Qualitative interpretation of galaxy spectra
  We describe a simple step-by-step guide to qualitative interpretation of
galaxy spectra. Rather than an alternative to existing automated tools, it is
put forward as an instrument for quick-look analysis, and for gaining physical
insight when interpreting the outputs provided by automated tools. Though the
recipe is of general application, it was developed for understanding the nature
of the Automatic Spectroscopic K-means based (ASK) template spectra. They
resulted from the classification of all the galaxy spectra in the Sloan Digital
Sky Survey data release 7 (SDSS-DR7), thus being a comprehensive representation
of the galaxy spectra in the local universe. Using the recipe, we give a
description of the properties of the gas and the stars that characterize the
ASK classes, from those corresponding to passively evolving galaxies, to HII
galaxies undergoing a galaxy-wide starburst. The qualitative analysis is found
to be in excellent agreement with quantitative analyses of the same spectra. A
number of byproducts follow from the analysis. There is a tight correlation
between the age of the stellar population and the metallicity of the gas, which
is stronger than the correlations between galaxy mass and stellar age, and
galaxy mass and gas metallicity. The galaxy spectra are known to follow a
1-dimensional sequence, and we identify the luminosity-weighted mean stellar
age as the affine parameter that describes the sequence. All ASK classes happen
to have a significant fraction of old stars, although spectrum-wise they are
outshined by the youngest populations. Old stars are metal rich or metal poor
depending on whether they reside in passive galaxies or in star-forming
galaxies.
