244 research outputs found
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
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
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
A translucent interstellar cloud at z=2.69: CO, H2 and HD in the line-of-sight to SDSS J123714.60+064759.5
We present the analysis of a sub-DLA system (log N(H^0)=20.0+/-0.15,
z_abs=2.69) toward SDSS J123714+064759 (z_em=2.78). Using the VLT/UVES and
X-shooter spectrographs, we detect H2, HD and CO molecules in absorption with
log N(H2,HD,CO)=(19.21,14.48,14.17). The overall metallicity of the system is
super-solar ([Zn/H]=+0.34) and iron is highly depleted ([Fe/Zn]=-1.39),
revealing metal-rich and dusty gas. The strongest H2 component does not
coincide with the centre of the HI absorption. This implies that the molecular
fraction in this component, f=2N(H2)/(2N(H2)+N(H^0)), is larger than the mean
molecular fraction =1/4 in the system. This is supported by the detection of
Cl^0 associated with this H2-component having N(Cl^0)/N(Cl^+)>0.4. Since Cl^0
is tied up to H2 by charge exchange reactions, this means that the molecular
fraction in this component is not far from unity. The size of the molecular
cloud is probably smaller than 1pc. Both the CO/H2=10^-5 and CO/C^0~1 ratios
for f>0.24 indicate that the cloud classifies as translucent, i.e., a regime
where carbon is found both in atomic and molecular form. The corresponding
extinction, Av=0.14, albeit lower than the definition of a translucent
sightline (based on extinction properties), is high for the observed H^0 column
density. This means that intervening clouds with similar local properties but
with larger column densities could be missed by current magnitude-limited QSO
surveys. The excitation of CO is dominated by radiative interaction with the
Cosmic Microwave Background Radiation (CMBR) and we derive Tex(CO)=10.5+0.8-0.6
K when TCMBR(z=2.69)=10.05 K is expected. The astration factor of deuterium
-with respect to the primordial D/H ratio- is only about 3. This can be the
consequence of accretion of unprocessed gas from the intergalactic medium onto
the associated galaxy. [abridged]Comment: 17 pages, 21 figures, 8 tables, accepted for publication in A&
HD molecules at high redshift: The absorption system at z=2.3377 towards Q 1232+082
We present a detailed analysis of the H_2 and HD absorption lines detected in
the Damped Lyman-alpha (DLA) system at z_abs=2.3377 towards the quasar
Q1232+082. We show that this intervening cloud has a covering factor smaller
than unity and covers only part of the QSO broad emission line region. The zero
flux level has to be corrected at the position of the saturated H_2 and
optically thin HD lines by about 10%. We accurately determine the Doppler
parameter for HD and CI lines (b = 1.86+/-0.20 km/s). We find a ratio
N(HD)/N(H_2)=(7.1 +3.7 -2.2)x10^-5 that is significantly higher than what is
observed in molecular clouds of the Galaxy. Chemical models suggest that in the
physical conditions prevailing in the central part of molecular clouds,
deuterium and hydrogen are mostly in their molecular forms. Assuming this is
true, we derive D/H = (3.6 +1.9 -1.1)x10^-5. This implies that the
corresponding baryon density of the Universe is \Omega_b h^2 = (0.0182 +0.0047
-0.0042). This value coincides within 1\sigma with that derived from
observations of the CMBR as well as from observations of the D/H atomic ratio
in low-metallicity QSO absorption line systems. The observation of HD at high
redshift is therefore a promising independent method to constrain \Omega_b.
This observation indicates as well a low astration factor of deuterium. This
can be interpreted as the consequence of an intense infall of primordial gas
onto the associated galaxy.Comment: 8 pages, 7 figures, 3 tables. Accepted for publication in MNRA
Probing the BLR in AGNs using time variability of associated absorption line
It is know that most of the clouds producing associated absorption in the
spectra of AGNs and quasars do not completely cover the background source
(continuum + broad emission line region, BLR). We note that the covering factor
derived for the absorption is the fraction of photons occulted by the absorbing
clouds, and is not necessarily the same as the fractional area covered. We show
that the variability in absorption lines can be produced by the changes in the
covering factor caused by the variation in the continuum and the finite light
travel time across the BLR. We discuss how such a variability can be
distinguished from the variability caused by other effects and how one can use
the variability in the covering factor to probe the BLR.Comment: 12 pages, latex(aaspp4.sty), 2 figures, (To appear in ApJ
Partial covering of emission regions of Q 0528-250 by intervening H clouds
We present an analysis of the molecular hydrogen absorption system at z = 2.811 in the spectrum of the blazar Q0528-250. We demonstrate that the
molecular cloud does not cover the background source completely. The partial
coverage reveals itself as a residual flux in the bottom of saturated H_2
absorption lines. This amounts to about (2.220.54)% of the continuum and
does not depend on the wavelength. This value is small and it explains why this
effect has not been detected in previous studies of this quasar spectrum.
However, it is robustly detected and significantly higher than the zero flux
level in the bottom of saturated lines of the Ly-alpha forest,
(-0.210.22)%. The presence of the residual flux could be caused by
unresolved quasar multicomponents, by light scattered by dust, and/or by
jet-cloud interaction. The H absorption system is very well described by a
two-component model without inclusion of additional components when we take
partial coverage into account. The derived total column densities in the H
absorption components A and B are logN(H)[cm] = 18.100.02 and
17.820.02, respectively. HD molecules are present only in component B.
Given the column density, logN(HD)= 13.330.02, we find
N(HD)/2N(H)=(1.480.10)x10, significantly lower than previous
estimations. We argue that it is crucial to take into account partial coverage
effects for any analysis of H bearing absorption systems, in particular
when studying the physical state of high-redshift interstellar medium.Comment: Accepted for MNRA
Neutral chlorine and molecular hydrogen at high redshift
Chlorine and molecular hydrogen are known to be tightly linked together in
the cold phase of the local interstellar medium through rapid chemical
reactions. We present here the first systematic study of this relation at high
redshifts using H-bearing damped Ly systems (DLAs) detected along
quasar lines of sight. Using high-resolution spectroscopic data from VLT/UVES
and Keck/HIRES, we report the detection of ClI in 9 DLAs (including 5 new
detections) out of 18 high- DLAs with Hcm
(including a new H detection at towards J21000641) and
present upper limits for the remaining 9 systems. We find a 5
correlation between (ClI) and (H) with only 0.2dex
dispersion over the range 18.1log(H)20.1, thus probing
column densities 10 times lower those seen towards nearby stars, roughly
following the relation (ClIH.
This relation between column densities is surprisingly the same at low and high
redshift suggesting that the physical and chemical conditions are similar for a
given H (or ClI) column density. In turn, the N({Cl\,I})/N({\rm
H_2}) ratio is found to be uncorrelated with the overall metallicity in the
DLA. Our results confirm that neutral chlorine is an excellent tracer of
molecule-rich gas and show that the molecular fraction or/and metallicity in
the H-bearing component of DLA could possibly be much higher than the
line-of-sight average values usually measured in DLAs.Comment: 5 pages, 3 figures, Accepted for publication in A&A Letter
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