2,667 research outputs found
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
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
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
Using machine learning to classify the diffuse interstellar bands
Using over a million and a half extragalactic spectra we study the
correlations of the Diffuse Interstellar Bands (DIBs) in the Milky Way. We
measure the correlation between DIB strength and dust extinction for 142 DIBs
using 24 stacked spectra in the reddening range E(B-V) < 0.2, many more lines
than ever studied before. Most of the DIBs do not correlate with dust
extinction. However, we find 10 weak and barely studied DIBs with correlations
that are higher than 0.7 with dust extinction and confirm the high correlation
of additional 5 strong DIBs. Furthermore, we find a pair of DIBs, 5925.9A and
5927.5A which exhibits significant negative correlation with dust extinction,
indicating that their carrier may be depleted on dust. We use Machine Learning
algorithms to divide the DIBs to spectroscopic families based on 250 stacked
spectra. By removing the dust dependency we study how DIBs follow their local
environment. We thus obtain 6 groups of weak DIBs, 4 of which are tightly
associated with C2 or CN absorption lines.Comment: minor changes, MNRAS accepte
Keck Echellette Spectrograph and Imager Observations of Metal-poor Damped Lyα Systems
We present the first results from a survey of SDSS quasars selected for strong H I damped Lyα (DLA) absorption with corresponding low equivalent width absorption from strong low-ion transitions (e.g., C II λ1334 and Si II λ1260). These metal-poor DLA candidates were selected from the SDSS fifth release quasar spectroscopic database, and comprise a large new sample for probing low-metallicity galaxies. Medium-resolution echellette spectra from the Keck Echellette Spectrograph and Imager spectrograph for an initial sample of 35 systems were obtained to explore the metal-poor tail of the DLA distribution and to investigate the nucleosynthetic patterns at these metallicities. We have estimated saturation corrections for the moderately underresolved spectra, and systems with very narrow Doppler parameters (b ≤ 5 km s^(–1)) will likely have underestimated abundances. For those systems with Doppler parameters b > 5 km s^(–1), we have measured low-metallicity DLA gas with [X/H] < –2.4 for at least one of C, O, Si, or Fe. Assuming non-saturated components, we estimate that several DLA systems have [X/H] < –2.8, including five DLA systems with both low equivalent widths and low metallicity in transitions of both C II and O I. All of the measured DLA metallicities, however, exceed or are consistent with a metallicity of at least 1/1000 of solar, regardless of the effects of saturation in our spectra. Our results indicate that the metal-poor tail of galaxies at z ~ 3 drops exponentially at [X/H] ≾ –3. If the distribution of metallicity is Gaussian, the probability of identifying interstellar medium gas with lower abundance is extremely small, and our results suggest that DLA systems with [X/H] < –4.0 are extremely rare, and could comprise only 8 × 10^(–7) of DLA systems. The relative abundances of species within these low-metallicity DLA systems are compared with stellar nucleosynthesis models, and are consistent with stars having masses of 30 M_⊙ < M * < 100 M_⊙. The observed ratio of [C/O] for values of [O/H] < –2.5 exceeds values seen in moderate metallicity DLA systems, and also exceeds theoretical nucleosynthesis predictions for higher mass Population III stars. We also have observed a correlation between the column density N(C IV) with [Si/H] metallicity, suggestive of a trend between mass of the DLA system and its metallicity
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
Characterizing the Low-Redshift Intergalactic Medium towards PKS1302-102
We present a detailed analysis of the intergalactic metal-line absorption
systems in the archival HST/STIS and FUSE ultraviolet spectra of the
low-redshift quasar PKS1302-102 (z_QSO = 0.2784). We supplement the archive
data with CLOUDY ionization models and a survey of galaxies in the quasar
field. There are 15 strong Lya absorbers with column densities logN_HI > 14. Of
these, six are associated with at least CIII 977 absorption (logN(C^++) > 13);
this implies a redshift density dN_CIII/dz = 36+13/-9 (68% confidence limits)
for the five detections with rest equivalent width W_r > 50 mA. Two systems
show OVI 1031,1037 absorption in addition to CIII (logN(O^+5) > 14). One is a
partial Lyman limit system (logN_HI = 17) with associated CIII, OVI, and SiIII
1206 absorption. There are three tentative OVI systems that do not have CIII
detected. For one OVI doublet with both lines detected at 3 sigma with W_r > 50
mA, dN_OVI/dz = 7+9/-4. We also search for OVI doublets without Lya absorption
but identify none. From CLOUDY modeling, these metal-line systems have
metallicities spanning the range -4 < [M/H] < -0.3. The two OVI systems with
associated CIII absorption cannot be single-phase, collisionally-ionized media
based on the relative abundances of the metals and kinematic arguments. From
the galaxy survey, we discover that the absorption systems are in a diverse set
of galactic environments. Each metal-line system has at least one galaxy within
500 km/s and 600 h^-1 kpc with L > 0.1 L_*.Comment: 21 pages in emulatepj form, 24 figures, 10 tables, accepted to Ap
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