1,518 research outputs found
Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255
Results of a careful analysis of the absorption systems with zabs = zem seen
toward the bright, z_em ~ 3.91, gravitationally lensed quasar APM 08279+5255
are presented. Two of the narrow-line systems, at z_abs = 3.8931 and z_abs =
3.9135, show absorptions from singly ionized species with weak or no NV and O V
absorptions at the same redshift. Absorption due to fine structure transitions
of C II and Si II (excitation energies corresponding to, respectively,
156m and 34m) are detected at z_abs = 3.8931. Excitation by IR
radiation is favored as the column density ratios are consistent with the shape
of APM 08279+5255 IR spectrum. The low-ionization state of the system favors a
picture where the cloud is closer to the IR source than to the UV source,
supporting the idea that the extension of the IR source is larger than ~ 200
pc. The absence of fine structure lines at z_abs = 3.9135 suggests that the gas
responsible for this system is farther away from the IR source. Abundances are
~ 0.01 and 1 at z_abs = 3.913 and 3.8931 and aluminum could be
over-abundant with respect to silicon and carbon by at least a factor of two
and five. All this suggests that whereas the \zabs = 3.8931 system is probably
located within 200 pc from the QSO and ejected at a velocity larger than 1000
kms^{-1}, the \zabs = 3.9135 system is farther away and part of the
host-galaxy. (abridged)Comment: 15 pages with 15 figures (psfiles), To appear in A&
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
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
The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems
We have searched for molecular hydrogen in damped Lyman-alpha (DLA) and
sub-DLA systems at z>1.8 using UVES at the VLT. Out of the 33 systems in our
sample, 8 have firm and 2 have tentative detections of associated H2 absorption
lines. Considering that 3 detections were already known from past searches, H2
is detected in 13 to 20 percent of the newly-surveyed systems. We report new
detections of molecular hydrogen at z=2.087 and 2.595 toward, respectively, Q
1444+014 and Q 0405-443, and also reanalyse the system at z=3.025 toward Q
0347-383. We find that there is a correlation between metallicity and depletion
factor in both our sample and also the global population of DLA systems (60
systems in total). The DLA and sub-DLA systems where H2 is detected are usually
amongst those having the highest metallicities and the largest depletion
factors. Moreover, the individual components where H2 is detected have
depletion factors systematically larger than other components in the profiles.
In two different systems, one of the H2-detected components even has
[Zn/Fe]>=1.4. These are the largest depletion factors ever seen in DLA systems.
All this clearly demonstrates the presence of dust in a large fraction of the
DLA systems. The mean H2 molecular fraction is generally small in DLA systems
and similar to what is observed in the Magellanic Clouds. From 58 to 75 percent
of the DLA systems have log f<-6. This can be explained if the formation rate
of H2 onto dust grains is reduced in those systems, probably because the gas is
warm (T>1000 K) and/or the ionizing flux is enhanced relative to what is
observed in our Galaxy.Comment: 21 pages, 16 figures, MNRA
About the Algebraic Solutions of Smallest Enclosing Cylinders Problems
Given n points in Euclidean space E^d, we propose an algebraic algorithm to
compute the best fitting (d-1)-cylinder. This algorithm computes the unknown
direction of the axis of the cylinder. The location of the axis and the radius
of the cylinder are deduced analytically from this direction. Special attention
is paid to the case d=3 when n=4 and n=5. For the former, the minimal radius
enclosing cylinder is computed algebrically from constrained minimization of a
quartic form of the unknown direction of the axis. For the latter, an
analytical condition of existence of the circumscribed cylinder is given, and
the algorithm reduces to find the zeroes of an one unknown polynomial of degree
at most 6. In both cases, the other parameters of the cylinder are deduced
analytically. The minimal radius enclosing cylinder is computed analytically
for the regular tetrahedron and for a trigonal bipyramids family with a
symmetry axis of order 3.Comment: 13 pages, 0 figure; revised version submitted to publication
(previous version is a copy of the original one of 2010
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
A collimated flow driven by radiative pressure from the nucleus of quasar Q~1511+091
High velocity outflows from quasars are revealed by the absorption signatures
they produce in the spectrum of the quasar. Clues on the nature and origin of
these flows are important for our understanding of the dynamics of gas in the
central regions of the Active Galactic Nucleus (AGNs) but also of the metal
enrichment of the intergalactic space. Line radiation pressure has often been
suggested to be an important process in driving these outflows, however no
convincing evidence has been given so far. Here we report observation of a
highly structured flow, toward Q~1511+091, where the velocity separations
between distinct components are similar to O VI, N V and C IV doublet
splittings with some of the profiles matching perfectly. This strongly favors
the idea that the absorbing clumps originate at similar physical location and
are driven by radiative acceleration due to resonance lines. The complex
absorption can be understood if the flow is highly collimated so that the
different optically thick clouds are aligned and cover the same region of the
background source. One component shows saturated H I Lyman series lines
together with absorptions from excited levels from C II and Si II but covers
only 40% of the source of continuum. The fact that clouds cover only part of
the small continuum source implies that the flow is located very close to it.Comment: 6 pages, 5 figures to appear in MNRA
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
The mass-metallicity relation for high-redshift damped Ly-alpha galaxies
We used our database of ESO VLT-UVES spectra of quasars to build up a sample
of 67 Damped Lyman-alpha (DLA) systems with redshifts 1.7<zabs<3.7. For each
system, we measured average metallicities relative to Solar, [X/H] (with either
X=Zn, S or Si), and the velocity widths of low-ionization line profiles, W1. We
find that there is a tight correlation between the two quantities, detected at
the 5sigma significance level. The existence of such a correlation, over more
than two orders of magnitude spread in metallicity, is likely to be the
consequence of an underlying mass-metallicity relation for the galaxies
responsible for DLA absorption lines. The best-fit linear relation is
[X/H]=1.35(\pm 0.11)\log W1 -3.69(\pm 0.18)$ with W1 expressed in km/s. While
the slope of this velocity-metallicity relation is the same within
uncertainties between the higher and the lower redshift bins of our sample,
there is a hint of an increase of the intercept point of the relation with
decreasing redshift. This suggests that galaxy halos of a given mass tend to
become more metal-rich with time. Moreover, the slope of this relation is
consistent with that of the luminosity-metallicity relation for local galaxies.
The DLA systems having the lowest metallicities among the DLA population would
therefore, on average, correspond to the galaxies having the lowest masses. In
turn, these galaxies should have the lowest luminosities among the DLA galaxy
population. This may explain the recent result that the few DLA systems with
detected Ly-alpha emission have higher than average metallicities.Comment: proceedings of IAU Colloquium No. 199, 2005, ``Probing Galaxies
through Quasar Absorption Lines'', P.R. Williams, C. Shu, B. Menard, ed
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