Laura Ferrarese
Rutgers University
lff@physics.rutgers.edu
Observational Evidence For
Supermassive Black Holes.
Lecture 1: Motivation
SIGRAV Graduate School in Contemporary
Relativity and Gravitational Physics
Lectures Outline
Lecture 1: Introduction and Motivation
Lecture 2: Stellar Dynamics
Lecture 3: Gas Dynamics
Lecture 4: AGNs and Reverberation Mapping
Lecture 5: Scaling Relations
Lecture 6: What the Future Might Bring
ALL LECTURES ARE ON-LINE:
http://www.physics.rutgers.edu/~lff/Como
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http://dipastro.pd.astro.it/bertola/astrofisica.html
Lecture 1: Outline

Motivation: Why Do We Think Supermassive Black Holes Exist, and Where
Should We Look if We Wanted to Find One?

The Mass Density in the Supermassive Black Holes Powering Quasar Activity

The Mass Density in the Supermassive Black Holes Powering Local AGNs

Supermassive Black Hole Detections
Historical Overview
 Although unrealized at the time, the first hint of the existence of supermassive black holes
was unveiled with:
 Karl Jansky’s 1932 discovery of radio emission from the Galactic center.
 Carl Seyfert’s 1943 discovery of the peculiar spiral galaxies which today carry his name.
 By the 1960, several hundred radio sources had been
discovered, and astronomers were struggling to find
optical counterparts.
 In 1960, Allan Sandage identified 3C48 with a single blue
point of light. In the two years after Sandage’s discovery of
the optical counterpart to 3C48, a half dozen such objects
were discovered; to distinguish them from radio galaxies,
for which the optical emission is clearly resolved, objects
like 3C48 were named quasi stellar radio objects or quasars.
3C 48
Karl Jansky
Historical Overview
Ground based optical
images of 3C273
Optical jet
Hubble Space Telescope
images of 3C273, revealing
the underlying galaxy
Quasars
 The spectral energy
distribution of quasars (and
AGNs in general) is
markedly non-stellar.
SED for 3C273:
green: contribution from the outer
jet
blue: contribution from the host
galaxy.
http://obswww.unige.ch/3c273/
Quasars
 The night after observing the optical counterpart to 3C48, Sandage took a spectrum, which
he described as “the weirdest spectrum I had ever seen”. The spectrum had several
emission lines, but none seemed to correspond to known elements.
 The impasse was broken by Maarten Schmidt in 1963.
Schmidt realized that the emission lines in the spectrum
of 3C273, were the very familiar hydrogen Balmer lines,
but redshifted by v/c = 0.158. It was soon realized that
all quasar spectra could be interpreted this way.
 Although controversial for a long time, it is now recognized
that quasar redshifts are cosmological.
Optical Spectrum of 3C273
QuickTime™ and a TIFF (Uncompress ed) decompress or are needed to s ee this picture.
Maarten Schmidt
Quasars
 3C273, and all quasars, show flux variability on timescales of hours to months (depending
on the frequency)
3C273: http://obswww.unige.ch/3c273/
Quasars
 ENERGY OUTPUT: At cosmological distances, quasars must be hundreds to many tens of
thousand times more luminous than an L* galaxy.
 In general, AGNs bolometric luminosity are of order 1044-1048 erg s-1
 In the Eddington approximation, this implies masses LE = 4 GMBH mp/T; and
assuming a typical quasar lifetime of order 107 yr  MBH > 106 M
 SIZE: The time variability sets very tight limits on the size of the emitting region, which
A
A


