Observations of GRBs at high redshift Tanvir, Nial R; Jakobsson, Páll
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
05/2007, Letnik:
365, Številka:
1854
Journal Article
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The extreme luminosity of gamma-ray bursts and their afterglows means they are detectable, in principle, to very high redshifts. Although the redshift distribution of gamma-ray bursts (GRBs) is ...difficult to determine, due to incompleteness of present samples, we argue that for Swift-detected bursts, the median redshift is between 2.5 and 3, with a few per cent probably at z>6. Thus, GRBs are potentially powerful probes of the era of reionization and the sources responsible for it. Moreover, it seems probable that they can provide constraints on the star-formation history of the Universe and may also help in the determination of the cosmological parameters.
We present the first results from our survey of stellar substructure in the outskirts of M31 using the Advanced Camera for Surveys on board the Hubble Space Telescope. We discuss the stellar ...populations associated with five prominent stellar overdensities discovered during the course of our panoramic ground-based imaging survey with the Isaac Newton Telescope Wide-Field Camera; a sixth pointing targets a region of "clean" halo. The color-magnitude diagrams, which contain between approximate to 10,000 and 90,000 stars and reach 4 mag below the horizontal branch, reveal clear variations in morphology between most fields, indicating that the age and/or metallicity mix of stars is not constant at large radius. This directly confirms the existence of large-scale population inhomogeneities within the halo of M31 and lends further support to the notion that M31 has formed, at least in part, through satellite accretions. We find a striking similarity between the populations of the giant stellar stream and those of another overdensity, the NE shelf, which lies northeast of the galaxy center. If these overdensities are associated with the same population, then the difference in their red clump magnitudes implies that the NE shelf lies in front of the stream by several tens of kiloparsecs, in good agreement with recent orbit calculations for the stream progenitor.
The two closest gamma-ray bursts so far detected (GRBs 980425 and 060218) were both under-luminous, spectrally soft, long-duration bursts with smooth, single-peaked light curves. Only of the order of ...100 GRBs have measured redshifts, and there are, for example, 2704 GRBs in the BATSE (Burst and Transient Source Experiment) catalogue alone. It is therefore plausible that other nearby GRBs have been observed but not identified as being relatively nearby. Here we search for statistical correlations between BATSE long-duration GRBs and galaxy samples with recession velocities v≤ 11 000 km s−1 (z= 0.0367, ≈155 Mpc) selected from two catalogues of nearby galaxies. We also examine the correlations using burst subsamples restricted to those with properties similar to the two known nearby bursts. Our results show correlation of the entire long-GRB sample to remain consistent with zero out to the highest radii considered whereas a subsample selected to be low fluence, spectrally soft, with smooth single-peaked light curves (177 bursts), demonstrates increased correlation with galaxies within ≈155 Mpc. The measured correlation (28 ± 16 per cent of the sample) suggests that BATSE observed between two and nine long-duration GRBs per year similar to, and from within similar distances to, GRBs 980425 and 060218. This implies an observed local rate density (to BATSE limits) of 700 ± 360 Gpc−3 yr−1 within 155 Mpc.
We have used the Hubble Space Telescope's Advanced Camera for Surveys (ACS) to detect and measure 65300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W ...photometry on these stars, we have determined their metallicity distribution function and constrained the types of stars present in this portion of Virgo's intracluster space. Based on the small number of stars detected that were brighter than the red giant branch (RGB) tip, we suggest that in this region, Virgo's intracluster stars are mostly old ( 10 Gyr). Through analysis of the RGB stars themselves, we determine that the population contains the full range of metallicities probed (-2.3 , M/H , 0.0). We also present evidence that the younger (,10 Gyr) component of the population is more metal-rich, with M/H > -0.5. The spatial distribution of the most metal-poor stars in the field shows significantly more structure than that of the metal-rich stars, indicating that the intracluster population is not well mixed. We discuss the implications that these observations have for the production of intracluster stars and the dynamical evolution of the Virgo Cluster.
GRB 090423 is the most distant spectroscopically confirmed source observed in the Universe. Using observations at 37.5 GHz, we place constraints on molecular gas emission in the CO (3-2) line from ...its host galaxy and immediate environs. The source was not detected either in line emission or in the rest-frame 850-μm continuum, yielding an upper limit of S
8mm= 9.3 μJy and M(H2) <4.3 × 109 M⊙ (3σ), applying standard conversions. This implies that the host galaxy of GRB 090423 did not possess a large reservoir of warm molecular gas but was rather modest either in star formation rate or in mass. It suggests that this was not an extreme starburst and hence that gamma ray bursts at high redshift trace relatively modest star formation rates, in keeping with the behaviour seen at lower redshifts. We do, however, identify a millimetre emission-line source in the field of GRB 090423. Plausible interpretations include a CO (1-0) emitting galaxy at z= 2.1, CO (2-1) at z= 5.2 and CO (3-2) at z= 8.3. Efforts to identify a counterpart for the molecular line emitter and to further characterize this source are continuing.
