234 research outputs found

### The absorption spectrum of the QSO PKS 2126-158 (z_em = 3.27) at high resolution

Spectra of the z_em = 3.268 quasar PKS 2126-158 have been obtained in the
range lambda lambda 4300-6620 A with a resolution R = 27000 and an average
signal-to-noise ratio s/n = 25 per resolution element. The list of the
identified absorption lines is given together with their fitted column
densities and Doppler widths. The modal value of the Doppler parameter
distribution for the Ly-alpha lines is = 25 km/s. The column density
distribution can be described by a power-law dn/dN \propto N^{-beta} with beta
= 1.5. 12 metal systems have been identified, two of which were previously
unknown. In order to make the column densities of the intervening systems
compatible with realistic assumptions about the cloud sizes and the silicon to
carbon overabundance, it is necessary to assume a jump beyond the He II edge in
the spectrum of the UV ionizing background at z = 3 a factor 10 larger than the
standard predictions for the integrated quasar contribution. An enlarged sample
of C IV absorptions (71 doublets) has been used to analyze the statistical
properties of this class of absorbers strictly related to galaxies. The column
density distribution is well described by a single power-law, with beta=1.64
and the Doppler parameter distribution shows a modal value b = 14 km/s. The two
point correlation function has been computed in the velocity space for the
individual components of C IV features. A significant signal is obtained for
scales smaller than 200-300 km/s, xi(30< Delta v < 90 km/s) = 32.71 +- 2.89. A
trend of decreasing clustering amplitude with decreasing column density is
apparent, analogously to what has been observed for Ly-alpha lines.Comment: 16 pages, LaTeX (l-aa.sty), 13 ps figures, included in text with
epsf, Fig. 1 can be retrieved at http://www.sissa.it/~dodorico/paper.html,
accepted by Astronomy and Astrophysic

### Ly-alpha forest: efficient unbiased estimation of second-order properties with missing data

Context. One important step in the statistical analysis of the Ly-alpha
forest data is the study of their second order properties. Usually, this is
accomplished by means of the two-point correlation function or, alternatively,
the K-function. In the computation of these functions it is necessary to take
into account the presence of strong metal line complexes and strong Ly-alpha
lines that can hidden part of the Ly-alpha forest and represent a non
negligible source of bias. Aims. In this work, we show quantitatively what are
the effects of the gaps introduced in the spectrum by the strong lines if they
are not properly accounted for in the computation of the correlation
properties. We propose a geometric method which is able to solve this problem
and is computationally more efficient than the Monte Carlo (MC) technique that
is typically adopted in Cosmology studies. The method is implemented in two
different algorithms. The first one permits to obtain exact results, whereas
the second one provides approximated results but is computationally very
efficient. The proposed approach can be easily extended to deal with the case
of two or more lists of lines that have to be analyzed at the same time.
Methods. Numerical experiments are presented that illustrate the consequences
to neglect the effects due to the strong lines and the excellent performances
of the proposed approach. Results. The proposed method is able to remarkably
improve the estimates of both the two-point correlation function and the
K-function.Comment: A&A accepted, 12 pages, 15 figure

### The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observations

A high resolution (R = 110.000), very high S/N (>600) spectrum of the
metal-poor turnoff star G271-162 has been obtained in connection with the
commissioning of UVES at VLT/Kueyen. Using both 1D hydrostatic and 3D
hydrodynamical model atmospheres, the lithium isotope ratio has been estimated
from the LiI 670.8 nm line by means of spectral synthesis. The necessary
stellar line broadening (1D: macroturbulence + rotation, 3D: rotation) has been
determined from unblended KI, CaI and FeI lines. The 3D line profiles agree
very well with the observed profiles, including the characteristic line
asymmetries. Both the 1D and 3D analyses reveal a possible detection of 6Li in
G271-162, 6Li/7Li = 0.02 +-0.01 (one sigma). It is discussed if the smaller
amount of 6Li in G271-162 than in the similar halo star HD84937 could be due to
differences in stellar mass and/or metallicity or whether it may reflect an
intrinsic scatter of the Li isotope ratio in the ISM at a given metallicity.Comment: 5 pages with 6 figures. Accepted as a letter in A&

