Sub-damped Lyman-alpha Systems Research Articles

SOAR imaging of sub-damped Lyman α systems at z < 1

We present deep ground based imaging of the environments of five QSOs that contain sub-Damped Lyman-alpha systems at z<1 with the SOAR telescope and SOI camera. We detect a clear surplus of galaxies in these small fields, supporting the assumption that we are detecting the galaxies responsible for the absorption systems. Assuming these galaxies are at the redshift of the absorption line systems, we detect luminous L>L* galaxies for four of the five fields within 10" of the QSO. In contrast to previous imaging surveys of DLA systems at these redshifts, which indicate a range of morphological types and luminosities for the host galaxies of the systems, the galaxies we detect in these sub-DLA fields appear to be luminous (L>L*). In the case of the absorber towards Q1009-0026 at z=0.8866 we have spectroscopic confirmation that the candidate galaxy is at the redshift of the absorber, at an impact parameter of ~35 kpc with a luminosity of 3 < L/L* < 8 depending on the magnitude of the K-correction. These observations are in concordance with the view that sub-DLAs may be more representative of massive galaxies than DLA systems. The environments of the absorbers span a range of types, from the inner disk of a galaxy, the periphery of a luminous galaxy, and the outskirts of interacting galaxies. The large impact parameters to some of the candidate galaxies suggest that galactic outflows or tidal tails are likely responsible for the material seen in absorption. We find a weak correlation between N(HI) and the impact parameter at the 2 sigma level, which may be expected from the heterogeneous population of galaxies hosting the absorption line systems and random orientation angles. In addition, we detect a possible gravitationally lensed image of the BL-Lac object Q0826-2230.

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 (DLA), 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 6 intervening quasar absorbers targeted. We derive F(H-alpha)=7.7+/-2.7*10^-17 erg/s/cm^2 (SFR=1.8+/-0.6 M_sun/yr) 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*10^-17 erg/s/cm^2 (SFR=2.9+/-1.0 M_sun/yr) 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 NII 6585/H-alpha ratio to derive the HII emission metallicities and compare them with the neutral gas H I 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)~10^-17 erg/s/cm^2 (corresponding to SFR~ 0.1 M_sun/yr at z~1 and ~1 M_sun/yr at z~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.

A SINFONI integral field spectroscopy survey for galaxy counterparts to damped Lyman α systems - II. Dynamical properties of the galaxies towards Q0302 − 223 and Q1009 − 0026★

Details of processes through which galaxies convert their gas into stars need to be studied in order to obtain a complete picture of galaxy formation. One way to tackle these phenomena is to relate the HI gas and the stars in galaxies. Here, we present dynamical properties of Damped and sub-Damped Lyman-alpha Systems identified in H-alpha emission with VLT/SINFONI at near infra-red wavelengths. While the DLA towards Q0302-223 is found to be dispersion-dominated, the sub-DLA towards Q1009-0026 shows clear signatures of rotation. We use a proxy to circular velocity to estimate the mass of the halo in which the sub-DLA resides and find M_halo=10^12.6 M_sun. We also derive dynamical masses of these objects, and find M_dyn=10^10.3 M_sun and 10^10.9 M_sun. For one of the two systems (towards Q0302-223), we are able to derive a stellar mass of M_*=10^9.5 M_sun from Spectral Energy Distribution fit. The gas fraction in this object is 1/3rd, comparable to similar objects at these redshifts. Our work illustrates that detailed studies of quasar absorbers can offer entirely new insights into our knowledge of the interaction between stars and the interstellar gas in galaxies.

