Arthur M. Wolfe

University of California, San Diego, San Diego, California, United States

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Publications (149)764.55 Total impact

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    ABSTRACT: Damped Ly-alpha absorbers (DLAs) are a well-studied class of absorption line systems, and yet the properties of their host galaxies remain largely unknown. To investigate the origin of these systems, we have conducted an imaging survey of 32 quasar fields with intervening DLAs between z~1.9-3.8, leveraging a technique that allows us to image galaxies at any small angular separation from the background quasars. In this paper, we present the properties of the targeted DLA sample, new imaging observations of the quasar fields, and the analysis of new and archival spectra of the background quasars. In a companion paper we use these data to obtain an unbiased census of the DLA host galaxy population(s) and to directly measure the in-situ star formation rates of gas-rich galaxies at z>2.
    04/2014;
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    ABSTRACT: We present evidence that the cosmological mean metallicity of neutral atomic hydrogen gas shows a sudden decrease at z > 4.7 down to \langle {Z}\rangle =-2.03^{+0.09}_{-0.11}, which is 6σ deviant from that predicted by a linear fit to the data at lower redshifts. This measurement is made possible by the chemical abundance measurements of eight new damped Lyα (DLA) systems at z > 4.7 observed with the Echellette Spectrograph and Imager on the Keck II Telescope, doubling the number of measurements at z > 4.7 to 16. Possible explanations for this sudden decrease in metallicity include a change in the physical processes that enrich the neutral gas within disks, or an increase of the covering factor of neutral gas outside disks due to a lower ultraviolet radiation field and higher density at high redshift. The later possibility would result in a new population of presumably lower metallicity DLAs, with an increased contribution to the DLA population at higher redshifts resulting in a reduced mean metallicity. Furthermore, we provide evidence of a possible decrease at z > 4.7 in the comoving metal mass density of DLAs, ρmetals(z)DLA, which is flat out to z ~ 4.3. Such a decrease is expected, as otherwise most of the metals from star-forming galaxies would reside in DLAs by z ~ 6. While the metallicity is decreasing at high redshift, the contribution of DLAs to the total metal budget of the universe increases with redshift, with DLAs at z ~ 4.3 accounting for ~20% as many metals as produced by Lyman break galaxies.
    01/2014; 782(2).
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    ABSTRACT: Measurements of the chemical evolution of damped Ly-alpha systems (DLAs) show a linear decrease in the metallicity of DLAs with increasing redshift out to 4.7. We present evidence for a sudden decrease or break in the chemical abundances of damped Ly-alpha systems (DLAs) at 5, doubling the previous sample size at this redshift. This break may represent a transition in the nature of DLAs above 4.7. Specifically, the lower UV radiation field and higher density at high redshift increases the neutral fraction of gas inside halos, such as the lower metallicity cold streams, thereby contributing more significantly to the DLA population. This would result in more low metallicity systems being classified as DLAs. Even with the observed break, we show that the metallicity 'floor' of ~1/600 solar continues out to 5, despite our sensitivity for finding DLAs with much lower metallicities. In addition, we find preliminary evidence suggesting that the the comoving HI mass density and the incidence of DLAs at 5 may be decreasing or flattening out, even though they are predicted to increase, due to an increase in the neutrality of cold accreting gas as we approach the epoch of reionization.
    01/2014;
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    ABSTRACT: We review the first results from our large KODIAQ (Keck Observatory Database of Ionized Absorbers toward Quasars) survey that aims to characterize the properties of the OVI-bearing gas associated with optically thick absorbers (Lyman limit systems and Damped Lya absorbers, LLSs and DLAs) at z>2. We searched the Keck observatory archive (KOA) for optically thick HI absorbers where the OVI doublet has little contamination and where the HI column density and the metallicity can be estimated to study the OVI properties, and baryon and metal budgets in the highly ionized phase as a function of these parameters. We assembled a sample of 20 absorbers that satisfy these criteria. Based on the comparison of the absorption profiles of the high ions (OVI, NV, CIV, SiIV), we show that the physics that govern the O VI and the other high ions are generally quite different, implying that the highly ionized gas in LLSs and DLAs is multiphase. All the properties of the OVI absorption imply that the OVI-bearing gas is not in collisional ionization equilibrium, but seems instead consistent with radiatively cooling flows of hot gas. The observed high OVI columns are also consistent with those seen in local starburst galaxies, making OVI associated with LLSs and DLAs a potential unique tracer of stellar activity on large scales at z>2.
