Deep Galaxy Surveys in the 9150 A Airglow Window

Abstract

We describe the current status of two complementary programs to search for objects with strong emission lines in a ~300 A gap, centered at 9150 A, in the strong airglow emission. Both programs are being carried out with LRIS on the Keck II telescope. The first of these uses broad-band and narrow-band filter photometry to select candidates, followed by multi-slit spectroscopy through the same narrow-band filter to limit the bandpass and allow a dense packing of slits. The second uses six parallel long slits to carry out a blind spectroscopic search through the filter isolating the 9150 A window. The total slit area covered ranges from 1 to 3.5 square arcmin per pointing, depending on slit width, and we can obtain 3-sigma detections of emission lines of <2 x 10^-18 erg/cm^2/s in a 12000 s observation with 1.5 arcsec slits. Because, for faint objects in both programs, we are most sensitive to strong lines with large equivalent widths, most of our detections will be restricted to a few specific emission lines at certain discrete redshifts. One of the more interesting possibilities is Ly-alpha at z~6.5. However, even with 12000 s integrations on the Keck II telescope, our narrow-band imaging does not pick up objects with emission-line fluxes less than 10^-17 erg/cm^2/s. With this limit, at z~6.5, we would pick up only the most luminous of the z>5 objects discovered so far. Our blind spectroscopic search potentially has a better chance of discovering such objects, but we have not yet found any definite examples in the limited area of the sky we have covered to date. We discuss the criteria for identifying Ly-alpha emission in noisy spectra and emphasize how high-ionization dwarf galaxies at low redshift can be mistaken for Ly-alpha candidates under certain conditions.