310 research outputs found
GRBs as Probes of Massive Stars Near and Far
Long-duration gamma-ray bursts are the manifestations of massive stellar
death. Due to the immense energy release they are detectable from most of the
observable universe. In this way they allow us to study the deaths of single
(or binary) massive stars possibly throughout the full timespan massive stars
have existed in the Universe. GRBs provide a means to infer information about
the environments and typical galaxies in which massive stars are formed. Two
main obstacles remain to be crossed before the full potential of GRBs as probes
of massive stars can be harvested: i) we need to build more complete and well
understood samples in order not to be fooled by biases, and ii) we need to
understand to which extent GRBs may be intrinsically biased in the sense that
they are only formed by a limited subset of massive stars defined by most
likely a restricted metallicity interval. We describe the status of an ongoing
effort to build a more complete sample of long-duration GRBs with measured
redshifts. Already now we can conclude that the environments of GRB progenitors
are very diverse with metallicities ranging from solar to a hundredth solar and
extinction ranging from none to A_V>5 mag. We have also identified a sightline
with significant escape of Lyman continuum photons and another with a clear
2175AA extinction bump.Comment: Invited review - in "Massive Stars as Cosmic Engines", IAU Symp. 250
(Kauai), ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University
Press), p. 443-456. Typos and refs correcte
Long Gamma-Ray Burst Host Galaxies and their Environments
In this book-chapter we first briefly discuss some basic observational issues
related to what a GRB host galaxy is (whether they are operationally well
defined as a class) and sample completeness. We then describe some of the early
studies of GRB hosts starting with statistical studies of upper limits done
prior to the first detections, the first host detection after the BeppoSAX
breakthrough and leading up to the current Swift era. Finally, we discuss the
status of efforts to construct a more complete sample of GRBs based on Swift
and end with an outlook. We only consider the host galaxies of long-duration
GRBs.Comment: 31 pages, 14 figures; Chapter 13 in "Gamma-Ray Bursts", eds. C.
Kouveliotou, R. A. M. J. Wijers, S. E. Woosley, Cambridge University Press,
201
A study of purely astrometric selection of extragalactic point sources with Gaia
Selection of extragalactic point sources, e.g. QSOs, is often hampered by
significant selection effects causing existing samples to have rather complex
selection functions. We explore whether a purely astrometric selection of
extragalactic point sources, e.g. QSOs, is feasible with the ongoing Gaia
mission. Such a selection would be interesting as it would be unbiased in terms
of colours of the targets and hence would allow selection also with colours in
the stellar sequence. We have analyzed a total of 18 representative regions of
the sky by using GUMS, the simulator prepared for ESAs Gaia mission, both in
the range of mag and mag. For each region we
determine the density of apparently stationary stellar sources, i.e. sources
for which Gaia cannot measure a significant proper motion. The density is
contrasted with the density of extragalactic point sources, e.g. QSOs, in order
to establish in which celestial directions a pure astrometric selection is
feasible. When targeting regions at galactic latitude
the ratio of QSOs to apparently stationary stars is above 50\% and when
observing towards the poles the fraction of QSOs goes up to about \%.
We show that the proper motions from the proposed Gaia successor mission in
about 20 years would dramatically improve these results at all latitudes.
Detection of QSOs solely from zero proper motion, unbiased by any assumptions
on spectra, might lead to the discovery of new types of QSOs or new classes of
extragalactic point sources.Comment: 4 pages, 4 figures, sent in and accepted for publishing to A&
VLT/X-Shooter Near-Infrared Spectroscopy and HST Imaging of Gravitationally-Lensed z~2 Compact Quiescent Galaxies
Quiescent massive galaxies at z~2 are thought to be the progenitors of
present-day massive ellipticals. Observations revealed them to be
extraordinarily compact. The determination of stellar ages, star formation
rates and dust properties via spectroscopic measurements has up to now only
been feasible for the most luminous and massive specimens (~3x M*). Here we
present a spectroscopic study of two near-infrared selected galaxies which are
close to the characteristic stellar mass M* (~0.9x M* and ~1.3x M*) and whose
observed brightness has been boosted by the gravitational lensing effect. We
measure the redshifts of the two galaxies to be z=1.71\pm0.02 and
z=2.15\pm0.01. By fitting stellar population synthesis models to their
spectro-photometric SEDs we determine their ages to be 2.4^{+0.8}_{-0.6} Gyr
and 1.7\pm0.3 Gyr, respectively, which implies that the two galaxies have
higher mass-to-light ratios than most quiescent z~2 galaxies in other studies.
