A wide field narrowband survey for star forming galaxies at different epochs

Abstract

Narrowband surveys are a well-established tool for finding star-forming galaxies at different epochs. This thesis presents the Wide Field Imager Lyman Alpha Search (WFILAS), a survey originally designed to find Lyman-a (Lya) emission-line galaxies at redshift z~5.7, and subsequently utilised to find Hydrogen-a (Ha) emitting galaxies at redshift z~0.24. The survey covers three 0.25 sq. deg. fields each observed in three narrowband filters, an intermediate band filter (encompassing all narrowband filters), and two broadband filters. A sample of seven luminous Lya-emitting galaxies was identified (Lya luminosity greater than or equal to 1.8e43 ergs), complementing existing surveys by further constraining the bright end of the Lya luminosity function. Three candidates identified in one of the three fields, the well-studied Chandra Deep Field South, were grouped together, supporting claims of an overdensity at this redshift by other groups. Two of the seven candidate Lya emitting galaxies have been confirmed through spectroscopy, one of which is the most luminous at this redshift to date. The spectra of both objects displayed the asymmetric line profiles common in Lya at these redshifts. Furthermore, tentative evidence of a second Lya component, redward of the Lya line was found. Additional high-resolution imaging showed that both objects were unresolved. Spectroscopic follow-up was used to determine the fraction of Ha-emitting galaxies in two of the fields from a total sample of 707 candidate emission line galaxies. This yielded two independent Ha luminosity functions and star formation densities at z ~ 0.24 following corrections for extinction, imaging and spectroscopic incompleteness. These values were found to agree with those of other recent surveys within the limits of uncertainty. A detailed error analysis found that both cosmic variance and differences in selection criteria remain the dominant sources of uncertainty between various Ha luminosity functions at z smaller than or equal to 0.4. While the star formation rates were consistent with the typical field galaxy densities probed by the fields, a tentative increase in star formation rate per galaxy with increasing density of star forming galaxies was found. This observation supports galaxy formation scenarios in which galaxy-galaxy interactions are triggers for star formation.