The fuelling of local supermassive black holes The fuelling of local supermassive black holes Silvia Pellegrini (Astronomy Dept., Bologna University, Italy) Silvia Pellegrini (Astronomy Dept., Bologna University, Italy)  Properties of hot gas flows in elliptical galaxies i.e., how an ISM is fed and behaves on galactic scale i.e., how an ISM is fed and behaves on galactic scale “Winds, Outflows and Inflows in X-ray Elliptical Galaxies”, “Winds, Outflows and Inflows in X-ray Elliptical Galaxies”, Ciotti, D’Ercole, Pellegrini and Renzini 1991, ApJ  The interplay between the hot ISM and the supermassive black hole (ubiquitous at the centers of the spheroids of the local Universe) new challenges for theories of galaxy formation and evolution [more recent work, also in coll. with G. Fabbiano et al. currently observable with Chandra [more recent work, also in coll. with G. Fabbiano et CfA, Cambridge, CfA, Cambridge, USA]

Einstein ’s observations showed that early-type galaxies contain a hot ISM (kT~ keV) in a largely varying quantity Fabbiano et al. 1992, ApJS Canizares et al. 1987, ApJ Hot gas in early type galaxies

Most recent catalog of the soft X-ray emission of early-type galaxies, comprehensive of all ROSAT observations (O’ Sullivan et al. 2001, MNRAS): stellar sources

Where does this hot ISM come from, and what is its history? Origin : evolving stars shed a considerable amount of mass, at a rate for a single-age passively evolving stellar population (Ciotti et al. 1991): valid in the age range from ~0.5 to over 15 Gyrs, fairly insensitive to the slope (1+x) of the IMF. History: mass losses are heated to X-ray temperatures by thermalization of the kinetic energy of the stellar motions of the SNIa’s ejecta Heating by SNIa’s is likely also secularly evolving (for the most recent modeling see Greggio 2005, A&A): directly from observations of the local Universe (e.g., Cappellaro et al. 1997) s is not known (surveys for high-z SN rates are being made, e.g., Dahlen et al. 2004) If s > or < 1.3 determines secular evolution of the specific heating of M (t) *. (Mathews 1990, Renzini 1990) =

With s > 1.3 (Ciotti et al. 1991), the specific heating is time-decreasing the hot gas flows evolve from winds (low Lx) to outflows/partial winds (intermediate Lx) to global inflows (large Lx), similar to cooling flows (Fabian 1994, ARAA) Only the most massive galaxies have been able to get to the inflow phase by the present epoch. Most of the galaxies are in a partial wind phase: gas is accreting only from within a stagnation radius, at a rate of up to 2 M  /yr ; it is outflowing from beyond it (Pellegrini & Ciotti 1998, A&A).

A few important open problems remain: 1) too much SNIa’s? The hot ISM abundances observed today seem only marginally consistent with those expected from this model (e.g., Humphrey & Buote 2005, ApJ) 2) too much accreted mass at the galactic center? Even ~ 1 M  / yr for a few Gyrs… it is predicted to reside in a phase with T gas <<100 K (Ferland, Fabian, & Johnstone 1994), mainly molecular. The ultimate fate of the cooling gas has been the subject of an extensive and contested debate (e.g., Fabian 1994): IT IS NOT OBSERVED., most likely 3) new, important ingredient: dark mass concentrations at the centers of spheroids in the local Universe (Kormendy & Richstone 1995, Magorrian et al. 1998), most likely supermassive black holes (SMBHs) with M BH ~ M ⊙ challenges posed by the SMBH ? What are the new challenges posed by the SMBH ?

The first challenge: There are tight correlations involving SMBHs and their host galaxies, as the M BH -  relation ( Gebhardt et al. 2000, Ferrarese & Merritt 2000 ) growth the birth, growth activity cycle and activity cycle of the SMBH formation and evolution of the spheroid

But also: low levels of activity : Nearby SMBHs are radiatively quiescent or show low levels of activity : L/L Edd ~1 in powerful AGNs L/L Edd < in Sgr A*, < in NGC4258, (the most securely measured SMBH masses) L/L Edd < in NGC1399, NGC4472, NGC4636 (elliptical galaxies, sites of the most luminous quasars) In a statistically complete sample (spectroscopic survey of 486 galaxies with B T <12.5 mag, Ho et al. 1997): only ~40% show “AGN”-like activity, mostly of low level (line emission could be explained by accretion) How can this radiative quiescence be reconciled with the presence of a SMBH + the accretion of <~ 2 M  /yr ?

