1,627 research outputs found
Molecular Hydrogen in a Damped Lyman-alpha System at z_abs=4.224
We present the direct detection of molecular hydrogen at the highest redshift
known today (z_abs=4.224) in a Damped Lyman-alpha (DLA) system toward the
quasar PSS J1443+2724. This absorber is remarkable for having one of the
highest metallicities amongst DLA systems at z_abs>3, with a measured iron
abundance relative to Solar of -1.12+/-0.10. We provide for the first time in
this system accurate measurements of NI, MgII, SII and ArI column densities.
The sulfur and nitrogen abundances relative to Solar, -0.63+/-0.10 and
-1.38+/-0.10 respectively, correspond exactly to the primary nitrogen
production plateau. H2 absorption lines are detected in four different
rotational levels (J=0, 1, 2 and 3) of the vibrational ground-state in three
velocity components with total column densities of log N(H2)=17.67, 17.97,
17.48 and 17.26 respectively. The J=4 level is tentatively detected in the
strongest component with log N(H2)~14. The mean molecular fraction is log
f=-2.38+/-0.13, with f=2N(H2)/(2N(H2)+N(HI)). We also measure log
N(HD)/N(H2)<-4.2. The excitation temperatures T_{01} for the two main
components of the system are 96 and 136 K respectively. We argue that the
absorbing galaxy, whose star-formation activity must have started at least
2-5x10^8 yrs before z=4.224, is in a quiescent state at the time of
observation. The density of the gas is small, n_H<=50 cm^{-3}, and the
temperature is of the order of T~90-180 K. The high excitation of neutral
carbon in one of the components can be explained if the temperature of the
Cosmic Microwave Background Radiation has the value expected at the absorber
redshift, T=14.2 K.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter
Testing Cosmological Models With A \lya Forest Statistic: The High End Of The Optical Depth Distribution
We pay particular attention to the high end of the \lya optical depth
distribution of a quasar spectrum. Based on the flux distribution
(Miralda-Escud\'e et al 1996), a simple yet seemingly cosmological model
-differentiating statistic, -- the cumulative probability of
a quasar spectrum with \lya optical depth greater than a high value
-- is emphasized. It is shown that two different models -- the cold dark matter
model with a cosmological constant and the mixed hot and cold dark matter
model, both normalized to COBE and local galaxy cluster abundance -- yield
quite different values of : 0.13 of the former versus 0.058 of
the latter for at . Moreover, it is argued that
may be fairly robust to compute theoretically because it does
not seem to depend sensitively on small variations of simulations parameters
such as radiation field, cooling, feedback process, radiative transfer,
resolution and simulation volume within the plausible ranges of the concerned
quantities. Furthermore, it is illustrated that can be
obtained sufficiently accurately from currently available observed quasar
spectra for , when observational noise is properly taken
into account. We anticipate that analyses of observations of quasar \lya
absorption spectra over a range of redshift may be able to constrain the
redshift evolution of the amplitude of the density fluctuations on
small-to-intermediate scales, therefore providing an independent constraint on
, and .Comment: ApJ Letters, in press, substantial changes have been made from the
last versio
21-cm absorption from galaxies at z ~ 0.3
We report the detection of 21-cm absorption from foreground galaxies towards
quasars, specifically z_gal = 0.3120 towards SDSS J084957.97+510829.0 (z_qso =
0.584; Pair-I) and z_gal = 0.3714 towards SDSS J144304.53+021419.3 (z_qso =
1.82; Pair-II). In both the cases, the integrated 21-cm optical depth is
consistent with the absorbing gas being a damped Lyman-\alpha (DLA) system. In
the case of Pair-I, strong Na I and Ca II absorption are also detected at z_gal
in the QSO spectrum. We identify an early-type galaxy at an impact parameter of
b ~ 14 kpc whose photometric redshift is consistent with that of the detected
metal and 21-cm absorption lines. This would be the first example of an
early-type galaxy associated with an intervening 21-cm absorber. The gas
detected in 21-cm and metal absorption lines in the outskirts of this luminous
red galaxy could be associated with the reservoir of cold H I gas with a low
level of star formation activity in the outer regions of the galaxy as reported
in the literature for z ~ 0.1 early-type galaxies. In the case of Pair-II, the
absorption is associated with a low surface brightness galaxy that, unlike most
other known quasar-galaxy pairs (QGPs) i.e. QSO sight lines passing through
disks/halos of foreground galaxies, is identified only via narrow optical
emission lines detected on top of the QSO spectra. Using SDSS spectra we infer
that the emission lines originate within ~ 5 kpc of the QSO sight line, and the
gas has metallicity [12+O/H] ~ 8.4 and star formation rate ~ 0.7-0.8 M_sun per
yr. The measured 21-cm optical depth can be reconciled with the N(H I) we
derive from the measured extinction (A_V=0.6) if either the H I gas is warm or
the extinction per hydrogen atom in this galaxy is much higher than the mean
value of the Small Magellanic Cloud. (Abridged)Comment: 8 pages, 7 figures, 3 tables (A&A in press
Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4
We present a study of the spatial coherence of the intergalactic medium
toward two pairs of high-redshift quasars with moderate angular separations
observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and
Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted
flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for
both pairs. This strongly suggests that at least some of the absorbing
structures span the 230-300/h_70 proper kpc transverse separation between
sightlines. We also statistically examine the similarity between paired spectra
as a function of transmitted flux, a measure which may be useful for comparison
with numerical simulations. In investigating the dependence of the correlation
functions on spectral characteristics, we find that photon noise has little
