Fermi Bubbles: echoes of the last quasar outburst? Sergei Nayakshin, University of Leicester Kastytis Zubovas, Andrew King (Leicester) Chris Power (U of Western Australia, Perth)
The Fermi Bubbles in the Milky Way Unlikely to be a static structure Su et al 2011
This talk Consider Cosmic Rays as a tracer population No model for gamma- ray emission What inflated the Fermi lobes?
Fermi lobes: who did it? SN mechanical energy per unit stellar mass SMBH mechanical energy Outflow velocities v_sn ~ (0.01-0.03) c E SN 0.001 M c 2 E BH 0.01 M c 2 v_bh ~ (0.1-1) c Both SN and BH could have done it
Why Sgr A* is a suspect The central 100 pc origin: Natural for BH feedback The bubbles are not likely to be quasi- static must be a recent, energetic event Sgr A* has probably been activated ~ 6 Myrs ago
Sgr A*: SMBH in the GC Eisenhauer et al 05 Young massive stars are best explained by star formation in situ (Paumard et al 2006). R. Genzel et al work
Stellar rings near Sgr A* (Genzel et al. 2003) view of the rings on the sky (also cf. Levin & Beloborodov 03, Paumard et al 06, Bartko et al 2009,2010, and also work by A. Ghez and collaborators)
Origin of Sgr A* young stars The gas was deposited by a rogue GMC (Yusef- Zadef 2008) Bonnell & Rice 2008 Hobbs & Nayakshin 09
Sgr A* must have been activated In all of the simulations 1000 to 10,000 Msun of low angular momentum material gets deposited inside ~ 0.03 pc Star formation there is not possible (N & Cuadra 05) The gas must have been accreted by Sgr A* (Alexander+ 2011, also cf. Yuri Levin s talk)
If Sgr A* made Fermi lobes, then how? Could be a jet Sgr A* has a weak jet now (Sera s talk) Other galaxies show AGN- driven jets Jets contain relativistic particles - - great for gamma- rays Guo & Mathews 2011
The jet scenario Jet had to be stopped. Model dependent. R = c t =2, 000 kpc Or the jet could be unrelated to the star formation event Why point perpendicular to the Galaxy plane? Jets in Seyfert galaxies point in random directions
The M- sigma relation: feedback not by a jet Ferrarese et al 2006 Black - - SMBH Red - - nuclear star clusters (NCs) There are massive BHs or Nuclear star clusters in most galaxies. Mass of these correlates with the host sigma
AGN Feedback: Momentum or Energy? SMBH can heat the gas energy feedback. Works well for systems when gas cannot cool (low gas densities, e.g., elliptical galaxies, galaxy clusters) SMBH can push the gas via an outflow momentum feedback. Works for systems accreting gas at high rates, such as quasars The most interesting case is the momentum feedback
Momentum outflow argument For an isothermal DM potential with velocity dispersion gas density at radius R (R) = f g 2 2 GR 2 Grav force (weight) at radius R AGN needs to produce outward force exceeding this to expel the gas
Momentum outflow argument Momentum outflow from a BH Ṗ = L Edd c = 4 GM BH apple Force balance 4 GM BH apple = 4f g G 4 King 2003, 2005 (Silk & Rees 98 had M ~ sigma^5) Note: v_out ~ 0.1 c outflows are seen in 40% of local AGN (Tombesi et al 2010)
M sigma relations Ferrarese et al 2006 Black - - SMBH Red - - nuclear star clusters (NCs) M < 10^8 Msun R ~ 3-10 pc NCs and SMBHs can be explained by same weight argument
Explains Nuclear Clusters too For massive stars Just need to count massive star fraction. For a young cluster with Salpeter s IMF Matches the observed offset well. McLaughlin, King & Nayakshin 2007
Sgr A* the quasar? Sgra A* hosted a star formation (M ~ 10,000 Msun) in the inner 0.5 pc ~ 6 Myrs ago (Paumard et al 2006). Is that much of gas accreted by Sgr A*, it would be near its Eddington luminosity
The model (Zubovas +11) Use same baseline model Spherically symmetric outflow The CMZ cannot be expelled H R L Edd c 0.1W CMZ The CMZ can focus the outflow perpendicular to the plane (Zubovas et al 2011)
Sgr A* power output Power output of the outflow Ė = 1 2Ṁv2 = v 2c L Edd 0.1L Edd Need a few 10^3 Msun to power the Fermi Lobes Comparable to mass needed to form young stars inside 0.5 pc Consistent with simulations of Hobbs & N 2009
Sgr A* feedback: numerical model Zubovas & Nayakshin, in prep. Use Nayakshin et al 2009 AGN feedback implementation Assume a spherical halo of diffuse gas And the Central Molecular Zone disc (Morris & Serabyn 1996) H/R = 1/8 to 1/4 Radial extent from 5 to 200 pc
Sgr A* feedback: numerical model Zubovas & Nayakshin, in prep. Zubovas & Nayakshin, in prep. Preliminary conclusions Consistent with analytical estimates of Zubovas + 2011 Activity for at least 10^5 yrs (at Eddington accretion rate) Note the outer thick shell - - the ROSAT X- ray walls?
200 100 0-100 -200-200 -100 0 100 200-1.5-1.0-0.5 0.0 0.5 1.0 4 5 6 7 8 9 10 200 100 0-100 -200-200 -100 0 100 200-2.5-2.0-1.5-1.0-0.5 0.0 4 5 6 7 8 9 10 Sgr A* feedback: Expanding Molecular Ring 200 200 100 100 z [ pc] 0 z [ pc] 0-100 -100-200 -200-200 -100 0 100 200 x [ pc] -200-100 0 100 200 x [ pc] -1.5-1.0-0.5 0.0 0.5 1.0 log Σ [g cm -2 ] -2.5-2.0-1.5-1.0-0.5 0.0 log Σ [g cm -2 ] 4 5 6 7 8 9 10 log T [K] 4 5 6 7 8 9 10 log T [K] The CMZ disc is compressed into a ring moving outward - - EMR??? E_ring ~ (H/R) E_FermiBubble - - roughly consistent
Conclusions: suggested scenario 6 Myrs ago, a GMC walked into the central pc Made the central star cluster Sgr A* was activated as a bright quasar for a short while Broad angle outflow focused by CMZ may explain the morphology of the Fermi Bubbles (and the expanding molecular ring?)