. fueling at a rate of M ~ M  /yr (typical from modeling) + standard, radiatively efficient disc (Shakura & Sunyaev 1978). L bol ~ 0.1 M c 2 ~ 5.7x erg/s are expected This radiative quiescence represents one of the most intriguing aspects of SMBHs in the local Universe [ It was already pointed out by Fabian & Canizares 1988 (Nature) ] WHY ARE MOST SMBHs NOW DORMANT By explaining WHY ARE MOST SMBHs NOW DORMANT we will also learn something about

 HOW do SMBHs GROW through COSMIC TIME ?  WHY and HOW the luminous AGNs SWITCHED OFF ?  Is there a link between the RADIATIVE QUIESCENCE and  the feedback mechanism responsible for the M BH -  relation?  the evolution of AGNs ?

Chandra Thanks to Chandra (=highest angular resolution ever), a few of these questions can be addressed [ both the hot ISM and the nuclear emission show up in the X-ray band ]. nuclei In particular, Chandra is optimal to study the nuclei of nearby galaxies: its angular resolution is ~40 pc at the Virgo distance (1pix=0".49 for ACIS, 0".13 for the HRC ) pointlike emission can be detected in galaxies out to Virgo distances, for sources of L x >~10 37 erg/s (i.e., that of faint X-ray binaries).

The nuclear regions of ~20 early-type galaxies (of distances from 11 to 49 Mpc) have been studied so far with the Chandra ACIS: NGC821, NGC4594, NGC4736, IC1459, IC4296, NGC4697, NGC5845, NGC3377, NGC4486B, NGC4564 [S. Pellegrini in collaboration with G. Fabbiano, A. Baldi, M. Elvis, R. Soria, A. CfA, Cambridge, USA] NGC1399, NGC4636, NGC4472, M87, CenA, NGC1316, NGC4261, M32, Galactic Center, NGC4649 (Loewenstein et al. 2001, Di Matteo et al. 2003, …) A summary + overall interpretation is given in Pellegrini 2005, ApJ The main question is: WHY LOCAL SMBHS ARE NOT BRIGHT ? NOT BRIGHT ?

Possible answers: Possible answers: (i.e., the reason for the difference with the more luminous, more distant, “classical” AGNs)  the M BH ? NO (respective M BH cover roughly the same range, Ho 2002).  the fuelling rate M ?  the radiative efficiency ?  the existence of activity cycles ?

nucleus of Sombrero A test case: the nucleus of Sombrero Nearby, bulge-dominated spiral: Distance=9.4 Mpc (1"=45 pc) M BH =10 9 M ⊙ (from HST, Kormendy et al. 1996) LINER nucleus (Pellegrini et al. 2003, ApJ) Adaptively smoothed Chandra ACIS-S image ( keV) with superimposed optical contours from the DSS.

With the unprecedented angular resolution we can accurately determine the true nuclear luminosity true ISM density and temperature close to the nucleus L X,nuc =2.2x10 40 erg/s... this is just ~10 -7 L Edd !!. what is the expected mass accretion rate M ? Bondi (1952) developed the theory for steady, spherically symmetrical accretion (on a star, by gas at rest at infinity) : The mass accretion rate he found:. M BONDI =  (GM) 2 c s -3   f(  )

If we replace: the starSMBH. M BONDI  M BH 2 T(r acc ) -3/2 n(r acc ) for the galactic nuclei For most of the ~ 20 galaxies : M BH derives from detailed dynamical modeling of HST data (e.g., Gebhardt et al. 2003), except for IC4296 (from M BH -  relation) T and n are derived reasonably close to r acc with Chandra infinitythe “accretion radius” GM BH c s 2 then r acc ~

temperature density By inward extrapolation of Chandra ACIS-S data and taking errorbars into account:. M BONDI ~ – M ⊙ /yr (Pellegrini et al. 2003) This means an accretion power. L acc ~ 0.1 M BONDI c 2 ~ (4.5-38) x erg/s ~ 200 x L bol observed For Sombrero r acc ~ 64 pc ~ 1.35 arcsec

Similar results are obtained for all the nuclei observed so far : Left: Chandra 0.3–10 keV ACIS-S3 image of the central area of IC1459 (D=29.2 Mpc). Right: Adaptively smoothed image of the same region. Contours are logarithmically spaced from to 20 counts pixel-1. The horizontal bar is 10 arcsec long. Fabbiano et al. 2003

