Galaxies of the $z \sim 2$ Universe. I. Grism-Selected Rest-Frame
  Optical Emission Line Galaxies
  Euclid, WFIRST, and HETDEX will make emission-line selected galaxies the
largest observed constituent in the $z > 1$ universe. However, we only have a
limited understanding of the physical properties of galaxies selected via their
Ly$\alpha$ or rest-frame optical emission lines. To begin addressing this
problem, we present the basic properties of $\sim 2,000$ AEGIS, COSMOS,
GOODS-N, GOODS-S, and UDS galaxies identified in the redshift range $1.90 < z <
2.35$ via their [O II], H$\beta$, and [O III] emission lines. For these $z \sim
2$ galaxies, [O III] is generally much brighter than [O II] and H$\beta$, with
typical rest-frame equivalent widths of several hundred Angstroms. Moreover,
these strong emission-line systems span an extremely wide range of stellar mass
($\sim 3$ dex), star-formation rate ($\sim 2$ dex), and [O III] luminosity
($\sim 2$ dex). Comparing the distributions of these properties to those of
continuum selected galaxies, we find that emission-line galaxies have
systematically lower stellar masses and lower optical/UV dust attenuations.
These measurements lay the groundwork for an extensive comparison between these
rest-frame optical emission-line galaxies and Ly$\alpha$ emitters identified in
the HETDEX survey.
