The RadioLoudRadioQuiet Dichotomy of AGN

1
The Radio-Loud/Radio-Quiet Dichotomy of AGN

• Brandon Kelly
• Astro 596

2
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 z0.158 (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

3
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
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Structure of RLQ
Picture from Marscher, 2005, Mem. S.A.It., 76
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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
• 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)
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Bimodality (cont)

• Ivezic 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
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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

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Comparison of RQQ/RLQ SEDs
Mean RQQ (Solid) and RLQ (Dashed) SED, normalized
at 1.25 ?m. From Elvis et al.(1994, ApJS, 95).
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Do RQQs Have Jets?

• Ulvestad et al.(2005, ApJ, 621) observed 5 RQQs
  with the VLBA
• Four of the images had unresolved radio cores,
  J08046459 has two-sided jet
• Concluded radio emission in RQQs due to weak jets
• Recent VLA observations also find evidence for
  jet-like outflows in RQQs (Leipski et al., 2006,
  AA, 455)

Figure from Ulvestad et al. (2005)
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Correlation of LR with LOIII

• 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

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Interpretation of the LO 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
• Implies
• Assuming Shakura-Sunyaev thin disk, jet velocity
  Keplerian velocity, and that the vertical
  magnetic field the azimuthal one, this implies
  that mass flux into the jet
• Then, correlation is expected if
• Similarity of slopes implies that mechanism that
  generates radio emission is similar for both, but
  RLQs have much larger constant of proportionality

12
Similar Bimodal Trend Seen with Eddington Ratio

• Similarly, Sikora et al. (2007, in press at ApJ,
  astro-ph/0604095) find a similar anti-correlation
  between radio-loudness and Eddington ratio, but
  with different normalization

13
Does the Fraction of RLQs Vary with Luminosity
and/or Redshift?

• Some studies have concluded that the radio-loud
  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
  1-dimensional correlations
• Jiang et al. (2007, in press at ApJ,
  astro-ph/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

14
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, AA, 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

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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 its more
  appropriate to compare the jet power with MBH,
  find significant correlation with

Figure from McLure Jarvis (2004, MNRAS, 353)
Figure from Liu et al.(2006)
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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 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, AA, 453) find that among
  early-type galaxies, RLQs inhabit core galaxies,
  whereas RQQs inhabit power-law galaxies

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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

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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.

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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 early-type
  galaxies, which, according to this evolutionary
  scenario, have higher BH spins

20
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

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Summary

• Quasars show a dichotomy in their radio emission,
  with some having powerful radio-emitting 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 galaxys accretion
  history