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 SDSS/DR7. As it is long 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. We compare the
Automatic Spectroscopic K-means based classification (ASK) with most of the
commonly used 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 fixed the morphological type. The
distributions of Hubble types given an ASK class are very skewed -- they
present long tails that go to the late morphological types for the red
galaxies, and to the early morphological types for the blue spectroscopic
classes. The scatter is not produced by problems in the classification, and it
remains when particular subsets are considered. A considerable fraction of the
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. It contains only a 3 % of blue ellipticals. The galaxies in the
green valley class (ASK~5) are mostly spirals, and the AGN class (ASK 6)
presents a large scatter of Hubble types from E to Sd. From redshift 0.25 to
now 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, a property of interest in studies requiring knowing the
intrinsic shape of a galaxy (e.g., weak lensing calibration).
