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, 156$\mu$m and 34$\mu$m) 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$Z_{\odot}$ 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 at z=1.973 toward Q0013-004: Dust depletion pattern in damped Lyman-alpha systems

We study the dust depletion pattern in eight well separated components of the z=1.973, logN(HI)=20.83, damped Lyman-alpha system toward Q0013-004, four of which have detectable H2 absorption. The apparent correlation between the abundance ratios [Fe/S] and [Si/S] in the components indicates that the abundance pattern is indeed due to dust-depletion. In particular, we find evidence for depletion similar to what is observed in cold gas of the Galactic disk ([Fe/Zn]=-1.59, Fe/S=-1.74, Zn/S=-0.15, [Si/S]=-0.85) in one of the weakest components in which molecular hydrogen is detected with logN(H2)=16.5. This is the first time such a large depletion is seen in a DLA system. This observation supports the possibility that current samples of DLA systems might be biased against the presence of cold and dusty gas along the line of sight. The overall metallicities of this peculiar DLA system in which OI and CII are spread over 1050 km/s are [P/H]=-0.64, [Zn/H]=-0.74 and [S/H]=-0.82 relative to solar. The sub-DLA system at z=1.96753 has [P/H]>0.06, [Zn/H]>-0.02 and [S/H]>-0.18. The overall molecular fraction is in the range -2.7<logf<-0.6. CO is not detected (logN(CO)/N(HI)<-8) and HD could be present at z=1.97380. We show that the presence of H2 is closely related to the physical conditions of the gas: high particle density together with low temperature. The observed excitation of high J H2 levels and the molecular fraction show large variations from one component to the other suggesting that the UV radiation field is highly inhomogeneous throughout the system. Gas pressure, estimated from CI absorptions, is larger than what is observed in the ISM of our Galaxy. All this is probably a consequence of intense star-formation activity in the vicinity of the absorbing gas. (Abridged)Comment: 9 pages, 8 figures, MNRAS Latex. Accepted for publication in MNRA

Clustering at high redshift: The connection between Lyman Alpha emitters and Lyman break galaxies

We present a physically motivated semi-analytic model to understand the clustering of high redshift Lyman Alpha Emitters (LAEs). We show that the model parameters constrained by the observed luminosity functions, can be used to predict large scale bias and angular correlation function of LAEs. These predictions are shown to reproduce the observations remarkably well. We find that average masses of dark matter halos hosting LAEs brighter than threshold narrow band magnitude ~ 25 are ~ 10^11 M_\odot. These are smaller than that of typical Lyman Break Galaxies (LBGs) brighter than similar threshold continuum magnitude by a factor ~ 10. This results in a smaller clustering strength of LAEs compared to LBGs. However, using the observed relationship between UV continuum and Lyman-alpha luminosity of LAEs, we show that both LAEs and LBGs belong to the same parent galaxy population with narrow band techniques having greater efficiency in picking up galaxies with low UV luminosity. We also show that the lack of evidence for the presence of the one halo term in the observed LAE angular correlation functions can be attributed to sub-Poisson distribution of LAEs in dark matter halos as a result of their low halo occupations.Comment: Accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:1208.209

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

Multiwavelength investigation of a near-solar metallicity sub-DLA at z =1.3647 towards PKS 0237-233

We searched for 21-cm absorption associated with the z_abs = 1.3647 absorption system toward PKS 0237-233 using the GMRT. A high quality UVES spectrum shows that C I and C I* are detected at this redshift together with C II*, Mg I, Mg II, Si II, Al II, Fe II and Mn II. The complex profiles, spread over ~300 km/s, are fitted with 21 Voigt profile components. None of these components are detected in 21-cm absorption down to a detection limit of \tau(3\sigma)\le 3x10^{-3} (or N(HI)/T_S <10^{17} cm ^-2 K^-1). We derive log N(HI)<19.30$\pm$0.30 using the Lyman alpha absorption line detected in the IUE spectrum of the quasar. Mg II, Si II and Al II column densities are consistent with near solar metallicity and we measure [O/H]>-0.33. Using photoionization models constrained by the fine-structure excitations of C I and C II, and the 21-cm optical depth, we show that the C I absorption arises predominantly either in WIM or WNM in ionization and thermal equilibrium with the meta-galactic UV background dominated by QSOs and star forming galaxies. The estimated thermal pressure of the gas is of the same order of magnitude over different velocity ranges through the absorption profile (2.6\le log [P/k cm^{-3} K]\le 4.0). The gas-phase metallicity corrected for ionization is Z>0.5 Z_\odot with a signature of Fe co-production elements being under abundant compared to \alpha-process elements by ~0.5 dex. At z>1.9, C I absorption is usually associated with H_2 absorption arising from cold gas in DLAs. This system and the z=2.139 toward Tol 1037-270 are the only two systems known which show that C I absorption can also be detected in warm gas provided the metallicity is high enough. Interestingly, both the systems are part of unusual concentrations of absorption lines.Comment: 12 pages, 9 figures, accepted for publication in MNRA

He II optical depth and UV escape fraction of galaxies

We study the effect of H I ionizing photons escaping from high-redshift (high-z) galaxies have on the He II ionizing ultraviolet background (UVB) radiation. While these photons do not directly interact with He II ions, we show that they play an important role, through radiative transport, in modifying the shape of He II ionizing part of UVB spectrum. Within the observed range of UV escape from galaxies, we show that the rapid increase in He II Lyman alpha effective optical depth at z~2.7 can naturally be explained by radiative transport effects. Therefore, the relationship between a well measured He II Lyman alpha effective optical depth and the redshift in the post-He II reionization era can be used to place additional constraints on the redshift evolution of UV escape from high-z galaxies. Our study also suggests that the escape fraction of H I ionizing photons from galaxies has an important role in the fluctuations of the He II ionizing UVB.Comment: Published in MNRAS Letters, replacement of a figure and minor text changes corresponding to published versio