Emission Line Galaxies in the SHARDS Frontier Fields I: Candidate
  Selection and the Discovery of Bursty H{\alpha} Emitters
  Emission line galaxies provide a crucial tool for the study of galaxy
formation and evolution, providing a means to trace a galaxy's star formation
history or metal enrichment, and to identify galaxies at a range of stellar
masses. In this paper we present a study of emission line galaxies in the
SHARDS Frontier Fields medium-band survey. Through detailed flux calibrations
we combine the first results of the SHARDS-FF survey with existing Hubble
Frontier Field data to select 1,098 candidate emission line galaxies from the
Hubble Frontier Filed clusters Abell 370 and MACS J1149.5+2223. Furthermore, we
implement this deep medium-band imaging to update photometric redshift
estimates and stellar population parameters and discover 38 predominantly
low-mass H{\alpha} emitters at redshifts 0.24 < z < 0.46. Overall, 27 of these
sources have corresponding UV data from the Hubble Space Telescope which allows
us to distinguish these sources and investigate the burstiness of their star
formation histories. We find that more than 50% of our sample show an
enhancement in H{\alpha} over UV, suggesting recent bursts in star formation on
time scales of a few, to tens of megayears. We investigate these sources and
find that they are typically low-mass disky galaxies with normal sizes. Their
structures and star formation suggest that they are not undergoing mergers but
are bursting due to alternative causes, such as gas accretion.
