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ABSTRACT: (Abridged) Aims. To probe the spectral energy distribution (SED) of the
ionizing background radiation at z ~ 2 and to specify the sources contributing
to the intergalactic radiation field. Methods. The spectrum of a bright quasar
HS1103+6416 (zem = 2.19) contains five successive metal-line absorption systems
at zabs = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically
thin and reveal multiple lines of different metal ions with the ionization
potentials lying in the extreme ultraviolet (EUV) range (1 Ryd to 0.2 keV). For
each system, the EUV SED of the underlying ionization field is reconstructed by
means of a special technique developed for solving the inverse problem in
spectroscopy. For the zabs = 1.8916 system, the analysis also involves the HeI
resonance lines of the Lyman series and the HeI 504 A continuum, which are seen
for the first time in any cosmic object except the Sun. Results. From one
system to another, the SED of the ionizing continuum changes significantly,
indicating that the intergalactic ionization field at z ~ 2 fluctuates at the
scale of at least Delta_z ~ 0.004. This is consistent with Delta_z ~ 0.01
estimated from HeII and HI Lyman-alpha forest measurements between the
redshifts 2 and 3.
03/2013;
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A. J. Barth,
A. Pancoast,
S. J. Thorman,
V. N. Bennert,
D. J. Sand,
W Li,
G. Canalizo,
A. V. Filippenko,
E. L. Gates,
J. E. Greene, [......],
C. E. Harris,
E. Y. Hsiao,
J Lee,
L. Lopez,
J. Rex,
N Suzuki,
J. R. Trump, D. Tytler,
G. Worseck,
H. M. Yesuf
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ABSTRACT: The Lick AGN Monitoring Project 2011 observing campaign was carried out over
the course of 11 weeks in Spring 2011. Here we present the first results from
this program, a measurement of the broad-line reverberation lag in the Seyfert
1 galaxy Mrk 50. Combining our data with supplemental observations obtained
prior to the start of the main observing campaign, our dataset covers a total
duration of 4.5 months. During this time, Mrk 50 was highly variable,
exhibiting a maximum variability amplitude of a factor of 4 in the U-band
continuum and a factor of 2 in the H-beta line. Using standard
cross-correlation techniques, we find that H-beta and H-gamma lag the V-band
continuum by tau_cen = 10.64(-0.93,+0.82) and 8.43(-1.28,+1.30) days,
respectively, while the lag of He II 4686 is unresolved. The H-beta line
exhibits a symmetric velocity-resolved reverberation signature with shorter
lags in the high-velocity wings than in the line core, consistent with an
origin in a broad-line region dominated by orbital motion rather than infall or
outflow. Assuming a virial normalization factor of f=5.25, the virial estimate
of the black hole mass is (3.2+-0.5)*10^7 solar masses. These observations
demonstrate that Mrk 50 is among the most promising nearby active galaxies for
detailed investigations of broad-line region structure and dynamics.
10/2011;
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A. J. Barth,
M. L. Nguyen,
M. A. Malkan,
A. V. Filippenko,
W Li,
V. Gorjian,
M. D. Joner,
V. N. Bennert,
J. Botyanszki,
S. B. Cenko, [......],
R. M. Rich,
J. M. Silverman,
G. H. Smith,
A. Sonnenfeld,
N Suzuki, D. Tytler,
J. L. Walsh,
J. -H. Woo,
Y Yang,
C. Zeisse
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ABSTRACT: The Seyfert 1 galaxy Zw 229-015 is among the brightest active galaxies being
monitored by the Kepler mission. In order to determine the black hole mass in
Zw 229-015 from H-beta reverberation mapping, we have carried out nightly
observations with the Kast Spectrograph at the Lick 3m telescope during the
dark runs from June through December 2010, obtaining 54 spectroscopic
observations in total. We have also obtained nightly V-band imaging with the
Katzman Automatic Imaging Telescope at Lick Observatory and with the 0.9m
telescope at the Brigham Young University West Mountain Observatory over the
same period. We detect strong variability in the source, which exhibited more
than a factor of 2 change in broad H-beta flux. From cross-correlation
measurements, we find that the H-beta light curve has a rest-frame lag of
3.86(+0.69,-0.90) days with respect to the V-band continuum variations. We also
measure reverberation lags for H-alpha and H-gamma and find an upper limit to
the H-delta lag. Combining the H-beta lag measurement with a broad H-beta width
of sigma = 1590+/-47 km/s measured from the root-mean-square variability
spectrum, we obtain a virial estimate of M_BH = 1.00(-0.24,+0.19)*10^7 solar
masses for the black hole in Zw 229-015. As a Kepler target, Zw 229-015 will
eventually have one of the highest-quality optical light curves ever measured
for any active galaxy, and the black hole mass determined from reverberation
mapping will serve as a benchmark for testing relationships between black hole
mass and continuum variability characteristics in active galactic nuclei.
