Download Lecture 16, AGN Evolution

AGN
• The term ``Active Galactic Nucleus’’ (AGN) refers to the
existence of energetic phenomena in the nuclei of galaxies
that cannot be attributed directly to stars
• AGN are classified based (in large part) on their luminosity
in different wavebands (especially the optical and radio)
Seyfert Galaxies
• In 1943, Carl Seyfert identified a class of spiral galaxies
w/unusually bright central regions (stellar appearing cores),
whose spectra are dominated by high-excitation nuclear
emission lines
Seyfert galaxies make up about 1% of spirals, so
there are numerous nearby examples; however, at
the time, they were viewed as oddities
Seyfert Galaxy NGC 7742
Discovery of Quasars
• In late 1950s, w/development of radio telescopes, some
celestial objects were found to emit profusely
• Catalogs compiled, largely at Cambridge (e.g., 3rd such
catalog known as “3C,” & objects in it given numerical
designation)
• The positions of some of these radio sources were
found to be coincident with objects that looked like
stars on normal photographs
The nature of these bluish-looking “radio stars” (a
typical star in the sky is red) was very uncertain
(they became known as quasi-stellar radio sources)
No known mechanism by which stars could produce so
much radio radiation. Moreover, the spectra showed
very strong broad emission lines at unidentified
wavelengths
• Maarten Schmidt made a breakthrough in 1963
– realized emission lines seen in the spectrum of
3C 273 were the hydrogen Balmer-series
emission lines
– but at the uncommonly large redshift z=0.158
• More disturbing than this vast distance was the
enormous luminosity implied
– 3C 273 was and remains the brightest known
quasar (B=13.1 mag)
– It is about 100 times as luminous as normal
bright spirals like the Milky Way or M31
• Probable importance of quasars was recognized
immediately, providing strong motivation to find more
• Subsequent searches for blue stars revealed a class of
“radio-quiet” quasars
– Optical spectra similar to those of quasars, yet radio
emission weak or absent
– Now called QSOs (“quasi-stellar objects”), but
common to use terms interchangeably
– About 10x more numerous than radio-loud quasars
AGN SEDs cannot be described as blackbodies
Reflects amount of energy
emitted in each equally
spaced interval on the
logarithmic frequency axis
Quasars are among the most luminous objects at every
wavelength at which they have been observed
Working model for AGN phenomenon is a `central engine’ that
consists of a hot accretion disk surrounding a supermassive
black hole. Energy is generated by gravitational infall of
material, which is heated to high temperatures in the disk
Radiation Force
• The energy produced propagates outwards & can
interact with infalling matter by absorption or scattering
• Through this interaction, the momentum of the
radiation is transferred to the matter, i.e., the infalling
matter experiences an outward-directed radiation force
• For matter to fall onto the SMBH at all, this radiation
force needs to be smaller than the gravitational force
Source of Radiation
• Assume
-Spherically symmetric emission
-Completely ionized hydrogen gas
• Outward energy flux at distance r from center is S=L/4πr2
• The momentum carried by a photon (energy E=hν) is E/c, so the
outward momentum flux, or pressure, is S/c= L/4πr2 c
• The outward radiation force on a single electron is then
obtained by multiplying by the cross-section for interaction with
a photon (Thomson scattering), frad= σTL/4πr2c
Eddington Limit
• Outward radiation force
"T L
f rad =
4!r 2c
• Force due to gravity per electron-proton pair (ignore electron
mass because nearly a factor of 2000 smaller than proton mass)
GM • m p
f grav =
r2
• Radiation pressure balances gravity when
f rad = f grav
Critical luminosity is called the Eddington limit
(bolometric, since σT is photon freq-independent)
! T L GM • m p
=
2
4"r c
r2
If L is larger than this value, the pressure due to
4"Gcm p
radiation exceeds the gravitational force at all radii,
LEdd =
M Edd and gas will be blown away
!T
LEdd
M•
" 1.3 !10
ergs / s
M Sun
38
• For accretion to occur at all, we need L<LEdd
• We can turn the above argument around: if a luminosity L is
observed, we conclude LEdd>L, or
M • > M Edd =
$T
L
LEdd " 8 !105 44
M Sun
4#Gcm p
10 ergs / s
• Thus, a lower limit for the mass of the SMBH can be derived
from the luminosity. For QSOs, typical masses are >108 MSun,
while Seyfert galaxies have lower limits of ~106 MSun
Fueling AGN
• The fundamental process at work in AGN is the conversion of
mass to energy, which is done with some efficiency ε
• The energy available for emission in radiation from a mass M is
E=εMc2
dE
2
&
L
=
=
!
