The Blast Wave Model for AGN Feedback: Effects on AGN Obscuration

Abstract

We compute the effect of the galactic absorption on AGN emission in a cosmological context by including a physical model for AGN feeding and feedback in a semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers for AGN accretion, and on expanding blast waves as a mechanism to propagate outwards the AGN energy injected into the interstellar medium at the center of galaxies. We first test our model against the observed number density of AGNs with different intrinsic luminosity as a function of redshift. The model yields a ''downsizing'' behavior in close agreement with the observed one for z<2. At higher redshifts, the model predicts an overall abundance of AGNs (including Compton-thick sources) larger than the observed Compton-thin sources by a factor around 2 for z>2 and L_X < 10^{44} erg/s. Thus, we expect that at such luminosities and redshifts about 1/2 of the total AGN population is contributed by Compton-thick sources. We then investigate the dependence of the absorbing column density N_H associated to cold galactic gas (and responsible for the Compton-thin component of the overall obscuration) on the AGN luminosity and redshift. We find that the absorbed fraction of AGNs with N_H>10^{22} cm^{-2} decreases with luminosity for z<1; in addition, the total (integrated over luminosity) absorbed fraction increases with redshift up to z around 2, and saturates to the value around 0.8 at higher redshifts. Finally, we predict the luminosity dependence of the absorbed fraction of AGNs with L_X< 3 10^{44} erg/s to weaken with increasing redshift. We compare our results with recent observations, and discuss their implications in the context of cosmological models of galaxy formation.Comment: 13 pages, accepted for publication in Astrophysical Journal; added 2 reference

X-shooter reveals powerful outflows in z ∼ 1.5 X-ray selected obscured quasi-stellar objects

“…We span a 2 orders of magnitude range in L(AGN) (10 45 -10 47 erg s −1 ) and no clear trend between these two quantities is seen in these data. This result is at odds with the results recently presented in , where a trend of the [OIII] width with the IR luminosity is seen in SDSS luminous Type 2 quasars, as expected for outflows driven by the radiation pressure of the quasar (e.g., Menci et al 2008). …”

“…While among less luminous sources (L bol < 10 45 erg s −1 ), nuclear activity and Star Formation (SF) can be regulated by local processes such as accretion triggered gas inflows or disk instabilities likely induced by minor mergers or galaxies encounters (see Ciotti & Ostriker 2007;Bournaud et al 2011;Cen 2012), among the most luminous sources (L bol > 10 46 erg s −1 ), major galaxy mergers are indicated as the culprit for such physical coupling, since they can efficiently funnel a large amount of gas into the nuclear region to feed (and obscure) the accreting SMBH (Menci et al 2008).…”

“…Menci et al 2008;Hopkins et al 2008). The first phase (i) is associated with rapid SMBH growth and efficient SF, in a dust-enshrouded, dense environment.…”

Black hole growth and starburst activity atz= 0.6–4 in the Chandra Deep Field South

“…On the contrary, unobscured AGN should be associated to galaxies with low or absent episodes of star-formation, given that most of the gas and dust responsible for the star formation has been blown away by the effect of AGN feedback. Many observational evidences (see Alexander et al 2005;Page et al 2004;Stevens et al 2006) and theoretical arguments (Menci et al 2008, and references therein) in favor of the evolutionary sequence do exist. These results challenge our 20-years old AGN view, in which the differences we see in different classes of sources -especially between "obscured" and "unobscured" ones -are simply due to orientations effects (Antonucci et al 1985;Antonucci 2003;Urry & Padovani 1995).…”

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

“…Even more important, ⋆ E-mail: roberto.dellaceca@brera.inaf.it there are now increasing evidence that the formation and growth of galaxies and their nuclear supermassive black holes (SMBHs, M BH > 10 6 M ⊙ ) are intimately related; feedback from actively accreting SMBH, AGN, must play a fundamental role in regulating both star formation and accretion throughout galaxy's history (e.g. Silk & Rees 1998, Granato et al 2004, Di Matteo et al 2005, Croton et al 2006, Hopkins et al 2008, Menci et al 2008, King 2010, Faucher-Giguère & Quataert 2012. Although the nature of this relationship is still poorly understood, there are hints that most of the SMBH accretion takes place during an obscured quasar phase.…”