50 research outputs found
The Keck+Magellan Survey for Lyman Limit Absorption I: The Frequency Distribution of Super Lyman Limit Systems
We present the results of a survey for super Lyman limit systems (SLLS;
defined to be absorbers with 19.0 <= log(NHI) <= 20.3 cm^-2) from a large
sample of high resolution spectra acquired using the Keck and Magellan
telescopes. Specifically, we present 47 new SLLS from 113 QSO sightlines. We
focus on the neutral hydrogen frequency distribution f(N,X) of the SLLS and its
moments, and compare these results with the Lyman-alpha forest and the damped
Lyman alpha systems (DLA; absorbers with log(NHI) >= 20.3 cm^-2). We find that
that f(N,X) of the SLLS can be reasonably described with a power-law of index
alpha = -1.43^{+0.15}_{-0.16} or alpha = -1.19^{+0.20}_{-0.21} depending on
whether we set the lower N(HI) bound for the analysis at 10^{19.0} cm^-2 or
10^{19.3}$ cm^-2, respectively. The results indicate a flattening in the slope
of f(N,X) between the SLLS and DLA. We find little evidence for redshift
evolution in the shape of f(N,X) for the SLLS over the redshift range of the
sample 1.68 < z < 4.47 and only tentative evidence for evolution in the zeroth
moment of f(N,X), the line density l_lls(X). We introduce the observable
distribution function O(N,X) and its moment, which elucidates comparisons of HI
absorbers from the Lyman-alpha through to the DLA. We find that a simple three
parameter function can fit O(N,X) over the range 17.0 <= log(NHI) <=22.0. We
use these results to predict that f(N,X) must show two additional inflections
below the SLLS regime to match the observed f(N,X) distribution of the
Lyman-alpha forest. Finally, we demonstrate that SLLS contribute a minor
fraction (~15%) of the universe's hydrogen atoms and, therefore, an even small
fraction of the mass in predominantly neutral gas.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journal. Revision
includes updated reference
Incidence of MgII absorbers towards Blazars and the GRB/QSO puzzle
In order to investigate the origin of the excess of strong MgII systems
towards GRB afterglows as compared to QSO sightlines, we have measured the
incidence of MgII absorbers towards a third class of objects: the Blazars. This
class includes the BL Lac object population for which a tentative excess of
MgII systems had already been reported. We observed with FORS1 at the ESO-VLT
42 Blazars with an emission redshift 0.8<z_em<1.9, to which we added the three
high z northern objects belonging to the 1Jy BL Lac sample. We detect 32 MgII
absorbers in the redshift range 0.35-1.45, leading to an excess in the
incidence of MgII absorbers compared to that measured towards QSOs by a factor
~2, detected at 3 sigma. The amplitude of the effect is similar to that found
along GRB sightlines. Our analysis provides a new piece of evidence that the
observed incidence of MgII absorbers might depend on the type of background
source. In front of Blazars, the excess is apparent for both 'strong' (w_
r(2796) > 1.0 A) and weaker (0.3 < w_r(2796) < 1.0 A) MgII systems. The
dependence on velocity separation with respect to the background Blazars
indicates, at the ~1.5 sigma level, a potential excess for beta = v/c ~0.1. We
show that biases involving dust extinction or gravitational amplification are
not likely to notably affect the incidence of MgII systems towards Blazars.
