Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.9703r&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #97.03; Bulletin of the American Astronomical Society, V
Astronomy and Astrophysics
Astronomy
Scientific paper
Analysis of the frequency and physical properties of galaxies with star-formation and AGN activity in different environments in the local universe is a cornerstone for understanding structure formation and galaxy evolution. We have undertaken a study of active galaxies in different galaxies surveys (15R, 2DF, SDSS) to explore the properties of these systems in different environments. We built new multi-wavelength catalogs gathering information on H-alpha, R-band, radio,far-infrared, and X-ray emission, as well as radio and optical morphologies, and have developed a classification scheme to compare different selection methods and to select accurately samples of radio emitting galaxies with AGN and star-forming activity. While alternative classification schemes do not lead to major differences for star-forming galaxies, we show that spectroscopic and photometric classifications of AGN lead to incomplete samples. In particular, a large population of AGN-containing galaxies with absorption-line spectra, and in many cases extended radio structures (jets, lobes), is missed in the standard Baldwin-Phillips-Terlevich emission-line classification of active galaxies. This class of objects accounts for roughly half of the radio AGN population. Similarly, for X-ray selected AGN in our sample, we find that absorption-line AGN account for half of the sample. Spectroscopically unremarkable, passive galaxies with AGN activity are not an exception, but the norm, and we show that although they exist in all environments, these systems preferentially reside in higher density regions. Because of the existence of this population, the fractional abundance of AGN strongly increases with increasing density, in contrast to the results based on emission-line AGN extracted from the 15R, Sloan and 2DF redshift surveys. Since emission-line radio AGN are mostly associated with late-type galaxies and absorption-line radio AGN with early-type galaxies, the trends found are connected to the well-known but poorly understood density-morphology relation.
Helfand David
Reviglio Pietro
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