03/2011;
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ABSTRACT: The current paradigm for the AGN phenomenon is a central engine that consists of an inflow of material accreting in the form of a disk onto a supermassive black hole. Observations in the UV and optical find high velocity ionized material outflowing from the black hole. We present results from Suzaku and XMM-Newton observations of a sample of intrinsic NAL quasars with high velocity outflows. Our derived values of the intrinsic column densities of the X-ray absorbers are consistent with an outflow scenario in which NAL quasars are viewed at smaller inclination angles than BAL quasars. We find that the distributions of alpha_ox and Dalpha_ox of the NAL quasars of our sample differ significantly from those of BAL quasars and SDSS radio-quiet quasars. The NAL quasars are not significantly absorbed in the X-ray band and the positive values of Dalpha_ox suggest absorption in the UV band. The positive values of Dalpha_ox of the intrinsic NAL quasars can be explained in a geometric scenario where our lines of sight towards the compact X-ray hot coronae of NAL quasars do not traverse the absorbing wind whereas lines of sight towards their UV emitting accretion disks do intercept the outflowing absorbers. Comment: 12 pages, 5 figures, accepted in New Astronomy Reviews
07/2009;
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ABSTRACT: We have produced a catalog of line identifications and equivalent width measurements for all absorption features in eight ultraviolet echelle quasar spectra. These spectra were selected as having the highest signal-to-noise among the HST/STIS spectra obtained with the E230M grating. We identify 56 metal-line systems toward the eight quasars, and present plots of detected transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the features in the Lya forest region, redward of the incidence of the Lyb forest, are metal lines. High ionization transitions are common. We see both O VI and C IV in 88 - 90% of the metal-line systems for which the spectra cover the expected wavelength. Si III is seen in 58%, while low ionization absorption in C II, Si II, and/or Al II is detected in 50% of the systems for which they are covered. This catalog will facilitate future studies of the Lya forest and of metal-line systems of various types. Comment: 13 pages, 2 figures, submitted to Monthly Notices of the Royal Astronomical Society, a complete version with the appendix and all figures is available at http://www.astro.psu.edu/users/misawa/pub/Paper/qalcat.pdf.gz
05/2007;
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ABSTRACT: We present statistical and monitoring results of narrow absorption lines that are physically related to quasars (i.e., intrinsic NALs). We use Keck/HIRES spectra of 37 optically bright quasars at z=2-4, and identify 150 NAL systems that contain 124 C IV, 12 N V, and 50 Si IV doublets. Among them, 39 are classified as intrinsic systems based on partial coverage analysis. At least 50% of quasars host intrinsic NALs. We identify two families of intrinsic systems based on their ionization state. Some intrinsic systems have detectable low-ionization NALs at similar velocities as higher-ionization NALs, although such low-ionization lines are rare in broad absorption line (BAL) systems. We also have observed an optically bright quasar, HS1603+3820, eight times with Subaru/HDS and HET/MRS over an interval of 4.2 years (1.2 years in the quasar rest frame), for the purpose of monitoring a variable C IV mini-BAL system. We find that all the troughs of the system vary in concert. However, no other correlations are seen between the variations of different profile parameters. We propose that the observed variations are either (i) a result of rapid continuum fluctuations, caused by a clumpy screen of variable optical depth located between the continuum source and the mini-BAL gas, or (ii) a result of variable scattering of continuum photons around the absorber.
