Abstract
The multiplexing capability of slitless spectroscopy is a powerful asset in
creating large spectroscopic datasets, but issues such as spectral confusion
make the interpretation of the data challenging. Here we present a new method
to search for emission lines in the slitless spectroscopic data from the 3D-HST
survey utilizing the Wide-Field Camera 3 on board the Hubble Space Telescope.
Using a novel statistical technique, we can detect compact (extended) emission
lines at 90% completeness down to fluxes of 1.5 (3.0) times 10^-17
erg/s/cm^2, close to the noise level of the grism exposures, for objects
detected in the deep ancillary photometric data. Unlike previous methods, the
Bayesian nature allows for probabilistic line identifications, namely redshift
estimates, based on secondary emission line detections and/or photometric
redshift priors. As a first application, we measure the comoving number density
of Extreme Emission Line Galaxies (restframe O III 5007 equivalent widths in
excess of 500 Angstroms). We find that these galaxies are more than 10 times
more common above z~1.5 than at z<0.5. With upcoming large grism surveys such
as Euclid and WFIRST, methods like the one presented here will be crucial for
constructing emission line redshift catalogs in an automated and
well-understood manner.
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