The evolution of neutral hydrogen over the past 11 Gyr via H i 21 cm absorption
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We present the results of a blind search for intervening HI 21 cm absorption toward 260 radio sources in the redshift range 0Keywords:
Hydrogen line
Lyman limit
Line-of-sight
Large inhomogeneities in neutral hydrogen in the universe can be detected at redshifts $z \leq 10$ using the redshifted 21cm line emission from atomic hydrogen. This paper reviews the expected evolution of neutral hydrogen and presents estimates for future surveys of HI at $z \approx 3$. We also discuss the possibility of detecting neutral hydrogen at higher redshifts.
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The Hubble Deep Field South (HDFS) has been recently selected and the observations are planned for October 1998. We present a high resolution (FWHM $\simeq 14$ \kms) spectrum of the quasar J2233--606 ($z_{em}\simeq2.22$) which is located 5.1 arcmin East of the HDFS. The spectrum obtained with the New Technology Telescope redward of the Lyman--$α$ emission line covers the spectral range 4386--8270 Å. This range corresponds to redshift intervals for CIV and MgII intervening systems of $z=1.83-2.25$ and $z=0.57-1.95$ respectively. The data reveal the presence of two complex intervening CIV systems at redshift $z=1.869$ and $z=1.943$ and two complex associated ($z_{abs} \approx z_{em}$) systems. Other two CIV systems at $z=1.7865$ and $z=2.077$, suggested by the presence of strong Lyman--$α$ lines in low resolution ground based and Hubble Space Telescope (HST) STIS observations (Sealey et al. 1998) have been identified. The system at $z=1.943$ is also responsible for the Lyman limit absorption seen in the HST/STIS spectrum. The main goal of the present work is to provide astronomers interested in the Hubble Deep Field South program with information related to absorbing structures at high redshift, which are distributed along the nearby QSO line of sight. For this purpose, the reduced spectrum, obtained from three hours of integration time, has been released to the astronomical community.
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The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m(R)~19, resulted in the discovery of thirty-one quasars with z > 4. High signal-to-noise optical spectrophotometry at 5A resolution has been obtained for the twenty-eight quasars easily accessible from the northern hemisphere. These spectra have been surveyed to create new samples of high redshift Lyman-limit systems, damped Lyman-alpha absorbers, and metal absorption systems (e.g. CIV and MgII). In this paper we present the spectra, together with line lists of the detected absorption systems. The QSOs display a wide variety of emission and absorption line characteristics, with 5 exhibiting broad absorption lines and one with extremely strong emission lines (BR2248-1242). Eleven candidate damped Ly-alpha absorption systems have been identified covering the redshift range 2.83.5). An analysis of the measured redshifts of the high ionization emission lines with the low ionization lines shows them to be blueshifted by 430+/-60 km/s. In a previous paper (Storrie-Lombardi et. al. 1994) we discussed the redshift evolution of the Lyman limit systems catalogued here. In subsequent papers we will discuss the properties of the Ly-alpha forest absorbers and the redshift and column density evolution of the damped Ly-alpha absorbers.
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We present a measurement of the mean intensity of the hydrogen-ionizing background radiation field at low redshift using 906 Ly-alpha absorption lines in 151 quasar spectra from the archives of the Faint Object Spectrograph on the Hubble Space Telescope. Using a maximum likelihood technique and the best estimates possible for each QSO's Lyman limit flux and systemic redshift, we find J(ν_{0})= 7.6^{+9.4}_{-3.0} x 10^{-23} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1} at 0.03 < z < 1.67. This is in good agreement with the mean intensity expected from models of the background which incorporate only the known quasar population. When the sample is divided into two subsamples, consisting of lines with z < 1 and z > 1, the values of J(ν_{0}) found are 6.5^{+38.}_{-1.6} x 10^{-23} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, and 1.0^{+3.8}_{-0.2} x 10^{-22} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, respectively, indicating that the mean intensity of the background is evolving over the redshift range of this data set. Relaxing the assumption that the spectral shapes of the sample spectra and the background are identical, the best fit HI photoionization rates are found to be 6.7 x 10^{-13} s^{-1} for all redshifts, and 1.9 x 10^{-13} s^{-1} and 1.3 x 10^{-12} s^{-1} for z < 1 and z > 1, respectively.