01/2007;
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ABSTRACT: The shape of the UV ionizing background is reconstructed from optically thin metal absorption-line systems identified in spectra of HE2347-4342, Q1157+3143, and HS1700+6416 in the redshift interval 1.8 < z < 2.9. The systems are analyzed by means of the Monte Carlo Inversion method completed with the spectral shape recovering procedure. The UVB spectral shape fluctuates at 2.4 < z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z < 1.8, the IGM becomes almost transparent both in the HI and HeII Lyman continua and the variability of the spectral shape comes from diversity of spectral indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered spectral shapes show intensity depression between 3 and 4 Ryd due to HeII Ly-alpha absorption in the IGM clouds (line blanketing) and continuous medium (true Gunn-Petersen effect). The mean HeII Ly-alpha opacity estimated from the depth of this depression corresponds within 1-2sigma to the values directly measured from the HI/HeII Ly-alpha forest towards the quasars studied. The observed scatter in eta = N(HeII)/N(HI) and anti-correlation between N(HI) and eta can be explained by the combined action of variable spectral softness and differences in the mean gas density between the absorbing clouds. Neither of the recovered spectral shapes show features which can be attributed to the putative input of radiation from soft sources like starburst galaxies. Comment: 20 pages, 20 figures. Accepted for publication in A&A
10/2006;
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ABSTRACT: We report on high resolution spectra of the bright QSO HS0747+4259 (zem = 1.90, V = 15.8) observed to search for intermediate redshift OVI absorption systems. The spectra were obtained by means of the Space Telescope Imaging Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs = 1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10 Ryd. All recovered UV ionizing spectra are characterized by the enhanced intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau (1996). This is in line with the observational evidence of a deficiency of strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV background shows significant local variations: the spectral shape estimated at z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the line of sight. No features favoring the input of stellar radiation to the ionizing background are detected, limiting the escape fraction of the galactic UV photons to f_esc < 0.05. Comment: 14 pages, 7 figures, 5 tables; accepted for publication in A&A
11/2005;
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ABSTRACT: We report the discovery of a further line of sight allowing detection of HeII Ly alpha absorption by the intergalactic medium. A HST/STIS survey of 32 bright z ~ 3 quasars selected from the Hamburg Quasar Surveys yielded one detection toward QSO 1157+3143 (z ~ 3, B ~ 17). A 10 orbit follow-up spectrum reveals a UV spectrum significantly supressed by two intervening Lyman limit systems at z=2.77 and 2.94, but with the continuum flux recovering sufficiently shortward of ~ 1700 A to allow study of the HeII absorption spectrum in the redshift range 2.75 < z < 2.97. The absorption is characterized by alternating voids and dense filament structures seen in both HeII and HI. Attempts to model the HeII opacity in terms of HI Ly alpha forest absorption are not successful in the voids, suggesting that HeII reionization is not complete between z=2.77 and 2.97 or that an optically thin Lyman limit system with z ~ 0.3 is responsible for the additional opacity. Comment: 6 pages, 4 figures, accepted by A&A
07/2005;
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T. Jena,
M. L. Norman, D. Tytler,
D. Kirkman,
N Suzuki,
A. Chapman,
C. Melis,
G. So,
B. W. O'Shea,
W. Lin,
D. Lubin,
P. Paschos,
D. Reimers,
E. Janknecht,
C. Fechner
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ABSTRACT: We present 40 fully hydrodynamical numerical simulations of the intergalactic gas that gives rise to the Ly-alpha forest. We make artificial spectra from each and measure three output parameters: the mean flux, a measure of the most common Ly-alpha line width (b-value), and the 1D power spectrum of the flux on scales from 0.01 - 0.1 s/km. We investigate how these output parameters are correlated with the input parameters that describe the simulations. These input parameters include the main cosmological parameters; and two astrophysical parameters gammah and gammahe. The parameter gammah controls the rate of ionization of H I, He I and He II while gammahe controls the rate of heating. We compare the output from the simulations to data from HIRES and UVES spectra and we give a new measurement of the flux power from these spectra at z=1.95. We find that simulations with a wide variety of sigma_8 values, from at least 0.8 - 1.1, can fit the small scale flux power and b-values when we adjust gammahe to compensate for the sigma_8 change. We can also use gammah to adjust the H I ionization rate to simultaneously match the mean flux. We derive scaling relations that give the output parameter values expected for a variety of input parameters with errors of 2% for optical depth and 1.4% for b-balues. We can break the degeneracy between gammahe and sigma_8 using large scale power or other data to fix sigma_8. Comment: Revised author list
12/2004;
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ABSTRACT: Detailed analysis of the spectral lines from two low redshift damped Lyman alpha absorbers (DLA) confirms that they are a heterogeneous population. Both systems have low metal abundances of approximately 0.05 to 0.1 solar. The abundance pattern of the DLA at z=1.15 towards HE 0515-4414 shows dust depletion comparable to that found in our Galaxy, while the system at z=0.68 towards HE 1122-1649 shows abundance ratios resembling metal-poor halo stars with no dust depletion, for a comparable HI column. Constraints for N/Fe and N/Si also hint at z=0.68 DLA as a galaxy with recent star formation. The trend of increasing CI/HI with decreasing z is discussed. Only weak absorption from highly-ionized species associated with the DLA at moderate z have been detected and it probably originates in regions distinct from the low ionization gas. The low-ion profiles show very complex structures and are too large to be explained by the rotation of a disk: the system at z=0.68 spans over approximately 300 km/s and the z=1.15 DLA presents substructure over more than 700 km/s, the largest velocity extent found up to date for a DLA.
