Transcript Probing the Central Regions of Active Galactic Nuclei

The Masses of Black Holes in
Active Galactic Nuclei
Bradley M. Peterson
The Ohio State University
Space Telescope Science Institute
1
12 January 2005
Principal Collaborators
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M. Bentz, C.A. Onken, R.W. Pogge (Ohio State)
L. Ferrarese (Herzberg Inst., Victoria)
K.M. Gilbert (Lick Obs.)
K. Horne (St. Andrews)
S. Kaspi, D. Maoz, H. Netzer (Tel-Aviv Univ.)
M.A. Malkan (UCLA)
D. Merritt (RIT)
S.G. Sergeev (Crimean Astrophys. Obs.)
M. Vestergaard (Steward Obs.)
A. Wandel (Hebrew Univ.)
2
Outline
1. How emission-line reverberation works
2. Results: what has worked, what hasn’t
3. (Brief) implications for the AGN broad-line
region (BLR)
4. Evidence for a virialized BLR
5. The AGN MBH-*. Relationship
6. The AGN MBH-L Relationship
7. Secondary (scaling) methods
8. Immediate prospects
3
Driving Force in AGNs
• Simple arguments suggest AGNs
are powered by supermassive
black holes
– Eddington limit requires M  106 M
• Requirement is that self-gravity
exceeds radiation pressure
– Deep gravitational potential leads to
accretion disk that radiates across
entire spectrum
• Accretion disk around a 106 – 108 M
black hole emits a thermal spectrum
that peaks in the UV
4
Driving Force in
AGNs
• UV/optical “big blue
bump” can plausibly
be identified with
accretion-disk
emission
“Big Blue Bump”
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~10 17 cm
6
Quasars
• Very luminous AGNs
were much more
common in the past.
• The “quasar era”
occurred when the
Universe was 1020% its current age.
• Where are they
now?
7
Supermassive Black Holes Are Common
• Supermassive black
holes are found in
galaxies with large central
bulge components.
• These are almost
certainly remnant black
holes from the quasar
era.
• To understand accretion
history, we need to
determine black-hole
demographics.
M 87, a giant elliptical
SMBH > 3109 M
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How Can We Measure Black-Hole
Masses?
• Virial mass measurements based on
motions of stars and gas in nucleus.
– Stars
• Advantage: gravitational forces only
• Disadvantage: requires high spatial resolution
– larger distance from nucleus  less critical test
– Gas
• Advantage: can be found very close to nucleus
• Disadvantage: possible role of non-gravitational
forces
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Virial Estimators
Source
Distance from
central source
3-10 RS
X-Ray Fe K
Broad-Line Region 200104 RS
Megamasers
4 104 RS
Gas Dynamics
8 105 RS
Stellar Dynamics 106 RS
In units of the Schwarzschild radius
RS = 2GM/c2 = 3 × 1013 M8 cm .
Mass estimates from the
virial theorem:
M = f (r V 2 /G)
where
r = scale length of
region
V = velocity dispersion
f = a factor of order
unity, depends on
details of geometry
and kinematics
10
NGC 4258
• The first and still
most reliable
measurement of a
black-hole mass in
an AGN is due to
megamaser motions
in NGC 4258.
• Radial velocities and
proper motions give
a mass 4 ×107M.
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Gas Motions in M84 Nucleus
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Reverberation Mapping
• Kinematics and
geometry of the BLR
can be tightly
constrained by
measuring the emissionline response to
continuum variations.
NGC 5548, the most closely
monitored Seyfert 1 galaxy
Continuum
Emission line
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Reverberation Mapping Concepts:
Response of an Edge-On Ring
• Suppose line-emitting
clouds are on a circular
orbit around the central
source.
• Compared to the signal
from the central source,
the signal from
anywhere on the ring is
delayed by light-travel
time.
• Time delay at position
(r,) is  = (1 + cos )r / c
 = r/c
 = r cos /c
The isodelay surface is
a parabola:
cτ
r
1  cos θ
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“Isodelay Surfaces”
All points
on an “isodelay
surface” have
the same extra
light-travel time
to the observer,
relative to
photons
from the
continuum
source.
 = r/c
 = r/c
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Velocity-Delay Map
for an Edge-On Ring
• Clouds at intersection of
isodelay surface and orbit
have line-of-sight velocities
V = ±Vorb sin.
