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

TL;DR

This study uses Tully-Fisher data to statistically analyze the optical depth of spiral galaxy disks, revealing they are relatively transparent in outer regions but appear opaque when viewed statistically.

Contribution

It provides a new statistical approach to distinguish true and apparent optical depths of spiral galaxy disks using the KLUN+ survey data.

Findings

Spiral galaxy disks are relatively optically thin (tauB=0.1) in outer regions.

Galaxies appear optically thick (tauB>1) when analyzed statistically.

Normalized distances help reduce selection effects in optical depth estimation.

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 tauB = 0.1, at least in the outermost regions, while they appear in general to be optically thick tauB > 1 when the apparent magnitude and average surface brightness are studied statistically.