the fuelling of local supermassive black holes the fuelling of local supermassive black holes silvia...

Click here to load reader

Post on 13-Jan-2016




0 download

Embed Size (px)


  • The fuelling of local supermassive black holes 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 Winds, Outflows and Inflows in X-ray Elliptical Galaxies, Ciotti, DErcole, 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 evolutioncurrently observable with Chandra [more recent work , also in coll. with G. Fabbiano et al. @ CfA, Cambridge, USA]

  • Einsteins observations showed that early-type galaxies contain a hot ISM (kT~0.3-0.8 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 SNIas ejecta Heating by SNIas 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 determinessecular evolution of thespecific 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 SNIas? 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 Tgas

  • The first challenge:

    There are tight correlations involving SMBHs and their host galaxies,

    as the MBH - relation (Gebhardt et al. 2000, Ferrarese & Merritt 2000)

    the birth, growth and activity cycle of the SMBH formation and evolution of the spheroid

  • But also:

    Nearby SMBHs are radiatively quiescent or show low levels of activity : L/LEdd ~1 in powerful AGNs L/LEdd

  • . fueling at a rate of M ~ 0.01-2 M/yr (typical from modeling) + standard, radiatively efficient disc (Shakura & Sunyaev 1978)

    . Lbol ~ 0.1 M c2 ~ 5.7x1043 - 1046 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) ]

    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 MBH - relation?

    the evolution of AGNs ?

  • 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 ].

    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 Lx>~1037 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. Siemiginowska @ 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 ?

  • Possible answers: (i.e., the reason for the difference with the more luminous, more distant, classical AGNs)

    the MBH ? NO (respective MBH cover roughly the same range, Ho 2002) . the fuelling rate M ? the radiative efficiency ? the existence of activity cycles ?

  • A test case: the nucleus of Sombrero Nearby, bulge-dominated spiral: Distance=9.4 Mpc (1"=45 pc) MBH=109M (from HST, Kormendy et al. 1996) LINER nucleus

    (Pellegrini et al. 2003, ApJ)Adaptively smoothed Chandra ACIS-S image (0.3-10 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 LX,nuc=2.2x1040 erg/s ... this is just ~10-7 LEdd !!

    . 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: . MBONDI = (GM)2 cs-3 f()

  • If we replace:

    the star SMBH . MBONDI MBH2 T(racc)-3/2 n(racc) for the galactic nuclei

    For most of the ~ 20 galaxies :MBH derives from detailed dynamical modeling of HST data (e.g., Gebhardt et al. 2003), except for IC4296 (from MBH- relation)

    T and n are derived reasonably close to racc with Chandra

    infinitythe accretion radius GMBH cs2

    then racc ~

  • temperaturedensity By inward extrapolation of Chandra ACIS-S data and taking errorbars into account: . MBONDI ~ 0.008 0.067 M/yr (Pellegrini et al. 2003)

    This means an accretion power . Lacc ~ 0.1 MBONDI c2 ~ (4.5-38) x 1043 erg/s ~ 200 x Lbol observed For Sombrero racc ~ 64 pc ~ 1.35 arcsec

  • Similar results are obtained for all the nuclei observed so far :

    Left: Chandra 0.310 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 0.011 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 ~ racc ) has been made with Chandra in Fornax and Virgo:Loewenstein et al . (2001)

    For all the cases studied so far :

    racc ~ 0.03 - 2.3 arcsec . the mass accretion rate MBONDI ~ 10-4 to 0.2 M/yr

    LX ~ 10-5 to 0.03 Lacc

  • 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 ~ racc (from ACIS-S image)

    the radiative efficiency ? . . . For M = MBONDI

  • . Lacc~ 0.1MBONDI c2 standard disc . . with M=MBONDI . . or M ~ 0.1 MBONDI - - - - - - LADAF/LEddCenAGal. Center

    . but for the whole sample of nuclei . with Lx and MBONDI measured with Chandra: The predicted luminosity is: . . LADAF/LEdd ( M / MEdd )2 Absence of the predicted trend and large scatter in Lx,nuc/LEdd Pellegrini (2005)

  • 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 ( MBONDI)

    . . SMBHs can be accreting at M >) x

  • Heating sources:

    radiative: inverse Compton scattering + photoionization heating of nuclear photons (Ciotti & Ostriker 2001, Sazonov et al. 2004)

    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 < tdyn )

    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 >~ 108 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 ~ 0.01-1 M/yr

    at the present epoch, and larger before. These can build up several 108 M - 109 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 MBONDI

  • Pellegrini (2005) Large scatter also in:

  • from the Chandra images :

    1. 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

    . The DECREASE in the fuelling rate M (due to the secular . 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)

    Are local dormant SMBHs really outflow dominated? (more direct observational evidence needed)