16 research outputs found
Deuterium Abundance in the Most Metal-Poor Damped Lyman alpha System: Converging on Omega_baryons
The most metal-poor DLA known to date, at z = 2.61843 in the spectrum of the
QSO Q0913+072, with an oxygen abundance only about 1/250 of the solar value,
shows six well resolved D I Lyman series transitions in high quality echelle
spectra recently obtained with the ESO VLT. We deduce a value of the deuterium
abundance log (D/H) = -4.56+/-0.04 which is in good agreement with four out of
the six most reliable previous determinations of this ratio in QSO absorbers.
We find plausible reasons why in the other two cases the 1 sigma errors may
have been underestimated by about a factor of two. The addition of this latest
data point does not change significantly the mean value of the primordial
abundance of deuterium, suggesting that we are now converging to a reliable
measure of this quantity. We conclude that = -4.55+/-0.03 and
Omega_b h^2 (BBN) = 0.0213+/-0.0010 (68% confidence limits). Including the
latter as a prior in the analysis of the five year data of WMAP leads to a
revised best-fitting value of the power-law index of primordial fluctuations
n_s = 0.956+/-0.013 (1 sigma) and n_s < 0.990 with 99% confidence. Considering
together the constraints provided by WMAP 5, (D/H)_p, baryon oscillations in
the galaxy distribution, and distances to Type Ia supernovae, we arrive at the
current best estimates Omega_b h^2 = 0.0224+/-0.0005 and n_s = 0.959+/-0.013.Comment: 13 pages, 8 Figures. Revised version following referee's comments.
Accepted for publication in Monthly Notices of the Royal Astronomical
Society. A few typos correcte
Diffuse Interstellar Bands in z < 0.6 CaII Absorbers
The diffuse interstellar bands (DIBs) probably arise from complex organic
molecules whose strength in local galaxies correlates with neutral hydrogen
column density, N(HI), and dust reddening, E(B-V). Since CaII absorbers in
quasar (QSO) spectra are posited to have high N(HI) and significant E(B-V),
they represent promising sites for the detection of DIBs at cosmological
distances. Here we present the results from the first search for DIBs in 9
CaII-selected absorbers at 0.07 < z_abs < 0.55. We detect the 5780Ang DIB in
one line of sight at z_abs = 0.1556; this is only the second QSO absorber in
which a DIB has been detected. Unlike the majority of local DIB sight-lines,
both QSO absorbers with detected DIBs show weak 6284Ang absorption compared
with the 5780Ang band. This may be indicative of different physical conditions
in intermediate redshift QSO absorbers compared with local galaxies. Assuming
that local relations between the 5780Ang DIB strength and N(HI) and E(B-V)
apply in QSO absorbers, DIB detections and limits can be used to derive N(HI)
and E(B-V). For the one absorber in this study with a detected DIB, we derive
E(B-V) = 0.23mag and log[N(HI)] >= 20.9, consistent with previous conclusions
that CaII systems have high HI column densities and significant reddening. For
the remaining 8 CaII-selected absorbers with 5780Ang DIB non-detections, we
derive E(B-V) upper limits of 0.1-0.3mag.Comment: 5 pages, 2 figures. Accepted to MNRAS Letter
The host galaxies of strong CaII QSO absorption systems at z<0.5
We present new imaging and spectroscopic observations of the fields of five
QSOs with very strong intervening CaII absorption systems at redshifts z<0.5
selected from the Sloan Digital Sky Survey. Recent studies of these very rare
absorbers indicate that they may be related to damped Lyman alpha systems
(DLAs). In all five cases we identify a galaxy at the redshift of the CaII
system with impact parameters up to ~24 kpc. In four out of five cases the
galaxies are luminous (L ~L*), metal-rich (Z ~Zsun), massive (velocity
dispersion, sigma ~100 km/s) spirals. Their star formation rates, deduced from
Halpha emission, are high, in the range SFR = 0.3 - 30 Msun/yr. In our
analysis, we paid particular attention to correcting the observed emission line
fluxes for stellar absorption and dust extinction. We show that these effects
are important for a correct SFR estimate; their neglect in previous low-z
studies of DLA-selected galaxies has probably led to an underestimate of the
star formation activity in at least some DLA hosts. We discuss possible links
between CaII-selected galaxies and DLAs and outline future observations which
will help clarify the relationship between these different classes of QSO
absorbers.Comment: Accepted for publication in MNRAS, 14 pages, 9 figures. Version with
full resolution images available at
http://www.ast.cam.ac.uk/~bjz/papers/Zych_etal_2007a.pd
C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems
This study focuses on some of the most metal-poor damped Lyman alpha
absorbers known in the spectra of high redshift QSOs, using new and archival
observations obtained with UV-sensitive echelle spectrographs on the Keck and
