Giovanni Vladilo

Ionization Properties and Elemental Abundances in Damped Lyα Systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of studying the ionization properties of damped Lyα systems (DLAs) from the analysis of the ratio R(Al+2/Al+) ≡ N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abundances are not severely influenced by ionization effects. We find a log R(Al+2/Al+)-log N(H0) anticorrelation that we use in conjunction with idealized photoionization equilibrium calculations to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bearing regions. The log R(Al+2/Al+)-log N(H0) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements, the corrections are generally below measurements errors, which is contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies in the Al recombination rates used in the photoionization calculations.

Dust and Elemental Abundances in Damped Lyα Absorbers

The effects of the dust on the determination of elemental abundances in damped Lyα (DLA) absorbers are investigated. Relations between the observed abundances measured in the gas phase and the overall abundances (gas plus dust) are derived as a function of dust-to-gas ratio, metallicity, element-to-element abundance pattern, average extinction coefficient of dust grains, and chemical composition of dust grains. A method is presented for determining dust-to-gas ratios, dust-to-metals ratios, and dust-corrected relative abundances in DLA absorbers by assuming dust of Galactic type and constant abundance ratios between iron-peak elements. The method is applied to a sample of 17 DLA absorbers with available Zn, Cr, and/or Fe measurements. The resulting dust-to-gas ratios are mostly distributed between 2% and 25% of the Galactic value, in good quantitative agreement with the results from reddening studies of QSOs with foreground DLA absorption. A correlation is found between dust-to-gas ratio and metallicity in DLA galaxies, with a typical dust-to-metals ratio of ≈ 60% of the Galactic value. The derived dust-to-metals ratios are then used to correct from the effects of dust the abundance ratios [Si/Fe], [S/Fe], [Ti/Fe], [Mn/Fe], and [Ni/Fe] available for a subsample of nine absorbers. The [α/Fe] ratios corrected from dust do not show the enhancement characteristic of metal-poor Galactic stars, but instead have essentially solar values, within ±0.2 dex. This suggests that the chemical history of DLA absorbers is different from that experienced by the Milky Way. Evidence that points to dwarf galaxies, rather than to spiral galaxies, as important contributors to the DLA phenomenon is summarized.

Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman α systems ,

We present 4 new measurements of nitrogen abundances and one upper limit in damped Ly α absorbers (DLAs) obtained by means of high resolution (FWHM � 7k m s −1 ) UVES/VLT spectra. In addition to these measurements we have compiled data from all DLAs with measurements of nitrogen and α-capture elements (O, S or Si) available in the literature, including all HIRES/Keck and UVES/VLT data for a total of 32 systems, i.e. the largest sample investigated so far. We find that (N/α) ratios are distributed in two groups: 75% of the DLAs show a mean value of (N/α) = −0.87 with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at (N/α) = −1.45 with an even lower dispersion of 0.05 dex. The high (N/α) �− 0.9 plateau is consistent with the one observed in metal-poor H  regions of blue compact dwarf (BCD) galaxies ((N/α) = −0.73 ± 0.13), while the (N/α) �− 1.5 values are the lowest ever observed in any astrophysical site. These low (N/α) ratios are real and are not due to ionization effects. They provide crucial evidence against the primary production of N by massive stars as being responsible for the plateau at −0.9/−0.7 dex observed in DLAs and BCD galaxies. The transition between the low-N ((N/α) �− 1.5) and high-N ((N/α) �− 0.9) DLAs occurs at a nitrogen abundance of (N/H) �− 2.8, suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The (N/α) �− 1.5 values and their low dispersion are consistent with a modest production of primary N in massive stars; however, due to the limited sample, specially for the low-N DLAs, we cannot exclude a primary origin in intermediate mass stars as responsible for the low N abundances observed. Nitrogen is an important element when attempting to under- stand the chemical evolution of galaxies. The synthesis of N in the CNO cycle during hydrogen burning is reasonably well understood, but the characteristics of the stars that produce this element (range of masses, stage of evolution, etc.) are not completely clear. N is mostly a secondary element, produced in the CNO cycle from seed C and O nuclei created in ear- lier generation of stars. Primary N is produced when freshly synthesized C in the helium-burning shell penetrates into the hydrogen-burning shell where it will be converted in primary N by means of the CNO cycle. In evolution models of intermedi- ate mass stars, thermal pulses occurring during the asymptotic giant branch AGB phase are responsible for the transport of

