Transcript The Radio-Loud/Radio-Quiet Dichotomy of AGN

The Radio-Loud/Radio-Quiet
Dichotomy of AGN
Brandon Kelly
Astro 596
History
• Radio emission was first distinguishing mark
of AGN
• 1963- Maarten Schmidt realized that bright
optical point source associated with radio
source 3C 273 had z=0.158 (The QSRSs,
‘quasars’)
• 1965- Allan Sandage finds that most quasars
are radio-quiet (QSG, QSO)
• Kellermann et al.(1989, AJ, 98) find
bimodality in radio-loudness, 5-10 times more
RQQs and RLQs
• Existence of bimodality still debated, ~10% of
quasars are radio-loud
Jets
• Kpc scales
• Radio- Synchrotron
Emission
• X-rays- Compton
upscattering
• Motivating question:
Why do some AGN
develop powerful with
strong radio emission,
while most have weak
or no jets?
Centaurus A
Red: Radio, Blue: X-ray
Structure of RLQ
Picture from Marscher, 2005, Mem.
S.A.It., 76
Is There a Bimodality in
Radio-Loudness?
• Kellermann et al.(1989) used
sample of 114 BQS sources with
VLA 6 cm observations, inferred
bimodality in radio loudness:
R  f Radio
fOptical /UV
• Is the observed bimodality real?
• Some have questioned its

existence,
e.g., White et
al.(2000, ApJS, 126), Cirasuolo
et al.(2003, MNRAS, 346)
• Selection effects are a concern
Kellermann et al.(1989, AJ, 98)
Bimodality (cont)
• Ivezić et al.(2004,
ASPC, 311) make
histogram of R for
~10,000 sources
detected by SDSS and
FIRST, find support for
bimodality
•
Argue that uncertainties
in K-corrections, other
errors will broaden
observed R-distribution
Bimodality implies something `triggers’ the
production of powerful jets
Bimodality and Dichotomy: Other
Issues
• Possibility of significant Doppler boosting can
make total LR a poor indicator of jet power
• Most the energy in the jet not radiated away,
but transported to lobes
• Doppler boosting can also affect Lopt
• Even if no bimodality in R, this does not imply
that there is no division between quasars with
powerful jets and quasars with weak jets
• Probably better to look for dichotomy using
more fundamental parameters, e.g., ratio of
jet power to disk luminosity, but
observationally difficult or impossible
Comparison of RQQ/RLQ
SEDs
Mean RQQ (Solid) and RLQ (Dashed) SED, normalized at 1.25 m. From
Elvis et al.(1994, ApJS, 95).
Do RQQs Have Jets?
• Ulvestad et al.(2005, ApJ,
621) observed 5 RQQs
with the VLBA
• Four of the images had
unresolved radio cores,
J0804+6459 has twosided jet
• Concluded radio emission
in RQQs due to weak jets
• Recent VLA observations
also find evidence for jetlike outflows in RQQs
(Leipski et al., 2006, A&A,
455)
Figure from Ulvestad et al. (2005)
Correlation of LR with LO[III]
• Correlation has
been found between
radio luminosity and
that of the O [III]
narrow emission line
• Xu et al. (1999, AJ,
118) find similar
slope for both RQQs
and RLQs
Interpretation of the L[O III]-LR
Correlation
• Xu et al. proposed the following explanation:
– Strong Observational Evidence that O [III] is a
good orientation-independent measure of AGN
intrinsic luminosity
Ý
– Implies L[O III ]  M
acc
– Assuming Shakura-Sunyaev thin disk, jet velocity
~ Keplerian velocity, and that the vertical magnetic
field ~ the azimuthal one, this implies that mass
Ýacc
flux into the jet  M

Ý

– Then, correlation is expected if LR  MJet
• Similarity of slopes implies that mechanism
that generates radio emission is similar for
both, but
 RLQs have much larger constant of
proportionality