    01/2014;
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    Regina A. Jorgenson, Arthur M. Wolfe
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    ABSTRACT: We present the first Keck/OSIRIS infrared IFU observations of a high redshift damped Lyman-alpha (DLA) galaxy detected in the line of sight to a background quasar. By utilizing the Laser Guide Star Adaptive Optics (LGSAO) to reduce the quasar PSF to FWHM~0.15 arcsec, we were able to search for and map the foreground DLA emission free from the quasar contamination. We present maps of the H-alpha and [OIII] $\lambda \lambda$ 5007, 4959 emission of DLA 2222-0946 at a redshift of z ~ 2.35. From the composite spectrum over the H-alpha emission region we measure a star formation rate of 9.5 $\pm$ 1.0 M$_{\odot}$ year$^{-1}$ and a dynamical mass, M$_{dyn}$ = 6.1 x 10$^9$ M$_{\odot}$. The average star formation rate surface density is < \Sigma_{SFR} > = 0.55 M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$, with a central peak of 1.7 M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$. Using the standard Kennicutt-Schmidt relation, this corresponds to a gas mass surface density of $\Sigma_{gas}$ = 243 M$_{\odot}$ pc$^{-2}$. Integrating over the size of the galaxy we find a total gas mass of M$_{gas}$ = 4.2 x 10$^9$ M$_{\odot}$. We estimate the gas fraction of DLA 2222-0946 to be $f_{gas}$ ~ 40%. We detect [NII]$\lambda$6583 emission at 2.5 sigma significance with a flux corresponding to a metallicity of 75% solar. Comparing this metallicity with that derived from the low-ion absorption gas ~6 kpc away, ~30% solar, indicates possible evidence for a metallicity gradient or enriched in/outflow of gas. Kinematically, both H-alpha and [OIII] emission show relatively constant velocity fields over the central galactic region. While we detect some red and blueshifted clumps of emission, they do not correspond with rotational signatures that support an edge-on disk interpretation.
    10/2013; 785(1).
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    ABSTRACT: We present evidence that the cosmological mean metallicity of neutral atomic hydrogen gas shows a sudden decrease at $z>4.7$ down to $< {\rm Z} >=-2.03^{+0.09}_{-0.11}$, which is $6\sigma$ deviant from that predicted by a linear fit to the data at lower redshifts. This measurement is made possible by the chemical abundance measurements of 8 new damped Ly-$\alpha$ (DLA) systems at $z>4.7$ observed with the Echellette Spectrograph and Imager on the Keck II telescope, doubling the number of measurements at $z>4.7$ to 16. The sudden decrease in metallicity is possibly due to the lower ultra-violet radiation field and higher density at high redshift increasing the neutral fraction of gas inside halos, such as cold flows. This would result in a new population of presumably lower metallicity DLAs, with an increased contribution to the DLA population at higher redshifts resulting in a reduced mean metallicity. While the comoving metal mass density of DLAs, $\rho_{\rm metals}(z)_{\rm DLA}$, is flat out to $z\sim4.3$, there is evidence of a possible decrease at $z>4.7$. Such a decrease is expected, as otherwise most of the metals from star-forming galaxies would reside in DLAs by $z\sim6$. While the metallicity is decreasing at high redshift, the contribution of DLAs to the total metal budget of the universe increases with redshift, with DLAs at $z\sim4.3$ accounting for $\sim20$% as many metals as produced by Lyman break galaxies.
