34 research outputs found
Bimodality in Damped Lyman alpha Systems
We report evidence for a bimodality in damped Ly systems (DLAs). Using [C II]
158 mu cooling rates, lc, we find a distribution with peaks at lc=10^-27.4 and
10^-26.6 ergs s^-1 H^-1 separated by a trough at lc^crit ~= lc < 10^-27.0 ergs
s^-1 H^-1. We divide the sample into low cool DLAs with lc < lc^crit and high
cool DLAs with lc > lc^crit and find the Kolmogorv-Smirnov probabilities that
velocity width, metallicity, dust-to-gas ratio, and Si II equivalent width in
the two subsamples are drawn from the same parent population are small. All
these quantities are significantly larger in the high cool population, while
the H I column densities are indistinguishable in the two populations. We find
that heating by X-ray and FUV background radiation is insufficient to balance
the cooling rates of either population. Rather, the DLA gas is heated by local
radiation fields. The rare appearance of faint, extended objects in the Hubble
Ultra Deep Field rules out in situ star formation as the dominant
star-formation mode for the high cool population, but is compatible with in
situ star formation as the dominant mode for the low cool population. Star
formation in the high cool DLAs likely arises in Lyman Break galaxies. We
investigate whether these properties of DLAs are analogous to the bimodal
properties of nearby galaxies. Using Si II equivalent width as a mass
indicator, we construct bivariate distributions of metallicity, lc, and areal
SFR versus the mass indicators. Tentative evidence is found for correlations
and parallel sequences, which suggest similarities between DLAs and nearby
galaxies. We suggest that the transition-mass model provides a plausible
scenario for the bimodality we have found. As a result, the bimodality in
current galaxies may have originated in DLAs.Comment: Accepted for publication in the Astrophysical Journal 18 pages 14
figure
A carbon-enhanced metal-poor damped Lyα system: probing gas from Population III nucleosynthesis?
We present high-resolution observations of an extremely metal-poor damped Lyα system (DLA), at z_(abs) = 2.340 0972 in the spectrum of the QSO J0035−0918, exhibiting an abundance pattern consistent with model predictions for the supernova yields of Population III stars. Specifically, this DLA has [Fe/H]≃−3, shows a clear ‘odd–even’ effect, and is C-rich with [C/Fe]=+1.53, a factor of ∼20 greater than reported in any other DLA. In analogy to the carbon-enhanced metal-poor stars in the Galactic halo (with [C/Fe] > +1.0), this is the first known case of a carbon-enhanced DLA. We determine an upper limit to the mass of ^(12)C, M(^(12)C) ≤ 200 M_⊙, which depends on the unknown gas density n(H); if n(H) > 1 cm^(−3) (which is quite likely for this DLA given its low velocity dispersion), then M(^(12)C) ≤ 2 M_⊙, consistent with pollution by only a few prior supernovae. We speculate that DLAs such as the one discovered here may represent the ‘missing link’ between the yields of Population III stars and their later incorporation in the class of carbon-enhanced metal-poor stars which show no enhancement of neutron-capture elements (CEMP-no stars)
Understanding Physical Conditions in High Redshift Galaxies through C I Fine Structure Lines: Data and Methodology
We probe the physical conditions in high redshift galaxies, specifically, the
Damped Lyman-alpha Systems (DLAs) using neutral carbon (CI) fine structure
lines and molecular hydrogen (H2). We report five new detections of CI and
analyze the CI in an additional 2 DLAs with previously published data. We also
present one new detection of H2 in a DLA. We present a new method of analysis
that simultaneously constrains \emph{both} the volume density and the
temperature of the gas, as opposed to previous studies that a priori assumed a
gas temperature. We use only the column density of CI measured in the fine
structure states and the assumption of ionization equilibrium in order to
constrain the physical conditions in the gas. We present a sample of 11 CI
velocity components in 6 DLAs and compare their properties to those derived by
the global CII* technique. The resulting median values for this sample are:
= 69 cm^{-3}, = 50 K, and = 3.86 cm^{-3} K, with
standard deviations, sigma_{n(HI)} = 134 cm^{-3}, sigma_T = 52 K, and
sigma_{log(P/k)} = 3.68 cm^{-3} K. This can be compared with the integrated
median values for the same DLAs : = 2.8 cm^{-3}, = 139 K, and
= 2.57 cm^{-3} K, with standard deviations sigma_{n(HI)} = 3.0
cm^{-3}, sigma_T = 43 K, and sigma_{log(P/k)} = 0.22 cm^{-3} K. Interestingly,
the pressures measured in these high redshift CI clouds are similar to those
found in the Milky Way. We conclude that the CI gas is tracing a
higher-density, higher-pressure region, possibly indicative of post-shock gas
or a photodissociation region on the edge of a molecular cloud. We speculate
that these clouds may be direct probes of the precursor sites of star formation
in normal galaxies at high redshift.Comment: Accepted for publication in Ap
The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System
As large optical quasar surveys for damped Lya become a reality and the study
of star forming gas in the early Universe achieves statistical robustness, it
is now vital to identify and quantify the sources of systematic error. Because
the nature of optically-selected quasar surveys makes them vulnerable to dust
obscuration, we have undertaken a radio-selected quasar survey for damped Lya
systems to address this bias. We present the definition and results of this
survey. We then combine our sample with the CORALS dataset to investigate the
HI column density distribution function f(N) of damped Lya systems toward
radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1
N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in
excellent agreement with that of optically-selected samples at low N(HI), an
important yet expected result given that obscuration should have negligible
effect at these gas columns. However, because of the relatively small size of
the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not
well constrained at large N(HI) and the first moment of the HI distribution
function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep
enough, however, that extrapolating it to higher column densities implies only
a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g
= 1.15 x 10^-3, agrees well with the results of optically-selected surveys.