UVES Observations of QSO 0000–2620: Oxygen and Zinc Abundances in the Damped Lyα Galaxy at zabs = 3.3901*

Observations of the QSO 0000-2620 with the Ultraviolet-Visual Echelle Spectrograph at the 8.2 m ESO Kueyen telescope are used for abundance analysis of the damped Lyα system at zabs = 3.3901. Several oxygen lines are identified in the Lyα forest, and a measure for the oxygen abundance is obtained at [O/H] = -1.85 ± 0.15 by means of the unsaturated O I λ925 and O I λ950 lines. This represents the most accurate O measurement in a damped Lyα galaxy so far. We have also detected Zn II λ2026 and Cr II λλ2056, 2062 redshifted at ≈8900 A and found abundances [Zn/H] = -2.07 ± 0.10 and [Cr/H] = -1.99 ± 0.09. Furthermore, previous measurements of Fe, Si, Ni, and N have been refined, yielding [Fe/H] = -2.04 ± 0.09, [Si/H] = -1.90 ± 0.08, [Ni/H] = -2.27 ± 0.09, and [N/H] = -2.68 ± 0.12. The abundance of the nonrefractory element zinc is the lowest among the damped Lyα systems, showing that the associated intervening galaxy is indeed in the early stages of its chemical evolution. The fact that the Zn abundance is identical to that of the refractory elements Fe and Cr suggests that dust grains have not formed yet. In this damped Lyα system the observed [O, S, Si/Zn, Fe, Cr] ratios, in whatever combination are taken, are close to solar (i.e., 0.1-0.2 dex) and do not show the [α-element/Fe] enhancement observed in Milky Way stars of comparable metallicity. The observed behavior supports a galaxy evolution model characterized by either episodic or low star formation rate rather than a Milky Way-type evolutionary model.

A SINFONI integral field spectroscopy survey for galaxy counterparts to damped Lyman α systems – I. New detections and limits for intervening and associated absorbers★

Detailed studies of damped and sub-damped Lyman alpha systems (DLAs), the galaxies probed by the absorption they produce in the spectra of background quasars, rely on identifying the galaxy responsible for the absorber with more traditional methods. Integral field spectroscopy provides an efficient way of detecting faint galaxies near bright quasars, further providing immediate redshift confirmation. Here, we report the detection of H alpha emission from a DLA and a sub-DLA galaxy among a sample of six intervening quasar absorbers targeted. We derive F(H alpha) = 7.7 +/- 2.7 x 10(-17) erg s(-1) cm(-2) (SFR = 1.8 +/- 0.6M(circle dot) yr(-1)) at impact parameter b = 25 kpc towards quasar Q0302 - 223 for the DLA at z(abs) = 1.009 and F(H alpha) = 17.1 +/- 6.0 x 10(-17) erg s(-1) cm(-2) (SFR = 2.9 +/- 1.0M(circle dot) yr(-1)) at b = 39 kpc towards Q1009 - 0026 for the sub-DLA at z(abs) = 0.887. These results are in line with low star formation rates previously reported in the literature for quasar absorbers. We use the [N II]lambda 6585/H alpha ratio to derive the H II emission metallicities and compare them with the neutral gas HI absorption metallicities derived from high-resolution spectra. In one case, the absorption metallicity is actually found to be higher than the emission line metallicity. For the remaining objects, we achieve 3 sigma limiting fluxes of the order F(H alpha) similar to 10(-17) erg s(-1) cm(-2) (corresponding to SFR similar to 0.1 M-circle dot yr(-1) at z similar to 1 and similar to 1 M-circle dot yr(-1) at z similar to 2), i.e. among the lowest that have been possible with ground-based observations. We also present two other galaxies associated with C IV systems and serendipitously discovered in our data.

