Extinction in Star-Forming Disk Galaxies from Inclination-Dependent Composite Spectra

TL;DR

This study investigates how galaxy inclination affects dust extinction in star-forming disk galaxies, revealing that extinction increases with inclination and varies between stellar continuum and HII regions, with implications for understanding dust distribution.

Contribution

It provides empirical measurements of inclination-dependent extinction in composite spectra and characterizes the extinction curve across optical wavelengths in disk galaxies.

Findings

Inclination increases stellar continuum extinction in g, r, i bands.

Extinction in highly inclined galaxies reaches 1.2 mag in g-band.

The Balmer decrement remains constant with inclination.

Abstract

Extinction in galaxies affects their observed properties. In scenarios describing the distribution of dust and stars in individual disk galaxies, the amplitude of the extinction can be modulated by the inclination of the galaxies. In this work we investigate the inclination dependency in composite spectra of star-forming disk galaxies from the Sloan Digital Sky Survey Data Release 5. In a volume-limited sample within a redshift range 0.065-0.075 and a r-band Petrosian absolute magnitude range -19.5 to -22 which exhibits a flat distribution of inclination, the inclined relative to face-on extinction in the stellar continuum is found empirically to increase with inclination in the g, r, and i bands. Within the central 0.5 intrinsic half-light radius of the galaxies, the g-band relative extinction in the stellar continuum for the highly-inclined objects (axis ratio b/a = 0.1) is 1.2 mag, agreeing with previous studies. The extinction curve of the disk galaxies is given in the restframe wavelengths 3700-8000 angstrom, identified with major optical emission and absorption lines in diagnostics. The Balmer decrement remains constant with inclination, suggesting a different kind of dust configuration and/or reddening mechanism in the HII region from that in the stellar continuum. One factor is shown to be the presence of spatially non-uniform interstellar extinction, presumably caused by clumped dust in the vicinity of the HII region.