The plane W(Na I) x W (Mg I) : effects of interstellar Na I in a sample of southern galaxies

Abstract

Galaxy spectra from a subsample of the Southern Sky Redshift Survey data bank were used to study the equivalent width plane for the lines Na I λ 5893 Å vs Mg I λ 5175 Å. We try to estimate how important the contribution of the interstellar gas for the sodium line is compared to that of the stellar population. The sample is made up of galaxies with morphological types from E to Se and are distributed up to radial velocities of 25 000 km s-ˡ , most of them smaller than 15 000 km s-ˡ. Most early type galaxies with dust lanes, particularly nearly edge-on SO's, present an enhancement of the Na I line. Inclined spiral galaxies tend to present enhanced Na I with respect to face-on spirals. This tendency, previously found in a smaller sample of galaxies limited to V≤6000 km s-ˡ,is now confirmed for more distant ones. In the large velocity sample we are observing the global bulge rather than the very nucleus; the persistence of the effect suggests that the scale height of the gas layer in the central disk can reach a considerable fraction of the bulge radius.