Very Strong Emission-Line Galaxies in the WISP Survey and Implications
  for High-Redshift Galaxies
  The WFC3 Infrared Spectroscopic Parallel Survey (WISP) uses the Hubble Space
Telescope (HST) infrared grism capabilities to obtain slitless spectra of
thousands of galaxies over a wide redshift range including the peak of star
formation history of the Universe. We select a population of very strong
emission-line galaxies with rest-frame equivalent widths higher than 200 A. A
total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the
180 arcmin^2 area we analyzed so far. After estimating the AGN fraction in the
sample, we show that this population consists of young and low-mass starbursts
with higher specific star formation rates than normal star-forming galaxies at
any redshift. After spectroscopic follow-up of one of these galaxies with
Keck/LRIS, we report the detection at z = 0.7 of an extremely metal-poor galaxy
with 12+Log(O/H)= 7.47 +- 0.11. The nebular emission-lines can substantially
affect the broadband flux density with a median brightening of 0.3 mag, with
examples producing brightening of up to 1 mag. The presence of strong emission
lines in low-z galaxies can mimic the color-selection criteria used in the z ~
8 dropout surveys. In order to effectively remove low redshift interlopers,
deep optical imaging is needed, at least 1 mag deeper than the bands in which
the objects are detected. Finally, we empirically demonstrate that strong
nebular lines can lead to an overestimation of the mass and the age of galaxies
derived from fitting of their SED. Without removing emission lines, the age and
the stellar mass estimates are overestimated by a factor of 2 on average and up
to a factor of 10 for the high-EW galaxies. Therefore the contribution of
emission lines should be systematically taken into account in SED fitting of
star-forming galaxies at all redshifts.
