Automated Selection and Characterization of Emission-Line Sources in Advanced Camera for Surveys Wide Field Camera Grism Data

We present complimentary techniques to find emission-line targets and measuretheir properties in a semi-automated fashion from grism observations obtainedwith the Advanced Camera for Surveys aboard the Hubble Space Telescope. Thefirst technique is to find all likely sources in a direct image, extract theirspectra and search them for emission lines. The second method is to look foremission-line sources as compact structures in an unsharp masked version of thegrism image. Using these methods we identify 46 emission-line targets in theHubble Deep Field North using a modest (3 orbit) expenditure of HST observingtime. Grism spectroscopy is a powerful tool for efficiently identifyinginteresting low luminosity, moderate redshift emission-line field galaxies. Thesources found here have a median i band flux 1.5 mag fainter than thespectroscopic redshift catalog of Cohen et al. They have redshift z <= 1.42,high equivalent widths (typically EW > 100{\AA}), and are usually less luminousthan the characteristic luminosity at the same redshift. The chief obstacle ininterpreting the results is line identification, since the majority of sourceshave a single emission line and the spectral resolution is low. Photometricredshifts are useful for providing a first guess redshift. However, even at thedepth of the state-of-the-art data used here, photometric errors can result inuncertainties in line identifications, especially for sources with i > ~24.5ABmag. Reliable line identification for the faintest emission-line galaxiesrequires additional ground-based spectroscopy for confirmation. Of particularconcern are the faint high EW [OII] emitters which could represent a stronglyevolving galaxy population if the possibility that they are mis-identifiedlower redshift interlopers can be ruled out. (Slightly abridged)Comment: AJ accepted. 27 pages, 4 tables, 11 figures. Uses emulateapj.cl