Brightness
must be smaller than the distance light can travel in that time:
 Even if the brightness changes at every point simultaneously, the change happening
at point A would reach us sooner than the change from point B. It will take the time
for light to travel from point B to point A for an observer to perceive the full change.
B
B
A B
A
B
Time
This implies that the emitting region is less than a few light weeks or days across.
Combined with the constraints on the mass, the implied central densities are of order
1015 Mpc-3
Quasars
 COHERENCE: jet stability and collimation over hundred of kiloparsecs in some objects
imply a stable energy source.
~ 1Mpc
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
AGNs
 RELATIVISTIC MOTIONS: one of the
greatest surprises provided by very-long
baseline interferometry (VLBI) observations
was the fact that some AGNs exhibit
motion along their jets with speeds which
appear to be several times faster than light.
Sequence of HST images showing blobs
in the M87 jet apparently moving at six
times the speed of light. The slanting
lines track the moving features.
5000 light years
 Energetics, sizes, densities,
coherence, and the presence of
relativistic motions imply that the
power supply is gravitational;
central engines are relativistic,
massive, compact and good
gyroscopes.
 A massive black hole is the
inevitable end result of nuclear
runaway
From Rees 1984, ARA&A 22, 471
The Relativistic Region
Evidence for a Strong Field Regime: 6.4 keV Fe K emission is the most compelling case
of the existence of an accretion disk at 3-10Rs from a central BH (Fabian et al. 1989,
1995; Nandra et al. 1997, Iwasawa et al. 1999). Line widths reach 105 km s-1
Potential way to constrain:
1) spin of the BH;
2) accretion rate;
3) central mass
(Fabian et al. 1989, Martocchia et al. 2000)
0.3 c
Composite Spectrum of 18 AGNs
observed with ASCA (Nandra et
al. 1997)
Where to Look: Punchline
 Quasars were much more common in the
past: the “quasar” era occurred when the
Universe was only 10-20% of its present
age.
 Simple arguments indicate that the
cumulative mass density in supermassive
black holes powering quasar activity is of
order
BH(QSO) ~ 3 - 4 105 M Mpc-3
 However, the mass density in
supermassive black holes at the centers of
local AGNs is a full two order of
magnitudes lower!
 Where have the quasars gone?
 The bulk of the mass connected with the
accretion in high redshift QSOs does not
reside in local AGNs.
 Remnants of past activity must be present
in a large number of quiescent galaxies.
Where to Look
 Our journey into SBH demographics stars from quasars: let’s try to follow their evolution
from the study of the luminosity function (number of quasars per unit comoving volume).
LOW REDSHIFTS (z < 2.3) (Boyle et al. 2000, MNRAS, 317, 1014):
The 2-degree field QSO Redshift survey includes redshifts for > 25000 18.25<B<20.85
QSOs in two 75° ××5° declination strips in the South Galactic Pole and in an equatorial
region at the North Galactic cap. Data were collected using the AAT Two-Degree Field
(2dF) multi-object spectrographic system, which allows up to 400 spectra to be
obtained at once.
 http://www.aao.gov.au/2df/
 http://www.2dfquasar.org/
HIGH REDSHIFTS (z > 3.5) (Fan et al. 2001, AJ, 121, 54):
The Sloan Digital Sky Survey First Data Release includes photometric data based on
five-band imaging observations of 2099 square degrees of sky. Based on these
photometric data, spectra were obtained for 150,000 galaxies and quasars. The survey
will ultimately cover 1/4 of the sky, and is currently 65% complete for imaging, and
44% complete for spectroscopy.
 http://www.sdss.org/
The 2dF Quasar Survey
Completeness
QSO distribution
THe SDSS Quasar Survey
 The LF is derived from 39 luminous QSOs over the range 3.6<z<5.0, and -27.5<M1450<-25.5.
The luminous quasar density decreases by a factor of ~ 6 from z =3.5 to z =5.0. The
luminosity function at the bright end is significantly flatter than the bright end luminosity
function found at z<3, suggesting that the quasar evolution from z=2 to z=5 cannot be
described as pure luminosity evolution (Fan et al. 2001, AJ, 121, 54).
 The survey has also detected 4 quasars at redshift > 6, including the current record holder
at z=6.48 (Fan et al. astro-ph/0301135)

