Abstract
Recent well resolved numerical simulations of AGN feedback have shown that its effects on the host galaxy may be not only negative but also positive. In the late gas poor phase, AGN feedback blows the gas away and terminates star formation. However, in the gas-rich phase(s), AGN outflows trigger star formation by over-compressing cold dense gas and thus provide positive feedback on their hosts. In this paper we study this AGN-triggered starburst effect. We show that star formation rate in the burst increases until the star formation feedback counteracts locally the AGN outflow compression. Globally, this predicts a strong nearly linear statistical correlation between the AGN and starburst bolometric luminosities in disc galaxies, L_* \propto L_{AGN}^{5/6}. The correlation is statistical only because AGN activity may fluctuate on short time scales (as short as tens of years), and because AGN may turn off but its effects on the host may continue to last until the AGN-driven outflow leaves the host, which may be up to 10 times longer than the duration of the AGN activity. The coefficient in front of this relation depends on the clumpiness and morphology of the cold gas in the galaxy. A "maximum starburst" takes place in am azimuthally uniform gas disc, for which we derive an upper limit of L_* \sim 50 times larger than L_{AGN} for typical quasars. For more clumpy and/or compact cold gas distributions, the starburst luminosity decreases. We also suggest that similar AGN-triggerred starbursts are possible in hosts of all geometries, including during galaxy mergers, provided the AGN is activated. Finally, we note that due to the short duration of the AGN activity phase the accelerating influence of AGN on starbursts may be much more common than observations of simultaneous AGN and starbursts would suggest.
Highlights
Supermassive black holes (SMBHs) are known to reside in the centres of most large galaxies
The first plot shows the time dependence of star formation rate M ∗, total mass of gas converted into stars and the total luminosity of the starburst
We have shown, using analytical arguments and a semianalytical model, that powerful AGN outflows in spiral galaxies may have a strong positive effect on the galaxy disc
Summary
Supermassive black holes (SMBHs) are known to reside in the centres of most large galaxies. The masses of these SMBHs correlate with a range of properties of their host spheroids, including luminosity (e.g. Magorrian et al 1998) and mass The correlation between SMBH mass (Mbh) and the velocity dispersion (σ) of its host spheroid, referred to below as a “Mbh–σ relation”, has been studied by many authors. Other types of outflows and radiation (e.g., Silk & Rees 1998; Fabian 1999; Ciotti & Ostriker 2001) are expected to drive the gas out of the host’s potential, limiting the SMBH mass and establishing the observed SMBH-galaxy correlations (King 2003, 2005)
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