195 research outputs found
A Survey of Weak MgII Absorbers at 0.4 < z < 2.4
We present results from a survey of weak MgII absorbers in the VLT/UVES
spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified
112 weak MgII systems within the redshift interval 0.4 < z < 2.4 with 86%
completeness down to a rest-frame equivalent width of W_r(2796) = 0.02A,
covering a cumulative redshift path length of deltaZ=77.3. From this sample, we
estimate that the number of weak absorbers per unit redshift dN/dz increases
from 1.06 +/- 0.04 at =1.9 to 1.76 +/- 0.08 at =1.2 and thereafter
decreases to 1.51 +/- 0.09 at =0.9 and 1.06 +/- 0.10 at =0.6. Thus we
find evidence for an evolution in the population of weak MgII absorbers, with
their number density peaking at z=1.2. We also determine the equivalent width
distribution of weak systems at =0.9 and =1.9. At 0.4 < z < 1.4, there is
evidence for a turnover from a powerlaw of the form n(W_r) \propto W_r^{-1.04}
at W_r(2796) < 0.1A. This turnover is more extreme at 1.4 < z < 2.4, where the
equivalent width distribution is close to an extrapolation of the exponential
distribution function found for strong MgII absorbers. Based on these results,
we discuss the possibility that some fraction of weak MgII absorbers,
particularly single cloud systems, are related to satellite clouds surrounding
strong MgII systems. These structures could also be analogs to Milky Way high
velocity clouds. In this context, the paucity of high redshift weak MgII
absorbers is caused by a lack of isolated accreting clouds on to galaxies
during that epoch.Comment: 14 pages, 11 figures, ApJ accepte
The Milky Way halo as a QSO absorption-line system. New results from an HST/STIS absorption-line catalogue of Galactic high-velocity clouds
We use archival UV absorption-line data from HST/STIS to statistically
analyse the absorption characteristics of the high-velocity clouds (HVCs) in
the Galactic halo towards more than 40 extragalactic background sources. We
determine absorption covering fractions of low- and intermediate ions (OI, CII,
SiIII, MgII, FeII, SiIII, CIV, and SiIV) in the range fc = 0.20 - 0.70. For
detailed analysis we concentrate on SiII absorption components in HVCs, for
which we investigate the distribution of column densities, b-values, and radial
velocities. Combining information for SiII and MgII, and using a geometrical
HVC model we investigate the contribution of HVCs to the absorption cross
section of strong MgII absorbers in the local Universe. We estimate that the
Galactic HVCs would contribute on average ~52 % to the total strong MgII cross
section of the Milky Way, if our Galaxy were to be observed from an exterior
vantage point. We further estimate that the mean projected covering fraction of
strong MgII absorption in the Milky Way halo and disc from an exterior vantage
point is fc(sMgII) = 0.31 for a halo radius of R = 61 kpc. These numbers,
together with the observed number density of strong MgII absorbers at low
redshift, indicate that the contribution of infalling gas clouds (i.e., HVC
analogues) in the halos of Milky Way-type galaxies to the cross section of
strong MgII absorbers is <34 %. These findings are in line with the idea that
outflowing gas (e.g., produced by galactic winds) in the halos of more actively
star-forming galaxies dominate the absorption-cross section of strong MgII
absorbers in the local Universe
A Survey of Analogs to Weak MgII Absorbers in the Present
We present the results of a survey of the analogs of weak MgII absorbers
(rest frame equivalent width W(2796) < 0.3 A) at 0 < z < 0.3. Our sample
consisted of 25 HST/STIS echelle quasar spectra (R = 45,000) which covered SiII
1260 and CII 1335 over this redshift range. Using those similar transitions as
tracers of MgII facilitates a much larger survey, covering a redshift
pathlength of g(z) = 5.3 for an equivalent width limit of MgII corresponding to
W(2796) > 0.02 A, with 30% completeness for the weakest lines. We find the
number of weak MgII absorber analogs with 0.02 < W(2796) < 0.3 to be dN/dz =
1.00 +/- 0.20 for 0 < z < 0.3. This value is consistent with cosmological
evolution of the population. We consider the expected effect on observability
of weak MgII absorbers of the decreasing intensity of the extragalactic
background radiation eld from z~1 to z~0. Assuming that all the objects that
produce absorption at z~1 are stable on a cosmological timescale, and that no
new objects are created, we would expect dN/dz of 2-3 at z~0. About 30-50% of
this z~0 population would be decendants of the parsec-scale structures that
produce single-cloud, weak MgII absorbers at z~1. The other 50-70% would be
lower density, kiloparsec-scale structures that produce CIV absorption, but not
detectable low ionization absorption, at z~1. We conclude that at least one,
and perhaps some fraction of both, of these populations has evolved away since
z~1, in order to match the z~0 dN/dz measured in our survey. This would follow
naturally for a population of transient structures whose generation is related
to star-forming processes, whose rate has decreased since z~1.Comment: 45 pages, 12 figures, 7 tables ApJ accepte
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