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

The fuelling of local The fuelling of local supermassive black holessupermassive black holes

Silvia PellegriniSilvia Pellegrini (Astronomy Dept., Bologna University, Italy)(Astronomy Dept., Bologna University, Italy)

Properties Properties ofof hot gas flows in elliptical galaxieshot gas flows in elliptical galaxies i.e., how an ISM is fed and behaves on galactic scalei.e., how an ISM is fed and behaves on galactic scale

“ “Winds, Outflows and Inflows in X-ray Elliptical GalaxiesWinds, Outflows and Inflows in X-ray Elliptical Galaxies”, ”, Ciotti, D’Ercole, Pellegrini and Renzini 1991, ApJ

The interplay between the hot ISM and the The interplay between the hot ISM and the supermassive supermassive

black hole black hole (ubiquitous at the centers of the spheroids of the local (ubiquitous at the centers of the spheroids of the local Universe)Universe)

new challenges for theories of galaxy formation and evolution currently observable with Chandra [more recent work , also in coll. with G. Fabbiano [more recent work , also in coll. with G. Fabbiano

et al. et al.

@ CfA, Cambridge, @ CfA, Cambridge, USA]USA]

Einstein’s observations showed that early-type galaxies contain a hot ISM (kT~0.3-0.8 keV)

in a largely varying quantity

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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 determinessecular evolution of the

specific heating of M (t)

* .

(Mathews 1990, Renzini 1990)

= QuickTime™ and aTIFF (LZW) decompressor








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 Tgas <<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.

3) new, important ingredient: dark mass concentrations at the centers of spheroids in the local Universe (Kormendy & Richstone 1995, Magorrian et al. 1998), , most likelymost likely

supermassive black holes (SMBHs) with MBH~106 -109 M⊙

What are the new challengeschallenges posed by the SMBH ?posed by the SMBH ?

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, growthgrowth

and activity cycleactivity cycle

of the SMBH

formation and

evolution

of the spheroid

But also:

Nearby SMBHs are radiatively quiescent or show low levels of activity : low levels of activity :

L/LEdd ~1 in powerful AGNs

L/LEdd <10-11 in Sgr A*,

<10-4 in NGC4258,

(the most securely measured SMBH masses)

L/LEdd <10-8 in NGC1399, NGC4472, NGC4636

(elliptical galaxies, sites of the most luminous quasars)

In a statistically complete sample (spectroscopic survey of 486 galaxies with BT<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 ~ 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 DORMANTWHY ARE MOST SMBHs NOW DORMANT we will also learn something about

HOW do SMBHs GROW through COSMIC TIME ?HOW do SMBHs GROW through COSMIC TIME ?

WHY and HOW the luminous AGNs SWITCHED OFF ?WHY and HOW the luminous AGNs SWITCHED OFF ?

Is there a link between the RADIATIVE QUIESCENCE andIs there a link between the RADIATIVE QUIESCENCE and

the feedback mechanism responsible for the MBH - relation?

the evolution of AGNs ?

Thanks to ChandraChandra (=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 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 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 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 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 luminositytrue nuclear luminosity

true ISM densitytrue ISM density and and temperaturetemperature close to the nucleus 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 MBH

2 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

infinity the “accretion radius”

GMBH

cs2

then

racc ~

temperature density

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 0.3–10 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

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: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 << MEdd 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 c2

standard

disc

. . with M=MBONDI . . or M ~ 0.1 MBONDI

- - - - - - LADAF/LEdd

CenA

Gal. Center



QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.

…. 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: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 << MBONDI (or >>)

x

Heating sources:

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

mechanicalmechanical: impact of collimated outflows: 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 << M* (racc) = the stellar mass loss rate within the

accretion radius racc , 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 :

1.1. 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?


are needed to see this picture. 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 (due to the secular .. decrease in Mdecrease 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)

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

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