Stellar velocity dispersions and emission line properties of
  SDSS-III/BOSS galaxies
  We perform a spectroscopic analysis of 492,450 galaxy spectra from the first
two years of observations of the Sloan Digital Sky Survey-III/Baryonic
Oscillation Spectroscopic Survey (BOSS) collaboration. This data set has been
released in the ninth SDSS data release, the first public data release of BOSS
spectra. We show that the typical signal-to-noise ratio of BOSS spectra is
sufficient to measure stellar velocity dispersion and emission line fluxes for
individual objects. The typical velocity dispersion of a BOSS galaxy is 240
km/s, with an accuracy of better than 30 per cent for 93 per cent of BOSS
galaxies. The distribution in velocity dispersion is redshift independent
between redshifts 0.15 and 0.7, which reflects the survey design targeting
massive galaxies with an approximately uniform mass distribution in this
redshift interval. The majority of BOSS galaxies lack detectable emission
lines. We analyse the emission line properties and present diagnostic diagrams
using the emission lines [OII], Hbeta, [OIII], Halpha, and [NII] (detected in
about 4 per cent of the galaxies). We show that the emission line properties
are strongly redshift dependent and that there is a clear correlation between
observed frame colours and emission line properties. Within in the low-z sample
around 0.15<z<0.3, half of the emission-line galaxies have LINER-like emission
line ratios, followed by Seyfert-AGN dominated spectra, and only a small
fraction of a few per cent are purely star forming galaxies. AGN and LINER-like
objects, instead, are less prevalent in the high-z sample around 0.4<z<0.7,
where more than half of the emission line objects are star forming. This is a
pure selection effect caused by the non-detection of weak Hbeta emission lines
in the BOSS spectra. Finally, we show that star forming, AGN and emission line
free galaxies are well separated in the g-r vs r-i target selection diagram.