    10/2013;
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    ABSTRACT: We present an overview of a 90-orbit Hubble Space Telescope treasury program to obtain near ultraviolet imaging of the Hubble Ultra Deep Field using the Wide Field Camera 3 UVIS detector with the F225W, F275W, and F336W filters. This survey is designed to: (i) Investigate the episode of peak star formation activity in galaxies at 1<z<2.5; (ii) Probe the evolution of massive galaxies by resolving sub-galactic units (clumps); (iii) Examine the escape fraction of ionizing radiation from galaxies at z~2-3; (iv) Greatly improve the reliability of photometric redshift estimates; and (v) Measure the star formation rate efficiency of neutral atomic-dominated hydrogen gas at z~1-3. In this overview paper, we describe the survey details and data reduction challenges, including both the necessity of specialized calibrations and the effects of charge transfer inefficiency. We provide a stark demonstration of the effects of charge transfer inefficiency on resultant data products, which when uncorrected, result in uncertain photometry, elongation of morphology in the readout direction, and loss of faint sources far from the readout. We agree with the STScI recommendation that future UVIS observations that require very sensitive measurements use the instrument's capability to add background light through a "post-flash". Preliminary results on number counts of UV-selected galaxies and morphology of galaxies at z~1 are presented. We find that the number density of UV dropouts at redshifts 1.7, 2.1, and 2.7 is largely consistent with the number predicted by published luminosity functions. We also confirm that the image mosaics have sufficient sensitivity and resolution to support the analysis of the evolution of star-forming clumps, reaching 28-29th magnitude depth at 5 sigma in a 0.2 arcsecond radius aperture depending on filter and observing epoch.
    The Astronomical Journal 05/2013; · 4.97 Impact Factor
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    ABSTRACT: Using a sample of 100 H I-selected damped Lyα (DLA) systems, observed with the High Resolution Echelle Spectrometer on the Keck I telescope, we present evidence that the scatter in the well-studied correlation between the redshift and metallicity of a DLA is largely due to the existence of a mass-metallicity relationship at each redshift. To describe the fundamental relations that exist between redshift, metallicity, and mass, we use a fundamental plane description, which is described by the following equation: [M/H] = (– 1.9 ± 0.5) + (0.74 ± 0.21) log Δv 90 – (0.32 ± 0.06) z. Here, we assert that the velocity width, Δv 90, which is defined as the velocity interval containing 90% of the integrated optical depth, traces the mass of the underlying dark matter halo. This description provides two significant improvements over the individual descriptions of the mass-metallicity correlation and metallicity-redshift correlation. Firstly, the fundamental equation reduces the scatter around both relationships by about 20%, providing a more stringent constraint on numerical simulations modeling DLAs. Secondly, it confirms that the dark matter halos that host DLAs satisfy a mass-metallicity relationship at each redshift between redshifts 2 through 5.
    The Astrophysical Journal 05/2013; 769(1):54. · 6.73 Impact Factor
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    ABSTRACT: As part of the KODIAQ program (supported through ADAP NASA grant NNX10AE84G.), we have begun a comprehensive search for metal line absorption in the spectra of 20 of the highest signal to noise spectra in the KODIAQ database. Our search selects absorption line systems based on the presence of observed absorption in triply ionized carbon (CIV). CIV is present in gas having a wide range of neutral hydrogen column density and total hydrogen density. As such, this database probes a wide range of astrophysical environments from the low density, highly ionized intergalactic medium up through the high density, mostly neutral ISM of galaxies. Our sample covers absorption over the range 1.6 < z < 4, a significant fraction of the age of the universe, and a period where significant changes in the galaxy population and the galaxy-circumgalactic medium interface take place. We present the current status of our survey and offer a preliminary interpretation of the results.
    01/2013;
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    ABSTRACT: Exploring the circumgalactic medium (CGM) between galaxies and the intergalactic medium is critical to our understanding of the formation and evolution of structures in the universe. OVI is a key tracer of the flows in the CGM. We have undertaken a large research program with the Keck Observatory Archive (KOA) to produce a comprehensive analysis of OVI (and other high and low ions) associated with optically thick absorbers (Lyman Limit systems -LLS, damped Lyα absorbers - DLA) seen in QSO spectra at z>2 observed by Keck/HIRES. Our early results imply that the O VI-bearing gas associated with LLS and DLA is a significant reservoir of baryons and metals at high z. I will present and discuss these new and exciting developments from our ADAP research program supported through NASA grant NNX10AE84G.