M
c
• The rate at which energy is emitted,
dt
gives the mass accretion rate
L
$
L
!3 '
!1
&
M = 2 ) 1.8 (10 %
M
yr
"
Sun
44
*c
*
10
ergs
/
s
&
#
• For ε~0.1, the accretion rate for even fairly high-luminosity
sources, say 1046 ergs/s, is only about 2 Msun/yr
Zoo of AGN
• Luminous AGN are classified as
• Seyfert galaxies (Type 1 and Type 2)
• Quasars
• BL Lacs
• Radio galaxies (‘broad line’ and ‘narrow line’ varieties)
• All are powered by accretion onto supermassive black holes,
so why are there so many classes? Are these all physically
distinct objects?
• Unified model seeks to explain the different classes as being
due to different orientations of intrinsically similar systems to
the observer’s line of sight
The Unified Model
The type of AGN that you see in the optical depends
on your line of sight
Around the accretion disk are relatively dense, fastmoving clouds of hot gas responsible for the broad
emission lines seen in unobscured AGN
Emission lines are “broad” due to differential Doppler
shifts from the bulk motions of the individual fastmoving line-emitting clouds
But, if you are viewing the AGN through a dust torus
that hides the accretion disk, then only slowermoving clouds farther from the black hole are visible,
and the emission lines are narrow
If you are viewing the disk face-on & a beam of radiation
is being produced, then the active galaxy is a BL Lac
object (also know as a blazar), which is featureless
No absorption
or emission lines
AGN Evolution
• We now know that dormant supermassive black holes
reside at the centers of almost all present-day galaxies
• Their presence has been detected through their gravitational
influence on neighboring stars and gas, which orbit the
holes
Milky Way
R. Genzel
• We also now know from local observations
that a tight relation exists between
– the mass of the black hole & the spread
of stellar velocities within the host galaxy
M• " !
4
– or, less perfectly, the mass of the black
hole & the mass of the host galaxy
Mbh / Mbulge = 0.001-0.002
This hints at a deep connection between the processes
that formed the stars in the host and the processes
that formed the central black hole
When Did These SMBHs Form?
Historic Answer:
Number of optically selected quasars peaks at z=2-3
We are here!
Time after Big Bang (Gyr)
Where Did They Go?
•
Galaxies in the past were closer together
 collisions were more common, pushing
gas into the black hole
•
Galaxies in the past had more gas
(not yet incorporated into stars)
 feeding frenzy for the black hole!
Indeed, often the galaxies hosting quasars are
observed to be interacting or merging w/other
galaxies
• However, as a quasar’s central black hole
consumed most of its surrounding gas, the
quasar probably faded w/time
• But the supermassive black hole itself
cannot be destroyed!
Quasar
Normal Galaxy
V
(Very Feeble)
So, did all the action really take place at early times?
Problem Here!
Optical surveys may miss a lot of sources
due to obscuration, which moves the light
into the far-infrared rather than the optical
Thus, the optical results may not be the
whole story
Accretion onto supermassive black holes can
be obscured by dust & gas that absorbs
optical light; why not try using X-rays?
C. Done
Giacconi’s 1962 rocket mission to
study X-rays from the Moon found
a diffuse uniform X-ray “glow”
from the sky
This X-ray Background (XRB)
radiation was the first cosmic
background radiation discovered
Only a small fraction of the total energy produced in the
universe emerges in X-rays, but X-ray surveys are a
window on black hole evolution
The Great Escape
• Early X-ray telescopes could only produce images in
low-energy (soft; 0.5-2 keV) X-rays
• Unfortunately, even soft X-ray surveys cannot find all
of the sources that make up the X-ray background due
to obscuration
• Thus, observations are also needed in high-energy
(hard) X-rays
• Hard X-ray photons can penetrate the dust and gas
cocoon that may be obscuring a black hole
Modellers predicted what one should find:
a population of Type 2 QSOs at z=2-3
Schmidt 1998
Fundamental
goal of the
Chandra X-ray
Observatory:
Resolve the
sources of the
2-8 keV
‘hard’ XRB
XMM-Newton
also observes
at these high
energies:
poorer angular
resolution than
Chandra, but
larger
telescope
collecting area
Essential Instrumentation
Chandra/XMM have revolutionized distant AGN
studies
Now possible
• to map the history of the AGN population using
hard X-ray surveys, and
• for the first time, to compare high-redshift &
low-redshift samples chosen in the same restframe hard energy (2-8 keV) band
Hard X-rays
• Can directly probe AGN activity
• Are uncontaminated by star formation processes at
the X-ray luminosities of interest (I will consider
any source with L2-8 keV>1042 ergs s-1 to be an AGN)
• Can detect all but the most absorbed sources (may
miss sources with NH>1024 cm-2)
Chandra or XMM?