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We have obtained high signal:to:noise optical spectroscopy at 5\AA\ resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-$\alpha$ absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with $\log$N(HI)$\ge17.5$, over the redshift range 0.01 $<$ z $<$ 5. The main results are: \begin{itemize} \item Lyman limit systems: The data are well fit by a power law $N(z) = N_0(1 + z)^{\gamma}$ for the number density per unit redshift. For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an $\Omega = 1$ Universe. We find $\gamma = 1.55$ ($\gamma = 0.5$ for no evolution) and $N_0 = 0.27$ with $>$99.7\% confidence limits for $\gamma$ of 0.82 \& 2.37. \item Damped \lya systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for $z>3$. Eleven candidate and three confirmed damped Ly$\alpha$ absorption systems, have been identified in the APM QSO spectra covering the redshift range $2.8\le z \le 4.4$ (11 with $z>3.5$). Combining the APM survey confirmed and candidate damped \lya absorbers with previous surveys, we find evidence for a turnover at z$\sim$3 or a flattening at z$\sim$2 in the cosmological mass density of neutral gas, $\Omega_g$. \end{itemize} The Lyman limit survey results are published in Storrie-Lombardi, et~al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.
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We have measured the redshift evolution of the density of Lyman limit systems (LLSs) in the intergalactic medium over the redshift range 0 < z < 6. We have used two new quasar samples to (1) improve coverage at z ∼ 1, with GALEX grism spectrograph observations of 50 quasars with 0.8 < zem < 1.3, and (2) extend coverage to z ∼ 6, with Keck ESI spectra of 25 quasars with 4.17 < zem < 5.99. Using these samples together with published data, we find that the number density of LLS per unit redshift, n(z), can be well fit by a simple evolution of the form n(z) = n3.5[(1 + z)/4.5]γ with n3.5 = 2.80 ± 0.33 and γ = 1.94+0.36−0.32 for the entire range 0 < z < 6. We have also reanalyzed the evolution of damped Lyα systems (DLAs) in the redshift range 4 < z < 5 using our high-redshift quasar sample. We find a total of 17 DLAs and sub-DLAs, which we have analyzed in combination with published data. The DLAs with show the same redshift evolution as the LLS. When combined with previous results, our DLA sample is also consistent with a constant ΩDLA = 9 × 10−4 from z = 2 to z = 5. We have used the LLS number density evolution to compute the evolution in the mean free path (mfp) of ionizing photons. We find a smooth evolution to z ∼ 6, very similar in shape to that of Madau et al. but about a factor of two higher. Recent theoretical models roughly match to the z < 6 data but diverge from the measured power law at z>6 in different ways, cautioning against extrapolating the fit to the mfp outside the measured redshift range.
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We present a measurement of the mean intensity of the hydrogen-ionizing background radiation field at low redshift using 906 Ly-alpha absorption lines in 151 quasar spectra from the archives of the Faint Object Spectrograph on the Hubble Space Telescope. Using a maximum likelihood technique and the best estimates possible for each QSO's Lyman limit flux and systemic redshift, we find J(\nu_{0})= 7.6^{+9.4}_{-3.0} x 10^{-23} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1} at 0.03 1, the values of J(\nu_{0}) found are 6.5^{+38.}_{-1.6} x 10^{-23} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, and 1.0^{+3.8}_{-0.2} x 10^{-22} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, respectively, indicating that the mean intensity of the background is evolving over the redshift range of this data set. Relaxing the assumption that the spectral shapes of the sample spectra and the background are identical, the best fit HI photoionization rates are found to be 6.7 x 10^{-13} s^{-1} for all redshifts, and 1.9 x 10^{-13} s^{-1} and 1.3 x 10^{-12} s^{-1} for z 1, respectively.
