866 research outputs found
Element Abundances at High-redshift: Magellan MIKE Observations of sub-Damped Lyman-alpha Absorbers at 1.7 < z <2.4
We present chemical abundance measurements from high-resolution observations
of 5 sub-damped Lyman-alpha absorbers at 1.7 < z < 2.4 observed with the
Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II
Clay telescope. Lines of Zn II, Mg I, Mg II, Al II, Al III, S II, Si II, Si IV,
C II, C II*, C IV, Ni II, Mn II and Fe II were detected and column densities
were determined. The metallicity of the absorbing gas, inferred from the nearly
undepleted element Zn, is in the range of < -0.95 to +0.25 dex for the five
absorbers in our sample, with three of the systems being near-solar or
super-solar. We also investigate the effect of ionisation on the observed
abundances using photoionisation modelling. Combining our data with other
sub-DLA and DLA data from the literature, we report the most complete existing
determination of the metallicity vs. redshift relation for sub-DLAs and DLAs.
We confirm the suggestion from previous investigations that sub-DLAs are, on
average, more metal-rich than DLAs and evolve faster. We also discuss relative
abundances and abundance ratios in these absorbers. The more metal-rich systems
show significant dust depletion levels, as suggested by the ratios [Zn/Cr] and
[Zn/Fe]. For the majority of the systems in our sample, the [Mn/Fe] vs. [Zn/H]
trend is consistent with that seen previously for lower-redshift sub-DLAs. We
also measure the velocity width values for the sub-DLAs in our sample from
unsaturated absorption lines of Fe II 2344, 2374, 2600 A, and examine where
these systems lie in a plot of metallicity vs. velocity dispersion. Finally, we
examine cooling rate vs. H I column density in these sub-DLAs, and compare this
with the data from DLAs and the Milky Way ISM. We find that most of the systems
in our sample show higher cooling rate values compared to those seen in the
DLAs.Comment: Accepted for publication in the Monthly Notices of The Royal
Astronomical Societ
A GBT Survey for HI 21 cm Absorption in the Disks and Halos of Low-Redshift Galaxies
We present an HI 21 cm absorption survey with the Green Bank Telescope (GBT)
of galaxy-quasar pairs selected by combining data from the Sloan Digital Sky
Survey (SDSS) and the Faint Images of the Radio Sky at Twenty-Centimeters
(FIRST) survey. Our sample consists of 23 sightlines through 15 low-redshift
foreground galaxy - background quasar pairs with impact parameters ranging from
1.7 kpc up to 86.7 kpc. We detected one absorber in the GBT survey from the
foreground dwarf galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc and
another possible absorber in our follow-up Very Large Array (VLA) imaging of
the nearby foreground galaxy, UGC 7408. Both of the absorbers are narrow (FWHM
of 3.6 and 4.8 km/s), have sub Damped Lyman alpha column densities, and most
likely originate in the disk gas of the foreground galaxies. We also detected H
I emission from three foreground galaxies, including UGC 7408. Although our
sample contains both blue and red galaxies, the two H I absorbers as well as
the H I emissions are associated with blue galaxies. We discuss the physical
conditions in the 21 cm absorbers and some drawbacks of the large GBT beam for
this type of survey.Comment: 15 pages, 6 figures, 3 table
Hubble Space Telescope Observations of Sub-Damped Lyman-alpha Absorbers at z < 0.5, and Implications for Galaxy Chemical Evolution
We report observations of four sub-damped Lyman-alpha (sub-DLA) quasar
absorbers at z<0.5 obtained with the Hubble Space Telescope Cosmic Origins
Spectrograph. We measure the available neutrals or ions of C, N, O, Si, P, S,
Ar, Mn, Fe, and/or Ni. Our data have doubled the sub-DLA metallicity samples at
z<0.5 and improved constraints on sub-DLA chemical evolution. All four of our
sub-DLAs are consistent with near-solar or super-solar metallicities and
relatively modest ionization corrections; observations of more lines and
detailed modeling will help to verify this. Combining our data with
measurements from the literature, we confirm previous suggestions that the
N(HI)-weighted mean metallicity of sub-DLAs exceeds that of DLAs at all
redshifts studied, even after making ionization corrections for sub-DLAs. The
absorber toward PHL 1598 shows significant dust depletion. The absorbers toward
PHL 1226 and PKS 0439-433 show the S/P ratio consistent with solar, i.e., they
lack a profound odd-even effect. The absorber toward Q0439-433 shows
super-solar Mn/Fe. For several sub-DLAs at z<0.5, [N/S] is below the level
expected for secondary N production, suggesting a delay in the release of the
secondary N or a tertiary N production mechanism. We constrain the electron
density using Si II* and C II* absorption. We also report different metallicity
vs. Delta V_90 relations for sub-DLAs and DLAs. For two sub-DLAs with
detections of emission lines from the underlying galaxies, our measurements of
the absorption-line metallicities are consistent with the emission-line
metallicities, suggesting that metallicity gradients are not significant in
these galaxies.Comment: 77 pages, 13 figures; accepted for publication in the Astrophysical
Journal. Submitted (in the original form) May 26, 2014; accepted Apr. 15,
201
Do Damped and Sub-damped Lyman-alpha Absorbers Arise in Galaxies of Different Masses?
