  Large and less-biased samples of star-forming galaxies are essential to
investigate galaxy evolution. H$\rm\alpha$ emission line is one of the most
reliable tracers of star-forming galaxies because its strength is directly
related to recent star formation. However, it is observationally expensive to
construct large samples of H$\rm\alpha$ emitters by spectroscopic or
narrow-band imaging survey at high-redshifts. In this work, we demonstrate a
method to extract H$\rm\alpha$ fluxes of galaxies at $z=2.1$-$2.5$ from $K_s$
broad-band photometry of ZFOURGE catalog. Combined with 25-39 other filters, we
estimate the emission line fluxes by SED fitting with stellar population models
that incorporate emission-line strengths. 2005 galaxies are selected as
H$\rm\alpha$ emitters by our method and their fluxes show good agreement with
previous measurements in the literature. On the other hand, there are more
H$\rm\alpha$ luminous galaxies than previously reported. The discrepancy can be
explained by extended H$\rm\alpha$ profiles of massive galaxies and a
luminosity dependence of dust attenuation, which are not taken into account in
the previous work. We also find that there are a large number of low-mass
galaxies with much higher specific star formation rate (sSFR) than expected
from the extrapolated star formation main sequence. Such low-mass galaxies
exhibit larger ratios between H$\rm\alpha$ and UV fluxes compared to more
massive high sSFR galaxies. This result implies that a ``starburst'' mode may
differ among galaxies: low-mass galaxies appear to assemble their stellar mass
via short-duration bursts while more massive galaxies tend to experience
longer-duration ($>10\ \mathrm{Myr}$) bursts.

Astrophysics of Galaxies