130 research outputs found
The Host Galaxies of Fast-Ejecta Core-Collapse Supernovae
Get PDFSpectra of broad-lined Type Ic supernovae (SN Ic-BL), the only kind of SN
observed at the locations of long-duration gamma-ray bursts (LGRBs), exhibit
wide features indicative of high ejecta velocities (~0.1c). We study the host
galaxies of a sample of 245 low-redshift (z<0.2) core-collapse SN, including 17
SN Ic-BL, discovered by galaxy-untargeted searches, and 15 optically luminous
and dust-obscured z<1.2 LGRBs. We show that, in comparison with SDSS galaxies
having similar stellar masses, the hosts of low-redshift SN Ic-BL and z<1.2
LGRBs have high stellar-mass and star-formation-rate densities. Core-collapse
SN having typical ejecta velocities, in contrast, show no preference for such
galaxies. Moreover, we find that the hosts of SN Ic-BL, unlike those of SN
Ib/Ic and SN II, exhibit high gas velocity dispersions for their stellar
masses. The patterns likely reflect variations among star-forming environments,
and suggest that LGRBs can be used as probes of conditions in high-redshift
galaxies. They may be caused by efficient formation of massive binary
progenitors systems in densely star-forming regions, or, less probably, a
higher fraction of stars created with the initial masses required for a SN
Ic-BL or LGRB. Finally, we show that the preference of SN Ic-BL and LGRBs for
galaxies with high stellar-mass and star-formation-rate densities cannot be
attributed to a preference for low metal abundances but must reflect the
influence of a separate environmental factor.Comment: Accepted by ApJ 9 May 2014 with only minor revision
On The Origin Of The Mass-Metallicity Relation For GRB Host Galaxies
Full text linkWe investigate the nature of the mass-metallicity (M-Z) relation for long
gamma-ray burst (LGRB) host galaxies. Recent studies suggest that the M-Z
relation for local LGRB host galaxies may be systematically offset towards
lower metallicities relative to the M-Z relation defined by the general star
forming galaxy (SDSS) population. The nature of this offset is consistent with
suggestions that low metallicity environments may be required to produce high
mass progenitors, although the detection of several GRBs in high-mass,
high-metallicity galaxies challenges the notion of a strict metallicity cut-off
for host galaxies that are capable of producing GRBs. We show that the nature
of this reported offset may be explained by a recently proposed
anti-correlation between the star formation rate (SFR) and the metallicity of
star forming galaxies. If low metallicity galaxies produce more stars than
their equally massive, high-metallicity counterparts, then transient events
that closely trace the SFR in a galaxy would be more likely to be found in
these low metallicity, low mass galaxies. Therefore, the offset between the GRB
and SDSS defined M-Z relations may be the result of the different methods used
to select their respective galaxy populations, with GRBs being biased towards
low metallicity, high SFR, galaxies. We predict that such an offset should not
be expected of transient events that do not closely follow the star formation
history of their host galaxies, such as short duration GRBs and SN Ia, but
should be evident in core collapse SNe found through upcoming untargeted
surveys.Comment: 6 pages, 4 figures, submitted to ApJ
On The Origin Of High Energy Correlations in Gamma-ray Bursts
Full text linkI investigate the origin of the observed correlation between a GRB's nuFnu
spectral peak Epk and its isotropic equivalent energy Eiso through the use of a
population synthesis code to model the prompt gamma-ray emission from GRBs. By
using prescriptions for the distribution of prompt spectral parameters as well
as the population's luminosity function and co-moving rate density, I generate
a simulated population of GRBs and examine how bursts of varying spectral
properties and redshift would appear to a gamma-ray detector here on Earth. I
find that a strong observed correlation can be produced between the source
frame Epk and Eiso for the detected population despite the existence of only a
weak and broad correlation in the original simulated population. The energy
dependance of a gamma-ray detector's flux-limited detection threshold acts to
produce a correlation between the source frame Epk and Eiso for low luminosity
GRBs, producing the left boundary of the observed correlation. Conversely, very
luminous GRBs are found at higher redshifts than their low luminosity
counterparts due to the standard Malquest bias, causing bursts in the low Epk,
high Eiso regime to go undetected because their Epk values would be redshifted
to energies at which most gamma-ray detectors become less sensitive. I argue
that it is this previously unexamined effect which produces the right boundary
of the observed correlation. Therefore, the origin of the observed correlation
is a complex combination of the instrument's detection threshold, the intrinsic
cutoff in the GRB luminosity function, and the broad range of redshifts over
which GRBs are detected. These simulations serve to demonstrate how selection
effects caused by a combination of instrumental sensitivity and the
cosmological nature of an astrophysical population can act to produce an
artificially strong correlation between observed properties.Comment: 9 pages, 12 figures, submitted to Ap
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