PDF《Astronomy》

2) galaxies is observationally a challenging task. Several obser- vational strategies have emerged in the past

15 years, each of them targeting a subset of the population of high-z galaxies, which is named after the selection technique, with some overlap in their properties. For example, because of their selection in Based on data obtained with MagE at the Clay telescope of the Las Campanas Observatory (CNTAC Prgm. ID 2011B-90) as well as X-shooter (Prgm. ID 286.A-5044) and UVES (archive: Prgm ID. 385.A-0778) at the Very Large Telescope of the European Southern Observatory. ?ux-limited surveys, Lyman-break galaxies (LBGs, Steidel et al. 2003) tend to probe the bright end of the high-redshift galaxy luminosity function. A fraction of LBGs are also Lyman-α emit- ters (LAEs, see e.g. Cowie &

Hu 1998). However, as most LAEs are selected based on the Ly α emission line, they tend to sample lower luminosities more e?ectively than colour-selected LBGs (Fynbo et al. 2003;

Kornei et al. 2010). Recent deep searches of LAEs seem indeed to have reached the faint end of the high-z luminosity function (e.g. Rauch et al. 2008;

Grove et al. 2009). In turn, the detection of damped Lyman-α systems (DLAs) in absorption against bright background sources (such as quasars or gamma-ray burst (GRB) afterglows) has the major advantage Article published by EDP Sciences A63, page

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A 540, A63 (2012) that it depends not on the luminosity of the associated galax- ies but on the cross-section of the neutral gas. These systems are selected from their large neutral hydrogen column densi- ties, N(H i) ≥

2 *

1020 cm?2 (Wolfe et al. 1986) and repre- sent the main reservoir of neutral hydrogen at high redshift (e.g. Noterdaeme et al. 2009b). The presence of associated heavy elements (e.g. Prochaska &

Wolfe 2002) and molecules (e.g. Petitjean et al. 2000;

Ledoux et al. 2003;

Noterdaeme et al. 2008), the evolution with redshift of the H i mass density in DLAs (Péroux et al. 2003;

Prochaska et al. 2005;

Noterdaeme et al. 2009b) and the detectability of DLAs over a wide range of redshifts make them the appropriate laboratories for study- ing the cosmological evolution of star formation activity in a luminosity-unbiased way. Although observational studies of DLAs have been pursued over

25 years, an important question that remains unanswered yet is the connection between DLAs and star-forming galaxies. Owing to their means of selection, most DLA galaxies proba- bly occupy the faint end of the luminosity function (e.g. Fynbo et al. 2008). Nevertheless, DLA galaxies can provide substantial contributions to the global star formation rate (SFR) density at high redshifts (Wolfe et al. 2003;

Srianand et al. 2005;

Rauch et al. 2008;

Rahmani et al. 2010). In addition, it has long been debated whether the kinematics of the absorbing gas, studied in terms of the velocity pro?les of metal absorption lines, is more representative of that of large rotating galactic discs (Prochaska &

Wolfe 1997) or small low-mass galactic clumps (Ledoux et al. 1998) that build up hierarchically to form the galaxies known today (e.g. Haehnelt et al. 1998). The detection of DLA galactic counterparts is therefore of great importance to shedding light on the nature of DLAs. Until recently, searches for direct emission from high- redshift intervening DLA galaxies have resulted mostly in non- detections (e.g. Bunker et al. 1999;

Kulkarni et al. 2000, 2006;

Lowenthal et al. 1995;

Christensen et al. 2009, as well as sev- eral unpublished works) with few cases being spectroscopically con?rmed by the detection of Ly-α emission (M?ller et al. 2002, 2004). Thankfully, improved sele........