SLICING THROUGH MULTICOLOR SPACE - GALAXY REDSHIFTS FROM BROAD-BAND PHOTOMETRY

As a means of better understanding the evolution of optically selected galaxies we consider the distribution of galaxies within the multicol or space U, B-J, R(F), and I-N. We find that they form an almost plana r distribution out to B-J = 22.5 and z < 0.3. The position of a galaxy within this plane is dependent on its redshift, luminosity, and spect ral type. While in the original U, B-J, R(F), and I-N space these prop erties are highly correlated, we can define an optimal rotation of the photometric axes that makes much of this information orthogonal. Fitt ing the observed spectroscopic redshifts with a quadratic function of the four magnitudes we show that redshifts for galaxies can be estimat ed to an accuracy better than Delta z = 0.05. This dispersion is due t o the photometric uncertainties within the photographic data. Assuming no galaxy evolution we derive a set of simulated galaxy fluxes in the U, J, F, and N passbands. Using these data we investigate how the red shift is encoded within the broadband magnitudes and the intrinsic dis persion of the photometric-redshift relation. We find that the signal that defines a galaxy's photometric redshift is not related to specifi c absorption or emission lines but comes from the break in the overall shape of the galaxy continuum at around 4000 Angstrom. Using high sig nal-to-noise photometric data we estimate that it is possible to achie ve an intrinsic dispersion of less than Delta z = 0.02. (C) 1995 Ameri can Astronomical Society.