SBHs in High Redshift Quasars
Fan et al. 2001, Boyle et al. 2000
QSO Mass Function (0.3 < z < 5)
(Soltan 1982, MNRAS, 200, 115; Chokshi &
Turner 1992, Small & Blandford 1992, Salucci
et al. 1998…)
1) Luminosity Function
(L,z)  (z)(L/L(z))
2) Integrated comoving energy density

radiat ion    L(L, z)dL
0 0
d
dz
dz
3) Integrated comoving mass density
BH 
10 rad
 0.1c 2
K
QSO ( M)  bol2
c
 

0 L
L' (L',z)
H 0 (1 z) m (1 z)  
3
dL'dz
SBHs in High Redshift Quasars
Ferrarese 2002 (astroph/0203047)
A note of caution:
 The magnitude limits of the 2dF and
SDSS samples correspond to Eddington
limits on the masses of 4.5107 M and
7.3108 M respectively.
 The quasar LF has no coverage in the
2.3 < z < 3.0 redshift range.

See also Yu & Tremaine 2002 (MNRAS
335, 965)
SBHs in Local AGNs
Local AGN Mass Function (0 < z < 0.2)
(Padovani et al. 1990, ApJ, 353, 438)
Need a way to estimate MBH in a complete
sample of galaxies:
Assume that the BLR clouds are gravitationally
bound:
MBH=v2r/G
with r = size of the Broad Line Region
measured from
 Reverberation mapping (Blandford &
McKee, Peterson 2001)
 Photoionization methods (Padovani et al.
1990; Wandel Peterson & Malkan 1998)
The bulk of the mass connected with the accretion of high z QSOs does not reside in local AGNs.
Remnants of past activity must be present in a large number of quiescent galaxies
How to Do It
 How can we constrain the masses of supermassive black holes?
 naively, we might think that the presence of a SBH will create a cusp in the brightness

profile of the host galaxy.
It does, but…..
From Kormendy & Richstone 1995, ARAA, 33, 581
How to Do It
Phenomenon:
Quiescent
Galaxies
Type 2
AGNs
Type 1
AGNs
Primary
Methods:
Stellar or
gas
dynamics
Water
Megamasers
Reverberation
Mapping
NGC4261 - HST/WFPC2
NGC205 - HST/ACS/HRC - 29X29 arcsec
NGC4258 (Seyfert 2)
Detections of SBHs in the Local
Universe
Implied
density
(M pc-3)
SBH Mass
(M)
Innermost radius
probed
Reverberation Mapping
Three dozen Seyfert 1s and quasars
5  106 to
5  108
a few light
days
~1012
Stellar Kinematics (proper motion):
Milky Way
3.7  106
0.008 pc
1  1017
Water Masers:
Type 2 AGNs (NGC 4258 & Circunus)
4  107
0.13 pc
1  1012
4  107 to
4  109
>0.4 pc, but
typically>3.5pc
< 1  107,
typically 104
Method
Kinematics of gas disks:
9 galaxies, mainly large ellipticals with low
luminosity AGNs
Stellar kinematics:
10 galaxies, mainly smaller, rotational supported
ellipticals
Detecting Supermassive Black Holes in Local
Galaxies

With the exception of the Iron K observations, every other technique used to
measure supermassive black holes masses probes regions well beyond the
strong field regime.
Source
X-Ray Fe K
Broad-Line Region
Proper Motion (MW)
Megamasers
Gas Dynamics
Stellar Dynamics
Distance from
central source
3-10 RS
600 RS
2000 RS
4 104 RS
8 105 RS
106 RS
In units of the Schwarzschild radius
RS = GM/c2 = 1.5  1013 M8 cm .
Preview: Scaling Relations
Suggested Readings
 Iron Kapha Line:
Reynolds & Nowak 2003, astro-ph/0212065
 SBH Demographics:
Soltan 1982, MNRAS, 200, 115
Ferrarese 2002, in ‘Current high-energy emission around
black holes’, Eds by C.-H. Lee and H.-Y. Chang. Singapore:
World Scientific Publishing, p.3, astro-ph/0203047
Yu & Tremaine 2002, MNRAS, 335, 965
 Quasar Luminosity Function:
Fan et al. 2001, AJ, 121, 54
Boyle et al. 2000, MNRAS, 317, 1014