903 research outputs found
The elusive HI-> H2 transition in high-z damped Lyman-alpha systems
We study the H2 molecular content in high redshift damped Lyman-alpha systems
(DLAs) as a function of the HI column density. We find a significant increase
of the H2 molecular content around log N(HI) (cm^-2)~21.5-22, a regime unprobed
until now in intervening DLAs, beyond which the majority of systems have log
N(H2) > 17. This is in contrast with lines of sight towards nearby stars, where
such H2 column densities are always detected as soon as log N(HI)>20.7. This
can qualitatively be explained by the lower average metallicity and possibly
higher surrounding UV radiation in DLAs. However, unlike in the Milky Way, the
overall molecular fractions remain modest, showing that even at a large N(HI)
only a small fraction of overall HI is actually associated with the
self-shielded H2 gas. Damped Lyman-alpha systems with very high-N(HI) probably
arise along quasar lines of sight passing closer to the centre of the host
galaxy where the gas pressure is higher. We show that the colour changes
induced on the background quasar by continuum (dust) and line absorption (HI
Lyman and H2 Lyman & Werner bands) in DLAs with log N(HI)~22 and metallicity
~1/10 solar is significant, but not responsible for the long-discussed lack of
such systems in optically selected samples. Instead, these systems are likely
to be found towards intrinsically fainter quasars that dominate the quasar
luminosity function. Colour biasing should in turn be severe at higher
metallicities.Comment: accepted for publication in A&A letter
Neutral atomic-carbon QSO absorption-line systems at z>1.5: Sample selection, HI content, reddening, and 2175 A extinction feature
We present the results of a search for cold gas at high redshift along QSO
lines-of-sight carried out without any a priori assumption on the neutral
atomic-hydrogen (HI) content of the absorbers. To do this, we systematically
looked for neutral-carbon (CI) 1560,1656 transition lines in low-resolution QSO
spectra from the SDSS database. We built up a sample of 66 CI absorbers with
redshifts 1.5<z<3.1 and equivalent widths 0.1<W_r(1560)<1.7 A. The completeness
limit of our survey is W_r,lim(1560)~0.4 A. CI systems stronger than that are
more than one hundred-times rarer than DLAs at z_abs=2.5. The number of CI
systems per unit redshift increases significantly below z=2. We suggest that
the CI absorbers are closely related to the process of star formation and the
production of dust in galaxies. We derive the HI content of the CI systems and
find that a majority of them are sub-DLAs with N(HI)~10^20 atoms cm^-2. The
dust content of these absorbers is yet significant as seen from the redder
optical colours of the background QSOs and their reddened SEDs. The overall
N(HI) distribution of CI systems is relatively flat however. As a consequence,
among the CI systems classifying as DLAs there is a probable excess of strong
DLAs with log N(HI)>21 compared to systematic DLA surveys. We study empirical
relations between W_r(CI), N(HI), E(B-V) and the strength of the 2175 A
extinction feature, the latter being detected in about 30% of the CI absorbers.
We show that the 2175 A feature is weak compared to Galactic lines-of-sight
exhibiting the same amount of reddening. This is probably the consequence of
current or past star formation in the vicinity of the CI systems. We also find
that the strongest CI systems tend to have the largest amounts of dust and that
the metallicity of the gas and its molecular fraction is likely to be high in a
large number of cases.Comment: Accepted for publication in Astronomy & Astrophysics Main Journal on
20 April 201
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
Parsec-scale structures and diffuse bands in a translucent interstellar medium at z 0.079
We present a detailed study of the QSO-galaxy pair [SDSS J163956.35+112758.7
(zq = 0.993) and SDSS J163956.38+112802.1 (zg = 0.079)] based on observations
carried out using the Giant Meterwave Radio Telescope (GMRT), the Very Large
Baseline Array (VLBA), the Sloan Digital Sky Survey (SDSS) and the ESO New
Technology Telescope (NTT). We show that the interstellar medium of the galaxy
probed by the QSO line of sight has near-solar metallicity (12+log(O/H) =
8.47+/-0.25) and dust extinction (E(B-V) 0.83+/-0.11) typical of what is
usually seen in translucent clouds. We report the detection of absorption in
the \lambda 6284 diffuse interstellar band (DIB) with a rest equivalent width
of 1.45+/-0.20\AA. Our GMRT spectrum shows a strong 21-cm absorption at the
redshift of the galaxy with an integrated optical depth of 15.70+/-0.13 km/s.