The advantage of Chandra for this kind of work is
its high positional accuracy, which minimizes
misidentifications of faint optical galaxy
counterparts
(Even with Chandra, about 20% of faint-end
optical identifications [R=24-26] will be
spurious)
Moreover, Chandra does not reach the confusion
limit, even with the deepest observations
CDF-N
CDF-N
12 Ms
Ms
503 sources
RED:
0.5-2 keV
GREEN:
2-4 keV
BLUE:
4-8 keV
D. sources
Alexander
370
0.13 sq deg
Differential X-ray Number Counts
CDF-N + HAWAII
S
COMBINED
-1.63
>80-90% of the 2-8 keV
X-ray background has
been resolved into
discrete sources
Cowie et al. 2002
CDF-S
S
-2.57
Uncertainties in XRB measurement are at 10-20% level, so
we cannot accurately determine the resolved fraction of the
XRB. Problematic for population synthesis modelers!
Hickox & Markevitch 2006
Contributions to the 2-8 keV XRB
Unexpected result: large
field-to-field variations in
the X-ray number counts
on the scale of a Chandra
image
COMBINED
CDF-N
CDF-S
SSA13
Need many Chandra fields to get average true number counts
Importance of Wide-Area Surveys
Large areas also needed to get necessary volume to
• Map low-redshift luminosity function
• Sample small number of very high-z sources
Can be done w/serendipitous pointings (ChaMP
strategy; e.g., Kim et al. 2004; Green et al. 2004)
or w/contiguous fields
Advantage of contiguous approach is that one can
also study hard X-ray source clustering
CLASXS
400 ks
525 sources
0.36 sq deg
Field-to-field variance of ~50% on solid angles of ~240 arcmin2
Yang et al. 2003
CDF-N
SSA13
All 9 LH fields
CDF-S
Yang et al. 2003
A Multiwavelength Approach
• X-ray observations are essential for finding hidden
black holes, but they are not sufficient for
• fully understanding the nature of the sources
• determining their distances (redshifts)
• We need observations at many different
wavelengths for this
Striking how modest the number of X-ray sources is
compared to the number of optical sources
Diverse assortment of counterparts to hard X-ray sources
Redshift Distribution of the
X-ray Sources
Above f(2-8 keV)~10-14 ergs cm-2 s-1, 80-90% of hard X-ray
sources have redshifts, while below this flux, ~60%
ASCA
CDF-N
CLASXS
CDF-S
Barger et al. 2005
All
All z
HEX
BLAGN
Lower X-ray flux sources are often optically faint and
red and hence difficult to measure redshifts for
A2390
CISCO + HST
I, J, H
20’ x 20’ shown
RJK
NIR Photometric Redshifts
With the addition of our NIR data and MIR data from Spitzer, we
are able to make reasonable photometric redshift estimates
However, they do not tell us the spectral type of the galaxy
producing the X-ray light
Spectroscopic samples are highly complete to R=24.5
Photometric redshifts increase overall identified fraction to about 85%
Photometric redshifts
Spectroscopic redshifts
Barger et al. 2005
Spectroscopic incompleteness at z>1.2 due to absence
of strong spectral features & faintness of sources at these zs
Photometric redshifts (open)
Barger et al. 2005
Large Scale Structure in CDF-N
Barger et al. 2002
Averaged Out When Include CLASXS Data
Steffen et al. 2004
Spatial Correlation Function
How correlated are the X-ray sources?
Do the hard X-ray sources predominantly lie in
highly clustered regions?
How does the correlation strength compare to that
of massive galaxies? (Alison Coil colloquium)
AGN
Steffen et al. 2004
Some Striking Results
First striking result: many of these X-ray identified AGN could not have
been picked out w/optical spectra alone---no AGN signatures!
Barger et al. 2002
Second striking result: median z~1, not the z~2-3 expected,
and lower X-ray flux sources are not QSOs (i.e., Lx>1044 ergs/s)
Uncertainties are 1σ median ranges
Redshift distribution in 3 xStandard redshift-luminosity relation
ray flux bins.
for a source with rest-frame 2-8 keV
44 ergs s-1
Notice that the
median
and
luminosity
of 10
range do not change much
with x-ray flux
Solid squares--spectroscopic
medians for full
sample
Open squares--spectroscopic
+photometric
medians for CDF-N
& CDF-S only
Individual sources in the CDF-N (8’ radius), where we
have spectroscopically identified 70% of the sources
Lrf(2-8 keV)=1044 ergs/s
1043 ergs/s
Red=spec-zs
Purple=phot-zs
Diamonds=unid
We now know the historic answer is not the full answer!