We find no direct evidence for active star-formation or AGN activity in either
of the two galaxies, based on the non-detection of emission lines. Based on the
derived redshifts and stellar ages we estimate the formation redshifts to be
z=4.3^{+3.4}_{-1.2} and z=4.3^{+1.0}_{-0.6}, respectively. We use the increased
spatial resolution due to the gravitational lensing to derive constraints on
the morphology. Fitting Sersic profiles to the de-lensed images of the two
galaxies confirms their compactness, with one of them being spheroid-like, and
the other providing the first confirmation of a passive lenticular galaxy at a
spectroscopically derived redshift z~2.Comment: accepted for publication in Ap
Consensus report on 25 years of searches for damped Ly galaxies in emission: Confirming their metallicity-luminosity relation at
Starting from a summary of detection statistics of our recent X-shooter
campaign, we review the major surveys, both space and ground based, for
emission counterparts of high-redshift damped Ly absorbers (DLAs)
carried out since the first detection 25 years ago. We show that the detection
rates of all surveys are precisely reproduced by a simple model in which the
metallicity and luminosity of the galaxy associated to the DLA follow a
relation of the form, , and the DLA cross-section follows a relation of the form
. Specifically, our spectroscopic campaign
consists of 11 DLAs preselected based on their equivalent width of SiII
to have a metallicity higher than [Si/H] > -1. The targets have
been observed with the X-shooter spectrograph at the Very Large Telescope to
search for emission lines around the quasars. We observe a high detection rate
of 64% (7/11), significantly higher than the typical 10% for random,
HI-selected DLA samples. We use the aforementioned model, to simulate the
results of our survey together with a range of previous surveys: spectral
stacking, direct imaging (using the `double DLA' technique), long-slit
spectroscopy, and integral field spectroscopy. Based on our model results, we
are able to reconcile all results. Some tension is observed between model and
data when looking at predictions of Ly emission for individual targets.
However, the object to object variations are most likely a result of the
significant scatter in the underlying scaling relations as well as
uncertainties in the amount of dust which affects the emission.Comment: 25 pages (7 of which in appendix), accepted for publication in MNRA
The nature of damped HI absorbers probed by cosmological simulations: satellite accretion and outflows
We use state-of-the-art cosmological zoom simulations to explore the
distribution of neutral gas in and around galaxies that gives rise to high
column density \ion{H}{i} \mbox{Ly-} absorption (formally, sub-DLAs and
DLAs) in the spectra of background quasars. Previous cosmological hydrodynamic
simulations under-predict the mean projected separations of these
absorbers relative to the host, and invoke selection effects to bridge the gap
with observations. On the other hand, single lines of sight (LOS) in absorption
cannot uniquely constrain the galactic origin. Our simulations match all
observational data, with DLA and sub-DLA LOS existing over the entire probed
parameter space ([M/H], kpc) at all redshifts
(). We demonstrate how the existence of DLA LOS at kpc from a massive host galaxy require high numerical resolution, and
that these LOS are associated with dwarf satellites in the main halo, stripped
metal-rich gas and outflows. Separating the galaxy into interstellar
("\ion{H}{i} disc") and circumgalactic ("halo") components, we find that both
components significantly contribute to damped \ion{H}{i} absorption LOS. Above
the sub-DLA (DLA) limits, the disc and halo contribute with and
per cent, respectively. Our simulations confirm analytical
model-predictions of the DLA-distribution at . At high redshift
() sub-DLA and DLAs occupy similar spatial scales, but on average
separate by a factor of two by . On whether sub-DLA and DLA LOS
sample different stellar-mass galaxies, such a correlation can be driven by a
differential covering-fraction of sub-DLA to DLA LOS with stellar mass. This
preferentially selects sub-DLA LOS in more massive galaxies in the low-
universe.Comment: 12 pages, 5 figures, submitted to MNRAS 29/01/201
The Optically Unbiased GRB Host (TOUGH) survey. IV. Lyman-alpha emitters
We report the results of a spectroscopic search for Lyman-alpha emission from
gamma-ray burst host galaxies. Based on the well-defined TOUGH sample of 69
X-ray selected Swift GRBs, we have targeted the hosts of a subsample of 20 GRBs
known from afterglow spectroscopy to be in the redshift range 1.8-4.5. We
detect Lya emission from 7 out of the 20 hosts, with the typical limiting
3sigma line flux being 8E-18 erg/cm2/s, corresponding to a Lya luminosity of
6E41 erg/s at z=3. The Lya luminosities for the 7 hosts in which we detect Lya
emission are in the range (0.6-2.3)E42 erg/s corresponding to star-formation
rates of 0.6-2.1 Msun/yr (not corrected for extinction). The rest-frame Lya
equivalent widths (EWs) for the 7 hosts are in the range 9-40A. For 6 of the 13
hosts for which Lya is not detected we place fairly strong 3sigma upper limits
on the EW (<20A), while for others the EW is either unconstrained or has a less
constraining upper limit. We find that the distribution of Lya EWs is
inconsistent with being drawn from the Lya EW distribution of bright Lyman
break galaxies at the 98.3% level, in the sense that the TOUGH hosts on average
have larger EWs than bright LBGs. We can exclude an early indication, based on
a smaller, heterogeneous sample of pre-Swift GRB hosts, that all GRB hosts are
Lya emitters. We find that the TOUGH hosts on average have lower EWs than the
pre-Swift GRB hosts, but the two samples are only inconsistent at the 92%
level. The velocity centroid of the Lya line is redshifted by 200-700 km/s with
respect to the systemic velocity, similar to what is seen for LBGs, possibly
indicating star-formation driven outflows from the host galaxies. There seems
to be a trend between the Lya EW and the optical to X-ray spectral index of the
afterglow (beta_OX), hinting that dust plays a role in the observed strength
and even presence of Lya emission. [ABRIDGED]Comment: ApJ accepted (v2: minor changes in the Subject headings and reference
list
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