A MIKE + UVES survey of sub-damped Lyman α systems atz&lt; 1.5

We have combined the results from our recent observations of Damped and sub-Damped Lyman-alpha systems with the MIKE and UVES spectrographs on the Magellan Clay and VLT Kueyen telescopes with ones from the literature to determine the N(HI)-weighted mean metallicity of these systems based both on Fe, a depleted element in QSO absorbers and the local ISM, and Zn a relatively undepleted element. In each case, the N(HI)-weighted mean metallicity is higher and shows faster evolution in sub-DLAs than the classical DLA systems. Large grids of photoionisation models over the sub-DLA \nhI range with CLOUDY show that the ionisation corrections to the abundances are in general small, however the fraction of ionized H can be up to ~90 per cent. The individual spectra have been shifted to the rest frame of the absorber and averaged together to determine the average properties of these systems at z<1.5. We find that the average abundance pattern of the Sub-DLA systems is similar to the gas in the halo of the Milky Way, with an offset of ~0.3 dex in the overall metallicity. Both DLAs and Sub-DLAs show similar characteristics in their relative abundances patterns, although the DLAs have smaller <[Mn/Zn]> as well as higher <[Ti/Zn]> and <[Cr/Zn]>. We calculate the contribution of sub-DLAs to the metal budget of the Universe, and find that the sub-DLA systems at z<1.5 contain a comoving density of metals Omega_met (3.5-15.8)x10^{5} M_sun Mpc^{-3}, at least twice the comoving density of metals in the DLA systems. The sub-DLAs do however track global chemical evolution models much more closely than do the DLAs, perhaps indicating that they are a less dust biased metallicity indicator of galaxies at high redshifts than the DLA systems.

Revisiting the origin of the high metallicities of sub-damped Lyman-alpha systems

Abstract Sub-damped Lyman-alpha systems (sub-DLAs) have previously been found to exhibit a steeper metallicity evolution than the classical damped Lyman-alpha systems (DLAs), evolving to close to solar metallicity by z∼ 1. From new high-resolution spectra of 17 sub-DLAs, we have increased the number of measurements of [Fe/H] at z &amp;lt; 1.7 by 25 per cent and compiled the most complete literature sample of sub-DLA and DLA abundances to date. We find that sub-DLAs are indeed significantly more metal-rich than DLAs, but only at z &amp;lt; 1.7; the metallicity distributions of sub-DLAs and DLAs at z &amp;gt; 1.7 are statistically consistent. We also present the first evidence that sub-DLAs follow a velocity width–metallicity correlation over the same velocity range as DLAs, but the relation is offset to higher metallicities than the DLA relation. On the basis of these results, we revisit the previous explanation that the systematically higher metallicities observed in sub-DLAs are indicative of higher host galaxy masses. We discuss the various problems that this interpretation encounters and conclude that in general sub-DLAs are not uniquely synonymous with massive galaxies. We rule out physically related sources of bias (dust, environment, ionization effects) and examine systematics associated with the selection and analysis of low-redshift sub-DLAs. We propose that the high metallicities of sub-DLAs at z &amp;lt; 1.7 that drives an apparently steep evolution may be due to the selection of most low-redshift sub-DLAs based on their high Mgii equivalent widths.

Metal-enriched plasma in protogalactic halos

We continue our recent work to characterize the plasma content of high-redshift damped and sub-damped Lyman-alpha systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey N V absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67<z<4.28 with unblended coverage of the N V doublet, using data from VLT/UVES, Keck/HIRES, and Keck/ESI. In DLAs, we find eight secure N V detections, four marginal detections, and 79 non-detections. The detection rate of N V in DLAs is therefore 13^{+5}_{-4}%. Two sub-DLA N V detections are found among a sample of 18, at a similar detection rate of 11^{+15}_{-7}%. We show that the N V detection rate is a strong function of metallicity, increasing by a factor of ~4 at [N/H]=[NI/HI]>-2.3. The N V and CIV component b-value distributions in DLAs are statistically similar, but the median b(N V) of 18 km/s is lower than the median b(O VI) of 25 km/s. Some ~20% of the N V components have b<10 km/s and thus arise in warm photoionized plasma at log (T/K)<4.92; local sources of ionizing radiation (as opposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining ~80% of (broad) N V components is unclear; models of radiatively-cooling collisionally-ionized plasma at log(T/K)=5.2-5.4 are fairly successful in reproducing the observed integrated high-ion column density ratios and the component line widths, but we cannot rule out photoionization by local sources. Finally, we identify several unusual DLAs with extremely low metallicity (<0.01 solar) but strong high-ion absorption [log N(N V)>14 or log N(O VI)>14.2] that present challenges to either galactic inflow or outflow models.