• Response time is
 = (1 + cos )r/c
• Circular orbit projects to an
ellipse in the (V, ) plane.
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Thick Geometries
• Generalization to a disk or
thick shell is trivial.
• General result is illustrated
with simple two ring system.
A multiple-ring system
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Observed Response of an
Emission Line
The relationship between the continuum and emission
can be taken to be:
L(V, t ) 
 (V,  )C(t   )d


Emission-line
light curve
“Velocity- Continuum
Delay Map” Light Curve
Velocity-delay map is observed line
response to a -function outburst
Simple
velocity-delay map
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Time after continuum outburst
“Isodelay surface”
20 light days
Broad-line region
as a disk,
2–20 light days
Black hole/accretion disk
Time
delay
Line profile at
current time delay
Two Simple Velocity-Delay Maps
Inclined Keplerian
disk
Randomly inclined
circular Keplerian orbits
The profiles and velocity-delay maps are superficially similar,
but can be distinguished from one other and from other forms.
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Recovering VelocityDelay Maps from
Real Data
Optical lines in Mrk 110
(Kollatschny 2003)
C IV and He II in NGC 4151
(Ulrich & Horne 1996)
• Existing velocity-delay maps are noisy and ambiguous
• In no case has recovery of the velocity-delay map been
a design goal for an experiment!
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Emission-Line Lags
• Because the data requirements are relatively modest,
it is most common to determine the cross-correlation
function and obtain the “lag” (mean response time):
CCF(t ) 
 ( )ACF(t   )d


Reverberation
Mapping Results
• Reverberation lags
have been measured
for 36 AGNs, mostly
for H, but in some
cases for multiple
lines.
• AGNs with lags for
multiple lines show
that highest
ionization emission
lines respond most
rapidly  ionization
stratification
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Time-Variable
Lags
  Lopt0.9
• 14 years of observing
the H response in
NGC 5548 shows that
lags increase with the
mean continuum flux.
• Measured lags range
from 6 to 26 days
• Best fit is   Lopt0.9
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Optical continuum flux
How Should the Lag Vary with Luminosity?
• Responsivity of a line
depends primarily on
ionizing flux and particle
density.
• Assuming wide range of
densities at all radii
implies that the radius of
peak responsivity should
depend primarily on
geometrical dilution:
  Lopt0.9
  L1/2
  L1/2
Hidden in this argument is that the flux must be the
ionizing flux.
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BLR Size vs.
Luminosity
  L1/2
Mean optical continuum flux
Optical flux
• UV varies more than
optical
•   Lopt0.9  (LUV 0.56 )
0.9  L
0.5
UV
  Lopt0.9
Lopt  LUV0.56
UV flux
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What Fine -Tunes the BLR?
• Why are the ionization parameter and
electron density the same for all AGNs?
• How does the BLR know precisely
where to be?
• Answer: gas is everywhere in the
nuclear regions. We see emission lines
emitted under optimal conditions.
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Locally optimally-emitting cloud (LOC) model
– Determined by where
physical conditions
(mainly flux and particle
density) give the largest
response for given
continuum increase.
• Emission in a particular
line comes
predominantly from
clouds with optimal
conditions for that line.
Ionizing flux
• The flux variations in
each line are
responsivity-weighted.
Particle density
Korista et al. (1997)
Evidence for a Virialized BLR
• Gravity is important
– Broad-lines show
virial relationship
between size of lineemitting region and
line width, r   2
– Yields measurement
of black-hole mass
Peterson et al. (2004)
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Virialized BLR
• The virial relationship
is best seen in the
variable part of the
emission line.
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Calibration of the Reverberation
Mass Scale
M = f (ccent 2 /G)
• Detemine scale
factor f that matches
AGNs to the
quiescent-galaxy
MBH-*. relationship
• Current best
estimate:
f = 5.5 ± 1.8
Ferrarese slope
Tremaine slope
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MBH-*. relationship
Other methods
Reverberation
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The AGN Mass–Luminosity Relationship
The AGN Mass–Luminosity Relationship
Lbol = 9L(5100 Å)
Luminosity Effects
• Average line spectra
of AGNs are
amazingly similar
over a wide range of
luminosity.