For gamma-ray burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: 1 h, 1 d, 3 d and 1 week after the burst, thanks to the afterglow ...remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe ii, and from metastable levels of Fe ii and Ni ii at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni ii
4F9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the ultraviolet radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems.
The brightness of gamma-ray burst (GRB) afterglows and their occurrence in young, blue galaxies make them excellent probes to study star-forming regions in the distant universe. We here elucidate ...dust extinction properties in the early universe through the analysis of the afterglows of all known z > 6 GRBs: GRB 09042,080913, and 050904, at z = 8.2, 6.69, and 6.295, respectively. We gather all available optical and near-infrared photometry, spectroscopy, and X-ray data to construct spectral energy distributions (SEDs) at multiple epochs. We then fit the SEDs at all epochs with a dust-attenuated power law or broken power law. We find no evidence for dust extinction in GRB 050904 and GRB 090423, with possible evidence for a low level of extinction in GRB 080913. We compare the high redshift GRBs to a sample of lower redshift GRB extinctions and find a lack of even moderately extinguished events (AV ~ 0.3) above z 4. In spite of the biased selection and small number statistics, this result hints at a decrease in dust content in star-forming environments at high redshifts.
Recent progress on the nature of short-duration gamma-ray bursts has shown that a fraction of them originate in the local Universe. These systems may well be the result of giant flares from soft ...gamma-repeaters (highly magnetized neutron stars commonly known as magnetars). However, if these neutron stars are formed via the core collapse of massive stars then it would be expected that the bursts should originate from predominantly young stellar populations, while correlating the positions of BATSE short bursts with structure in the local Universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars formed via the core collapse of massive stars, they also form via accretion-induced collapse following the merger of two white dwarfs, one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via white dwarf–white dwarf (WD–WD) mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of production of magnetars by core collapse is proportional to the star formation rate, the rate of production via WD–WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD–WD mergers may produce soft gamma-repeater giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short-duration burst population at higher redshift.
The Stellar Halo and Outer Disk of M33 McConnachie, Alan W; Chapman, Scott C; Ibata, Rodrigo A ...
The Astrophysical journal,
08/2006, Letnik:
647, Številka:
1
Journal Article
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We present first results from a Keck DEIMOS spectroscopic survey of red giant branch (RGB) stars in M33. The radial velocity distributions of the stars in our fields are well described by three ...Gaussian components, corresponding to a candidate halo component with an uncorrected radial velocity dispersion of s 50 km s super(-1), a candidate disk component with a dispersion s 16 km s super(-1), and a third component offset from the disk by 650 km s super(-1), but for which the dispersion is not well constrained. By comparing our data to a model of M33 based on its H I rotation curve, we find that the stellar disk is offset in velocity by 625 km s super(-1) from the H I disk, consistent with the warping that exists between these components. The spectroscopic metallicity of the halo component is Fe/H -1.5, significantly more metal-poor than the implied metallicity of the disk population (Fe/H -0.9), which also has a broader color dispersion than the halo population. These data represent the first detections of individual stars in the halo of M33 and, despite being 610 times less massive than M31 or the Milky Way, all three of these disk galaxies have stellar halo components with a similar metallicity. The color distribution of the third component is different from the disk and the halo but is similar to that expected for a single, coeval, stellar population, and could represent a stellar stream. More observations are required to determine the true nature of this intriguing third kinematic component in M33.
It has been suspected for nearly 50 years that galaxy clusters contain a population of intergalactic stars ripped from the galaxies during cluster formation, or when the galactic orbits pass through ...the cluster centre. Observational support for theexistence of such a stellar population is provided both by measurements of the diffuse light in clusters, and by the recent detection of planetary nebulae with positions or velocities far removed from any observable cluster galaxy,. But estimates for the mass of the diffuse population and its distribution relative to the cluster galaxies are still highly uncertain. Here we report the direct detection of intergalactic stars in deep images of a blank field in the Virgo cluster. The data suggest that these stars form approximately one-tenth of the total stellar mass of the cluster. We observe a relatively homogeneous distribution of stars, with evidence of a slight gradient towards M87.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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