Dust and Nebular Emission. I. Models for Normal Galaxies

TL;DR

This paper introduces a comprehensive model for nebular emission in star-forming galaxies that incorporates dust effects, enabling improved interpretation of observational data and more accurate star formation rate estimations across multiple wavelengths.

Contribution

The authors develop a new model integrating nebular emission with dust effects into spectrophotometric analysis, enhancing the understanding of star formation indicators in dusty galaxies.

Findings

Age selective extinction explains the lack of correlation between Halpha and UV attenuation.

FIR/UV ratio provides the most reliable UV attenuation estimate for modest SFR galaxies.

SFR estimates from UV corrected by FIR/UV ratio have minimal uncertainty.

Abstract

We present a model for nebular emission in star forming galaxies, which takes into account the effects of dust reprocessing. The nebular emissions have been computed with CLOUDY and then included into GRASIL, our spectrophotometric code specifically developed for dusty galaxies. The interface between nebular emission and population synthesis is based on a set of pre-computed HII region emission models covering a wide range of physical quantities. Concerning the extinction properties of normal star forming galaxies, we are able to interpret the observed lack of correlation between the attenuation measured at Halpha and in the UV band as a consequence of age selective extinction. We also find that, for these galaxies with modest SFR, the ratio FIR/UV provides the best constraints on the UV attenuation. Our model also allows to deal with different SFR estimators in a consistent way, from the UV to radio wavelengths, and to discuss the uncertainties arising from the different physical conditions encountered in star forming galaxies. We provide our best estimates of SFR/luminosity calibrations, together with their expected range of variation. It results that SFR derived through Halpha, even when corrected for extinction using the Balmer decrement, is affected by important uncertainties due to age selective extinction. Another remarkable result is that SFR from UV luminosity corrected by means of the ratio FIR/UV has a small uncertainty. Finally, our model provides a calibration of SFR from radio luminosity; we are also able to reproduce the observed FIR/radio ratio.