3,282 research outputs found
Metallicities and dust content of proximate damped Lyman alpha systems in the Sloan Digital Sky Survey
Composite spectra of 85 proximate absorbers (log N(HI)>20 and velocity
difference between the absorption and emission redshift, dv<10,000 km/s) in the
SDSS are used to investigate the trends of metal line strengths with velocity
separation from the QSO. We construct composites in 3 velocity bins: dv<3000
km/s, 30006000 km/s, with further sub-samples to
investigate the metal line dependence on N(HI) and QSO luminosity. Low (e.g.
SiII and FeII) and high ionization (e.g. SiIV and CIV) species alike have
equivalent widths (EWs) that are larger by factors of 1.5 -- 3 in the dv<3000
km/s composite, compared to the dv>6000 km/s spectrum. The EWs show an even
stronger dependence on dv if only the highest neutral hydrogen column density
(log N(HI)>20.7) absorbers are considered. We conclude that PDLAs generally
have higher metallicities than intervening absorbers, with the enhancement
being a function of both dv and N(HI). It is also found that absorbers near
QSOs with lower rest-frame UV luminosities have significantly stronger metal
lines. We speculate that absorbers near to high luminosity QSOs may have had
their star formation prematurely quenched. Finally, we search for the signature
of dust reddening by the PDLAs, based on an analysis of the QSO continuum
slopes relative to a control sample and determine a limit of E(B-V)<0.014 for
an SMC extinction curve. This work provides an empirical motivation for
distinguishing between proximate and intervening DLAs, and establishes a
connection between the QSO environment and galaxy properties at high redshifts.Comment: Accepted for publication in MNRA
The Last Eight-Billion Years of Intergalactic SiIV Evolution
We identified 24 SiIV absorption systems with z <~ 1 from a blind survey of
49 low-redshift quasars with archival Hubble Space Telescope ultraviolet
spectra. We relied solely on the characteristic wavelength separation of the
doublet to automatically detect candidates. After visual inspection, we defined
a sample of 20 definite (group G = 1) and 4 "highly-likely" (G = 2) doublets
with rest equivalent widths W_r for both lines detected at > 3 sigma. The
absorber line density of the G = 1 doublets was dN_SiIV/dX = 1.4+0.4/-0.3 for
log N(Si+3) > 12.9. The best-fit power law to the G = 1 frequency distribution
of column densities f(N(Si+3)) had normalization k = (1.2+0.5/-0.4) x 10^-14
cm2 and slope alpha = -1.6+0.3/-0.3. Using the power-law model of f(N(Si+3)),
we measured the Si+3 mass density relative to the critical density: Omega(Si+3)
= (3.7+2.8/-1.7) x 10^-8 for 13 < log N(Si+3) < 15. From Monte Carlo sampling
of the distributions, we estimated our value to be a factor of 4.8+3.0/-1.9
higher than the 2 . From a simple linear fit to
Omega(Si+3) over the age of the Universe, we estimated a slow and steady
increase from z = 5.5 --> 0 with dOmega/dt_age = (0.61+/-0.23) x 10^-8 Gyr^-1.
We compared our ionic ratios N(Si+3)/N(C+3) to a 2 < z < 4.5 sample and
concluded, from survival analysis, that the two populations are similar, with
median = 0.16.Comment: 18 pages, 9 figures, 4 tables, added figures and new analysis,
results have changed, accepted to Ap
The survival of gas clouds in the Circumgalactic Medium of Milky Way-like galaxies
Observational evidence shows that low-redshift galaxies are surrounded by
extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM).
To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we
performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas
clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4
cm^-3) coronal medium, typical of Milky Way-like galaxies at large
galactocentric distances (~ 50-150 kpc). We explored the effects of different
initial values of relative velocity and radius of the clouds. Our simulations
were performed on a two-dimensional grid with constant mesh size (2 pc) and
they include radiative cooling, photoionization heating and thermal conduction.