### Simulating cosmic metal enrichment by the first galaxies

We study cosmic metal enrichment via AMR hydrodynamical simulations in a (10
Mpc/h)$^3$ volume following the Pop III-Pop II transition and for different Pop
III IMFs. We have analyzed the joint evolution of metal enrichment on galactic
and intergalactic scales at z=6 and z=4. Galaxies account for <9% of the
baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e.
regions with extremely low density ($\Delta$<1), (b) the true intergalactic
medium (IGM, 1<$\Delta$<10) and (c) the circumgalactic medium (CGM,
10<$\Delta<10^{2.5}$), the interface between the IGM and galaxies. By z=6 a
galactic mass-metallicity relation is established. At z=4, galaxies with a
stellar mass $M_*=10^{8.5}M_\odot$ show log(O/H)+12=8.19, consistent with
observations. The total amount of heavy elements rises from
$\Omega^{SFH}_Z=1.52\, 10^{-6}$ at z=6 to 8.05 $10^{-6}$ at z=4. Metals in
galaxies make up to ~0.89 of such budget at z=6; this fraction increases to
~0.95 at z=4. At z=6 (z=4) the remaining metals are distributed in
CGM/IGM/voids with the following mass fractions: 0.06/0.04/0.01
(0.03/0.02/0.01). Analogously to galaxies, at z=4 a density-metallicity
($\Delta$-Z) relation is in place for the diffuse phases: the IGM/voids have a
spatially uniform metallicity, Z~$10^{-3.5}$Zsun; in the CGM Z steeply rises
with density up to ~$10^{-2}$Zsun. In all diffuse phases a considerable
fraction of metals is in a warm/hot (T>$10^{4.5}$K) state. Due to these
physical conditions, CIV absorption line experiments can probe only ~2% of the
total carbon present in the IGM/CGM; however, metal absorption line spectra are
very effective tools to study reionization. Finally, the Pop III star formation
history is almost insensitive to the chosen Pop III IMF. Pop III stars are
preferentially formed in truly pristine (Z=0) gas pockets, well outside
polluted regions created by previous star formation episodes.Comment: 23 pages, 18 figures, 3 tables, Accepted for publication in MNRA

### High resolution spectroscopy of the three dimensional cosmic web with close QSO groups

We study the three-dimensional distribution of matter at z~2 using high
resolution spectra of QSO pairs and simulated spectra drawn from cosmological
hydro-dynamical simulations. We present a sample of 15 QSOs, corresponding to
21 baselines of angular separations evenly distributed between ~1 and 14
arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at
the European Southern Observatory-Very Large Telescope (ESO-VLT). The observed
correlation functions of the transmitted flux in the HI Lya forest transverse
to and along the line of sight are in agreement, implying that the distortions
in redshift space due to peculiar velocities are relatively small and - within
the relatively large error bars - not significant. The clustering signal is
significant up to velocity separations of ~300 km/s, corresponding to about 5
h^{-1} comoving Mpc. Compatibility at the 2 sigma level has been found both for
the Auto- and Cross-correlation functions and for the set of the Cross
correlation coefficients. The analysis focuses in particular on two QSO groups
of the sample. Searching for alignments in the redshift space between Lya
absorption lines belonging to different lines of sight, it has been possible to
discover the presence of a wide HI structures extending over about ten Mpc in
comoving space, and give constraints on the sizes of two cosmic under-dense
regions in the intergalactic medium.Comment: Accepted by MNRAS, version matching the published on

### The clustering properties of the Lyman-alpha clouds

We analyze the clustering properties of a high-resolution (~10 km/s) sample
of about 1600 Lyman-alpha lines in the spectra of 15 quasars, obtained in the
framework of an ESO key-programme with the addition of literature data. The
two-point correlation function in the velocity space shows a significant signal
on small velocity scales (Delta v < 300 km/s) with amplitude and significance
increasing with increasing column density. The correlation scale at z~3 turns
out to be 200 - 300 h_{50}^{-1} kpc. A trend of increasing correlation with
decreasing redshift is apparent. The existence of over- and under-densities on
scales of a few tens of Megaparsec is confirmed with a high confidence level
and a number of possible structures are identified. The present observations
are found to be consistent with models of gravitationally induced correlations.
A continuity scenario between Lyman-alpha and metal systems emerges, with a
suggested physical association between the Lyman-$\alpha$ clouds with log
N_{HI} > 14 and the halos of protogalactic systems.Comment: 9 pages, 5 figures, MNRAS-Latex. Accepted for publication in the
Monthly Not. Roy. Astro. So

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