OPAQUE MATTER IN SPIRAL GALAXIES. COSMOLOGICAL CONSEQUENCES

Abstract

ABSTRACT The luminosity function plays a direct role in several points of cosmological interest, like the magnitud and redshift galaxy number counts, the determination of the mean luminosity density in a given volume and the determination of the spatial two point correlation function from the knowledge of the angular correlation function. In this work, we have related the optical thickness of the galactic disk with some cosmological observations. We assume that the absorbing material appears in a epoch zd and obtain the absolute luminosity function corrected for the dust effects and study some cosmological consequences of this correction. Our main results are: 1.Luminosity function: As an effect of the opacity, an inclination i different from zero modifies the apparent luminosity of the galaxies and leads to a wrong estimation of the absolute luminosity. The corrections that must be applied depend on how the luminosities vary with inclination and therefore, in how to assign the corrected distribution function for the variable associated to the galaxy inclination. We have used a distribution function for the variable tau = |cos i|, assuming that the galaxies are uniformly distributed in a region of space, with the variable i (inclination) uniformly distributed. We have checked out that this hypothesis is reasonable for a pair of samples but further work must be done with larger and more complete samples in order to comfirm or to choose a more suitable distribution function to the variable mu and get more conclusions about the modifications in the luminosity function due to the opacity effects. We have found that the opacity modifies the luminosity function in the sense of increasing the number of more bright galaxies and keeping almost the same the number of faint galaxies. 2.Magnitude and redshift number counts: The modifications in the luminosity function affects directly the galaxy counts Nm and Nz. We have found for Nm in the B band (blue) that moderate opacities with tau = 1.2-1.3 reproduce quite well the observed excess detected in faint magnitudes. We remember that the tau =1.3 is the value adopted by Valentijn (1990) for the disk opacity based in statistical studies. The ratio between the infrared and optical emission LFIR/LOPT gives 0.3 for tau =1.3, which is in agreement with the mean value observed by IRAS. In the Nz counts we found that the effect of a moderate opacity is to produce an excess in the number of the observed galaxies at high redshifts bringing the predictions and the observations close together. We found that the best fit is obtained for the 20.0 - 21.5 apparent magnitud band. In the far infrared band (IRAS) the observed effect in Nz due to a small opacity tau =0.3, is to improve the fit between the theoretical predictions and the observational data without the need for a empirical evolution law for the luminosity function. The corresponding extinction due to this opacity is not incompatible with the values of the observed extinction curve given by Rowan-Robinson1987 which are model dependent. Although more and better observations are needed to clarify this point. 3. Estimate of the tridimensional correlation scale length. As a result of the corrections due to the effect of the absorbing matter, we obtain a new estimation of the tridimensional correlation scale lenght: ro = 10 ± h-1 Mpc, that is 1.9 times greater than the former. This value is in better agreement for instance with the estimations of Martinez et al. (1993), that predicts an increase of ro from 4.6 to 8 Mpc for samples of 40-80 Mpc deep.