    01/2013;
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    ABSTRACT: We present intial results from the UVUDF project: a Cycle 19 HST Treasury program that obtained ultraviolet imaging of the Hubble Ultra Deep Field. The data consist of 30 orbits with WFC3/UVIS in each of F225W, F275W, and F336W, divided between three epochs. The science goals of the UVUDF project are to: (i) Investigate the episode of peak star formation activity in galaxies at 1 < z < 2.5; (ii) Study the star formation properties of moderate redshift starburst galaxies; (iii) Probe the evolution of massive galaxies by resolving sub-galactic units (clumps); (iv) Examine the escape fraction of ionizing radiation from galaxies at 2-3; and (v) Measure the star formation rate efficiency of neutral atomic-dominated hydrogen gas at 1-3. In addition to the first UVUDF science results, we discuss technical lessons learned that may be of interst in planning future deep UV surveys with HST.
    01/2013;
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    ABSTRACT: We present chemical abundance measurements for damped Ly-alpha systems (DLAs) out to 5 observed with the Keck telescopes. We determine the evolution of the cosmic metallicity of neutral gas, and find a continued decrease in the metallicity of DLAs with increasing redshift, improving the significance of the trend and extending it to higher redshifts. We find a possible break in the metallicity evolution of DLAs at z>4.7, and present the latest results from our current program targeting z>4.7 DLAs. We also show that the metallicity 'floor' of ~1/600 solar continues out to 5, despite our sensitivity for finding DLAs with much lower metallicities. In addition, we find that the metallicity distribution and the alpha/Fe ratios of z>2 DLAs are consistent with those of halo stars. It is therefore possible that the halo stars in the Milky Way formed out of DLA gas.
    01/2013;
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    ABSTRACT: The circumgalactic medium (CGM) between galaxies and the intergalactic medium can be sensitively explored by strong HI absorbers (log N(HI)>15) seen in the spectra of QSOs at redshifts near and above the peak of galaxy formation, z>1.5. At low redshift, OVI has been shown to be a key tracer of flows in the CGM, but it is poorly constrained at z>2 because of the OVI doublet’s location within the Lyman alpha forest. Therefore, surveys for OVI at z>2 in high HI column density systems have been small in size and vary greatly in terms of HI abundance, metallicity, and other parameters. In order to better constrain the properties and behavior of OVI and other so-called high ions (CIV, SiIV, NV) at z>2, we have undertaken a comprehensive search for OVI in strong HI absorbers using the Keck Observatory Archive as part of the KODIAQ research effort (supported through ADAP NASA grant NNX10AE84G). The resulting sample of absorbers is the largest yet constructed at high redshift, particularly for OVI. We present the current status of this survey and offer a preliminary interpretation of the physical processes that give rise to the high ions at high redshift through a comparison of the relative abundances of the high ions at high z and the Milky Way.
    01/2013;
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    ABSTRACT: We present chemical abundance measurements for 47 damped Lyα (DLA) systems, 30 at z > 4, observed with the Echellette Spectrograph and Imager and the High Resolution Echelle Spectrometer on the Keck telescopes. H I column densities of the DLAs are measured with Voigt profile fits to the Lyα profiles, and we find an increased number of false DLA identifications with Sloan Digital Sky Survey at z > 4 due to the increased density of the Lyα forest. Ionic column densities are determined using the apparent optical depth method, and we combine our new metallicity measurements with 195 from previous surveys to determine the evolution of the cosmic metallicity of neutral gas. We find the metallicity of DLAs decreases with increasing redshift, improving the significance of the trend and extending it to higher redshifts, with a linear fit of –0.22 ± 0.03 dex per unit redshift from z = 0.09-5.06. The metallicity "floor" of 1/600 solar continues out to z ~ 5, despite our sensitivity for finding DLAs with much lower metallicities. However, this floor is not statistically different from a steep tail to the distribution. We also find that the intrinsic scatter of metallicity among DLAs of ~0.5 dex continues out to z ~ 5. In addition, the metallicity distribution and the α/Fe ratios of z > 2 DLAs are consistent with being drawn from the same parent population with those of halo stars. It is therefore possible that the halo stars in the Milky Way formed out of gas that commonly exhibits DLA absorption at z > 2.
