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5 November 2010

Jet-powered molecular hydrogen emission from radio galaxies

Authors: P. Ogle et al.
Link to the paper: arXiv:1009.4533

The authors examine a sample of 55 radio galaxies with z < 0.22 and find 31% percent of those to exhibit particularly strong mid-IR emission from warm H2 (T = 100 - 1500 K, with masses in this phase typically around some 108, up to 2 x 1010 solar masses), putting them in a newly (re)defined class of "molecular hydrogen emission galaxies" (MOHEGs). They argue that the jets in these sources may interact with the cold H2 in these galaxies and shock-heat it, resulting in the observed emission. They find that X-rays originating from the AGN are incapable of powering the H2 emission, but that cosmic rays may still be an alternative explanation if a cosmic ray pressure 25 times higher than thermal pressure is deemed to be reasonable.

The radio MOHEGs show only low to moderate star formation rates (< 3 solar masses per year) and much less 7.7 ┬Ám PAH emission then normal star forming galaxies with respect to the mid-IR continuum (a factor of 10 - 100 lower). The H2 luminosity does not show much correlation with radio power, but several sources in clusters with X-ray cavities indicate that the ratio between H2 luminosity and kinetic jet power may lie around 10-4 to some 10-3. Most (14 of 17) radio MOHEGs belong to close galaxy pairs, groups or cluster and the authors conjecture that this environment or past gas-rich mergers may deliver large quantities of gas to the galaxies.

The authors conclude that jet-driven outflows may be responsible for the emission, although the details are yet very unclear. The jet powers, however, would more than sufficient for this.


  1. It is unclear to me why these galaxies should be put into a new class of galaxies: Fig. 11 and 12 rather suggest that these galaxies are an extension of a continuous distribution of galaxies where, however, the jets seem to be responsible for their more extreme behaviour. A more continuous transition would also be supported by jets not simply being on or off, but rather showing a continuous spectrum of radio silent to radio loud (and similarly, of course, in their mechanical effect).

    What is also unclear to me is the statement in 1.2 that extreme H_2 emission has been found in a variety of galaxy types so far. By what mechanism are these powered then, and why could those mechanisms not be at work in radio MOHEGs?

    The potential difficulty of efficiently coupling the jet power to the host galaxy ISM, however, does not seem very critical to me. The mentioned level of efficiency is still much lower than the mechanical feedback from the jet on the denser ISM phases of a few percent (which we find in our jet-ISM simulations). This still leaves enough room for energy deposition on other phases and inefficient coupling, and turbulent motion in the jet cocoon will probably provide feedback on a sufficient level even after the jet has broken out of the galaxy.

  2. The author Patrick Ogle5 November 2010 at 19:36

    Regarding whether these galaxies with large H2/PAH should belong in a distinct class, I believe they should. They clearly stand apart from normal star forming galaxies in this ratio. From an astrophysical point of view, very large ~unity warm/cold H2 mass ratios indicate that a large injection of mechanical energy is dramatically disturbing the molecular ISM. This in turn appears to suppress star-formation efficiency by a factor 10-50 (see Nesvadba 10). On the other hand I agree that there is not a distinct separation but rather a continuum in the H2/PAH ratio and corresponding warm/cold H2 fraction, corresponding to varying degree of disturbance in the ISM by the radio jet. The selection of H2/PAH=0.04 as the dividing line is somewhat arbitrary, but does roughly correspond to the maximum value of this ratio obtained in normal star forming galaxies and PDR models of the same.

    Regarding the 2nd comment, there can be a variety of mechanical energy inputs that heat the molecular ISM in various classes of galaxies. Radio galaxy samples have a very high fraction >30% of MOHEGs. They are perhaps the best case, where there is weak star formation and weak nuclear X-ray emission, where we can isolate the mechanical energy source (the radio jet). In the case of dusty ellipticals, we think they are quiescent radio galaxies where the radio jet is currently off but heated the ISM in the recent past. This needs further study to confirm. H2 emission from colliding galaxies such as Stephan's Quintet has been convincingly argued to be powered by shocks from the colliding ISM (Appleton 06, Cluver 10). Late stage ULIRG galaxy mergers such as NGC 6240 may be powered by the kinetic energy of the merger, or by starburst driven winds. The jury is also out on radio quiet AGNs. Perhaps sub-relativistic winds are important.

    I agree with your last statement that only a small fraction (0.001) of the radio power has to be extracted to power the H2, and it may not be difficult to do this via the jet cocoon. However, consider the case of 3C 326 where the jet cocoon is 2 Mpc across and has long since overrun the entire galaxy. Will the jet-ISM coupling be as efficient in such a case?

    Thanks for your interesting discussion and invitation to comment.
    Patrick Ogle

  3. For a large galaxy as 3C 326, I agree that the jet-ISM coupling may not be sufficient anymore. However, the radio source is very asymmetric and has a strange lobe morphology (the big blob on the east and the long feature on the west, which doesn't look like a normal" cocoon / lobes to me...) and I find it very difficult to speculate on this. Also, our simulations don't reach the Mpc scale. It definitely is a challenge for any indirect (via cocoon) interaction with the galaxy.