impact for S/N >~ 10 per resolution element. However, the agreement between the
autocorrelation along the line sight and the crosscorrelation between
sightlines, a potential test of cosmological geometry, depends significantly on
instrumental resolution. Finally, we present an inventory of metal lines. These
include a a pair of strong C IV systems at z ~ 3.4 appearing only toward
Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.Comment: 33 pages, 13 figures, submitted to Ap
Large-scale Correlation of Mass and Galaxies with the Lyman-alpha Forest Transmitted Flux
We present predictions of the correlation between the Lyman-alpha forest
absorption in quasar spectra and the mass within \sim 5 Mpc/h (comoving) of the
line of sight, using fully hydrodynamic and hydro-PM numerical simulations of
the cold dark matter model supported by present observations. The observed
correlation based on galaxies and the Lya forest can be directly compared to
our theoretical results, assuming that galaxies are linearly biased on large
scales. Specifically, we predict the average value of the mass fluctuation,
, conditioned to a fixed value of the Lya forest transmitted flux
delta_F, after they have been smoothed over a 10 Mpc/h cube and line of sight
interval, respectively. We find that /sigma_m as a function of
delta_F/sigma_F has a slope of 0.6 at this smoothing scale, where sigma_m and
sigma_F are the rms dispersions (this slope should decrease with the smoothing
scale). We show that this value is largely insensitive to the cosmological
model and other Lya forest parameters. Comparison of our predictions to
observations should provide a fundamental test of our ideas on the nature of
the Lya forest and the distribution of galaxies, and can yield a measurement of
the bias factor of any type of galaxies that are observed in the vicinity of
Lya forest lines of sight.Comment: Submitted to ApJ, 41 page
Multi-epoch intra-night optical monitoring of 8 radio-quiet BL Lac candidates
For a new sample of 8 weak-line-quasars (WLQs) we report a sensitive search
in 20 intranight monitoring sessions, for blazar-like optical flux variations
on hour-like and longer time scale (day/month/yearlike). The sample consists
exclusively of the WLQs that are not radioloud and have either been
classified as `radio-weak probable BL Lac candidates' and/or are known to have
exhibited at least one episode of large, blazarlike optical variability.
Whereas only a hint of intranight variability is seen for two of these WLQs,
J104833.5620305.0(z = 0.219) and J133219.6622715.9 (z = 3.15),
statistically significant internight variability at a few per cent level is
detected for three of the sources, including the radio-intermediate WLQ
J133219.6622715.9 (z = 3.15) and the well known bonafide radioquiet
WLQs J121221.5534128.0 (z = 3.10) and WLQ J153259.9003944.1 (z = 4.62).
In the restframe, this variability is intra-day and in the farUV band. On
the time scale of a decade, we find for three of the WLQs large brightness
changes, amounting to 1.6550.009, 0.1630.010 and 0.1440.018 mag,
for J104833.5620305.0, J123743.1630144.9 and J232428.4144324.4,
respectively. Whereas the latter two are confirmed radio-quiet WLQs, the
extragalactic nature of J104833.5620305.0 remains to be well established,
thanks to the absence of any feature(s) in its available optical spectra. The
present study forms a part of our ongoing campaign of intranight optical
monitoring of radio quiet weak-line quasars, in order to improve the
understanding of this enigmatic class of Active Galactic Nuclei and to look
among them for a possible tiny, elusive population of radio-quiet BL Lacs.Comment: Accepted to MNRAS. 12 pages, 1 figure, 4 Tabl
High-Resolution Spectroscopy from 3050 to 10000 A of the HDF-S QSO J2233-606 with UVES at the ESO VLT
We report on high-resolution observations () of the Hubble
Deep Field South QSO J2233-606 obtained with the VLT UV-Visual Echelle
Spectrograph (UVES). We present spectral data for the wavelength region \AA. The ratio of the final spectrum is about 50 per
resolution element at 4000 \AA, 90 at 5000 \AA, 80 at 6000 \AA, 40 at 8000 \AA.
Redshifts, column densities and Doppler widths of the absorption features have
been determined with Voigt-profile fitting. A total of 621 lines have been
measured. In particular 270 Ly-alpha lines, 41 Ly-beta and 24 systems
containing metal lines have been identified. Together with other data in the
literature, the present spectrum confirms that the evolution of the number
density of Ly-alpha lines with \huno has an upturn at .Comment: 34 pages Latex, with 3 PostScript figures. Astronomical Journal, in
press. A few revised upper limit
Temperature and Kinematics of CIV Absorption Systems
We use Keck HIRES spectra of three intermediate redshift QSOs to study the
physical state and kinematics of the individual components of CIV selected
heavy element absorption systems. Fewer than 8 % of all CIV lines with column
densities greater than 10^{12.5} cm^{-2} have Doppler parameters b < 6 km/s. A
formal decomposition into thermal and non-thermal motion using the simultaneous
presence of SiIV gives a mean thermal Doppler parameter b_{therm}(CIV) = 7.2
km/s, corresponding to a temperature of 38,000 K although temperatures possibly
in excess of 300,000 K occur occasionally. We also find tentative evidence for
a mild increase of temperature with HI column density. Non-thermal motions
within components are typically small (< 10 km/s) for most systems, indicative
of a quiescent environment. The two-point correlation function (TPCF) of CIV
systems on scales up to 500 km/s suggests that there is more than one source of
velocity dispersion. The shape of the TPCF can be understood if the CIV systems
are caused by ensembles of objects with the kinematics of dwarf galaxies on a
small scale, while following the Hubble flow on a larger scale. Individual high
redshift CIV components may be the building blocks of future normal galaxies in
a hierarchical structure formation scenario.Comment: submitted to the ApJ Letters, March 16, 1996 (in press); (13 Latex
pages, 4 Postscript figures, and psfig.sty included
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