• Exception: Baldwin
Effect
– Relative to continuum,
C IV 1549 is weaker
in more luminous
objects
– Origin unknown
SDSS composites, by luminosity
Vanden Berk et al. (2004)
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BLR Scaling with Luminosity
• Suppose, to first order,
AGN spectra look the
same:
r  L0.69 ± 0.05
Q( H)
L
U

2
4 r nH c nH r 2
 Same ionization
parameter
 Same density
r  L1/2
Radius-luminosity relationship
from reverberation data
(Peterson et al. 2004)
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Secondary Mass
Indicators
• Reverberation masses
serve as an anchor for
related AGN mass
determinations (e.g.,
based on photoionization
modeling)
– Will allow exploration of AGN
black hole demographics
over the history of the
Vestergaard (2002)
Universe.
based on scaling relationship r  L0.7
and C IV line width
M = f (ccent 2 /G)  L0.7 2
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Narrow-Line Widths as
a Surrogate for *
• Narrow-line widths
and * are
correlated
– The narrow-line
widths have been
used to estimate
black-hole mass,
based on the MBH* correlation
– Limitations imposed
by angular
resolution, non-virial
component (jets)
Narrow [O III] FWHM
Shields et al. (2003)
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Estimating AGN Black Hole Masses
Phenomenon:
Primary
Methods:
Fundamental
Empirical
Relationships:
Secondary
Mass
Indicators:
Application:
Quiescent
Galaxies
Stellar, gas
dynamics
Type 2
AGNs
Megamasers
BL Lac
objects
2-d
RM
1-d
RM
AGN MBH – *
MBH – *
Fundamental
plane:
e , r e   *
 MBH
Type 1
AGNs
[O III] line width
V  *  MBH
Low-z AGNs
Broad-line width V
& size scaling with
luminosity
R  L0.7
 MBH
High-z AGNs
Next Crucial
Step
• Obtain a high-fidelity
velocity-delay map for
at least one line in one
AGN.
– Cannot assess
systematic uncertainties
without knowing
geometry/kinematics of
BLR.
– Even one success would
constitute “proof of
concept”.
BLR with a spiral wave and its
velocity-delay map in three emission lines
(Horne et al. 2004)
Requirements to Map the BLR
• Extensive simulations based on realistic behavior.
• Accurate mapping requires a number of characteristics
(nominal values follow for typical Seyfert 1 galaxies):
–
–
–
–
High time resolution ( 0.2 day)
Long duration (several months)
Moderate spectral resolution ( 600 km s-1)
High homogeneity and signal-to-noise (~100)
Program
No. Sources
Time Resolution
Duration
Spectral Resolution
Homogeneity
Signal/Noise Ratio
AGN Watch
AGN Watch AGN Watch AGN Watch
CTIO/
NGC 5548
NGC 4151 NGC 7469
(other)
OSU OSU
IUE 89 HST 93 Opt IUE Opt IUE Opt IUE
Opt Opt Opt
1
1
1
1
1
1
1
3
5
8
2
LAG
Opt
5
Wise Wise/
1988 SO PG
Opt
Opt
3
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A program to obtain a velocity-delay map is not
much more difficult than what has been done already!
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10 Simulations Based on HST/STIS Performance
Each step increases the
experiment duration by 25 days
Accuracy of Reverberation Masses
• Without knowledge of the
BLR kinematics and
geometry, it is not even
possible to estimate how
large the systematic errors
might be (e.g., lowinclination disk could have
a huge projection
correction).
– However, superluminal jet
implies that 3C 120 is nearly
face-on
– Simple disks alone do not
work
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Accuracy of Reverberation Masses
• AGNs masses follow
same MBH-*
relationship as normal
galaxies
• Scatter around MBH-*
indicates that
reverberation masses
are accurate to better
than 0.5 dex.
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Summary
• Good progress has been made in using reverberation
mapping to measure BLR radii and corresponding
black hole mases.
– 36 AGNs, some in multiple emission lines
• Reverberation-based masses appear to be accurate
to a factor of about 3.
– Direct tests and additional statistical tests are in progress.
• Scaling relationships allow masses of many quasars
to be estimated easily
– Uncertainties typically ~1 dex at this time
• Full potential of reverberation mapping has not yet
been realized.
– Significant improvements in quality of results are within
reach.
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