Far-infrared and radio continuum study of nuclear activity

Far-infrared and radio continuum study of nuclear activity

Scientific Results

Far-infrared and radio continuum study of nuclear activity

We present a study of the nuclear activity in a well-defined sample of the most isolated galaxies (total sample: n = 1050, complete subsample: n = 719) in the local Universe traced by their far-infrared (FIR) and radio continuum emission.


With this aim we have used the well-known radio continuum-FIR correlation to select radio-excess galaxies that are candidates to host an active galactic nucleus (AGN), as well as the FIR colours to find obscured AGN-candidates. We also used the existing information on nuclear activity in the Véron-Cetty catalogue and in the NASA Extragalactic Database.


Results

A final catalogue of AGN-candidate galaxies has been produced that will provide a baseline for studies on the dependence of activity on the environment.

Fig. 1 Radio versus FIR luminosity for the complete subsample (n = 710). We show the correlation as a solid line and the 5 times radioexcess and FIR-excess levels as dashed lines. The galaxies above the upper-dashed line are the radio-excess galaxies.

 
Our sample is mostly radio quiet, consistent with its high content of late-type galaxies. At most 1.5% of the galaxies show a radio excess with respect to the radio-FIR correlation, and this fraction even goes down to less than 0.8% after rejection of back/foreground sources using FIRST.
 
We find that the fraction of FIR colour selected AGN-candidates is 28% with a lower limit of 7% Our final catalogue contains 89 AGN candidates and is publicly available on the AMIGA web page. A comparison with the results from the literature shows that the AMIGA sample has the lowest ratio of AGN candidates, both globally and separated into early and late types. Field galaxies as well as poor cluster and group environments show intermediate values, while the highest rates of AGN candidates are found in the central parts of clusters and in pair/merger dominated samples. For all environments, early-type galaxies show a higher ratio of radio-excess galaxies than late types, as can be expected, since massive elliptical galaxies are the usual hosts of powerful radio continuum emission.
 
We conclude that the environment plays a crucial and direct role in triggering radio nuclear activity and not only via the density-morphology relation. Isolated, early-type galaxies show a particularly low level of activity at radio wavelengths hence constituting the most nurture-free population of luminous early-type galaxies.