Radio-AGN Feedback

AGN and galaxy co-evolution

We now know (through a series of very complicated or mildly complicated observations) that most galaxies actually host a supermassive black hole at their centers. These monsters have grown to their current mass through the accretion of matter from their surroundings.

Interestingly, we find that there is a tight positive relation between the current mass of black holes and the current mass (or more accurately central velocity dispersion) of their host galaxies. In other words, the bigger the galaxy, the bigger the black hole. This implies that there must be a sort of communication between the central black hole and the galaxy that is hosting it. That is described by the general (aka vague) term “co-evolution”.

AGN feedback

It is therefore natural to consider what may be the mechanism or process that allows these two components, at very different scales (both in terms of size, mass, and energetics), to communicate.

One such mechanism or process is called AGN feedback. According to the AGN feedback paradigm, during the growth phase of the black hole (i.e., the active galactic nucleus phase), part of the energy emitted from the AGN is injected into the host galaxy and in effect prohibits the formation of new stars. That is, the galaxy that is hosting the black hole stops growing.

There are different flavors of AGN feedback, the two main being quasar-mode feedback and radio-mode feedback. The first is usually associated with black holes that are accreting very efficiently and therefore are producing copious amounts of radiation. The latter one relates to the low-luminosity cousins of quasars, the radio-AGN, which although do not emit the same amounts of radiation, instead eject powerful well-collimated outflows that carry with them high momentum. In some cases both mechanisms can be found in the same object (i.e., a radio-loud quasar!).

Radio-AGN feedback

In this work we are studied the star-formation properties of radio-AGN host galaxies [3]. The goal of this study was to uncover the possible signatures of negative or positive feedback. To do this we used photometric information at many different wavelengths, ranging from the ultraviolet to the far-infrared. By studying the broadband spectral energy distributions of radio sources, we looked at the properties of both the central accreting supermassive black hole, as well as the characteristics of the host galaxy and its star-formation component. For the first time, this was done for a sample of faint radio-sources (extending to sub-mJy levels).

We found evidence for two different kinds of feedback at play in our sample. First we showed that there is a broad correlation between the AGN and SF components luminosity. By comparing our results with models of jet or outflow induced SF, we could rule out this as an explanation to this correlation. This implies that the observed correlation is either a result of selection effects or, perhaps more intriguingly, implies a feedback in the opposite direction. That is to say that it is the SF that is feeding the central supermassive black hole and therefore results to more luminous AGN being hosted in host galaxies with higher SF.

Second we showed that for a given redshift and AGN luminosity range, radio-AGN with higher radio luminosities show reduced star-formation rates per unit of stellar mass. In other words, the more powerful the radio-jet in an AGN, the more the SF appears suppressed in its host galaxy. Interestingly, when we compare the star-formation rates per unit of stellar mass for these radio-loud AGN, they are in agreement with normal star-forming galaxies at their respective redshifts. Thus we concluded that although the radio-jet works towards suppressing star-formation, it does not quench it but rather regulates it.