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