Optical versus infrared studies of dusty galaxies and active galactic nuclei – I. Nebular emission lines

Optical nebular emission lines are commonly used to estimate the star formation rate of galaxies and the black hole accretion rate of their central active nuclei. The accuracy of the conversion from line strengths to physical properties depends upon the accuracy to which the lines can be corrected for dust attenuation. For studies of single galaxies with normal amounts of dust, most dust corrections result in the same derived properties within the errors. However, for statistical studies of populations of galaxies, or for studies of galaxies with higher dust contents, such as might be found in some classes of ‘transition’ galaxies, significant uncertainty arises from the dust attenuation correction. In this paper, we compare the strength of the predominantly unobscured mid-infrared [Ne_(II)] λ 15.5 μm+[Ne_(III)] λ 12.8 μm emission lines to the optical Hα emission lines in four samples of galaxies: (i) ordinary star-forming galaxies (80 galaxies); (ii) optically selected dusty galaxies (11); (iii) ultraluminous infrared galaxies (6); and (iv) Seyfert 2 galaxies (20). We show that a single dust attenuation curve applied to all samples can correct the Hα luminosity for dust attenuation to a factor better than 2. Similarly, we compare [O_(IV)] and [O_(III)] luminosities to find that [O_(III)] can be corrected to a factor better than 3. This shows that the total dust attenuation suffered by the active galactic nucleus narrow-line region is not significantly different from that suffered by the star-forming H_(II) regions in the galaxy. We provide explicit dust attenuation corrections, together with errors, for [O_(II)], [O(III)] and Hα. The best-fitting average attenuation curve is slightly greyer than the Milky Way extinction law, indicating either that external galaxies have slightly different typical dust properties from those of the Milky Way or that there is a significant contribution from scattering. Finally, we uncover an intriguing correlation between silicate absorption and Balmer decrement, two measures of dust in galaxies, which probe entirely different regimes in optical depth