Revisiting the optical depth of spiral galaxies using the Tully-Fisher B relation

¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 21500 Piikkiö, Finland e-mail: eskank@utu.fi
² GEPI/CNRS URA1757, 92195 Meudon Principal Cedex, France
³ LPCE/CNRS UMR6115, 3A Av. de la recherche scientifique, 45071 Orléans Cedex 02, France

Received: 2 April 2008
Accepted: 17 September 2008

Abstract

Aims. We attempt to determine the optical depth of spiral galaxy disks by a statistical study of new Tully-Fisher data from the ongoing KLUN+ survey, and to clarify the difference between the true and apparent behavior of optical depth.

Methods. By utilizing so-called normalized distances, a subsample of the data is identified to be as free from selection effects as possible. For these galaxies, a set of apparent quantities are calculated for face-on positions using the Tully-Fisher diameter and magnitude relations. These values are compared with direct observations to determine the mean value of the parameter C describing the optical depth.

Results. The present study suggests that spiral galaxy disks are relatively optically thin 0.1, at least in the outermost regions, while they appear in general to be optically thick > 1 when the apparent magnitude and average surface brightness are studied statistically.