The dust obscuration bias in damped Lyman

We present a new study of the effects of quasar obscuration on the statistics of Damped Ly α (DLA) systems. We show that the extinction of any Galactic or extragalactic H i region, Aλ, increases linearly with the column density of zinc, with a turning point ∂Aλ/∂(logNZn) = 1, above which background sources are suddenly obscured. We derive a relation Aλ = Aλ(NH,Z,z) between the extinction of a DLA system and its H i column density, NH, metallicity, Z, fraction of iron in dust, fFe(Z), and redshift, z. From this relation we estimate the fraction of DLA systems missed as a consequence of their own extinction in magnitude-limited surveys. We derive a method for recovering the true frequency distributions of NH and Z in DLAs, fNH and fZ, using the biased distributions measured in the redshift range where the observations have sufficient statistics (1.8 ≤ z ≤ 3). By applying our method we find that the well-known empirical thresholds of DLA column densities, NZn < 10 13.1 atoms cm −2 and NHi < 10 22 atoms cm −2 , can be successfully explained in terms of the obscuration effect without tuning of the local dust parameters. The obscuration has a modest effect on the distribution of quasar apparent magnitudes, but plays an important role in shaping the statistical distributions of DLAs. The exact estimate of the bias is still limited by the paucity of the data (� 40 zinc measurements at 1.8 ≤ z ≤ 3). We find that the fraction of DLAs missed as a consequence of obscuration is ∼30% to 50%, consistent with the results of surveys of radio-selected quasars. By modelling the metallicity distribution with a Schechter function we find that the mean metallicity can be ∼ 5t o 6 times higher than the value commonly reported for DLAs at z ∼ 2.3.

\alpha

Relative elemental abundances, and in particular the α/Fe ratio, are important diagnostic tools of the chemical evolution of damped Lyα (DLA) systems. The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the α/Fe ratio. We report six new determinations of sulphur abundance in DLA systems at zabs ≥ 2 with already-known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high-redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [α/Fe] enhancement observed in metal-poor stars of the Milky Way at a comparable level of metallicity ([Zn/H] ≈ -1). The behavior of DLA data is consistent with a general trend of a decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the α/Fe ratio in the course of the chemical evolution of DLA systems. However, in contrast to what is observed in our Galaxy, the α/iron peak ratio seems to attain solar values when the metallicity is still low ([Zn/H] ≤ -1) and to decrease below solar values at higher metallicities. The behavior of the α/Fe ratio challenges the frequently adopted hypothesis that high-redshift DLA systems are progenitors of spiral galaxies and instead favors an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low-redshift DLA systems, where the candidate DLA galaxies show a variety of morphological types, including dwarf and low surface brightness galaxies and only a minority of spirals.

systems

We study the nature of damped Lyman α systems (DLAs) by means of a comparison between the observed abundances and models of chemical evolution of galaxies of different morphological type. In particular, we compare for the first time the abundance ratios as functions of metallicity and redshift with dust-corrected data. We have developed detailed models following the evolution of several chemical elements (H, D, He, C, N, O, Ne, Mg, Si, S, Fe, Ni and Zn) for elliptical, spiral and irregular galaxies. Each of the models is calibrated to reproduce the main features of a massive elliptical, the Milky Way and the Large Magellanic Cloud (LMC), respectively. In addition, we also run some models for dwarf irregular starburst galaxies. All the models share the same up to date nucleosynthesis prescriptions but differ in their star formation histories. The role of supernovae of different types (II, Ia) is studied in each galaxy model. Our main conclusions are as follows. (i) When dust depletion is taken into account, most of the claimed α/Fe overabundances disappear and DLAs show solar or subsolar abundance ratios. (ii) The majority of DLAs can be explained either by discs of spirals observed at large galactocentric distances or by irregular galaxies, such as the LMC, or by starburst dwarf irregulars observed at different times after the last burst of star formation. (iii) Elliptical galaxies cannot be DLA systems because they reach too high a metallicity at early times and their abundance ratios show overabundances of α-elements relative to Fe over a large range of [Fe/H]. (iv) The observed neutral gas cosmic evolution is compared with our predictions but no firm conclusions can be drawn in the light of the available data.

