OMEGA – OSIRIS mapping of emission-line galaxies in A901/2-IV. Extinction of star formation estimators with inclination

Wolf, C., Weinzirl, T., Aragon Salamanca, A., Gray, M. E., Del Pino, B. R., Chies Santos, A. L., Bamford, S. P., Bohm, A., Harborne, K. 2019. OMEGA – OSIRIS mapping of emission-line galaxies in A901/2-IV. Extinction of star formation estimators with inclination. Monthly Notices of the Royal Astronomical Society 480, 3, 3788-3799, DOI: 10.1093/mnras/sty2112

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 (`IRV) 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 discs in Hubble Space Telescope images. As a novelty, the H alpha- and separately the N II emission are measured by tunable-filter imaging and encompass galaxies in their entirety. At galaxy stellar masses above log M-*/M-circle dot greater than or similar to 10 we find trends in the UV and Ha mass-to-light ratio that suggest an inclination-induced attenuation from face-on to edge-on of similar to 1 mag and similar to 0.7 mag in UV and H alpha, respectively, implying that star formation rates of edge-on galaxies would be underestimated by similar to 2.5 x in UV and similar to 2 x in Ha. We find the luminosities in UV and Ha 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-circle dot less than or similar to 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.

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