A Magellan IMACS spectroscopic search for Lyα-emitting galaxies at redshift 5.7

We present results from a blind, spectroscopic survey for z similar to 5: 7 Ly alpha-emitting galaxies (LAEs) using the Inamori Magellan Areal Camera and Spectrograph. A total of similar to 200 arcmin(2) were observed in the COSMOS and LCIRS fields using a narrowband filter, which transmits between atmospheric emission lines at 8190 8, and a mask with 100 long slits. This observing technique provides higher emission-line sensitivity than narrowband imaging and probes larger volumes than strong lensing. We find 170 emission-line galaxies and identify their redshifts spectroscopically. We confirm three LAEs, the first discovered using multislit narrowband spectroscopy. Their line profiles are narrow, but fitted models suggest intrinsic, unattenuated widths similar to 400 km s(-1) FWHM. The red wing of the line profiles presents features consistent with galactic winds. The star formation rates of these galaxies are at least 5-7 M circle dot yr(-1) and likely a factor of 2 higher. We estimate that the number density of L >= 5 x 10(42) erg s(-1) LAEs is 9.0(-4)(+12) x 10(-5) h(70)(3) Mpc(-3) at redshift 5.7 and constrain the Schechter function parameters describing this population. Galaxies fainter than our detection limit may well be the primary source of ionizing photons at z similar to 6. We argue, however, that the break luminosity L*, Ly alpha is not yet well constrained. If this break luminosity is near our detection limit, and somewhat lower than previous estimates, then the detected LAE population could be responsible for ionizing the intergalactic gas at redshift z similar to 6. We discuss the potential of multislit narrowband spectroscopy for deeper emission-line surveys.