Follow-up VLBA observations show that the background radio source is resolved
into three components with a maximum projected separation of 89 pc at the
redshift of the galaxy. One of these components is too weak to provide useful
HI 21-cm absorption information. The integrated HI optical depth towards the
other two components are higher than that measured in our GMRT spectrum and
differ by a factor 2. By comparing the GMRT and VLBA spectra we show the
presence of structures in the 21-cm optical depth on parsec scales. We discuss
the implications of such structures for the spin-temperature measurements in
high-z damped Lyman-alpha systems. The analysis presented here suggests that
this QSO-galaxy pair is an ideal target for studying the DIBs and molecular
species using future observations in optical and radio wavebands.Comment: 10 pages, 8 figures, 2 tables, accepted for publication in MNRA
Detection of 21-cm, H2 and Deuterium absorption at z>3 along the line-of-sight to J1337+3152
We report the detection of 21-cm and H2 absorption lines in the same DLA
system (log N(HI)=21.36+-0.10) at zabs=3.17447 towards SDSSJ133724+315254
(z=3.174). We estimate the spin temperature of the gas to be, Ts~600 K,
intermediate between the expected values for cold and warm neutral media. This
suggests that the HI absorption originates from a mixture of different phases.
The total molecular fraction is low, f=10^-7, and H2 rotational level
populations are not in equilibrium. The average abundance of the alpha-elements
is, [S/H]=-1.45. N and Fe are found underabundant with respect to
alpha-elements by ~1.0 dex and ~0.5 dex respectively. Using photoionization
models we conclude that the gas is located more than 270 kpc away from the QSO.
While the position of 21-cm absorption line coincides with the H2 velocity
profile, their centroid are shifted by 2.7+-1.0 km/s from each other. However,
the position of the strongest metal absorption component matches the position
of the 21-cm absorption line within 0.5 km/s. From this, we constrain the
variation of the combination of fundamental constants x=alpha^2 Gp/mu, Delta
x/x=-(1.7+-1.7)x10^-6. This system is unique as we can at the same time have an
independent constrain on mu using H2 lines. However only Werner band absorption
lines are seen and the range of sensitivity coefficients is too narrow to
provide a stringent constraint: Delta mu/mu <= 4.0x10^-4. The VLT/UVES spectrum
reveals another DLA at zabs=3.16768 with log N(HI)=20.41+-0.15 and low
metallicity, [Si/H]=-2.68+-0.11. We derive log N(DI)/N(HI)=-(4.93+-0.15) in
this system. This is a factor of two smaller than the value expected from the
best fitted value of Omega_b from the WMAP 5 yr data. This confirms the
presence of astration of deuterium even at very low metallicity. [abridged]Comment: 14 pages, 11 figures, 4 tables, accepted for publication in MNRA
SDSS J092712.64+294344.0: recoiling black hole or merging galaxies?
We report long-slit spectroscopic observations of SDSS J092712+294344
carried-out at the recently commissioned 2m telescope in IUCAA Girawali
Observatory, India. This AGN-like source is known to feature three sets of
emission lines at zem = 0.6972, 0.7020 and 0.7128. Different scenarios such as
a recoiling black hole after asymmetric emission of gravitational waves, binary
black holes and possible merging systems are proposed for this object. We test
these scenarios by comparing our spectra with that from the Sloan Digital Sky
Survey (SDSS), obtained 4 years prior to our observations. Comparing the
redshifts of [OIII]4960,5008 we put a 3 sigma limit on the relative
acceleration to be less than 32 km s^-1 yr^-1 between different emitting
regions. Using the 2D spectra obtained at different position angles we show
that the [OIII]5008 line from the zem = 0.7128 component is extended beyond the
spectral point spread function. We infer the linear extent of this line
emitting region is ~8 kpc. We also find a tentative evidence for an offset
between the centroid of the [OIII]5008 line at zem = 0.7128 and the QSO trace
when the slit is aligned at a position angle of 299 degrees. This corresponds
to the zem = 0.7128 system being at an impact parameter of ~1 kpc with respect
to the zem = 0.6972 in the north west direction. Based on our observations we
conclude that the binary black hole model is most unlikely. The spatial extent
and the sizes are consistent with both black hole recoil and merging scenarios.Comment: accepted for publication in MNRAS Letter
Search for cold gas in z>2 damped Lyman-alpha systems: 21-cm and H_2 absorption
(Abridged) We present the results of a systematic GBT and GMRT survey for
21-cm absorption in a sample of 10 DLAs at 2<z_abs<3.4. Analysis of L-band VLBA
images of the background QSOs are also presented. We detect 21-cm absorption in
only one DLA (at z_abs = 3.1745 towards J1337+3152). Combining our data with
