1,071 research outputs found
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
Mg II ABSORPTION AT 2 < z < 6 WITH MAGELLAN/FIRE. II. A LONGITUDINAL STUDY OF HI, METALS, AND IONIZATION IN GALACTIC HALOS
We present a detailed study of H I and metals for 110 Mg II absorption systems discovered at 1.98 12 Gyr). We observe a significant strengthening in the characteristic N(H I) for fixed Mg II equivalent width as one moves toward higher redshift. Indeed, at our sample's mean [bar over z] = 3.402, all Mg II systems are either damped Lyα absorbers (DLAs) or sub-DLAs, with 40.7% of systems exceeding the DLA threshold (compared to 16.7% at [bar over z] = 0.927). We set lower limits on the metallicity of the Mg II systems where we can measure H I; these results are consistent with the full DLA population. The classical Mg II systems (W[λ2796 over 0] = 0.3-1.0 A), which preferentially associate with sub-DLAs, are quite metal rich at ~0.1 solar. We applied quantitative classification metrics to our absorbers to compare with low-redshift populations, finding that weak systems are similar to classic Mg II absorbers at low redshift. The strong systems either have very large Mg II and Fe II velocity spreads implying non-virialized dynamics or are more quiescent DLAs. There is tentative evidence that the kinetically complex systems evolve in similar fashion to the global star formation rate. We speculate that if weaker Mg II systems represent accreting gas as suggested by recent studies of galaxy-absorber inclinations, then their high metal abundance suggests re-accretion of recently ejected material rather than first-time infall from the metal-poor intergalactic medium, even at early times.National Science Foundation (U.S.) (Grant AST-0908920)National Science Foundation (U.S.) (Grant AST-1109115)Massachusetts Institute of Technology. Undergraduate Research Opportunities Progra
Constraints on the Universal CIV Mass Density at z~6 from Early IR Spectra Obtained with the Magellan FIRE Spectrograph
We present a new determination of the intergalactic CIV mass density at 4.3 <
z < 6.3. Our constraints are derived from high signal-to-noise spectra of seven
quasars at z > 5.8 obtained with the newly commissioned FIRE spectrograph on
the Magellan Baade telescope, coupled with six observations of northern objects
taken from the literature. We confirm the presence of a downturn in the CIV
abundance at =5.66 by a factor of 4.1 relative to its value at =4.96, as
measured in the same sightlines. In the FIRE sample, a strong system previously
reported in the literature as CIV at z=5.82 is re-identified as MgII at z=2.78,
leading to a substantial downward revision in for these prior
studies. Additionally we confirm the presence of at least two systems with
low-ionization CII, SiII, and OI absorption but relatively weak signal from
CIV. The latter systems systems may be of interest if the downward trend in
at high redshift is driven in part by ionization effects.Comment: 14 pages, 6 figures, 2 tables. Submitted to Ap
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
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