Bright Active Galactic Nuclei cause high velocity ionised wind
11 March 2015
At the centre of most (if not all) galaxies lies a supermassive black hole. In in most cases, as in our own galaxy, it is is dormant. This means that it is not currently accreting matter. However, in some galaxies it is active: it is accreting matter and, as a result, radiating an enormous amount of energy across the electromagnetic spectrum. These so-called active galactic nuclei (AGN) likely play an important role in the lifetime of galaxies due to the close observed correlations between the mass of supermassive black holes and their galaxy's central bulge of stars. Such correlations suggest that they evolve and grow together.
A process called feedback is often invoked as the mechanism behind this correlation. AGN feedback is a broad term for all the effects that the presence of an accreting black hole will have on its host galaxy. In order to better understand feedback, and hence galaxy evolution, CAASTRO PhD student Rebecca McElroy (University of Sydney) and her colleagues selected an extreme sample of 17 local (z < 0.11) highly luminous type II AGN (log(L[O III]/L_Sun) > 8.7) with very high accretion rates where the effects of feedback should be most pronounced. They used the Anglo-Australian Telescope’s AAOmega spectrograph to observe these galaxies by integral field spectroscopy (IFS) – a technique that allows for hundreds of spectra to be taken across the spatial projection of a galaxy at once. At the median redshift of the sample they covered a field of view of 915 square kiloParsec and were able to record the kinematics and emission in different regions of the galaxy – both near and far from the AGN.
By fitting the emission lines present in the spectra across each galaxy the researchers were able to find signatures of outflows coming from the AGN in all 17 galaxies observed. These outflows were distinguished by their high velocity, velocity dispersion, and ionisation. In the majority of galaxies there was also evidence of shocks produced as the outflows pass through the surrounding gas. The research team could rule out radio jets as the cause for the outflows and concluded that the outflows come from these high luminosity AGN and are directly impacting the surrounding Interstellar Medium within the galaxies.
Rebecca McElroy, Scott M. Croom, Michael Pracy, Rob Sharp, I-Ting Ho, Anne M. Medling in MNRAS 446 (2015): "IFU observations of luminous type II AGN - I. Evidence for ubiquitous winds"