VLT telescopes. The weakness and simple velocity structure of the absorption
lines in these systems allows us to measure the abundances of several elements,
and in particular those of C, N, and O, a group that is difficult to study in
DLAs of more typical metallicities. We find that when the oxygen abundance is
less than about 1/100 of solar, the C/O ratio in high redshift DLAs and
sub-DLAs matches that of halo stars of similar metallicity and shows higher
values than expected from galactic chemical evolution models based on
conventional stellar yields. Furthermore, there are indications that at these
low metallicities the N/O ratio may also be above simple expectations and may
exhibit a minimum value, as proposed by Centurion and her collaborators in
2003. Both results can be interpreted as evidence for enhanced production of C
and N by massive stars in the first few episodes of star formation, in our
Galaxy and in the distant proto-galaxies seen as QSO absorbers. The higher
stellar yields implied may have an origin in stellar rotation which promotes
mixing in the stars' interiors, as considered in some recent model
calculations. We briefly discuss the relevance of these results to current
ideas on the origin of metals in the intergalactic medium and the universality
of the stellar initial mass function.Comment: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Dust depletion, chemical uniformity and environment of CaII H&K quasar absorbers
CaII 3934,3969 absorbers, which are likely to be a subset of damped Lyman
alpha systems, are the most dusty quasar absorbers known, with an order of
magnitude more extinction in E(B-V) than other absorption systems. There is
also evidence that CaII absorbers trace galaxies with more ongoing
star-formation than the average quasar absorber. Despite this, relatively
little is known in detail about these unusual absorption systems. Here we
present the first high resolution spectroscopic study of 19 CaII quasar
absorbers, in the range 0.6=0.2A. Their general
depletion patterns are similar to measurements in the warm halo phase of the
Milky Way and Magellanic Clouds ISM. Dust depletions and alpha-enrichments
profiles of sub-samples of 7 and 3 absorbers, respectively, are measured using
a combination of Voigt profile fitting and apparent optical depth techniques.
Deviations in [Cr/Zn]~0.3+-0.1dex and in [Si/Fe]>~0.8+-0.1dex are detected
across the profile of one absorber, which we attribute to differential dust
depletion. The remaining absorbers have <0.3dex (3sigma limit) variation in
[Cr/Zn], much like the general DLA population, though the dustiest CaII
absorbers remain relatively unprobed in our sample. A limit on electron
densities in CaII absorbers, n_e<0.1cm^-3, is derived using the ratio of
neutral and singly ionised species, assuming a MW-like radiation field. These
electron densities may imply hydrogen densities sufficient for the presence of
molecular hydrogen in the absorbers. The CaII absorber sample comprises a wide
range of velocity widths, v_90=50-470km/s, and velocity structures, thus a
range of physical models for their origin, from simple discs to galactic
outflows and mergers, would be required to explain the observations.Comment: Minor revision after publication proofs to ensure arXiv version of
paper matches MNRAS published versio
A downturn in intergalactic CIV as redshift 6 is approached
We present the results of the largest survey to date for intergalactic metals
at redshifts z > 5, using near-IR spectra of nine QSOs with emission redshifts
z(em) > 5.7. We find, for the first time, a change in the comoving mass density
of C IV ions as we look back to redshifts z > 5. At a mean = 5.76, we
deduce Omega(C IV)=(4.4+/-2.6)x10^(-9) which implies a drop by a factor of
about 3.5 compared to the value at z < 4.7, after accounting for the differing
sensitivities of different surveys. The observed number of C IV doublets is
also lower by a similar factor, compared to expectations for a non-evolving
column density distribution of absorbers. These results point to a rapid
build-up of intergalactic C IV over a period of only 300 Myr; such a build-up
could reflect the accumulation of metals associated with the rising levels of
star formation activity from z = 9 indicated by galaxy counts, and/or an
increasing degree of ionisation of the intergalactic medium (IGM), following
the overlap of ionisation fronts from star-forming regions. If the value of
Omega(C IV) we derive is typical of the IGM at large, it would imply a
metallicity Z(IGM) = 10^(-4) Z(Sun). The early-type stars responsible for
synthesising these metals would have emitted only about one Lyman continuum
photon per baryon prior to z = 5.8; such a background is insufficient to keep
the IGM ionised and we speculate on possible factors which could make up the
required shortfall.Comment: 15 pages, 8 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society. Typo in astro-ph abstract fixed. Paper
unchange