The Millennium Galaxy Catalogue: The severe attenuation of bulge flux by dusty spiral discs

TL;DR

This study uses the Millennium Galaxy Catalogue to show that dust in spiral galaxy discs significantly dims bulge light, affecting galaxy appearance and energy balance, with implications for galaxy classification and dust modeling.

Contribution

It demonstrates that galaxy discs are optically thick in their centers and quantifies how dust attenuates bulge flux depending on inclination, providing new insights into galaxy dust effects.

Findings

Bulge flux can be attenuated by 50% to 95% due to dust.

Galaxy discs are optically thick in their central regions.

The absorbed starlight energy matches the observed far-IR emission.

Abstract

Using the Millennium Galaxy Catalogue we quantify the dependency of the disc and bulge luminosity functions on galaxy inclination. Using a contemporary dust model we show that our results are consistent with galaxy discs being optically thick in their central regions (tau_B^f=3.8+/-0.7). As a consequence the measured B-band fluxes of bulges can be severely attenuated by 50% to 95% depending on disc inclination. We argue that a galaxy's optical appearance can be radically transformed by simply removing the dust, e.g. during cluster infall, with mid-type galaxies becoming earlier, redder, and more luminous. Finally we derive the mean photon escape fraction from the integrated galaxy population over the 0.1micron to 2.1 micron range, and use this to show that the energy of starlight absorbed by dust (in our model) is in close agreement with the total far-IR emission.