We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLA
absorbers in quasar spectra differ intrinsically in metallicity, and whether
they could arise in galaxies of different masses. Using the recent measurements
of the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirm
that sub-DLAs have higher mean metallicities than DLAs, especially at . We find that the intercept of the metallicity-redshift relation
derived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. We
also show that, while there is a correlation between the metallicity and the
rest equivalent width of Mg II or Fe II for DLAs,
no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe II
selection criteria employed in the discovery of both types of systems at lower
redshifts, the difference between metallicities of DLAs and sub-DLAs appears to
be real and not an artefact of selection. This conclusion is supported by our
simulations of Mg II and Fe II lines for a wide
range of physical conditions. On examining the velocity spreads of the
absorbers, we find that sub-DLAs show somewhat higher mean and median velocity
spreads (), and an excess of systems with km
s, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend for
sub-DLAs appears to be steeper and closer to the trend for Galactic bulge and
thick disk stars, possibly suggesting different stellar populations. The
absorber data appear to be consistent with galaxy down-sizing. The data are
also consistent with the relative number densities of low-mass and high-mass
galaxies. It is thus plausible that sub-DLAs arise in more massive galaxies on
average than DLAs.Comment: 27 pages, 5 figures, 4 tables. Accepted for publication in New
Astronom
Evidence for Cold Accretion: Primitive Gas Flowing onto a Galaxy at z~0.274
We present UV and optical observations from the Cosmic Origins Spectrograph
on the Hubble Space Telescope and Keck of a z= 0.27395 Lyman limit system (LLS)
seen in absorption against the QSO PG1630+377. We detect H I absorption with
log N(HI)=17.06\pm0.05 as well as Mg II, C III, Si III, and O VI in this
system. The column densities are readily explained if this is a multi-phase
system, with the intermediate and low ions arising in a very low metallicity
([Mg/ H] =-1.71 \pm 0.06) photoionized gas. We identify via Keck spectroscopy
and Large Binocular Telescope imaging a 0.3 L_* star-forming galaxy projected
37 kpc from the QSO at nearly identical redshift (z=0.27406, \Delta v = -26
\kms) with near solar metallicity ([O/ H]=-0.20 \pm 0.15). The presence of very
low metallicity gas in the proximity of a near-solar metallicity, sub-L_*
galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A
search of the literature reveals that such low metallicity LLSs are not
uncommon. We found that 50% (4/8) of the well-studied z < 1 LLSs have
metallicities similar to the present system and show sub-L_* galaxies with rho
< 100 kpc in those fields where redshifts have been surveyed. We argue that the
properties of these primitive LLSs and their host galaxies are consistent with
those of cold mode accretion streams seen in galaxy simulations.Comment: Accepted for publication in the Astrophysical Journa
The Evolution of Damped Lyman-alpha Absorbers: Metallicities and Star Formation Rates
The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide
powerful probes of the evolution of metals, gas, and stars in galaxies. One
major obstacle in trying to understand the evolution of DLAs and sub-DLAs has
been the small number of metallicity measurements at z < 1.5, an epoch spanning
\~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope
and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5.
Combining our results with those at higher redshifts from the literature, we
find that the global mean metallicity of DLAs does not rise to the solar value
at low redshifts. These surprising results appear to contradict the near-solar
mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of
cosmic chemical evolution models based on the global star formation history.
Finally, we discuss direct constraints on the star formation rates (SFRs) in
the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and
other emission-line studies in the literature. A large fraction of the observed
heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially
below the global mean SFR, consistent with the low metallicities observed in
the spectroscopic studies.Comment: 6 pages,3 figures, To appear in "Probing Galaxies through Quasar
Absorption Lines", Proceedings IAU Colloquium 199, 2005, Eds. P. R. Williams,
C. Shu, and B. Menar
The COS Absorption Survey of Baryon Harbors (CASBaH): Warm-hot Circumgalactic Gas Reservoirs Traced by Ne VIII Absorption
We survey the highly ionized circumgalactic media (CGM) of 29 blindly
selected galaxies at 0.49 < z_(gal) < 1.44 based on high-S/N ultraviolet
spectra of z > 1 QSOs and the galaxy database from the COS Absorption Survey of
Baryon Harbors (CASBaH). We detect the Ne VIII doublet in nine of the galaxies,
and for gas with N(Ne VIII) > 10^13.3 cm^-2 (> 10^13.5 cm^-2), we derive a Ne
VIII covering fraction f_c = 75 +15/-25% (44 +22/-20%) within impact parameter
(rho) < 200 kpc of M_* = 10^(9.5-11.5) Msol galaxies and f_c = 70 +16/-22% (f_c
= 42 +20/-17%) within rho < 1.5 virial radii. We estimate the mass in Ne
VIII-traced gas to be M_gas(Ne VIII) > 10^9.5 Msol (Z/Zsol)^-1, or 6-20% of the
expected baryonic mass if the Ne VIII absorbers have solar metallicity.
Ionizing Ne VII to Ne VIII requires 207 eV, and photons with this energy are
scarce in the CGM. However, for the median halo mass and redshift of our
sample, the virial temperature is close to the peak temperature for the Ne VIII
ion, and the Ne VIII-bearing gas is plausibly collisionally ionized near this
temperature. Moreover, we find that photoionized Ne VIII requires cool and
low-density clouds that would be highly underpressured (by approximately two
orders of magnitude) relative to the putative, ambient virialized medium,
complicating scenarios where such clouds could survive. Thus, more complex
(e.g., non-equilibrium) models may be required; this first statistical sample
of Ne VIII absorber/galaxy systems will provide stringent constraints for
future CGM studies.Comment: Published in ApJL, Volume 877, Issue 2, Article L2
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