Before we discuss searches for distant galaxies, provide an overview of the main results and discuss briefly some remaining questions, we shall summarize the basic observational techniques used to identify high-redshift galaxies.
The main search techniques for high-z galaxies can be classified into the two following
1. The Lyman-break or dropout technique, which selects galaxies over a certain redshift interval by measuring the Lyman break, which is the drop of the galaxy flux in the Lyman continuum (at A < 912 A) of the Lya break (shortwards of Lya) for z > 4-5 galaxies (cf. above). This method requires the detection of the galaxy in several (sometimes only two, but generally more) broad-band filters.
2. Emission-line searches (targeting Lya or other emission lines). Basically three different techniques may be used: (1) narrow-band (NB) imaging (two-dimensional) e.g. of a wide field selecting a specific redshift interval with the transmission of the
NB filter; (2) long-slit spectroscopy (one-dimensional) for "blind searches" e.g. along critical lines in lensing clusters; and (3) observations with integral field units (three-dimensional) allowing one to explore all three spatial directions (two-dimensional imaging plus redshift). The first one is currently the most-used technique. In practice, and to increase the reliability, several methods are often combined.
Surveys/searches are being carried out in blank fields or targeting deliberately gravitational-lensing clusters allowing one to benefit from gravitational magnification from the foreground galaxy cluster. For galaxies at z<7 the Lyman break and Lya are found in the optical domain. Near-IR ( > 1-|im) observations are necessary to locate z > 7 galaxies.
The status in 1999 of search techniques for distant galaxies has been summarized by Stern & Spinrad (1999). For more details on searches and galaxy surveys see Chapter 2 in this volume.
4.4.2 Distant Lya emitters Most of the distant known Lya emitters (LAEs) have been found through narrow-band imaging with the SUBARU telescope, thanks to its wide-field imaging capabilities. z ~ 6.5-6.6 LAE candidates are e.g. selected combining the three following criteria: an excess in a narrow-band filter (NB921) with respect to the continuum flux estimated from the broad z' filter, a 5a detection in this NB filter, and an ¿-dropout criterion (e.g. i — z' > 1.3) making sure that these objects exhibit a Lya break. Until recently 58 such LAE candidates had been found, with 17 of them confirmed subsequently by spectroscopy (Taniguchi et al. 2005; Kashikawa et al. 2006). The Hawaii group has found approximately 14 LAEs at z ~ 6.5 (Hu et al. 2005; Hu & Cowie 2006). The current record-holder as the most-distant galaxy with a spectroscopically confirmed redshift of z = 6.96 is one detected by Iye et al. (2006). Six candidate Lya emitters between z = 8.7 and 10.2 were recently proposed by Stark et al. (2007) using blind long-slit observations along the critical lines in lensing clusters.
LAEs have for example been used with SUBARU to trace large-scale structure at z = 5.7 thanks to the large field of view (Ouchi et al. 2005).
Overall, quite little is known about the properties of NB-selected LAEs, their nature and their relation to other galaxy types (LBGs and others, but see Section 4.4.3), since most of them - especially the most distant ones - are detected in very few bands, i.e. their SEDs are poorly constrained. The morphology of the highest-z LAEs is generally compact, indicating ionized gas with spatial extent of ~2-4 kpc or less (e.g. Taniguchi et al. 2005; Pirzkal et al. 2006).
Although they have (SFRs) of typically (2-50)M0 yr—1, the SFR density (SFRD) of LAEs is only a fraction of that of LBGs at all redshifts. For example, at z ~ 5-6.5, Taniguchi et al. (2005) estimate the SFRD from Lya emitters as SFRD (LAE) ~0.01 x SFRD(LBG), or up to 10% of SFRD(LBG) at best, allowing for LF corrections. At the highest z this value could be typically three times higher if the IGM transmission of ~30% estimated by Dijkstra et al. (2006c) applies. Shimasaku et al. (2006) have found a similar space density or UV LF for LAEs and LBGs at z ~ 6, and argue that LAEs contribute at least 30% of the SFRD at this redshift.
The typical masses of LAEs are still unknown and being debated. For example, Lai et al. (2007) find stellar masses of M* ~ (109- 1010)M0 for three LAEs at z ~ 5.7, whereas Prizkal et al. (2006) find much lower values of M* ~ (106-108)M0 for their sample of z ~ 5 Lya galaxies. Finkelstein et al. (2006) find masses between the two ranges for z ~ 4.5 LAEs. Selection criteria may explain some of these differences; e.g. the Lai et al. objects were selected for their detection at 3.6 and 4.5 |im with the Spitzer telescope. Mao
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