RELATIONSHIP BETWEEN HUBBLE TYPE AND SPECTROSCOPIC CLASS IN LOCAL GALAXIES

We compare the Hubble type and the spectroscopic class of the galaxies with spectra in the Sloan Digital Sky Survey Data Release 7. As has long been known, elliptical galaxies tend to be red whereas spiral galaxies tend to be blue; however, this relationship presents a large scatter, which we measure and quantify in detail for the first time. We compare the Automatic Spectroscopic K-means-based classification (ASK) with most of the commonly used morphological classifications. Despite the degree of subjectivity involved in morphological classifications, all of them provide consistent results. Given a spectral class, the morphological type wavers with a standard deviation between 2 and 3 T types, and the same large dispersion characterizes the variability of spectral classes given a morphological type. The distributions of Hubble types for each ASK class are very skewed-they present long tails that extend to late morphological types in the red galaxies and to early morphological types in the blue spectroscopic classes. The scatter is not produced by problems with the classification and it remains when particular subsets are considered-low and high galaxy masses, low and high density environments, barred and non-barred galaxies, edge-on galaxies, small and large galaxies, or when a volume-limited sample is considered. A considerable fraction of red galaxies are spirals (40%-60%), but they never present very late Hubble types (Sd or later). Even though red spectra are not associated with ellipticals, most ellipticals do have red spectra: 97% of the ellipticals in the morphological catalog by Nair & Abraham used here for reference belong to ASK 0, 2, or 3; only 3% of the ellipticals are blue. The galaxies in the green valley class (ASK 5) are mostly spirals, and the active galactic nuclei class (ASK 6) presents a large scatter of Hubble types from E to Sd. We investigate variations with redshift using a volume-limited subsample mainly formed by luminous red galaxies. From redshift 0.25 to the present, the galaxies redden from ASK 2 to ASK 0, as expected from the passive evolution of their stellar populations. Two of the ASK classes (1 and 4) gather edge-on spirals, and they may be useful in studies requiring knowledge of the intrinsic shape of a galaxy (e. g., weak-lensing calibration).