Finally we discuss the physical conditions required for these absorbers to be
gas entrained by the powerful Blazar jets. More realistic numerical modelling
of jet-ambient gas interaction is required to reach any firm conclusions as
well as repeat observations at high spectral resolution of strong MgII
absorbers towards Blazars in both high and low states.Comment: 14 pages, 8 figures. Accepted for publication in A&
UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers
We study the Gamma Ray Burst (GRB) environment and intervening absorbers by
analyzing the optical absorption features produced by gas surrounding the GRB
or along its line of sight. We analyzed high resolution spectroscopic
observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken
with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum
illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which
has at least four components in the main absorption system. We detect strong
FeII, SiII, and NiII excited absorption lines associated with the bluemost
component only. In addition to the host galaxy, at least two more absorbers
lying along the line of sight to the afterglow have been detected in the
redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost
component in the host galaxy, we derive information about its distance from the
site of the GRB explosion. We do so by assuming that the excited absorption
lines are produced by indirect UV pumping, and compare the data with a time
dependent photo-excitation code. The distance of this component is found to be
280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify
two additional MgII absorbers, one of them with a rest frame equivalent width
larger than 1A. The distance between the GRB and the absorber measured in this
paper confirms that the power of the GRB radiation can influence the conditions
of the interstellar medium up to a distance of at least several hundred pc. For
the intervening absorbers, we confirm the trend that on average one strong
intervening system is found per afterglow, as has been noted in studies
exhibiting an excess of strong MgII absorbers along GRB sightlines compared to
quasars.Comment: 8 Pages, 7 ps figures, A&A in pres
Non variability of intervening absorbers observed in the UVES spectra of the "naked-eye" GRB080319
The aim of this paper is to investigate the properties of the intervening
absorbers lying along the line of sight of Gamma-Ray Burst (GRB) 080319B
through the analysis of its optical absorption features. To this purpose, we
analyze a multi-epoch, high resolution spectroscopic observations (R=40000,
corresponding to 7.5 km/s) of the optical afterglow of GRB080319B (z=0.937),
taken with UVES at the VLT. Thanks to the rapid response mode (RRM), we
observed the afterglow just 8m:30s after the GRB onset when the magnitude was R
~ 12. This allowed us to obtain the best signal-to-noise, high resolution
spectrum of a GRB afterglow ever (S/N per resolution element ~ 50). Two further
RRM and target of opportunity observations were obtained starting 1.0 and 2.4
hours after the event, respectively. Four MgII absorption systems lying along
the line of sight to the afterglow have been detected in the redshift range 0.5
< z < 0.8, most of them showing a complex structure featuring several
components. Absorptions due to FeII, MgI and MnII are also present; they appear
in four, two and one intervening absorbers, respectively. One out of four
systems show a MgII2796 rest frame equivalent width larger than 1A. This
confirms the excess of strong MgII absorbers compared to quasars, with dn/dz =
0.9, ~ 4 times larger than the one observed along quasar lines of sight. In
addition, the analysis of multi-epoch, high-resolution spectra allowed us to
exclude a significant variability in the column density of the single
components of each absorber. Combining this result with estimates of the size
of the emitting region, we can reject the hypothesis that the difference
between GRB and QSO MgII absorbers is due to a different size of the emitting
regions.Comment: 10 pages, 15 ps figures, submitted to MNRA
High-Redshift Metals. II. Probing Reionization Galaxies with Low-Ionization Absorption Lines at Redshift Six
We present a survey for low-ionization metal absorption line systems towards
17 QSOs at redshifts z_em=5.8-6.4. Nine of our objects were observed at high
resolution with either Keck/HIRES or Magellan/MIKE, and the remainder at
moderate resolution with Keck/ESI. The survey spans 5.3 < z_abs < 6.4 and has a
pathlength interval \Delta X=39.5, or \Delta z=8.0. In total we detect ten
systems, five of which are new discoveries. The line-of-sight number density is
consistent with the combined number density at z~3 of DLAs and sub-DLAs, which
comprise the main population of low-ionization systems at lower redshifts. This
apparent lack of evolution may occur because low ionization systems are hosted
by lower-mass halos at higher redshifts, or because the mean cross section of
low-ionization gas at a given halo mass increases with redshift due to the
higher densities and lower ionizing background. The roughly constant number
density notably contrasts with the sharp decline at z > 5.3 in the number
density of highly-ionized systems traced by C IV. The low-ionization systems at
z~6 span a similar range of velocity widths as lower-redshift sub-DLAs but have
significantly weaker lines at a given width. This implies that the
mass-metallicity relation of the host galaxies evolves towards lower
