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Register a new userDusty MgII absorbers: population statistics, extinction curves and gamma-ray burst sightlines
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Jon Budzynski MA MSci
Publication date
2011
fields
Physics
and research's language is
English
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We present a new determination of the dust content and near-ultraviolet/optical extinction curves associated with a sample of ~8300 strong (equivalent width > 1A) Mg II absorbers, with redshifts 0.4<z<2.2, identified in Sloan Digital Sky survey (SDSS) spectra of quasars. Taking into account the selection effects that result from dust extinction, including the reduction in the signal-to-noise ratio of an absorber appearing in a reddened quasar spectrum, we find a stronger dependence of E(B-V) on absorber rest equivalent width (EW) than in other published work. The dependence of the median reddening on EW can be reproduced by a power-law model: E(B-V)=.8+/-3*10-4 * EW^(3.48+/-0.3) for 1.0A<EW<5.0A. Observed Mg II samples, derived from flux-limited quasar surveys, are shown to suffer from significant incompleteness at the level of 24+/-4 per cent for absorbers with EW>1A and 34+/-2 per cent for absorbers with EW>2A. Direct determination of the shape of the near-ultraviolet extinction curves for absorbers as a function of E(B-V) show evidence for systematic changes in the form of the extinction curves. At low E(B-V) (>0.05), the extinction curve is well represented by a Small Magellanic Cloud-like extinction curve. For intermediate E(B-V)s (<0.2), approximately a third of MgII absorbers show evidence for a 2175A feature similar to that of the Large Magellanic Cloud. For the small number of high E(B-V) (>0.3) absorbers, the majority of which exhibit strong CaII 3935,3970 absorption, there is evidence for a 2175A feature as strong as that found in the Milky Way. Application of the new results on the dust content of strong Mg II absorbers shows that dusty absorbers can account for a significant proportion, up to a factor of two, of the observed overdensity of absorbers seen towards Gamma-Ray Burst (GRB) sightlines, compared to sightlines towards quasars in flux-limited samples. (Abridged)
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In 2006, Prochter et al. reported a statistically significant enhancement of very strong Mg II absorption systems intervening the sightlines to gamma-ray bursts (GRBs) relative to the in- cidence of such absorption along quasar sightlines. This counterintuitive result, has inspired a diverse set of astrophysical explanations (e.g. dust, gravitational lensing) but none of these has obviously resolved the puzzle. Using the largest set of GRB afterglow spectra available, we reexamine the purported enhancement. In an independent sample of GRB spectra with a survey path 3 times larger than Prochter et al., we measure the incidence per unit redshift of $geq 1$AA rest-frame equivalent width Mg II absorbers at $z approx 1$ to be l(z)= 0.18 $pm$ 0.06. This is fully consistent with current estimates for the incidence of such absorbers along quasar sightlines. Therefore, we do not confirm the original enhancement and suggest those results suffered from a statistical fluke. Signatures of the original result do remain in our full sample (l(z) shows an $approx 1.5$ enhancement over l(z)QSO), but the statistical significance now lies at $approx 90%$ c.l. Restricting our analysis to the subset of high-resolution spectra of GRB afterglows (which overlaps substantially with Prochter et al.), we still reproduce a statistically significant enhancement of Mg II absorption. The reason for this excess, if real, is still unclear since there is no connection between the rapid afterglow follow-up process with echelle (or echellette) spectrographs and the detectability of strong Mg II doublets. Only a larger sample of such high-resolution data will shed some light on this matter.
The composition and amount of interstellar dust within gamma-ray burst (GRB) host galaxies is of key importance when addressing selection effects in the GRB redshift distribution, and when studying the properties of their host galaxies. As well as the implications for GRB research, probing the dust within the high-z hosts of GRBs also contributes to our understanding of the conditions of the interstellar medium and star-formation in the distant Universe. Nevertheless, the physical properties of dust within GRB host galaxies continues to be a highly contended issue. In this paper we explore the mean extinction properties of dust within the host galaxies of a sample of 17 GRBs with total host galaxy visual extinction Av<1 (<Av>=0.4), covering a redshift range z=0.7-3.1. We find the average host extinction curve to have an ultraviolet slope comparable to that of the LMC, but with little evidence of a 2175Angs dust extinction feature as observed along Milky Way and LMC sightlines. We cannot at present rule out the presence of a 2175Angs feature, and both the standard SMC and LMC extinction curves also provide good fits to our data. However, we can reject an extinction curve that has a UV slope as flat as the mean Milky Way extinction curve, whilst also having a 2175Angs feature as prominent as seen in the mean Milky Way extinction curve. This is in contrast to the clear detection of a 2175Angs bump and the flatter extinction curves of some more heavily extinguished GRBs (Av>1), which may be indicative of there being a dependence between dust abundance and the wavelength dependence of dust extinction, as has been previously speculated.