Chemical Evolution of Damped Lyα Galaxies: The [S/Zn] Abundance Ratio at Redshift ≥2*

Context. Absorption-line systems detected in quasar spectra can be used to compare the value of the fine-structure constant, , measured today on Earth with its value in distant galaxies. In recent years, some evidence has emerged of small temporal and also spatial variations in on cosmological scales. These variations may reach a fractional level of 10 ppm (parts per million). Aims. To test these claims we are conducting a Large Program of observations with the Very Large Telescope’s Ultraviolet and Visual Echelle Spectrograph (UVES), and are obtaining high-resolution (R 60 000) and high signal-to-noise ratio (S=N 100) UVES spectra calibrated specifically for this purpose. Here we analyse the first complete quasar spectrum from this programme, that of HE 2217 2818. Methods. We applied the many multiplet method to measure in five absorption systems towards this quasar: zabs = 0:7866, 0.9424, 1.5558, 1.6279 , and 1.6919. Results. The most precise result is obtained for the absorber at zabs = 1:6919 where 3 Feii transitions and Alii 1670 have high S/N and provide a wide range of sensitivities to . The absorption profile is complex with several very narrow features, and it requires 32 velocity components to be fitted to the data. We also conducted a range of tests to estimate the systematic error budget. Our final result for the relative variation in in this system is = = +1:3 2:4stat 1:0sys ppm. This is one of the tightest current bounds on -variation from an individual absorber. A second, separate approach to the data reduction, calibration, and analysis of this system yielded a slightly di erent result of 3.8 2:1stat ppm, possibly suggesting a larger systematic error component than our tests indicated. This approach used an additional 3 Feii transitions, parts of which were masked due to contamination by telluric features. Restricting this analysis to the Feii transitions alone and using a modified absorption profile model gave a result that is consistent with the first approach, = = +1:1 2:6stat ppm. The four other absorbers have simpler absorption profiles, with fewer and broader features, and o er transitions with a narrower range of sensitivities to . They therefore provide looser bounds on = at the > 10 ppm precision level.

Chemical evolution and nature of damped Lyman α systems

We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z ∼ 6. This paper focuses on the properties of metals at high redshift traced, in particular, by the C IV doublet absorption systems. Six objects were observed with resolutions �27 and 34 km s −1 in the visual, and 37.5 and 53.5 km s −1 in the near-infrared. We detected 102 C IV lines in the range: 4.35 <z< 6.2 of which 27 are above z ∼ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si IV doublets associated with the C IV at z 5. The column density distribution function of the C IV line sample is observed to evolve in redshift for z 5.3, with respect to the normalization defined by low-redshift (1.5 <z <4) C IV lines. This behaviour is reflected in the redshift evolution of the C IV cosmic mass density, � C IV, of lines with column density in the range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor of ∼ 2f or z 5.3. Considering only the stronger C IV lines (13.8 < log N(C IV) < 15), � C IV gently rises by a factor of ∼10 between z � 6.2 and z � 1.5 with a possible flattening towards z ∼ 0. The increase is well fitted by a power law: � C IV = (2 ± 1) × 10 −8 [(1 + z)/4] −3.1±0.1 .A n insight into the properties of the C IV absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C IV ,S iIV and C II absorptions with the output of a set of CLOUDY photoionization models. As already claimed by cosmological simulations, we find that C IV is a good tracer of the metallicity in the low-density intergalactic medium (IGM) gas at z ∼ 5−6 while at z ∼ 3 it arises in gas with overdensity δ ∼ 100.