Detalles de publicación
PP 011020
Relationship between Hubble type and spectroscopic class in local galaxies
Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain (1),
Departamento de Astrofisica, Universidad de La Laguna, Tenerife, Spain (2),
GEPI, Paris-Meudon Observatory 5, Place Jules Janssen, Meudon, Paris,
France (3), Universite Paris Diderot, Paris Cedex 13, France (4)
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. We investigate variations with redshift using a
volume limited subsample. 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, and they may be useful
in studies requiring knowing the intrinsic shape of a galaxy (e.g., weak lensing
calibration).
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. We investigate variations with redshift using a
volume limited subsample. 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, and they may be useful
in studies requiring knowing the intrinsic shape of a galaxy (e.g., weak lensing
calibration).