    The Astrophysical Journal 07/2012; 755(2):89. · 6.73 Impact Factor
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    ABSTRACT: We present details of a 90-orbit HST treasury program to obtain Ultraviolet (UV) imaging of the Hubble Ultra Deep Field (UDF) using the WFC3-UVIS detector with the F225W, F275W, and F336W filters. These UV images will reach point source detection limits of AB=29 at 10 sigma. This survey is designed to: (i) Investigate the episode of peak star formation activity in galaxies at 1 < z < 2.5. (ii) Study the star formation properties of moderate redshift starburst galaxies. (iii) Probe the evolution of massive galaxies by resolving sub-galactic units (clumps). (iv) Examine the escape fraction of ionizing radiation from galaxies at z 2-3. (v) Measure the star formation rate efficiency of neutral atomic-dominated hydrogen gas at z 1-3. We will present preliminary results from the UVUDF team based on the first observations (beginning March 2012).
    05/2012;
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    ABSTRACT: By comparing the widths of absorption lines from OI, SiII and FeII in the redshift z=2.076 single-component damped Lyman alpha absorption system in the spectrum of Q2206-199 we establish that these absorption lines arise in Warm Neutral Medium gas at ~12000 +/- 3000K. This is consistent with thermal equilibrium model estimates of ~ 8000K for the Warm Neutral Medium in galaxies, but not with the presence of a significant cold component. It is also consistent with, but not required by, the absence of CII* fine structure absorption in this system. Some possible implications concerning abundance estimates in narrow-line WNM absorbers are discussed.
    Monthly Notices of the Royal Astronomical Society 02/2012; 422(2). · 5.52 Impact Factor
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    ABSTRACT: A re-analysis of the Stokes-parameter spectra obtained of the z=0.692 21 cm absorption line toward 3C 286 shows that our original claimed detection of Zeeman splitting by a line-of-sight magnetic field, B_los = 87 microgauss is incorrect. Because of an insidious software error, what we reported as Stokes V is actually Stokes U: the revised Stokes V spectrum indicates a 3-sigma upper limit of B_los < 17 microgauss. The correct analysis reveals an absorption feature in fractional polarization that is offset in velocity from the Stokes I spectrum by -1.9 km/s. The polarization position-angle spectrum shows a dip that is also significantly offset from the Stokes I feature, but at a velocity that differs slightly from the absorption feature in fractional polarization. We model the absorption feature with 3 velocity components against the core-jet structure of 3C 286. Our chisquare minimization fitting results in components with differing (1) ratios of H I column density to spin temperature, (2) velocity centroids, and (3) velocity dispersions. The change in polarization position angle with frequency implies incomplete coverage of the background jet source by the absorber. It also implies a spatial variation of the polarization position angle across the jet source, which is observed at frequencies higher than the 839.4 MHz absorption frequency. The multi-component structure of the gas is best understood in terms of components with spatial scales of ~100 pc comprised of hundreds of low-temperature (T < 200 K) clouds with linear dimensions of about 1 pc.
    The Astrophysical Journal 03/2011; 733(1). · 6.73 Impact Factor
  • Marc Rafelski, A. Wolfe, H. Chen
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    ABSTRACT: We present evidence for spatially extended low surface brightness emission around Lyman break galaxies (LBGs), suggesting the presence of in situ star formation in gas associated with LBGs. We measure the emission in the V-band image of the Hubble Ultra Deep Field, corresponding to the z 3 rest-frame FUV light which is a sensitive measure of Star Formation Rates (SFRs). The outskirts of z 3 LBGs are too faint to accurately measure emission individually, therefore we create a stacked image of 48 z 3 LBGs that are compact, symmetrical, and isolated. We develop a theoretical framework to connect this emission around LBGs to the expected emission from surrounding neutral HI gas at high redshift, i.e., damped Lyman alpha systems (DLAs), using the Kennicutt Schmidt (KS) relation. Under the hypothesis that the observed FUV emission in the outskirts of LBGs is from in situ star formation in atomic dominated gas, we find that the SFR efficiency in neutral atomic-dominated hydrogen gas at z 3 is between factors of 10 and 50 lower than predictions based on the local KS relation. We also find that the metals produced by the in situ star formation in the outskirts of LBGs yield metallicities comparable to those of DLAs. This is a possible solution to the 'Missing Metals' problem, as long as the high metallicity gas in the cores of LBGs does not significantly contaminate the DLAs. In addition, we find that the covering fraction of DLA gas is sufficient to explain the emission in the outskirts of LBGs, while the covering fraction of molecular gas is not adequate. Therefore, our results are consistent with the idea of gas in spatially extended DLAs encompassing compact LBGs that could be the fuel source for star formation in the LBGs.
