-phase GRBs are the only
backlighting bright enough for X-ray absorption studies of the IGM at high
redshift.Comment: Accepted for publication in ApJL. 5 pages with emulateapj, 3 figure
A multi-colour study of the dark GRB 000210 host galaxy and its environment
We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark
gamma-ray burst (GRB) of February 10, 2000. These observations represent the
most exhaustive photometry given to date of any GRB host galaxy. A grid of
spectral templates have been fitted to the Spectral Energy Distribution (SED)
of the host. The derived photometric redshift is z=0.842^+0.054_-0.042, which
is in excellent agreement with the spectroscopic redshift (z=0.8463+/-0.0002)
proposed by Piro et al. (2002) based on a single emission line. Furthermore, we
have determined the photometric redshift of all the galaxies in an area of
6'x6' around the host galaxy, in order to check for their overdensity in the
environment of the host. We find that the GRB 000210 host galaxy is a
subluminous galaxy (L ~ 0.5+/-0.2 L*), with no companions above our detection
threshold of 0.18+/-0.06 L*. Based on the restframe ultraviolet flux a star
formation rate of 2.1+/-0.2 Solar Masses per year is estimated. The best fit to
the SED is obtained for a starburst template with an age of 0.181^+0.037_-0.026
Gyr and a very low extinction (Av~0). We discuss the implications of the
inferred low value of Av and the age of the dominant stellar population for the
non detection of the GRB 000210 optical afterglow.Comment: 10 pages with 4 encapsulated PostScript figures included. Accepted
for publication in Astronomy & Astrophysic
The optical/near-IR spectral energy distribution of the GRB 000210 host galaxy
We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy.
Fitting a grid of spectral templates to its Spectral Energy Distribution (SED),
we derived a photometric redshift (z=0.842\+0.0540.042) which is in excellent
agreement with the spectroscopic one (z=0.8463+/-0.0002; Piro et al. 2002). The
best fit to the SED is obtained with a blue starburst template with an age of
0.181\+0.0370.026 Gyr. We discuss the implications of the inferred low value of
Av and the age of the dominant stellar population for the non detection of the
GRB 000210 optical afterglow.Comment: 4 pages, 1 figure, contribution to the Rome 2002 GRB worksho
NGC 2770 - a supernova Ib factory?
NGC 2770 has been the host of three supernovae of Type Ib during the last 10
years, SN 1999eh, SN 2007uy and SN 2008D. SN 2008D attracted special attention
due to the serendipitous discovery of an associated X-ray transient. In this
paper, we study the properties of NGC 2770 and specifically the three SN sites
to investigate whether this galaxy is in any way peculiar to cause a high
frequency of SNe Ib. We model the global SED of the galaxy from broadband data
and derive a star-formation and SN rate comparable to the values of the Milky
Way. We further study the galaxy using longslit spectroscopy covering the major
axis and the three SN sites. From the spectroscopic study we find subsolar
metallicities for the SN sites, a high extinction and a moderate star-formation
rate. In a high resolution spectrum, we also detect diffuse interstellar bands
in the line-of-sight towards SN 2008. A comparison of NGC 2770 to the global
properties of a galaxy sample with high SN occurance (at least 3 SN in the last
100 years) suggests that NGC 2770 is not particularly destined to produce such
an enhancement of observed SNe observed. Its properties are also very different
from gamma-ray burst host galaxies. Statistical considerations on SN Ib
detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe
detected in 10 years. The high number of rare Ib SNe in this galaxy is
therefore likely to be a coincidence rather than special properties of the
galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is
highly starforming, has a very low mass and one of the lowest metallicities
detected in the nearby universe as derived from longslit spectroscopy. In the
most metal poor part, we even detect Wolf-Rayet features, against the current
models of WR stars which require high metallicities.Comment: 15 pages, 10 figures, submitted to Ap
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