  We discovered a sample of 250 Ly-Alpha emitting (LAE) galaxies at z=2.1 in an
ultra-deep 3727 A narrow-band MUSYC image of the Extended Chandra Deep
Field-South. LAEs were selected to have rest-frame equivalent widths (EW) > 20
A and emission line fluxes > 2.0 x 10^(-17)erg /cm^2/s, after carefully
subtracting the continuum contributions from narrow band photometry. The median
flux of our sample is 4.2 x 10^(-17)erg/cm^2/s, corresponding to a median Lya
luminosity = 1.3 x 10^(42) erg/s 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 GALEX.
At luminosity>1.3 x 10^42 erg/s, the equivalent width distribution is unbiased
and is represented by an exponential with scale-length of 83+/-10 A. Above this
same luminosity threshold, we find a number density of 1.5+/-0.5 x 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_(sun) /yr. The
median rest frame UV slope, parametrized by B-R, is that typical of dust-free,
0.5-1 Gyr old or moderately dusty, 300-500 Myr old populations. Approximately
40% of the sample occupies the z~2 star-forming galaxy locus in the UVR two
color diagram. Clustering analysis reveals that LAEs at z=2.1 have
r_0=4.8+/-0.9 Mpc and 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_(sun))=11.5^(+0.4)_(-0.5), which are among of the lowest-mass halos yet
probed at this redshift. We used the Sheth-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.