metallicities at higher redshifts. These systems lack strong Si IV and C IV,
which are common among lower-redshift DLAs and sub-DLAs. This is consistent,
however, with a similar decrease in the metallicity of the low- and
high-ionization phases, and does not necessarily indicate a lack of nearby,
highly-ionized gas. The high number density of low-ionization systems at z~6
suggests that we may be detecting galaxies below the current limits of
i-dropout and Ly-alpha emission galaxy surveys. These systems may therefore be
the first direct probes of the `typical' galaxies responsible for hydrogen
reionization.Comment: 15 pages, 15 figures, submitted to Ap
Super-Solar Super Lyman Limit Systems
We present abundance measurements for two super Lyman Limit systems (SLLS;
quasar absorption line systems with 10^19 cm^-2 < N_HI < 10^20.3 cm^-2)
selected from a set of metal-strong absorbers in the Sloan Digital Sky Survey
quasar database. After applying estimate corrections for photoionization
effects, we derive gas-phase metallicities of [M/H]=+0.7 +/- 0.2 dex for the
SLLS at z=1.7749 toward SDSS0927+5621 and [M/H]=+0.05 +/- 0.1 dex for the SLLS
at z=1.7678 toward SDSS0953+5230. The former exhibits among the highest gas
metallicity of any astrophysical environment and its total metal surface
density exceeds that of nearly every known damped Lya system. The properties of
these absorbers -- high metallicity and large velocity width (> 300 km/s) --
resemble those of gas observed in absorption in the spectra of bright,
star-forming galaxies at high redshift. We discuss the metal mass density of
the SLLS based on these observations and our ongoing SLLS survey and argue that
a conservative estimate to the total metal budget at z=2 is greater than 15% of
the total, suggesting that the metal-rich LLS may represent the dominant metal
reservoir in the young universe.Comment: 6 pages, 2 figures. Submitted to ApJL; Revised June 22, 200
On the Incidence of Strong MgII Absorbers Along GRB Sightlines
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 = 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.Comment: 5 pages, 3 figures. Submitted to ApJ
Dusty MgII absorbers: population statistics, extinction curves and gamma-ray burst sightlines
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)Comment: 21 pages, 20 figures, 2 tables, accepted for publication in MNRA
The distribution of equivalent widths in long GRB afterglow spectra
The extreme brightness of gamma-ray burst (GRB) afterglows and their simple
spectral shape make them ideal beacons to study the interstellar medium of
their host galaxies through absorption line spectroscopy. Using 69
low-resolution GRB afterglow spectra, we conduct a study of the rest-frame
equivalent width (EW) distribution of features with an average rest-frame EW
larger than 0.5 A. To compare an individual GRB with the sample, we develop EW
diagrams as a graphical tool, and we give a catalogue with diagrams for the 69
spectra. We introduce a line strength parameter (LSP) that allows us to
quantify the strength of the absorption features as compared to the sample by a
single number. Using the distributions of EWs of single-species features, we
derive the distribution of column densities by a curve of growth (CoG) fit. We
find correlations between the LSP and the extinction of the GRB, the UV
brightness of the host galaxies and the neutral hydrogen column density.
However, we see no significant evolution of the LSP with the redshift. There is
a weak correlation between the ionisation of the absorbers and the energy of
the GRB, indicating that, either the GRB event is responsible for part of the
ionisation, or that galaxies with high-ionisation media produce more energetic
GRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger than
those seen in QSO intervening damped Lyman-alpha (DLA) systems and slightly
more ionised. In particular we find larger excess in the EW of CIV1549 relative
to QSO DLAs, which could be related to an excess of Wolf-Rayet stars in the
environments of GRBs. From the CoG fitting we obtain an average number of
components in the absorption features of GRBs of 6.00(-1.25,+1.00). The most
extreme ionisation ratios in our sample are found for GRBs with low neutral
hydrogen column density, which could be related to ionisation by the GRB
emission.Comment: 37 pages, 31 figures, 15 tables. Accepted for publication in Astonomy
and Astrophysic
The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts
A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently
selected by Salvaterra et al. (2011). The sample has a high level of
completeness in redshift (91%). We derive here the intrinsic absorbing X-ray
column densities of these GRBs making use of the Swift X-ray Telescope data.
This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is
consistent with the distribution of the column densities derived from the total
sample of GRBs with redshift. We find a mild increase of the intrinsic column
density with redshift. This can be interpreted as due to the contribution of
intervening systems along the line of sight. Making use of the spectral index
connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the
relation of the intrinsic column density and the GRB `darkness'. We find that
there is a very tight correlation between dark GRBs and high X-ray column
densities. This clearly indicates that the dark GRBs are formed in a metal-rich
environment where dust must be present.Comment: MNRAS, 6 pages, 3 figures, 1 tabl