We discovered a sample of 250 Lyα emitting (LAE) galaxies at z 2.1 in an ultra-deep 3727 ? narrow-band MUSYC image of the Extended Chandra Deep Field-South. The LAEs were selected to have rest-frame equivalent widths (EWs) 20 ? and emission-line fluxes F Lyα2.0 × 10–17 erg cm–2 s–1, after carefully subtracting the continuum contributions from narrow-band photometry. The median emission-line flux of our sample is F Lyα = 4.2 × 10–17 erg cm–2 s–1, corresponding to a median Lyα luminosity L Lyα = 1.3 × 1042 erg s–1 at z 2.1. At this flux, our sample is ≥90% complete. Approximately 4% of the original NB-selected candidates were detected in X-rays by Chandra, and 7% were detected in the rest-frame far-UV by the Galaxy Evolution Explorer; these objects were eliminated to minimize contamination by active galactic nuclei and low-redshift galaxies. At L Lyα ≥ 1.3 × 1042 erg s–1, the EW distribution is unbiased and is represented by an exponential with scale-length 83 ±10 ?. Above this same luminosity threshold, we find a number density of 1.5 ± 0.5 × 10–3 Mpc–3. Neither the number density of LAEs nor the scale-length of their EW distribution show significant evolution from z 3 to z 2. We used the rest-frame UV luminosity to estimate a median star formation rate of 4 M ☉ yr–1. The median rest-frame UV slope, parameterized by the color B – R, is typical of dust-free, 0.5-1 Gyr old or moderately dusty, 300-500 Myr old population. Approximately 30% of our sample is consistent with being very young (age 100 Myr) galaxies without dust. Approximately 40% of the sample occupies the z ~ 2 star-forming galaxy locus in the UVR two-color diagram, but the true percentage could be significantly higher taking into account photometric errors. Clustering analysis reveals that LAEs at z 2.1 have r 0 = 4.8 ± 0.9 Mpc, corresponding to a bias factor b = 1.8 ± 0.3. This implies that z 2.1 LAEs reside in dark matter halos with median masses log(M/M ☉) = 11.5+0.4 –0.5, which are among the lowest mass halos yet probed at this redshift. We used the Sheth and Tormen conditional mass function to study the descendants of these LAEs and found that their typical present-day descendants are local galaxies with L* properties, like the Milky Way.
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