The Radio-Loud/Radio-Quiet 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 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

3. 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? Jets Centaurus A Red: Radio, Blue: X-ray

4. Structure of RLQ Picture from Marscher, 2005, Mem. S.A.It., 76

5. 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)

6. 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

7. 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

8. Comparison of RQQ/RLQ SEDs Mean RQQ (Solid) and RLQ (Dashed) SED, normalized at 1.25 m. From Elvis et al.(1994, ApJS, 95).

9. 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 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, A&A, 455) Figure from Ulvestad et al. (2005)

10. 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

11. 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 • 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, 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

15. 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 Figure from McLure & Jarvis (2004, MNRAS, 353) Figure from Liu et al.(2006)

16. 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

17. 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

18. 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.

19. 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

21. 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 galaxy’s accretion history