A homogeneous sample of sub-damped Lyman alpha systems - I. Construction of the sample and chemical abundance measurements

In this first paper of a series, we report on the use of quasar spectra obtained with the high-resolution Ultraviolet-Visual Echelle Spectrograph (UVES) and available through the European Southern Observatory (ESO) Very Large Telescope (VLT) archive to build the first homogeneous and unbiased sample of sub-damped Lyman a (sub-DLA) systems, absorbers with Hi column densities >10 1 9 cm - 2 but lower than the classical definition of damped Lyman a systems (2 x 10 2 0 cm - 2 ). A systematic investigation of the properties of these systems and a comparison with those of the DLAs (Paper II of this series) is expected to provide new clues on the association of high column density absorbers with galaxies and on the overall evolution of the neutral hydrogen gas mass and metal content in the Universe. In the spectra of 22 quasars which were found suitable for a sub-DLA search, we identified 12 sub-DLAs and one borderline case between the DLA and sub-DLA systems in the redshift interval z = 1.8-4.3. We measured the column densities of H I and of up to 16 additional ions of low, intermediate and high ionization: O I, C II, C IV, Si II, Si iv, N I, S II, Mg I, Mg II, Al II, Al III, Fe II, Fe III, Ni II, Zn II and Cr II. We further investigated the significance of the ionization corrections in the determination of the chemical abundances from the low-ionization ions in the sub-DLA HI column density range. Using the predictions of different ion ratios as a function of the ionization parameter computed using the CLOUDY software package, we have estimated that with the exception of one case, the ionization corrections to the abundances of nine systems for which we were able to constrain the ionization parameter are lower than 0.2 dex for all of the elements except Al II and Zn n down to H I column densities of log N(HI)=19.3 cm - 2 . Finally, we present the first sub-DLA chemical abundance data base, which contains the abundance measurements of 11 different elements (O, C, Si, N, S, Mg, Al, Fe, Ni, Zn and Cr). We took advantage of the lower H I column densities in sub-DLAs to measure, in particular, the O and C abundances using lines that are normally saturated in DLAs.

A homogeneous sample of sub-damped Lyman alpha systems - II. Statistical, kinematic and chemical properties

Damped Lyman α systems (DLAs), with N(H i) > 2 × 1020 atom cm−2, observed in the spectra of quasars have allowed us to quantify the chemical content of the Universe over cosmological scales. Such studies can be extended to lower column densities, in the sub-DLA range [1019 < N(H i) < 2 × 1020 atom cm−2], which are systems believed to contain a large fraction of the neutral hydrogen at z > 3.5. In this paper, we use a homogeneous sample of sub-DLAs from the European Southern Oberservatory (ESO) Ultraviolet–Visual Echelle Spectrograph (UVES) archives presented in Paper I, to determine observationally for the first time the shape of the column density distribution, ƒ(N), down to N(H i) = 1019 atom cm−2. The results are in good agreement with the predictions from Péroux et al. We also present the kinematic and clustering properties of this survey of sub-DLAs, which appear to be marginally different from the DLAs. We compare low- and high-ionization transition widths and find that the properties of the sub-DLAs span roughly the parameter space of DLAs. We also find hints of an increase of metallicity in systems with larger velocity widths in the metal lines, although the statistical significance of this result is low. Then we analyse the chemical content of this sample in conjunction with a compilation of abundances from 72 DLAs taken from the literature. As previously reported, the individual metallicities traced by [Fe/H] of these systems evolve mildly with redshift. Moreover, we analyse the H i column-density-weighted mean abundance, which is believed to be an indicator of the metallicity of the Universe. Although the number statistics is limited in the current sample, the results suggest a slightly stronger evolution of this quantity in the sub-DLA range. The effect is predominant at z < 2 and most of the evolution observed lies in this redshift range. Observational arguments support the hypothesis that the evolution we probe in the sub-DLA range is not due to their lower dust content. Therefore, these systems might be associated with a different class of objects, which better trace the overall chemical evolution of the Universe. Finally, we present abundance ratios of [Si/Fe], [O/Fe], [C/Fe] and [Al/Fe] for sub-DLAs in conjunction with DLA measurements from the literature. The elemental ratios in sub-DLAs are comparable with those from DLAs. It is difficult to decipher whether the observed values are the effect of nucleosynthesis or are due to differential dust depletion. The metallicities are compared with two different sets of models of galaxy evolution in order to provide constraints on the morphology of quasar absorbers.