09/2000;
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ABSTRACT: We present a new analysis of the deuterium absorption at z = 2.504 towards the quasar Q1009+2956 using the mesoturbulent model which accounts for possible correlations in the large scale velocity field. We obtain a slightly higher deuterium-to-hydrogen ratio D/H = (3.5 - 5.0)*10^{-5} as compared with the recent measurement D/H = (3.3 - 4.5)*10^{-5} (68% C.L.) performed by Burles & Tytler (1998b) using the usual microturbulent approximation which assumes that the velocity field is uncorrelated. Other mesoturbulent calculations of the D-abundances at z = 3.572 towards Q1937--1009 and at z = 0.701 towards Q1718+4807 (the systems showing `low' and possibly `high' D/H values, respectively, in the microturbulent approaches) agree with the present one within the errors of measurements. Thus, the mesoturbulent analysis does not reveal any spatial variations of D/H and supports the standard homogeneous model of big bang nucleosynthesis.
Astronomical and Astrophysical Transactions 01/2000; 19(3-4):385-396.
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ABSTRACT: We analyse absorption lines in optical and ultraviolet spectra of the
bright (V=15.8, z=2.19) QSO HS 1103+6416. High-resolution (FWHM =8 km
s(-1) ) optical spectra have been obtained with the Keck 10 m telescope
in the range from 3180 to 5780 Angstroms. Ultraviolet observations in
the range from 1150 to 3280 Angstroms were performed with the FOS and
the GHRS onboard the Hubble Space Telescope (HST). In this paper we
concentrate our discussion on a complex Lyman limit system (LLS) at
z=1.89. Absorption lines by carbon, silicon and aluminum in the optical
spectra reveal a complex velocity structure with at least 11 components
spanning a velocity range of 200 km s(-1) . From the Lyman limit in the
ultraviolet spectra we derive a total neutral hydrogen column density of
log N(H i) =17.46 cm(-2) . Column densities of heavy elements in the
individual components were derived by Voigt profile fitting. The eleven
components can be subdivided roughly into three groups: Components 2, 3
and 6 with radial velocities v = -129... -95 km s(-1) with low
ionization (L), components 4, 5, 7, 8 (v = -75... +2) with intermediate
ionization (I), and components 1, 9, 10, 11 (v = -129, +34... +57) with
high ionization (H). In order to study the ionization and abundances in
these systems we compare the observed column densities with
photoionization models. The observed absorption in the optical data can
be explained by individual clouds with slightly varying metal abundances
photoionized by slightly different radiation fields. Highly ionized
components favour the extragalactic radiation field as calculated by
Haardt & Madau (\cite{Haardt96}) while the components of low and
intermediate ionization are better reproduced with a harder ionizing
radiation field. Obviously local sources like stars can therefore be
excluded as the main ionizing sources. Observational parameters for HST
spectra of HS 1103+6416. <~bel{obs} Detector/Grating Exposure time
Resolution Observed range Date Offset S/N_{subs{max}} [s] FWHM
[Angstroms] [Angstroms] [Angstroms] AMBER/G270H 5336 2 2223-3277 Oct 31
1995 0.5 46 AMBER/G190H 8628 1.44 1572-2311 Oct 31 1995 1.24 21
DET1/G140L 17408 0.77 1415-1700 Jul 9 1996 0.66 8 DET1/G140L 22739 0.77
1150-1436 Jul 9 1996 0.66 12 Abundances in components L and I appear to
be slightly different from those in the high ionization component H. In
L and I we find roughly [C/H] = -0.9 while H has [C/H] = -1.2,
consistent with the expectation that in a galaxy or groups of galaxies
the abundances in the higher ionized `Halo' component are lower. The
relative element abundances are also different. While in components L
and I [Si/C] ~ 0.2, barely significant, and [S/C] and [O/C] ~ 0 within
the uncertainties, component H shows [Si/C] = 0.5 and in addition [O/C]
and [S/C] = 0.4 (both from HST spectra). [Al/C] measurable only in L and
I is always ~ 0. The tendency of enhanced alpha element (O, Si, S)
abundances at low C abundance is consistent with what is known from
nucleosynthesis theory (SNII dominant at the beginning of galactic
evolution), from metal deficient stars in our galaxy and from QSO
absorption line systems. If all components were ionized by the same
radiation field the relative overabundances of O and S in the highly
ionized components would be even larger. We show that HS 1103+6416 will
offer in the future for the first time the possibility to measure the
cosmic He abundance at high redshift. Detailed calculations of He i
