Considerable details have been learned about the physical properties and chemical enrichment in neutral atomic gas from DLA studies. To apply the knowledge of DLAs for a better understanding of distant galaxies, it is necessary to first identify DLA galaxies and compare them with the general galaxy population. Searches for DLA galaxies are challenging, because distant galaxies are faint and because the relatively small extent of high-N(H I) gas around galaxies places the absorbing galaxies at small angular distances from the bright background QSOs. Based on a well-defined H I size-mass relation observed in local H I galaxies (e.g., Broeils and Rhee 1997, Verheijen and Sancisi 2001, Swaters et al 2002), the characteristic projected separation (accounting for weighting by cross section) between a DLA and an L* absorbing galaxy is ≈ 16 kpc and smaller for lower-mass galaxies. At z = 1−2, a projected distance of 16 kpc corresponds to an angular separation of ≲ 2″, and greater at lower and higher redshifts.
While fewer DLAs are known at z ≲ 1 (see Sect. 2), a large number ( ≈ 40) of these low-redshift DLAs have their galaxy counterparts (or candidates) found based on a combination of photometric and spectroscopic techniques (e.g., Chen and Lanzetta 2003, Rao et al 2003, Rao et al 2011, Péroux et al (2016)). It has been shown based on this low-redshift DLA galaxy sample that DLAs probe a representative galaxy population in luminosity and colour. DLA galaxies are consistent with an H I cross-section selected sample with a large fraction of DLAs found at projected distance d ≳ 10 kpc from the absorbing galaxies (e.g., Chen and Lanzetta 2003, Rao et al 2011). In addition to regular disk galaxies, two DLAs have been found in a group environment (e.g., Bergeron and Boissé 1991, Chen and Lanzetta 2003, Kacprzak et al 2010, Péroux et al 2011), suggesting that stripped gas from galaxy interactions could also contribute to the incidence of DLAs. The low-redshift DLA sample is expected to continue to grow dramatically with new discoveries from the SDSS (e.g., Straka et al 2015). In contrast, the search for DLA galaxies at z > 2 has been less successful despite extensive efforts (e.g., Warren et al 2001, Møller et al 2002, Péroux et al 2012, Fumagalli et al 2015). To date, only ≈ 12 DLA galaxies have been found at z > 2 (Krogager et al 2012, Fumagalli et al 2015).
In addition to a general characterization of the DLA galaxy population, individual DLA and galaxy pairs provide a unique opportunity to probe neutral gas in the outskirts of distant galaxies. Figure 4 shows two examples of constraining the kinematics and chemical enrichment in the outskirts of neutral disks from combining resolved optical imaging and spectroscopy of the galaxy with an absorption-line analysis of the DLA. In the first example (top row), a DLA of log N(H I) = 19.7 is found at d = 7.6 kpc from an L* galaxy at z = 0.101, which also exhibits widespread CO emission in the disk (Neeleman et al 2016b). The galaxy disk is resolved in the ground-based r-band image (upper-left panel), which enables accurate measurements of the disk inclination and orientation (Chen et al 2005). While the observed N(H I) falls below the nominal threshold of a DLA, the gas is found to be largely neutral (e.g., Chen et al 2005, Som et al 2015). In addition, abundant H2 is detected in the absorbing gas (Muzahid et al 2015). Optical spectra of the galaxy clearly indicate a strong velocity shear along the disk, suggesting an organized rotation motion (Chen et al 2005) which is confirmed by recent CO observations (Neeleman et al 2016b). At the same time, the DLA is resolved into two components of comparable ionic column densities (Som et al 2015) but an order of magnitude difference in N(H2) (Muzahid et al 2015). A rotation curve of the gaseous disk extending beyond 10 kpc (top-centre panel) can be established based on the observed velocity shear (vobs) and deprojection onto the disk plane following
 |
(3) |
and
 |
(4) |
where R is the galactocentric radius along the disk, v is the deprojected rotation velocity, i is the inclination angle of the disk, and φ is the azimuthal angle from the major axis of the disk where the DLA is detected (Chen et al 2005, see also Steidel et al 2002 for an alternative formalism). For the two absorbing components in this DLA, it is found that the component with a lower N(H2) appears to be co-rotating with the optical disk (lower DLA data point), while the component with stronger N(H2) appears to be counter-rotating, possibly due to a satellite (upper DLA data point). Comparing the ISM gas-phase metallicity and the metallicity of the DLA shows a possible gas metallicity gradient of Δ Z / Δ R = −0.02 dex kpc−1 out to R ≈ 14 kpc.