the data from the literature (a sample of 28 DLAs) and assuming the measured
core fraction at milliarcsecond scale to represent the gas covering factor, we
find that the HI gas in DLAs at z> 2 is predominantly constituted by WNM. The
detection rate of 21-cm absorption seems to be higher for systems with higher
N(HI) or metallicity. However, no clear correlation is found between the
integrated 21-cm optical depth (or spin temperature) and either N(HI),
metallicity or velocity spread of the low ionization species. There are 13 DLAs
in our sample for which high resolution optical spectra covering the expected
wavelength range of H_2 absorption are available. We report the detection of
H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS
0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack
of 21-cm and H_2 absorption in these systems can be explained if most of the HI
in these DLAs originate from low density high temperature gas. In one case we
have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both
species are detected but do not originate from the same velocity component. In
the remaining 2 cases 21-cm absorption is not detected despite the presence of
H_2 with evidence for the presence of cold gas. All this is consistent with the
idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc)
and contain only a small fraction (i.e typically <10%) of the total N(HI)
measured in the DLAs.Comment: Accepted for publication in MNRA
An investigation of the line of sight towards QSO PKS 0237-233
We present a detailed analysis of absorption systems along the line of sight
towards QSO PKS 0237-233 using a high resolution spectrum of signal-to-noise
ratio (SNR) ~ 60-80 obtained with the Ultraviolet and Visual Echelle
Spectrograph mounted on the Very Large Telescope. This line of sight is known
to show a remarkable overdensity of CIV systems that has been interpreted as
revealing the presence of a supercluster of galaxies. A detailed analysis of
each of these absorption systems is presented. In particular, for the z_abs =
1.6359 (with two components of logN(HI) = 18.45, 19.05) and z_abs = 1.6720
(logN(H I) = 19.78) sub-Damped Ly-alpha systems (sub-DLAs), we measure accurate
abundances (resp. [O/H] = -1.63(0.07) and [Zn/H] = - 0.57(0.05) relative to
solar). While the depletion of refractory elements onto dust grains in both
sub-DLAs is not noteworthy, photoionization models show that ionization effects
are important in a part of the absorbing gas of the sub-DLA at z_abs = 1.6359
(HI is 95 percent ionized) and in part of the gas of the sub-DLA at z_abs =
1.6359. The CIV clustering properties along the line of sight is studied in
order to investigate the nature of the observed overdensity. We conclude that
despite the unusually high number of CIV systems detected along the line of
sight, there is no compelling evidence for the presence of a single unusual
overdensity and that the situation is consistent with chance coincidence.Comment: Accepted for publication in MNRAS. 23 pages, 16 figures, 12 table
Consensus report on 25 years of searches for damped Ly galaxies in emission: Confirming their metallicity-luminosity relation at
Starting from a summary of detection statistics of our recent X-shooter
campaign, we review the major surveys, both space and ground based, for
emission counterparts of high-redshift damped Ly absorbers (DLAs)
carried out since the first detection 25 years ago. We show that the detection
rates of all surveys are precisely reproduced by a simple model in which the
metallicity and luminosity of the galaxy associated to the DLA follow a
relation of the form, , and the DLA cross-section follows a relation of the form
. Specifically, our spectroscopic campaign
consists of 11 DLAs preselected based on their equivalent width of SiII
to have a metallicity higher than [Si/H] > -1. The targets have
been observed with the X-shooter spectrograph at the Very Large Telescope to
search for emission lines around the quasars. We observe a high detection rate
of 64% (7/11), significantly higher than the typical 10% for random,
HI-selected DLA samples. We use the aforementioned model, to simulate the
results of our survey together with a range of previous surveys: spectral
stacking, direct imaging (using the `double DLA' technique), long-slit
spectroscopy, and integral field spectroscopy. Based on our model results, we
are able to reconcile all results. Some tension is observed between model and
data when looking at predictions of Ly emission for individual targets.
However, the object to object variations are most likely a result of the
significant scatter in the underlying scaling relations as well as
uncertainties in the amount of dust which affects the emission.Comment: 25 pages (7 of which in appendix), accepted for publication in MNRA
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