absorption using the multicomponent model which explains the metal lines
shows consistency with the observed first seven series members of the He
i 584, 537, 522 Angstroms ... series for a helium abundance Y=0.24, the
expected cosmic He abundance from Big Bang nucleosynthesis modified by
stellar nucleosynthesis at ~ 1/10 solar metallicity. The presence of O i
and possibly O vi absorption cannot be explained by our photoionization
models and might hint at the existence of additional mainly neutral
components with relatively low H i column density and further ionization
mechanisms like, e.g., collisional ionization. Based on observations
with the NASA/ESA Hubble Space Telescope, obtained at the Space
Telescope Science Institute, which is operated by Aura, Inc., under NASA
contract NAS 5--26\,555. Optical data presented herein were obtained at
the W.M.\ Keck Observatory, which is operated as a scientific
partnership among the California Institute of Technology, the University
of California and the National Aeronautics and Space Administration. The
Observatory was made possible by the generous financial support of the
W.M.\ Keck Foundation.
Astronomy and Astrophysics 01/1999; 342:395-407. · 4.59 Impact Factor
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01/1999;
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ABSTRACT: We present our measurements of the deuterium to hydrogen ratio (D/H) in QSO absorption systems, which give D/H = 3.40 0.25 10-5 based on analysis of four independent systems. We discuss the properties of two systems which provide the strongest constraints on D/H. We outline the systematic effects involved in measurements of D/H and introduce a sophisticated method of analysis which properly accounts for these effects.
Space Science Reviews 03/1998; 84(1):65-75. · 3.61 Impact Factor
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ABSTRACT: We present the highest quality Lyman Alpha forest spectra published to date, from the QSO HS 1946+7658. The distribution of H I column densities is a power law of slope -1.5 from Log N = 12.1 - 14. This power law can extend to N = 0, because lines weaker than Log N = 12.1 do not have a large H I optical depth. Low column lines with Log N > 9 could account for all observed He II absorption, but lines with Log N > 12 alone are unlikely to do so. The b distribution between 20 and 60 km/sec is a Gaussian with a mean of 23 km/sec (less than reported in past at this z), and a sigma b of 14 km/sec. We report no evolution in the Lyman alpha forest (except the number of lines), because Lu et al. (1997) found the same N and b distributions at = 3.7. We see lines with 14 < b < 20 km/sec and b > 80 km/sec that cannot be accounted for by noise or blending effects. We discover that the lower cutoff in the b distribution varies with N, from b = 14 km/sec at Log N = 12.5 to b = 22 km/sec at Log N = 14.0, but otherwise b and N are not correlated. We see no Lyman Alpha line clustering above 50 \kms, in disagreement with previous results from lower signal to noise data, but we do see a 3 sigma clustering signal at 25 - 50 km/sec among lines with Log N > 13.6 Comment: (Minor changes including new identifications for two weak lines) 46 pages including 16 Figures, Latex Table 1 Available at http://nately.ucsd.edu/~david . To appear in ApJ
01/1997;
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ABSTRACT: The primordial ratio of deuterium to hydrogen nuclei (D/H), created as a result of the Big Bang, provides the most sensitive measure of the cosmological density of baryons. Measurements of the D/H ratio in the interstellar medium of our Galaxy place a strict lower limit on the primordial ratio, because processing of gas by stars reduces the abundance of deuterium relative to hydrogen. Absorption of radiation from distant quasars by intervening clouds of gas offers a means of probing D/H ratios at large redshifts, where the effects of stellar processing should be negligible. Measurements on one absorption system have indicated an extremely high primordial abundance ratio of 24 x 10(-5). Here we report a measurement of the D/H ratio in another high-redshift absorption system, and obtain a value that is an order of magnitude lower than that reported previously. The measured ratio of 2.3 x 10(-5) is consistent with that in the interstellar medium (after allowing for Galactic chemical evolution), and indicates that the absorption spectra on which the earlier estimates are based may have been subject to strong contamination. We calculate a baryon density that is 5% of the critical density required to close the Universe.
