47 research outputs found
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 Incidence of Low-Metallicity Lyman-Limit Systems at z~3.5: Implications for the Cold-Flow Hypothesis of Baryonic Accretion
Cold accretion is a primary growth mechanism of simulated galaxies, yet
observational evidence of "cold flows" at redshifts where they should be most
efficient (-4) is scarce. In simulations, cold streams manifest as
Lyman-limit absorption systems (LLSs) with low heavy-element abundances similar
to those of the diffuse IGM. Here we report on an abundance survey of 17 H
I-selected LLSs at -4.4 which exhibit no metal absorption in SDSS
spectra. Using medium-resolution spectra obtained at Magellan, we derive
ionization-corrected metallicities (or limits) with a Markov-Chain Monte Carlo
sampling that accounts for the large uncertainty in measurements
typical of LLSs. The metal-poor LLS sample overlaps with the IGM in metallicity
and is best described by a model where are drawn from the
IGM chemical abundance distribution. These represent roughly half of all LLSs
at these redshifts, suggesting that 28-40 of the general LLS population at
could trace unprocessed gas. An ancillary sample of ten LLSs without
any a priori metal-line selection is best fit with of
metallicities drawn from the IGM. We compare these results with regions of a
moving-mesh simulation; the simulation finds only half as many baryons in
IGM-metallicity LLSs, and most of these lie beyond the virial radius of the
nearest galaxy halo. A statistically significant fraction of all LLSs have low
metallicity and therefore represent candidates for accreting gas; large-volume
simulations can establish what fraction of these candidates actually lie near
galaxies and the observational prospects for detecting the presumed hosts in
emission.Comment: 19 pages, 17 figures; Submitted to ApJ; Corrected figure 16
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
Machine Learning Uncovers the Universe's Hidden Gems: A Comprehensive Catalogue of CIV Absorption Lines in SDSS DR12
We assemble the largest CIV absorption line catalogue to date, leveraging
machine learning, specifically Gaussian processes, to remove the need for
visual inspection for detecting CIV absorbers. The catalogue contains
probabilities classifying the reliability of the absorption system within a
quasar spectrum. Our training set was a sub-sample of DR7 spectra that had no
detectable CIV absorption in a large visually inspected catalogue. We used
Bayesian model selection to decide between our continuum model and our
absorption-line models. Using a random hold-out sample of 1301 spectra from all
of the 26,030 investigated spectra in DR7 CIV catalogue, we validated our
pipeline and obtained an 87% classification performance score. We found good
purity and completeness values, both ~80%, when a probability of ~95% is used
as the threshold. Our pipeline obtained similar CIV redshifts and rest
equivalent widths to our training set. Applying our algorithm to 185,425
selected quasar spectra from SDSS DR12, we produce a catalogue of 113,775 CIV
doublets with at least 95% confidence. Our catalogue provides maximum a
posteriori values and credible intervals for CIV redshift, column density, and
Doppler velocity dispersion. We detect CIV absorption systems with a redshift
range of 1.37 5.1, including 33 systems with a redshift larger than 5
and 549 absorbers systems with a rest equivalent width greater than 2 A at more
than 95% confidence. Our catalogue can be used to investigate the physical
properties of the circumgalactic and intergalactic media.Comment: 18 pages, 25 figures, 3 table
PREDOMINANTLY LOW METALLICITIES MEASURED IN A STRATIFIED SAMPLE OF LYMAN LIMIT SYSTEMS AT Z = 3.7
We measured metallicities for 33 z = 3.4–4.2 absorption line systems drawn from a sample of H i-selected-Lyman limit systems (LLSs) identified in Sloan Digital Sky Survey (SDSS) quasar spectra and stratified based on metal line features. We obtained higher-resolution spectra with the Keck Echellette Spectrograph and Imager, selecting targets according to our stratification scheme in an effort to fully sample the LLS population metallicity distribution. We established a plausible range of H i column densities and measured column densities (or limits) for ions of carbon, silicon, and aluminum, finding ionization-corrected metallicities or upper limits. Interestingly, our ionization models were better constrained with enhanced α-to-aluminum abundances, with a median abundance ratio of [α/Al] = 0.3. Measured metallicities were generally low, ranging from [M/H] = −3 to −1.68, with even lower metallicities likely for some systems with upper limits. Using survival statistics to incorporate limits, we constructed the cumulative distribution function (CDF) for LLS metallicities. Recent models of galaxy evolution propose that galaxies replenish their gas from the low-metallicity intergalactic medium (IGM) via high-density H i "flows" and eject enriched interstellar gas via outflows. Thus, there has been some expectation that LLSs at the peak of cosmic star formation (z ≈ 3) might have a bimodal metallicity distribution. We modeled our CDF as a mix of two Gaussian distributions, one reflecting the metallicity of the IGM and the other representative of the interstellar medium of star-forming galaxies. This bimodal distribution yielded a poor fit. A single Gaussian distribution better represented the sample with a low mean metallicity of [M/H] ≈ −2.5.Massachusetts Institute of Technology. Undergraduate Research Opportunities ProgramNational Science Foundation (U.S.) (Award AST-1109915
The Last Eight-Billion Years of Intergalactic CIV Evolution
We surveyed the HST UV spectra of 49 low-redshift quasars for z < 1 CIV
candidates, relying solely on the characteristic wavelength separation of the
doublet. After consideration of the defining traits of CIV doublets (e.g.,
consistent line profiles, other associated transitions, etc.), we defined a
sample of 38 definite (group G = 1) and five likely (G = 2) doublets with rest
equivalent widths W_r for both lines detected at >= 3 sigma. We conducted
Monte-Carlo completeness tests to measure the unblocked redshift (dz) and
co-moving pathlength (dX) over which we were sensitive to CIV doublets of a
range of equivalent widths and column densities. The absorber line density of
(G = 1+2) doublets is dN/dX = 4.1+0.7/-0.6 for log N(C^+3) >= 13.2, and dN/dX
has not evolved significantly since z = 5. The best-fit power-law to the G = 1
frequency distribution of column densities f(N(C^+3)) = k(N(C^+3)/N_0)^alpha_N
has coefficient k = (0.67+0.18/-0.16) x 10^-14 cm^2 and exponent alpha_N =
-1.50+0.17/-0.19, where N_0 = 10^14 cm^-2. Using the power-law model of
f(N(C^+3)), we measured the C^+3 mass density relative to the critical density:
Omega(C^+3) = (6.20+1.82/-1.52) x 10^-8 for 13 <= log N(C^+3) <= 15. This value
is a 2.8+/-0.7 increase in Omega(C^+3) compared to the error-weighted mean from
several 1 < z < 5 surveys for CIV absorbers. A simple linear regression to
Omega(C^+3) over the age of the Universe indicates that Omega(C^+3) has slowly
but steadily increased from z = 5 --> 0, with dOmega(C^+3)/dt_age =
(0.42+/-0.2) x 10^-8 Gyr^-1.Comment: 61 pages; accepted by ApJ; includes changes based on referee repor