It is widely believed that the cool gas clouds traced by MgII absorption, within a velocity offset of 5000 km/s relative to the background quasar are mostly associated with the quasar itself, whereas the absorbers seen at larger velocity offsets towards us are intervening absorber systems and hence their existence is completely independent of the background quasar. Recent evidence by Bergeron et al. (2011, hereinafter BBM) has seriously questioned this paradigm, by showing that the number density of intervening MgII absorbers towards the 45 blazars in their sample is nearly 2 times the expectation based on the MgII absorption systems seen towards normal QSOs. Given its serious implications, it becomes important to revisit this finding, by enlarging the blazar sample and subjecting it to an independent analysis. Here, we first report the outcome of our re-analysis of the available spectroscopic data for the BBM sample itself. Our analysis of the BBM sample reproduces their claimed factor of 2 excess of dN/dz along blazar sightlines, vis-a-vis normal QSOs. We have also assembled a ~3 times larger sample of blazars, albeit with moderately sensitive optical spectra. Using this sample together with the BBM sample, our analysis shows that the dN/dz of the MgII absorbers statistically matches that known for normal QSO sightlines. Further, the analysis indicates that associated absorbers might be contributing significantly to the estimated dN/dz upto offset speeds Delta v ~0.2c relative to the blazar.
We report on a survey for strong (rest equivalent width W_r >= 1A), intervening MgII systems along the sightlines to long-duration gamma-ray bursts (GRBs). The GRB spectra which comprise the survey have a heterogeneous mix of resolution and wavelength coverage, but we implement a strict, uniform set of search criteria to derive a well-defined statistical sample. We identify 15 strong MgII absorbers along 14 GRB sightlines (nearly every sightline exhibits at least one absorber) with spectra covering a total pathlength Delta z = 15.5 at a mean redshift <z> = 1.1. In contrast, the predicted incidence of such absorber systems along the same path length to quasar sightlines is only 3.8. The roughly four times higher incidence along GRB sightlines is inconsistent with a statistical fluctuation at greater than 99.9% c.l. Several effects could explain the result: (i) dust within the MgII absorbers obscures faint quasars giving a lower observed incidence along quasar sightlines; (ii) the gas is intrinsic to the GRB event; (iii) the GRB are gravitationally lensed by these absorbers. We present strong arguments against the first two effects and also consider lensing to be an unlikely explanation. The results suggest that at least one of our fundamental beliefs on absorption line research is flawed.
Because massive, low-metallicity population III (PopIII) stars may produce very powerful long gamma-ray bursts (LGRBs), high-redshift GRB observations could probe the properties of the first stars. We analyze the correlation between early PopIII stars and LGRBs by using cosmological N-body/hydrodynamical simulations, which include detailed chemical evolution, cooling, star formation, feedback effects and the transition between PopIII and more standard population I/II (PopII/I) stars. From the Swift observed rate of LGRBs, we estimate the fraction of black holes that will produce a GRB from PopII/I stars to be in the range 0.028<f_{GRB}<0.140, depending on the assumed upper metallicity of the progenitor. Assuming that as of today no GRB event has been associated to a PopIII star, we estimate the upper limit for the fraction of LGRBs produced by PopIII stars to be in the range 0.006<f_{GRB}<0.022. When we apply a detection threshold compatible with the BAT instrument, we find that the expected fraction of PopIII GRBs (GRB3) is ~10% of the full LGRB population at z>6, becoming as high has 40% at z>10. Finally, we study the properties of the galaxies hosting our sample of GRB3. We find that the average metallicity of the galaxies hosting a GRB3 is typically higher than the critical metallicity used to select the PopIII stars, due to the efficiency in polluting the gas above such low values. We also find that the highest probability of finding a GRB3 is within galaxies with a stellar mass <10^7 Msun, independently from the redshift.
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