Extremely Strong Emission-Line Galaxies In The WISP Survey And Implications For High-redhift Galaxies

The WFC3 Infrared Spectroscopic Parallel Survey (WISP) uses the Hubble Space Telescope 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 emissionline galaxies with rest-frame equivalent widths higher than 200 u A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin 2 area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). 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. After estimating the AGN fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission-lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that 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 one magnitude deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow Hubble Space Telescope imaging surveys. 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. Subject headings: galaxies: evolution – galaxies: statistics – galaxies: high-redshift – infrared: galaxies – surveys – cosmology: observations