While our results indicate that dust obscuration is likely not a major issue
for surveys of damped Lya systems, we estimate that a radio-selected sample of
approximately 100 damped Lya systems will be required to obtain the precision
necessary to absolutely confirm an absence of dust bias.Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 200
Spectral Polarization of the Redshifted 21 cm Absorption Line Toward 3C 286
A re-analysis of the Stokes-parameter spectra obtained of the z=0.692 21 cm
absorption line toward 3C 286 shows that our original claimed detection of
Zeeman splitting by a line-of-sight magnetic field, B_los = 87 microgauss is
incorrect. Because of an insidious software error, what we reported as Stokes V
is actually Stokes U: the revised Stokes V spectrum indicates a 3-sigma upper
limit of B_los < 17 microgauss. The correct analysis reveals an absorption
feature in fractional polarization that is offset in velocity from the Stokes I
spectrum by -1.9 km/s. The polarization position-angle spectrum shows a dip
that is also significantly offset from the Stokes I feature, but at a velocity
that differs slightly from the absorption feature in fractional polarization.
We model the absorption feature with 3 velocity components against the core-jet
structure of 3C 286. Our chisquare minimization fitting results in components
with differing (1) ratios of H I column density to spin temperature, (2)
velocity centroids, and (3) velocity dispersions. The change in polarization
position angle with frequency implies incomplete coverage of the background jet
source by the absorber. It also implies a spatial variation of the polarization
position angle across the jet source, which is observed at frequencies higher
than the 839.4 MHz absorption frequency. The multi-component structure of the
gas is best understood in terms of components with spatial scales of ~100 pc
comprised of hundreds of low-temperature (T < 200 K) clouds with linear
dimensions of about 1 pc.Comment: Accepted for Publication by the Astrophysical Journa
Constraining the Size of the Circumgalactic Medium Using the Transverse Autocorrelation Function of C IV Absorbers in Paired Quasar Spectra
The circumgalactic medium (CGM) plays a vital role in the formation and
evolution of galaxies, acting as a lifeline between galaxies and the
surrounding intergalactic medium (IGM). In this study we leverage a unique
sample of quasar pairs to investigate the properties of the CGM with absorption
line tomography. We present a new sample of medium resolution Keck/ESI,
Magellan/MagE, and VLT/XSHOOTER spectra of 29 quasar pairs at redshift . We supplement the sample with additional spectra of 32 pairs from the
literature, creating a catalog of 61 quasar pairs with angular separations
between 1.7" and 132.9" and projected physical separations () between
14 kpc and 887 kpc. We construct a catalog of 906 metal-line absorption
doublets of C IV () with equivalent widths ranging
from 6 m{\AA} m{\AA}. The best fit linear model to
the log-space equivalent width frequency distribution () of the sample yields coefficients of and
. To constrain the projected extent of C IV, we calculate the
transverse autocorrelation function. The flattening of the autocorrelation
function at low provides a lower limit for the coherence length of
the metal enriched CGM - on the order of 200 comoving kpc. This
physical size constraint allows us to refine our understanding of the metals in
the CGM, where the extent of C IV in the CGM depends on gas flows, feedback,
timescale of metal injection and mixing, and the mass of the host galaxies.Comment: 19 pages, 8 figures, 2 tables, Accepted for publication by The
Astronomical Journa