HI in AGN

If you are interested on this results, have a look at some of the papers I wrote (and see the references there for earlier papers):

Tracing the extreme interplay between radio jets and the ISM in IC5063 – A&A 580, 1, Morganti, Raffaella; Oosterloo, Tom; Oonk, Raymond J. B.; Frieswijk, Wilfred; Tadhunter, Clive
 dailyimage-001-3-3

“Jet acceleration of the molecular outflow in the Seyfert galaxy IC5063” published in Nature (Tadhunter, Morganti, Rose, Oonk, Oosterloo, 2014, doi:10.1038/nature13520).

 

See also my paper Science 341, 1082 (2013): fig1-005Morganti, Fogasy, Paragi, Oosterloo,Orienti “Radio Jets Clearing the Way Through a Galaxy: Watching Feedback in Action
Get a (free) copy: Abstract; Reprint; Full Text

 

The presence of neutral hydrogen in the region surrounding the active galactic nuclei (AGN) is known since many years. This gas can be studied (at the radio wavelengths) via 21-cm absorption line produced by HI located in front of a strong continuum source. This gas is now known to be associated with different structures.
Neutral hydrogen can be found in tori very close to the AGN (see cartoon). Although it has been generally assumed that the tori are composed of dusty molecular clouds, it is now clear that, under certain conditions, they can partly consist of atomic hydrogen. The HI can also be located in  larger-scale circumnuclear disks (with sizes ranging from 0.1 and 1 kpc). These structures are similar to the optical nuclear disks detected in a large number of early-type galaxies (both radio-loud and radio-quiet). These disks (mainly detected by HST) can be seen either in ionized gas or due to their strong dust absorption.
The neutral hydrogen can also be associated with more disturbed structures, like bridges or tails left over from recent mergers. The origin of activity in galaxies is often explained as triggered by merger and/or interaction processes. HI is often seen associated with all these phenomena.
However, quite unexpectedly, neutral hydrogen has recently been found also to be associated with fast (up to 2000 km/s) gaseous outflows emerging from the central regions of radio galaxies.

What causes these fast outflows?
Gas outflows can be a result of the interplay between the enormous amount of energy released from the active nucleus and the interstellar medium around it. Fast nuclear outflows of ionised gas appear to be a relatively common phenomena in active galactic nuclei. They are mainly detected in optical, UV and X-ray observations. Gas outflow of neutral atomic gas came, instead, as a surprise.
Why gas outflows are important? Gas outflows associated with AGN provide energy feedback into the interstellar medium (ISM) that can profoundly affect the evolution of the central engine as well as that of the host galaxy. The mass-loss rate from these outflows can be a substantial fraction of the accretion rate needed to power the AGN. Thus, they are an important factor in the evolution of the host galaxy.
A number of mechanisms have been suggested to explain the fast outflows of neutral hydrogen. One proposed mechanism is the interaction of the radio plasma with the interstellar medium. In order to understand whether this is indeed likely to occur, and what are the physical conditions, direct information about the exact location of the outflow is needed (in general using observations with high spatial resolution).
Radio data that can answer this question are now available and they represent the core of this research project. Studies of this kind have been done so far only for a few objects and the aim of the project is to considerably enlarge the sample.  The goal is to locate where the broad absorption is occurring and to study the similarities with the outflows detected in the ionised gas. This is crucial  information if we want to understand the mechanism that is actually producing the outflows.

 

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