Friday, June 26th

Stellar velocity dispersions and emission line properties of SDSS-III/BOSS galaxies

D. Thomas (ICG Portsmouth), O. Steele (ICG Portsmouth), C. Maraston (ICG Portsmouth), J. Johansson (MPA), A. Beifiori (MPE), J. Pforr (NOAO), G. Strombaeck(ICG Portsmouth), C. A. Tremonti (Wisconsin-Madison), D. Wake (Yale), D. Bizyaev (APO), A. Bolton (Utah), H. Brewington (APO), J. R. Browstein (Utah), J. Comparat (Marseille), J. P. Kneib (Marseille), E. Malanushenko (APO), V. Malanushenko (APO), D. Oravetz (APO), K. Pan (APO), J. K. Parejko (Yale), D. P. Schneider (UPenn), A. Shelden (APO), A. Simmons (APO), S. Snedden (APO), M. Tanaka (Tokyo), B. A. Weaver (NYU), R. Yan (NYU)
(Submitted on 25 Jul 2012)

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 is released in the ninth SDSS data release in July 2012, 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 ~240 km/s. The typical error in the velocity dispersion measurement 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. The majority of BOSS galaxies lack detectable emission lines. We analyse the emission line properties for a subsample below z=0.45. For this subset 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. In general, the fraction of star forming galaxies decreases and the fraction of AGN increases with increasing redshift, mostly owing to selection effects. Within in the low-z sample (LOWZ), the majority of emission-line galaxies have some AGN component, the fraction of purely star forming galaxies at z>0.15 only being a few per cent. The fraction of star-forming galaxies among the emission-line galaxies within the high-z sample (CMASS), instead, is ~20 per cent. We show that these objects typically have blue observed g-r colours and are well separated in the g-r vs r-i target selection diagram.

A new scaling relation for HII regions in spiral galaxies: unveiling the true nature of the mass-metallicity relation

F. F. Rosales-Ortega, S. F. Sánchez, J. Iglesias-Páramo, A. I. Díaz, J. M. Vílchez, J. Bland-Hawthorn, B. Husemann, D. Mast
(Submitted on 26 Jul 2012)

We demonstrate the existence of a -local- relation between galaxy surface mass density, gas metallicity, and star-formation rate density using spatially-resolved optical spectroscopy of HII regions in the local Universe. One of the projections of this distribution, -the local mass-metallicity relation- extends over three orders of magnitude in galaxy mass density and a factor of eight in gas metallicity. We explain the new relation as the combined effect of the differential radial distributions of mass and metallicity in the discs of galaxies, and a selective star-formation efficiency. We use this local relation to reproduce -with remarkable agreement- the total mass-metallicity relation seen in galaxies, and conclude that the latter is a scale-up integrated effect of a local relation, supporting the inside-out growth and downsizing scenarios of galaxy evolution.

Comments: Accepted for publication in Astrophysical Journal Letters, 5 pages, 4 figures
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1207.6216v1 [astro-ph.CO]
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