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 (SDSS) 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, despite being low, is sufficient to measure stellar velocity dispersion and emission line fluxes for individual objects. We show that the typical velocity dispersion of a BOSS galaxy is similar to 240 km s(-1). The typical error in the velocity dispersion measurement is 14 per cent, and 93 per cent of BOSS galaxies have velocity dispersions with an accuracy of better than 30 per cent. 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. We show that emission lines can be measured on BOSS spectra. However, the majority of BOSS galaxies lack detectable emission lines, as is to be expected because of the target selection design towards massive galaxies. We analyse the emission line properties and present diagnostic diagrams using the emission lines [O II], H beta, [OIII], H alpha and [N II] (detected in about 4 per cent of the galaxies) to separate star-forming objects and active galactic nuclei (AGN). 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 (LOWZ) around 0.15 < z < 0.3, half of the emission line galaxies have low-ionization nuclear emission-line region (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 (CMASS) 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 H beta 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 versus r - i target selection diagram.