We found that for large clouds (radii larger than 250 pc) the cool gas survives
for very long time (larger than 250 Myr): despite that they are partially
destroyed and fragmented into smaller cloudlets during their trajectory, the
total mass of cool gas decreases at very low rates. We found that thermal
conduction plays a significant role: its effect is to hinder formation of
hydrodynamical instabilities at the cloud-corona interface, keeping the cloud
compact and therefore more difficult to destroy. The distribution of column
densities extracted from our simulations are compatible with those observed for
low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI)
once we take into account that OVI covers much more extended regions than the
cool gas and, therefore, it is more likely to be detected along a generic line
of sight.Comment: 12 pages, 10 figures. Accepted for publication in MNRA
A High Signal-to-Noise Ratio Composite Spectrum of Gamma-ray Burst Afterglows
We present a composite spectrum of 60 long duration gamma-ray burst (GRB)
afterglows with redshifts in the range 0.35<z<6.7 observed with low resolution
optical spectra. The composite spectrum covers the wavelength range 700-6600 A
in the rest frame and has a mean signal-to-noise ratio of 150 per 1 A pixel and
reaches a maximum of ~300 in the range 2500-3500 A. Equivalent widths are
measured from metal absorption lines from the Lya line to ~5200 A, and
associated metal and hydrogen lines are identified between the Lyman break and
Lya line. The average transmission within the Lyman forest is consistent with
that found along quasar lines of sight. We find a temporal variation in fine
structure lines when dividing the sample into bursts observed within 2 hours
from their trigger and those observed later. Other lines in the predominantly
neutral gas show variations too, but this is most likely a random effect caused
by weighting of individual strong absorption lines and which mimics a temporal
variation. Bursts characterized with high or low prompt GRB energy release
produce afterglows with similar absorption line strengths, and likewise for
bursts with bright or faint optical afterglows. Bursts defined as dark from
their optical to X-ray spectral index have stronger absorption lines relative
to the optically bright bursts. The composite spectrum has strong CaII and MgII
absorption lines as commonly found in dusty galaxies, however, we find no
evidence for dust or a significant molecular content based on the non-detection
of diffuse interstellar bands. Compared to starburst galaxy spectra, the GRB
composite has much stronger fine structure lines, while metal absorption lines
are weaker.Comment: Accepted for publication in ApJ, 24 page
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
The nature of proximate damped Lyman alpha systems
We present high resolution echelle spectra of 7 proximate damped Lyman alpha
(PDLA) systems whose relative velocity separation from the background quasar is
Delta V < 3000 km/s. Combining our sample with a further 9 PDLAs from the
literature we compare the chemical properties of the proximate systems with a
control sample of intervening DLAs. Taken at face value, the sample of 16 PDLAs
exhibits a wide range of metallicities, ranging from Z ~ 1/3 Z_sun down to Z ~
1/1000 Z_sun, including the DLA with the lowest N(SiII)/N(HI) yet reported in
the literature. We find several pieces of evidence that indicate enhanced
ionization and the presence of a hard ionizing spectrum in PDLAs which lead to
properties that contrast with the intervening DLAs, particularly when the N(HI)
is low. The abundances of Zn, Si and S in PDLAs with log N(HI) > 21, where
ionization corrections are minimized, are systematically higher than the
intervening population by a factor of around 3. We also find possible evidence
for a higher fraction of NV absorbers amongst the PDLAs, although the
statistics are still modest. 6/7 of our echelle sample show high ionization
species (SiIV, CIV, OVI or NV) offset by >100 km/s from the main low ion
absorption. We analyse fine-structure transitions of CII* and SiII* to
constrain the PDLA distance from the QSO. Lower limits range from tens of kpc
up to >160 kpc for the most stringent limit. We conclude that (at least some)
PDLAs do exhibit different characteristics relative to the intervening
population out to 3000 km/s (and possibly beyond). Nonetheless, the PDLAs
appear distinct from lower column density associated systems and the inferred
QSO-absorber separations mean they are unlikely to be associated with the QSO
host. We speculate that the PDLAs preferentially sample more massive galaxies
in more highly clustered regions of the high redshift universe.Comment: Accepted for publication in MNRAS. Abstract abridged
Probing the IGM/Galaxy Connection IV: The LCO/WFCCD Galaxy Survey of 20 Fields Surrounding UV Bright Quasars
We publish the survey for galaxies in 20 fields containing ultraviolet bright
quasars (with z_em 0.1 to 0.5) that can be used to study the association
between galaxies and absorption systems from the low-z intergalactic medium
(IGM). The survey is magnitude limited (R~19.5 mag) and highly complete out to
10' from the quasar in each field. It was designed to detect dwarf galaxies (L
~ 0.1 L*) at an impact parameter rho 1Mpc (z=0.1) from a quasar. The complete
sample (all 20 fields) includes R-band photometry for 84718 sources and
confirmed redshifts for 2800 sources. This includes 1198 galaxies with 0.005 <
z < (z_em - 0.01) at a median redshift of 0.18, which may associated with IGM
absorption lines. All of the imaging was acquired with cameras on the Swope 40"
telescope and the spectra were obtained via slitmask observations using the
WFCCD spectrograph on the Dupont 100" telescope at Las Campanas Observatory
(LCO). This paper describes the data reduction, imaging analysis, photometry,
and spectral analysis of the survey. We tabulate the principal measurements for
all sources in each field and provide the spectroscopic dataset online.Comment: Accepted to the Astrophysical Journal Supplements; 20 pages, only 6
figures shown in this version. See
http://www.ucolick.org/~xavier/WFCCDOVI/index.html for a full-length
manuscript and other supportive materia
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