    01/2011;
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    Marc Rafelski, Arthur M. Wolfe, Hsiao-Wen Chen
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    ABSTRACT: We present evidence for spatially extended low surface brightness emission around Lyman break galaxies (LBGs) in the V band image of the Hubble Ultra Deep Field, corresponding to the z~3 rest-frame FUV light, which is a sensitive measure of star formation rates (SFRs). We find that the covering fraction of molecular gas at z~3 is not adequate to explain the emission in the outskirts of LBGs, while the covering fraction of neutral atomic-dominated hydrogen gas at high redshift is sufficient. We develop a theoretical framework to connect this emission around LBGs to the expected emission from neutral H I gas i.e., damped Lyman alpha systems (DLAs), using the Kennicutt-Schmidt (KS) relation. Working under the hypothesis that the observed FUV emission in the outskirts of LBGs is from in situ star formation in atomic-dominated hydrogen gas, the results suggest that the SFR efficiency in such gas at z~3 is between factors of 10 and 50 lower than predictions based on the local KS relation. The total SFR density in atomic-dominated gas at z~3 is constrained to be ~10% of that observed from the inner regions of LBGs. In addition, the metals produced by in situ star formation in the outskirts of LBGs yield metallicities comparable to those of DLAs, which is a possible solution to the "Missing Metals" problem for DLAs. Finally, the atomic-dominated gas in the outskirts of galaxies at both high and low redshifts has similar reduced SFR efficiencies and is consistent with the same power law.
    The Astrophysical Journal 11/2010; 736(1). · 6.73 Impact Factor
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    ABSTRACT: [ABRIDGED] We examine the velocity structure in the gas associated with \ion{H}{1} in the damped Ly$\alpha$ absorption system at redshift $z=1.7764$ towards the QSO $1331+170$ using 21cm data, optical and STIS spectra. We find at least two, and possibly three, components showing \ion{C}{1} lines. One of these has Doppler parameter $b=0.55${\kms}, corresponding to a kinetic temperature of 220K if the broadening is thermal. We re-examine the H$_2$ analysis undertaken by \citet{Cui05} using the neutral carbon velocity structure, and find a model which is, consistent with a mixture of collisional and background radiation excitation of the observed H$_2$ rotational levels. For singly ionized heavy elements we find eight components covering a velocity range of $\sim 110$ {\kms}. The \ion{H}{1} structure is expected to follow some combination of the singly ionized and neutral gas, but the 21cm absorption profile is considerably different. This may be because of the different extent and brightness distributions of the radio and optical background sources, and so the spin temperature derived by comparing the Ly$\alpha$ and 21cm line strengths has little physical meaning. The neutral and singly ionized heavy element line profiles also show significant differences, and so the dominant components in each appear to be physically distinct. Attempts to use the range of atomic masses to separate thermal and turbulent components of their Doppler widths were not generally successful. The velocity structure in all ionization stages up to $+3$, apart from the neutral heavy elements, is sufficiently complex that it is difficult to separate out the corresponding velocity components for different ionization levels and determine their column densities. Comment: 49 pages, 8 figures, 6 tables. Accepted for publication in MNRAS
    Monthly Notices of the Royal Astronomical Society 10/2010; · 5.52 Impact Factor

Publication Stats

5k Citations
764.55 Total Impact Points

Institutions

  • 1996–2014
    • University of California, San Diego
      • • Center for Astrophysics and Space Sciences (CASS)
      • • Department of Physics
      San Diego, California, United States
  • 2003–2008
    • California Institute of Technology
      Pasadena, California, United States
  • 2005
    • Yale University
      • Department of Astronomy
      New Haven, Connecticut, United States
  • 1994–2003
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, CA, United States
  • 2001
    • University of New South Wales
      • School of Physics
      Kensington, New South Wales, Australia
  • 1987–1993
    • Space Telescope Science Institute
      Baltimore, Maryland, United States