Nature 06/1996; 381(6579):207-9. · 36.28 Impact Factor
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04/1995; 27:848.
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ABSTRACT: We present optical spectra of QSO HS~1946+7658 with either high resolution (FWHM=10 km/s) or high signal to noise ratio (SNR=40-100). We find 113 Lyman alpha and six metal line systems. The metal systems at Zabs=2.844 and 3.050 have complex velocity structures. We find that the system at 2.844 is a damped Ly-a absorption system, with neutral hydrogen column density of logN(HI)=20.2+/-0.4, and it is the cause of the Lyman limit break at 3520\AA. The metal abundance in the gas phase of the system is [M/H]=-2.6+/-0.3, with a best estimate of [M/H]= -2.8, with ionization parameter Gamma=-2.75, from a photoionization model. We show that the abundance ratios rule out appreciable dust in this DLA system. We do not see the enhancement of O over C reported in Nature by Reimers last years. We see CII*(1335) offset by 15 km/s with respect to CII(1334), presumably because the gas density varies from 2 - 8 cm(-3) with changing velocity in the DLA system. These densities imply that the damped component is 6 - 25 pc thick, which is reasonable for a single cloud in a cold spiral disk. We see a 2.6sigma lack of Ly-a forest lines well away from the QSO redshift, which may be a chance fluctuation. We also see a correlation between column density N(HI) and Doppler parameter b for 96 unsaturated Ly-a forest absorption lines, and although this correlation persists in the 36 Ly-a lines which lie in regions where SNR=8-16, we agree with Rauch etal (1993) that it is probably a bogus effect of low SNR. We show that there are few low-b Ly-a forest lines in moderate SNR data. Comment: (Plain Tex, 46 pages and 21 figures. To get 21 postscript files for figures ftp to 132.239.146.152 and login as ftp in directory /pub/fan). To appear in the Astrophysical Journal Supplements
02/1994;
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ABSTRACT: Recently Lu presented tentative evidence for C IV lines in QSO Lyman-alpha forest systems with strong lines. We have performed a similar search for C IV in our 10 km/sec echelle spectra of the bright QSO HS~1946+7658. We shifted the spectra to align the expected positions of the C IV lines in 65 Lyman-alpha systems, then added them. The resulting composite spectrum, equivalent to 390 hours of exposure time on the Lick 3-m telescope, has a signal-to-noise ratio of 80 per 0.025 Angstrom in the rest frame of the absorbers. We do not see any C IV lines down to a $2\sigma$ limit of W$(1548) \leq 1.4 $~m\AA, about one-fifth of the strength of the lines seen by Lu. The C IV lines which Lu saw must be restricted to rare Lyman-alpha systems with large H I column densities $\geq 10^{14}$~cm$^{-2}$, which are too rare to show C IV in our sample. More common Lyman-alpha systems with H I column densities of $10^{13}$ -- $10^{14}$~cm$^{-2}$ do not show C IV lines. If their ionization is H/H~I $= 10^4$ then they have [C/H] $\leq -2.0$. Comment: Plain Tex, 9 pages and 2 figures. To get postscript files for figures ftp to 132.239.146.152 and login as ftp in directory pub. Send preprint request to Betty Travell at btravell@ucsd.edu. To appear in the Astrophysical Journal Letters
01/1994;
