8,636 research outputs found
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
CII* Absorption in Damped Lyman Alpha Systems: (II) A New Window on the Star Formation History of the Universe
Starting from the SFR per unit physical area, determined for DLAs using the C
II* method, we obtain the SFR per unit comoving volume at 3, and
find that it agrees with that for the Lyman Break Galaxies (LBGs). Though the
mass of produced stars indicated by the SFRs is consistent with the current
densities of known stellar populations, the mass of metals produced by =2.5
is 30 times larger than detected in absorption in DLAs. The most likely
solutions to this ``missing metals'' problem is that star formation occurs in
compact bulge regions. We search for evidence of feedback and find no
correlations between the SFR per unit area and N(H I), but possible
correlations between SFR per unit area and low-ion velocity width and SFR per
unit area and metal abundance. We show that (a) the correlation between cooling
rate and dust-to-gas ratio is positive evidence for grain photoelectric
heating, (b) the CMB does not significantly populate the C II excited
fine-structure states, and (c) the ratio of CII* to resonance-line optical
depths is a sensitive probe of the multi-phase structure of the DLA gas. We
address recent arguments that DLAs are comprised only of WNM gas, and show them
to be inconclusive. Despite the rough agreement between SFR per unit comoving
volume for DLAs and LBGs, current evidence indicates these are distinct
populations
Velocity-Metallicity Correlation for high-z DLA Galaxies: Evidence for a Mass-Metallicity Relation?
We used our database of VLT-UVES quasar spectra to build up a sample of 70
Damped Lyman-alpha (DLA) or strong sub-DLA systems with total neutral hydrogen
column densities of log N(HI)>20 and redshifts in the range 1.7<z_abs<4.3. For
each of the systems, we measured in an homogeneous manner the metallicities
relative to Solar, [X/H] (with X=Zn, or S or Si), and the velocity widths of
low-ionization line profiles, Delta V. We provide for the first time evidence
for a correlation between DLA metallicity and line profile velocity width,
which is detected at the 6.1sigma significance level. This confirms the trend
previously observed in a much smaller sample by Wolfe & Prochaska (1998). The
best-fit linear relation is [X/H]=1.55(\pm 0.12) log Delta V -4.33(\pm 0.23)
with Delta V expressed in km/s. The slope of the DLA velocity-metallicity
relation is the same within uncertainties between the higher (z_abs>2.43) and
the lower (z_abs<2.43) redshift halves of our sample. However, the two
populations of systems are statistically different. There is a strong redshift
evolution in the sense that the mean metallicity and mean velocity width
increase with decreasing redshift. We argue that the existence of a DLA
velocity-metallicity correlation, over more than a factor of 100 spread in
metallicity, is probably the consequence of an underlying mass-metallicity
relation for the galaxies responsible for DLA absorption lines. Assuming a
simple linear scaling of the galaxy luminosity with the mass of the dark-matter
halo, we find that the slope of the DLA velocity-metallicity relation is
consistent with that of the luminosity-metallicity relation derived for local
galaxies. [...] abridged.Comment: 10 pages, 4 figures, A&A in pres
Monopole Dynamics in N=2 super Yang-Mills Theory From a Threebrane Probe
The BPS states of N=2 super Yang-Mills theory with gauge group SU(2) are
constructed as non-trivial finite-energy solutions of the worldvolume theory of
a threebrane probe in F theory. The solutions preserve 1/2 of N=2 supersymmetry
and provide a worldvolume realization of strings stretching from the probe to a
sevenbrane. The positions of the sevenbranes correspond to singularities in the
field theory moduli space and to curvature singularities in the supergravity
background. We explicitly show how the UV cut off of the effective field theory
is mapped into an IR cut off in the supergravity. Finally, we discuss some
features of the moduli spaces of these solutions.Comment: 26 pages, harvmac, some equations corrected in section 3. No
conclusions changed, references added. Apologies to those not previously
referenced, final version to appear in Nucl. Phys.
Dynamics of the spontaneous breakdown of superhydrophobicity
Drops deposited on rough and hydrophobic surfaces can stay suspended with gas
pockets underneath the liquid, then showing very low hydrodynamic resistance.
