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

A design approach for integrated CMOS LC-tank oscillators using bifurcation analysis

Electrical oscillators play a decisive role in integrated transceivers for wired and wireless communication systems. In this context the study of fully integrated differential VCOs has received attention. In this paper formulas for investigations of the stability as well as the amplitude of CMOS LC tank oscillators are derived, where an overall model of nonlinear gain elements is used. By means of these results we are able to present an improved design approach which gives a deeper insight into the functionality of LC tank VCOs

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

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

Resolving The ISM Surrounding GRBs with Afterglow Spectroscopy

We review current research related to spectroscopy of gamma-ray burst (GRB) afterglows with particular emphasis on the interstellar medium (ISM) of the galaxies hosting these high redshift events. These studies reveal the physical conditions of star-forming galaxies and yield clues to the nature of the GRB progenitor. We offer a pedagogical review of the experimental design and review current results. The majority of sightlines are characterized by large HI column densities, negligible molecular fraction, the ubiquitous detection of UV pumped fine-structure transitions, and metallicities ranging from 1/100 to nearly solar abundance.Comment: Conference procedings for Gamma Ray Bursts 2007 November 5-9, 2007 Santa Fe, New Mexico (8 pages, 4 figures

The Molecular Hydrogen Deficit in Gamma-Ray Burst Afterglows

Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveal the absence of molecular hydrogen absorption lines, a surprising result in light of their large neutral hydrogen column densities and the detection of H$_2$ in similar, more local star-forming regions like 30 Doradus in the Large Magellanic Cloud (LMC). Observational evidence further indicates that the bulk of the neutral hydrogen column in these sight lines lies 100 pc beyond the progenitor and that H$_2$ was absent prior to the burst, suggesting that direct flux from the star, FUV background fields, or both suppressed its formation. We present one-dimensional radiation hydrodynamical models of GRB host galaxy environments, including self-consistent radiative transfer of both ionizing and Lyman-Werner photons, nine-species primordial chemistry with dust formation of H$_2$, and dust extinction of UV photons. We find that a single GRB progenitor is sufficient to ionize neutral hydrogen to distances of 50 - 100 pc but that a galactic Lyman-Werner background is required to dissociate the molecular hydrogen in the ambient ISM. Intensities of 0.1 - 100 times the Galactic mean are necessary to destroy H$_2$ in the cloud, depending on its density and metallicity. The minimum radii at which neutral hydrogen will be found in afterglow spectra is insensitive to the mass of the progenitor or the initial mass function (IMF) of its cluster, if present.Comment: 12 pages, 7 figures, accepted for Ap

Probing the IGM/Galaxy Connection V: On the Origin of Lya and OVI Absorption at z<0.2

We analyze the association of galaxies to Lya and OVI absorption, the most commonly detected transitions in the low-z intergalactic medium (IGM), in the fields of 14 quasars with z_em = 0.06-0.57. Confirming previous studies, we observe a high covering fraction for Lya absorption to impact parameter rho = 300kpc: 33/37 of our L>0.01L* galaxies show Lya equivalent width W_Lya>50mA. Galaxies of all luminosity L>0.01L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ~10^(10.5 +/- 0.3) Msun for a constant N_H. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lya absorbers (W_Lya > 300mA) but cannot reproduce the majority of observed lines in the Lya forest. We conclude that the majority of Lya absorption with W_Lya=30-300mA occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ~400kpc. Regarding OVI, we observe a near unity covering fraction to rho=200kpc for L>0.1L* galaxies and to rho = 300kpc for sub-L* (0.1 L*<L<L*) galaxies. Similar to our Lya results, stronger OVI systems (W_OVI > 70mA) arise in the virialized halos of L>0.1L* galaxies. Unlike Lya, the weaker OVI systems (W_OVI~30mA) arise in the extended CGM of sub-L* galaxies. The majority of OVI gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L~0.3L*, and rarely originates in the so-called warm-hot IGM (WHIM) predicted by cosmological simulations.Comment: Submitted to the Astrophysical Journal; 26 pages, 9 figures. See http://www.ucolick.org/~xavier/WFCCDOVI/index.html for mor