What about other galactic nuclei ? STUDY OF THE CIRCUMNUCLEAR REGIONS (r ~ 100 pc ~ r acc ) has been made with Chandra in Fornax and Virgo: Loewenstein et al. (2001) For all the cases studied so far : r acc ~ arcsec. the mass accretion rate M BONDI ~ to 0.2 M  /yr L X ~ to 0.03 L acc

Possible answers: Possible answers: (i.e., the reason for the difference with the more luminous, more distant, “classical” AGNs).  the fuelling rate M ? Nuclei are NOT fuel starved at r ~ r acc (from ACIS-S image)  the radiative efficiency ?... For M = M BONDI << M Edd the very popular solution of an Advection Dominated Accretion Flow is possible (ADAF; e.g., Narayan & Yi 1995 ) This has an extremely low radiative output : the ions decouple from the electrons and carry most of the accretion energy into the SMBH

. L acc ~ 0.1M BONDI c 2 standard disc.. with M=M BONDI.. or M ~ 0.1 M BONDI L ADAF /L Edd CenA Gal. Center …. but for the whole sample of nuclei. with Lx and M BONDI measured with Chandra: The predicted luminosity is:.. L ADAF /L Edd  ( M / M Edd ) 2 Absence of the predicted trend and large scatter in L x,nuc /L Edd Pellegrini (2005)

Possible answers: Possible answers:.  the fuelling rate M ? NO  the radiative efficiency ? NOT a successful general explanation  accretion is not a steady process: the ISM is heated. the Bondi theory does not apply ( M BONDI ).... SMBHs can be accreting at M >) x

Heating sources:  radiative: inverse Compton scattering + photoionization heating of nuclear photons of nuclear photons (Ciotti & Ostriker 2001, Sazonov et al. 2004) mechanical: impact of collimated outflows  mechanical: impact of collimated outflows (Binney & Tabor 1995; Di Matteo et al. 2003; Omma & Binney 2004) From detailed numerical simulations: these can trigger outbursts of activity (radiative/mechanical energy is released on  < t dyn ) until accretion is stopped, so that nuclear activity is switched off then gas cooling and accretion resume → → activity cycles

Important advantage of these ‘‘ feedback modulated ’’ accretion flows: SMBHs of masses >~ 10 8 M  already grow from high radiative efficiency accretion during the optically bright QSO phase (Yu & Tremaine 2002). The gas flows fed by stellar mass losses typically produce (from modelling without a central SMBH):. a central fuelling at a rate of M ~ M  /yr at the present epoch, and larger before. These can build up several 10 8 M  M  if undisturbed for many Gyrs… Feedback-modulated accretion can displace gas far from the galactic center and even remove it from the galaxy (e.g., Ostriker & Ciotti 2005).

Any observational evidence for intermittent accretion ? The T profile is rising towards the nucleus in Sombrero, instead of decreasing as in “cooling flow” models... The estimated M BONDI << M * (r acc ) = the stellar mass loss rate within the accretion radius r acc, in NGC821 (Fabbiano et al. 2004) and possibly few other galaxies under study the gas flow is not in a steady state

Pellegrini (2005) Large scatter also in:

from the Chandra images : the central  x(R) in M87,NGC1399, NGC4472, NGC4636 is “flat” : not possible to reproduce with standard steady inflowing gasdynamical solutions (Loewenstein et al. 2001) (Ciotti & Ostriker 2001) Energy is being deposited ?

2. a case of “disturbances”: NGC4636 (Jones et al. 2002) Sharp arm-like features, hotter than surrounding gas: produced by shocks driven by nuclear outburst? Filament across the nucleus, hotter than the surrounding gas (Fabbiano et al. 2004) 3. a case of central heating?

Conclusions Local SMBHs are not [all] bright AGNs because accretion follows activity cycles : this solution seems unavoidable (from numerical simulations) is the only one that can account for a variety of observational evidence (Chandra) does not imply large amounts of cooled off gas as long-lasting inflows (“cooling flows”) + ADAFs. Material can even be removed from the central regions.

Final speculations/questions. (due to the secular The DECREASE in the fuelling rate M (due to the secular. decrease in M * ) decrease in M * ) caused the peaks of activity to become more rare and less intense What is the outburst timescale and recurrence period? (also as a function of cosmological time (also as a function of cosmological time ) Are local dormant SMBHs really outflow dominated? (more direct observational evidence needed) (more direct observational evidence needed)