Download The fuelling of local supermassive black holes

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, 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
currently observable with Chandra [more recent work , also in coll. with G. Fabbiano et al.
@ CfA, Cambridge, USA]
Hot gas in early type galaxies
Einstein’s observations showed
that early-type galaxies contain
a hot ISM (kT~0.3-0.8 keV)
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in a largely varying quantity
Fabbiano et al. 1992, ApJS
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Canizares et al.
1987, ApJ
Most recent catalog of the soft X-ray emission of early-type galaxies, comprehensive of
all ROSAT observations (O’ Sullivan et al. 2001, MNRAS):
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Where does this hot ISM come from, and what is its history?
Origin: evolving stars shed a considerable amount of mass, at a rate
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for a single-age passively evolving stellar population (Ciotti et al. 1991):
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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
(Mathews 1990, Renzini 1990)
 of the SNIa’s ejecta
Heating by SNIa’s is likely also secularly evolving (for the most recent modeling see Greggio 2005, A&A):
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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)
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).
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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 likely
supermassive black holes (SMBHs)
with MBH~106 -109 M⊙
What are the new challenges 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, 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 <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 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,
.
 the fuelling rate M ?
 the radiative efficiency ?
 the existence of activity cycles ?
Ho 2002)
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
Adaptively smoothed Chandra
ACIS-S image (0.3-10 keV)
with superimposed optical
contours from the DSS.
(Pellegrini et al. 2003, ApJ)
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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:
infinity
the star
SMBH
the “accretion radius”
.
then
MBONDI  MBH2 T(racc)-3/2 n(racc)
For most of the ~ 20 galaxies
racc ~
GMBH
cs2
for the galactic nuclei
:
• 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
For Sombrero racc ~ 64 pc ~ 1.35 arcsec
density
temperature
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
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
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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 = 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
The predicted luminosity is:
. .
LADAF/LEdd ( M / MEdd )2
…. but for the whole sample of nuclei
.
with Lx and MBONDI measured with Chandra:
Pellegrini (2005)
CenA
.
.
with M=MBONDI
.
.
or M ~ 0.1 MBONDI
- - - - - - LADAF/LEdd
Qu i ck Ti m e ™ an d a
TI F F ( LZ W) d ec om pr es so r
ar e n ee de d t o s ee t h is pi ct ur e .
Absence of the
predicted trend
and
large scatter
in Lx,nuc/LEdd
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Gal. Center
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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
x
.
the Bondi theory does not apply ( MBONDI)
.
.
SMBHs can be accreting at M << MBONDI (or >>)
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 << 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
Large scatter also in:
Qu i ck Ti m e ™ an d a
TI F F ( LZ W) d ec om pr es so r
ar e n ee de d t o s ee t h is pi ct u re .
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Qu i ck Ti m e ™ an d a
TI F F ( LZ W) d ec om pr es so r
ar e n ee de d t o s ee t h is pi ct u re .
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Pellegrini (2005)
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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)
Energy is being deposited ?
(Ciotti & Ostriker 2001)
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?
3. a case of central heating?
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Filament across the nucleus,
hotter than the surrounding gas
(Fabbiano et al. 2004)
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)