When this superhydrophobic state breaks down, the subsequent wetting process
can show different dynamical properties. A suitable choice of the geometry can
make the wetting front propagate in a stepwise manner leading to {\it
square-shaped} wetted area: the front propagation is slow and the patterned
surface fills by rows through a {\it zipping} mechanism. The multiple time
scale scenario of this wetting process is experimentally characterized and
compared to numerical simulations.Comment: 7 pages, 5 figure
High matter density peaks from UVES observations of QSO pairs: correlation properties and chemical abundances
We study the transverse clustering properties of high matter density peaks as
traced by high column density absorption systems (either Lyman limit systems
characterized by N(HI)> 2 x 10^{17} cm^{-2} or CIV systems with W_{r}> 0.5 A)
at redshifts between 2 and 3 with UVES spectra of two QSO pairs (UM680/UM681 at
56 arcsec angular separation and Q2344+1228/Q2343+1232 at 5 arcmin angular
separation) and a QSO triplet (Q2139-4433/Q2139-4434/Q2138-4427 at 1, 7 and 8
arcmin angular separation). We find 3 damped Ly-alpha systems (N(HI)> 2 x
10^{20} cm^{-2}): 2 coinciding with strong metal systems in the nearby line of
sight and 1 matching the emission redshift of the paired QSO; plus 7 Lyman
limit systems: 4 forming two matching couples and 3 without a corresponding
metal system within ~3000 km/s in the coupled line of sight. In summary, we
detect five out of ten matching systems within 1000 km/s, indicating a highly
significant overdensity of strong absorption systems over separation lengths
from ~1 to 8 h^{-1} Mpc. The observed coincidences could arise in gas due to
starburst-driven superwinds associated with a quasar or a galaxy, or gas
belonging to large scale structures like filaments or sheets. We also determine
chemical abundance ratios for three damped Ly-alpha systems. In particular, for
the damped system at z ~ 2.53788 in the spectrum of Q2344+1228, new estimates
of the ratios O/Fe, C/Fe are obtained: [C/Fe]<0.06, [O/Fe]<0.2. They indicate
that O and C are not over-solar in this system.Comment: 14 pages, 15 figures, accepted for publication in A&
A new constraint on cosmological variability of the proton-to-electron mass ratio
Exotic cosmologies predict variability of the fundamental physical constants
over the cosmic time. Using the VLT/UVES high resolution spectra of the quasar
Q0347-3819 and unblended electronic - vibrational - rotational lines of the H2
molecule identified at z = 3.025 we test possible changes in the proton - to -
electron mass ratio mu_0 = m_p/m_e over the period of 11 Gyr. We obtained a new
constraint on the time - averaged variation rate of mu_0 of |d mu /d t /mu_0| <
5 10^{-15} yr^{-1} (1 sigma c.l.). The estimated 1 sigma uncertainty interval
of the |Delta mu/mu_0| ratio of about 0.004% implies that since the time when
the H2 spectrum was formed at z = 3.025, mu_0 has not changed by more than a
few thousands of a percent.Comment: 5 pages, 3 figures, a revised version accepted by MNRA
XMM-Newton and Swift observations prove GRB 090709A to be a distant, standard, long GRB
GRB 090709A is a long gamma-ray burst (GRB) discovered by Swift, featuring a
bright X-ray afterglow as well as a faint infrared transient with very red and
peculiar colors. The burst attracted a large interest because of a possible
quasi-periodicity at P=8.1 s in the prompt emission, suggesting that it could
have a different origin with respect to standard, long GRBs. In order to
understand the nature of this burst, we obtained a target of opportunity
observation with XMM-Newton. X-ray spectroscopy, based on XMM-Newton and Swift
data, allowed us to model the significant excess in photoelectric absorption
with respect to the Galactic value as due to a large column density (about
6.5E+22 cm^-2) in the GRB host, located at z=4.2. Such a picture is also
consistent with the infrared transient's properties. Re-analysis of the prompt
emission, based on INTEGRAL and on Swift data, excludes any significant
modulation at P=8.1 s. Thus, we conclude that GRB 090709A is a distant,
standard, long GRB.Comment: 7 pages, 7 figures, 2 tables; MNRAS, in press. (V3: minor changes to
match the final version.
Herd-level risk factors of bovine tuberculosis in England and Wales after the 2001 foot-and-mouth disease epidemic
We present the results of a 2005 caseâcontrol study of bovine tuberculosis (bTB) breakdowns in English and Welsh herds. The herd management, farming practices, and environmental factors of 401matched pairs of case and control herds were investigated to provide a picture of herd-level risk factors in areas of varying bTB incidence. A global conditional logistic regression model, with region-specific variants, was used to compare case herds that had experienced a confirmed bTB breakdown to contemporaneous control herds matched on region, herd type, herd size, and parish testing interval. Contacts with cattle from contiguous herds and sourcing cattle from herds with a recent history of bTB were associated with an increased risk in both the global and regional analyses. Operating a farm over several premises, providing cattle feed inside the housing, and the presence of badgers were also identified as significantly associated with an increased bTB risk. Steps taken to minimize cattle contacts with neighboring herds and altering trading practices could have the potential to reduce the size of the bTB epidemic. In principle, limiting the interactions between cattle and wildlife may also be useful; however this study did not highlight any specific measures to implement
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