A contribution to the selection of emission-line galaxies using narrowband filters in the optical airglow windows

Emission-line galaxies are an invaluable tool for understanding the evolution of galaxies in the universe. Imaging deep fields with narrowband filters allows not only the selection of these objects, but also inferences about the line flux and the equivalent width of the emission line, with some assumptions. The narrowband filter technique provides homogeneous samples of galaxies in small comoving volumes in the sky. We present an analysis of the selection of emission-line galaxies using narrowband filters. Different methods of observation are considered: broadband - narrowband filters, two narrowband filters, and two broadband and one narrowband filter. We also study the effect of several lines simultaneously entering the filters ( as in the case of Ha). In each case, the equations used to obtain the equivalent width and line flux from the photometry are obtained. Candidate emission-line objects are selected by their color excess in a color-magnitude diagram. For different narrowband filters, we compute the mean colors of stars and galaxies, showing that in addition to galaxies, some types of stars can be selected with certain filter sets. We show how to compute the standard deviation of the colors of the objects, even in the usual case in which there are not enough objects to determine the standard deviation from the data. We also present helpful equations to compute the narrow-and broadband exposure times in order to obtain minimal dispersion in the flux ratios of both bands, with a minimum total exposure time.