Newly discovered moderate luminosity X-ray sources peak at lower z,
and there are many more of them
1043-1044 ergs/s
1044-1045 ergs/s
(X-ray analog of optical quasars)
Barger & Cowie 2005
Hard X-ray Luminosity Functions
When computing rest-frame hard (2-8 keV)
X-ray luminosity functions, one of the
interesting things is to make a comparison
with optically-selected quasar samples
To do that, one needs to use optical
spectroscopic classifications to determine
the broad-line AGN luminosity functions
separately
Optical Spectral Classifications
of the X-ray Sources
Most groups use the four optical spectral classes of
Szokoly et al. 2004, which are crude by the
standards of optical AGN specialists:
absorbers,
star formers,
high-excitation sources (HEX),
broad-line AGN (BLAGN; FWHM>2000 km/s)
The HEX sources are quite easily distinguished from
the BLAGN and from the low excitation sources
weak Hβ
narrow CIV
Cowie & Barger
Noise is dominated by the noisiest spectrum
The star formers have strong Hβ and do not show signs
of [NeV] or of broad underlying Balmer lines
68 sources show no emission lines at all (absorbers)
strong Hβ
These would not have been picked out as AGN from their optical spectra
What are the X-ray colors by
optical spectral class?
High-Energy Spectra
• X-ray power-law fits are generally of the form
"!
"!
PE ( photons / s / keV ) # E # $
since photons per second is close to the measured
quantity of counts per second
• In units of energy flux
F" % PE ( photons / s / keV ) & h" (ergs / photon) % " #$+1 % " #!
α is usually referred to as the `energy index’ and Γ (=
α+1) is called the `photon index’
• AGNs are well-fit by a photon index of 1.8-2
BLAGN are nearly all soft and show essentially no visible absorption
in X-rays, consistent w/our understanding of them as unobscured
2 x 1021 cm-2
! = 1.8
Barger et al. 2005
All the other AGN are well-described by a power-law spectrum with
photoelectric absorption spread over a wide range of NH
3 x 1022 cm-2
! = 1.8
Open squares---absorbers and star formers
Solid squares---high-excitation signatures
Triangles---unidentified sources
Barger et al. 2005
ALL
LOCAL
BLAGNs
(RXTE)
Sazonov & Revnivtsev (2004)
Barger et al. 2005
As move to lower z, all the sources are decreasing in L while the LFs are maintaining the
same shapes; if drifted the x-axis, plots would look very much the same from z=1.2 to z=0
(RXTE)
Sazonov & Revnivtsev (2004)
Sanity Check
Good agreement
between the
optical and X-ray
selected LFs!
Our optical
spectroscopic
classification of
BLAGN is
consistent with
that of groups
doing direct
optical selection
Richards et al. 2005
Higher Redshift Intervals
Incompleteness larger here, but
phot-zs (triangles) indicate unids
mostly lie in z=1.5-3 interval
Shapes no longer wellrepresented by the maximum
likelihood fits to the z=0-1.2
HXLFs computed at z=1 (blue
curves): Thus, PLE does not
continue beyond z=1.2
There are fewer low-L sources
than one would expect; thus, the
light density is more dominated
by higher L sources at these z
The Steffen Effect
BLAGN dominate the number
densities at the higher X-ray
luminosities
This therefore says that
almost all luminous objects
are unobscured
BLAGN
Since there is a substantial
fraction of non-BLAGN
sources at the lower
luminosities, there must be
some luminosity dependence
on the obscuration: the
simple unified model is not
adequate
Accretion History of the Universe
We can now infer the present-day
supermassive black hole mass density
accreted by AGN and compare it with the
locally measured value (Soltan 1982)
(From the standard theory of accretion onto black holes, a maximum
efficiency of 6% for accretion onto a non-rotating BH is derived)
BCs
2-8 keV comoving energy density production rate drops
rapidly from z=1 to z=0; peak is in interval z=0.8-1.2
Open=spectroscopic sources
Solid=all, including phot-zs; ‘no ids’ put at z=3
Negative values mean nuclear
dominated
Cumulative growth of AGNs from Chandra (red curve)
compared with the cumulative SFH
Both form most of their mass at late times (z~1). If AGN feedback
has a significant effect, the relative histories can help diagnose that
• Thus, there is room for the obscured accretion that
we see with the Chandra observations (only ½ to
¼ of the SMBH mass density was fabricated in
broad-line AGN)
• One remaining issue is whether there additional
high-obscured sources that we are missing in the
2-8 keV sample
• Within the uncertainties, there could be a
comparable contribution from this type of source,
though not much more
Summary
• X-ray data are consistent with many of the non-BLAGN
(the dominant population) having high column densities
• AGN evolve very rapidly to z~1.2, consistent with pure
luminosity evolution to that redshift
• z~1 is where the bulk of the supermassive black hole
population forms
• Simple unified model is not correct, whether one uses
X-ray color or optical spectral classification---there are
far fewer low X-ray luminosity unobscured sources than
obscured sources
The End