OMEGA - OSIRIS Mapping of Emission-line Galaxies in A901/2: IV. - Extinction of Star-Formation Estimators with Inclination

TL;DR

This study investigates how galaxy inclination affects star-formation rate indicators in different spectral passbands, revealing significant attenuation effects in UV and Hα for massive galaxies, but not in FIR, impacting SFR estimates.

Contribution

It provides the first comprehensive analysis of inclination effects on multiple star-formation tracers using tunable-filter imaging across entire galaxies.

Findings

Inclination causes 1 mag attenuation in UV and 0.7 mag in Hα for massive galaxies.

Star-formation rates in edge-on galaxies are underestimated by approximately 2.5 times in UV and 2 times in Hα.

No significant inclination effect is observed in the FIR 24μ band, indicating optical thinness in FIR.

Abstract

We study the effect of inclination on the apparent brightness of star-forming galaxies in spectral passbands that are commonly used as star-formation indicators. As diagnostics we use mass-to-light ratios in three passbands: the UV continuum at 280 nm, the H$\alpha$ emission line, and the FIR 24$\mu$-band. We include a study of inclination trends in the IR/UV ratio ("IRX") and the IR/H$\alpha$ ratio. Our sample comprises a few hundred galaxies from the region around the clusters Abell 901/902 with deep data and inclinations measured from outer disks in Hubble Space Telescope images. As a novelty, the H$\alpha$- and separately the NII-emission are measured by tunable-filter imaging and encompass galaxies in their entirety. At galaxy stellar masses above log $M_*/M_\odot > 10$ we find trends in the UV and H$\alpha$ mass-to-light ratio that suggest an inclination-induced attenuation from face-on to edge-on of $\sim 1$ mag and $\sim 0.7$ mag in UV and H$\alpha$, respectively, implying that star-formation rates of edge-on galaxies would be underestimated by $\sim 2.5\times$ in UV and $\sim 2\times$ in H$\alpha$. We find the luminosities in UV and H$\alpha$ to be well correlated, but the optical depth of diffuse dust that causes inclination dependence appears to be lower for stars emitting at 280 nm than for gas clouds emitting Balmer lines. For galaxies with log $M_*/M_\odot < 9.7$, we find no measurable effect at $>0.1$ mag. The absence of an inclination dependence at 24$\mu$ confirms that the average galaxy is optically thin in the FIR.