Similar Bimodal Trend Seen with
Eddington Ratio
Similarly, Sikora et al. (2007,
in press at ApJ, astroph/0604095) find a similar
anti-correlation between
radio-loudness and
Eddington ratio, but with
different normalization
Does the Fraction of RLQs Vary with
Luminosity and/or Redshift?
• Some studies have concluded that the radioloud fraction (RLF) drops with increasing
redshift or decreasing optical/UV luminosity
• Artificial correlation of L and z from flux limit
makes it difficult to interpret simple 1dimensional correlations
• Jiang et al. (2007, in press at ApJ, astroph/0611453) find that the RLF increases with
increasing L2500 and decreasing z
• Unable to conclude if this is a statement
about the radio-loud tail, or about the entire
distribution
Differences in X-ray Properties
• Early Einstein observations suggested RLQs have
flatter X-ray spectra
• Difference in X-ray spectral slopes depends on radio
spectral slope, confirmed with ASCA and BeppoSAX
data
• RLQs also tend to be more X-ray loud, probably
additional component from Jet
• RLQs tend to have a weaker reflection component
• Galbiati et al. (2005, A&A, 430) analyzed sample of
25 RLQs with XMM data and concluded:
– Average X-ray spectra slope is same for both RLQs and
RQQs
– ‘Blazars’ have larger spread in X-ray spectra slope, spread is
similar for ‘non-blazar’ RLQs and RQQs
Dependence of R on Black Hole Mass
• Many studies have found that RLQs
tend to have slightly larger MBH, but
very poor correlation
• Liu et al. (2006, ApJ, 637) argue that
it’s more appropriate to compare the
jet power with MBH, find significant
correlation with
PJet  M1.220.09
BH

Figure from Liu et al.(2006)
Figure from McLure &
Jarvis (2004, MNRAS, 353)
Differences In Host Galaxy Morphology
• Many authors have found that RLQs tend to be found
in massive ellipticals and richer environments, but
RQQs can be in elliptical or disk galaxies
• Recently, Best et al. (2005, MNRAS, 362) studied a
sample of 2215 radio-loud AGN (0.03 < z < 0.3) from
the SDSS, find that the radio-loud fraction increases
with either stellar or black hole mass
• Also find that large, concentrated galaxies are more
likely to house a RLQ, and that RLQs are more prefer
richer environments
• Capetti (2006, A&A, 453) find that among early-type
galaxies, RLQs inhabit core galaxies, whereas RQQs
inhabit power-law galaxies
Origin of RLQ/RQQ Dichotomy: The
Spin Paradigm
• Various bimodalities suggest fundamental
difference between RLQ and RQQ, i.e., what
turns on RLQs?
• Difference in spin is a popular mechanism
(See Sikora et al., astro-ph/0604095 for
further discussion and references)
• If jet is powered by the Blandford-Znajek (BZ)
mechanism, then jet production likely related
to spin of black hole
• In this case, the power that can be extracted
is
2
2
2
PBZ  B M BH a
Spin Paradigm (cont)
• Mergers of galaxies can lead to varying
distributions of BH spin
• Sikora et al.(2007) suggest the following
revised spin paradigm:
– If the accretion history of a galaxy consists of
multiple accretion events of small mass and
random orientation of angular momentum vectors,
then this can lead to a preference for low BH spins
– If the accretion history of a galaxy underwent at
least on major merger, then the galaxy accretes a
large amount of gas. In this case the accretion
disk will align with the BH spin, and spin the BH
up.
Support for Revised Spin Paradigm
• Fueling of AGN is disk galaxies may be via
accretion of molecular clouds
• Observations indicate short life-times of
individual accretion events in Seyfert galaxies
• BH growth in giant ellipticals likely occurs via
major mergers
• BH accretes enough gas to align BH and
accretion disk angular momentum vectors,
spins up BH
• RLQs preferentially found in massive earlytype galaxies, which, according to this
evolutionary scenario, have higher BH spins
More to the Story Than Just Spin
• Ye & Wang (2005, MNRAS, 357)
developed a toy model
combining the BZ mechanism
with magnetic coupling of the
accretion disk
• Find that radio-loudness
depends strongly on spin, central
concentration of B-lines, and
inner radius of the disk
Summary
• Quasars show a dichotomy in their radio
emission, with some having powerful radioemitting jets (radio-loud), and most (~90%)
having weak or no jets (radio-quiet)
• RLQs and RQQs show similar correlations
with [O III] luminosity and Eddington ratio, but
with different normalizations
• RLQs tend to inhabit massive early type
galaxies and live in richer environments
• RLQ/RQQ dichotomy may be related to
dependence of black hole spin on the host
galaxy’s accretion history