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We present a survey for optically thick Lyman limit absorbers at z < 2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs), increasing the number of cataloged LLSs at z < 2.6 by a factor of ∼10. We compile a statistical sample of 50 τLLS ⩾ 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z) = dn/dz, at these redshifts is well described by a single power law, l(z)∝(1 + z)γ, with γ = 1.33 ± 0.61 at z < 2.6, or with γ = 1.83 ± 0.21 over the redshift range 0.2 ⩽ z ⩽ 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ∼1.5 over the ∼0.6 Gyr from z = 4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ∼8 Gyr from z = 2.6 to 0.25. We show that the column density distribution function, f(NH i), at low redshift is not well fitted by a single power-law index (f(NH i) ∝ N−βH i) over the column density range 13 ⩽ log NH i ⩽ 22 or log NH i ⩾ 17.2. While low- and high-redshift f(NH i) distributions are consistent for log NH i>19.0, there is some evidence that f(NH i) evolves with z for log NH i ≲ 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ∼9 from z ∼ 5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L* galaxies.
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view Abstract Citations (73) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of High-Redshift Lyman-Limit Absorption Systems Lanzetta, Kenneth M. Abstract The results of a new spectroscopic survey for high-redshift Lyman-limit absorption systems are presented. These results are based on recent observations of high-redshift QSOs by Lanzetta and coworkers, from which a total of 35 Lyman-limit systems from the spectra of 52 QSOs are identified. These new data are combined with existing observations of low- and high-redshift Lyman-limit systems in order to determine the properties of the absorbers over the redshift range 0.36 <= z <= 4.11. The most striking result to emerge from this new study is that on the basis of this data set the rate of incidence of the Lyman-limit systems with z >~ 2.5 is found to evolve strongly with redshift in the sense that the product of the number density per unit comoving volume and the absorption cross section increased with increasing redshift. Over the redshift range 2.5 <~Z <~ 3.7 the observed redshift distribution is well fitted by n(z) = n_0_(1+z)^y^ with Y = 5.7 +/- 1.9. (In contrast, the rate of incidence of the Lyman-limit systems is virtually constant over the redshift range 0.35 <~Z <~2.5.) If the observed evolution indicates intrinsic evolution of the absorbers, this result suggests that the evolution detected previously for the C IV-selected absorbers over a similar redshift range is most naturally interpreted as evolution of the ionization level of the absorbers rather than as a chemical enrichment effect, and model photoionisation calculations indicate that the observed evolution of the absorption systems may be explained if the ionization parameters of the absorbers increases with decreasing redshift by a factor of ~3 within the redshift interval 2.5 <~z <~ 3.7. This might arise as the result of a systematic evolution of the intensity of the diffuse background ultraviolet radiation field over the same red-shift interval or might result from an increased contribution to the radiation field from "local" sources, such as would occur if the absorbers underwent a phase of massive star formation. The data are also used to investigate the H I column density distribution over the column density range 17.2 <= log [N(H I)/cm^-2]<=21.8 and to examine the multiple- component structure of the absorbing complexes. As in previous studies, the H I column density distribution is found to be well fitted by a power-law form, and an attempt is made to use model photoionization calculations to estimate the total column density distribution. The fractional displacement δ of the observed half-intensity point from the wavelength of the Lyman limit predicted from the absorption redshift is found to be correlated with the equivalent width of the C II λ1334 absorption line, which suggests that δ is available in a statistical sense as a probe of the multiple- component structure of the absorbing complexes. Implications of these results are discussed. Publication: The Astrophysical Journal Pub Date: July 1991 DOI: 10.1086/170164 Bibcode: 1991ApJ...375....1L Keywords: Absorption Cross Sections; Absorption Spectra; Lyman Spectra; Quasars; Red Shift; Density Distribution; Galactic Structure; H I Regions; Optical Thickness; Astrophysics; COSMOLOGY; GALAXIES: EVOLUTION; GALAXIES: STRUCTURE; QUASARS full text sources ADS | data products NED (103) SIMBAD (59) MAST (1)
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