 |
Figure 4. Neutral gas kinematics and metallicity revealed by the presence of a DLA in the outskirts of two L* galaxies (adapted from Chen et al 2005). The top row presents a DLA found at d = 7.6 kpc from a disk galaxy at z = 0.101, which also exhibits widespread CO emission in the disk (Neeleman et al 2016b). The bottom row presents a DLA at d = 38 kpc from an edge-on disk at z = 0.525. Deep r-band images of the galaxies are presented in the left panels, which display spatially resolved disk morphologies and enable accurate measurements of the inclination and orientation of the optical disk. The middle panels present the optical rotation curves deprojected along the disk plane (points in shaded area) based on the inclination angle determined from the optical image of each galaxy (Eq. 3 & 4). If the DLAs occur in extended disks, the corresponding galactocentric distances of the two galaxies from Eq. (3) are R = 13.6 kpc (top) and R = 38 kpc (bottom). The DLA in the top panel is resolved into two components of comparable ionic column densities (Som et al 2015) but an order of magnitude difference in N(H2) (Muzahid et al 2015). The component with a lower N(H2) appears to be co-rotating with the optical disk (lower DLA data point), while the component with stronger N(H2) appears to be counter-rotating, possibly due to a satellite (upper DLA data point). The DLA in the bottom panel displays simpler gas kinematics consistent with an extended rotating disk out to ≈ 40 kpc. The right panels present the metallicity gradient observed in the gaseous disks based on comparisons of ISM gas-phase metallicity and metallicity of the DLA beyond the optical disks. In both cases, the gas metallicity declines with increasing radius according to Δ Z / Δ R = −0.02 dex kpc−1. |
The bottom row of Fig. 4 presents a DLA at d = 38 kpc from an edge-on disk at z = 0.525. A strong velocity shear is also seen along the disk of this L* galaxy. Because the QSO sightline occurs along the extended edge-on disk, Eq. (3) and (4) directly lead to R ≈ d and v ≈ vobs for this system. This DLA galaxy presents a second example for galaxies with an extended rotating disk out to ≈ 40 kpc. At the same time, the deep r-band image (lower-left panel) from HST suggests that the disk is warped near the QSO sightline, which is also reflected by the presence of a disturbed rotation velocity at R > 5 kpc (bottom-centre panel). The metallicity measured in the gas phase (bottom-right panel) displays a similar gradient of Δ Z / Δ R = −0.02 dex kpc−1 to the galaxy at the top, which is also comparable to what is seen in the ISM of nearby disk galaxies (e.g., Zaritsky et al 1994, van Zee et al 1998, Sánchez et al 2014). A declining gas-phase metallicity from the inner ISM to neutral gas at larger distances appears to hold for most DLA galaxies at z ≲ 1 and the declining trend continues into ionized halo gas traced by strong LLS of N(H I) = 1019−20 cm−2 (e.g., Péroux et al 2016).
At z > 2, spatially resolved observations of ISM gas kinematics become significantly more challenging, because the effective radii of L* galaxies are typically re = 1−3 kpc (e.g., Law et al 2012), corresponding to ≲ 0.3″, and smaller for fainter or lower-mass objects. Star-forming regions in these distant galaxies are barely resolved in ground-based, seeing-limited observations (e.g., Law et al 2007, Förster Schreiber et al 2009, Wright et al 2009). Beam smearing can result in significant bias in interpreting the observed velocity shear and distributions of heavy elements (e.g., Davies et al 2011, Wuyts et al 2016). However, accurate measurements can be obtained to differentiate ISM metallicities of DLA galaxies from metallicities of neutral gas beyond the star-forming regions. Using the small sample of known DLA galaxies at z ≳ 2, a metallicity gradient of Δ Z / Δ R = −0.02 dex kpc−1 is also found in these distant star-forming galaxies (Christensen et al 2014, Jorgenson and Wolfe 2014).