Understanding Jets from Sources Straddling the Fanaroff-Riley Divide
Preeti Kharb, Ethan Stanley, Matthew Lister, Herman Marshall, Chris O'Dea, Stefi Baum
aa r X i v : . [ a s t r o - ph . GA ] N ov Extragalactic Jets from every angleProceedings IAU Symposium No. 313, 2015F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska, eds. c (cid:13) Understanding Jets from Sources Straddlingthe Fanaroff-Riley Divide
Preeti Kharb , Ethan Stanley , Matthew Lister , Herman Marshall ,Chris O’Dea , and Stefi Baum , Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560034, Indiaemail: [email protected] Dept. of Physics and Astronomy, 525 Northwestern Avenue, West Lafayette, IN 47907, USA MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue,Cambridge, MA 02139, USA University of Manitoba, Winnipeg, MB R3T 2N2 Canada Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
Abstract.
Results from
Chandra-HST-VLA observations of 13 hybrid sources are presented.Data from ten sources in the literature are analysed along with new data from three hybridblazars belonging to the MOJAVE sample. Studies of such hybrid sources displaying both FRIand FRII jet characteristics could provide the key to resolving the long-standing Fanaroff-Rileydichotomy issue. A majority of the 13 hybrid sources show FRII-like total radio powers, i.e.,they are “hybrid” in radio morphology but not in total radio power. VLBI observations of tenof the 13 sources show that the X-ray jet is on the same side as the one-sided VLBI jet. X-raysare therefore emitted from relativistically-boosted approaching jets. This is consistent with theX-ray emission being IC/CMB in origin in the majority of sources. It is not completely clearfrom our study that asymmetries in the surrounding medium can create hybrid sources. Hybridradio morphologies could also be indicative of intrinsically asymmetric jets.
Keywords. galaxies: active, galaxies: jets, quasars: general, BL Lacertae objects: general
1. Background
The Fanaroff-Riley (FR) dichotomy has been a long-standing open question in activegalactic nuclei (AGN) astrophysics. It was in 1974 that B. Fanaroff and J. Riley pointedout that sources which possessed radio jets spanning tens to hundreds of kiloparsecsseemed to exhibit primarily two radio morphologies: the FR type I sources had broadjets that flared on scales of a kiloparsec with the brightest emission coming from near theradio cores and fading away with distance into diffuse radio plumes/lobes, while the FRtype II sources had collimated jets that terminated in regions of high surface brightnesscalled “hot spots” with the back-flowing plasma or plasma left behind by the advancingjet forming the radio lobes (Fanaroff & Riley 1974; Bridle & Perley 1984). FRIs weretherefore termed “edge-darkened” while FRIIs were “edge-brightened” (see Figure 1).The total radio power of the FRI/FRII sources also differed, with the cleanest separationoccurring at low radio frequencies: the FRI sources had radio luminosities at 178 MHzbelow ≈ × W Hz − , while the FRII sources had 178 MHz luminosities abovethis value. It was shown in the 1990s that the FRI/FRII division was also a functionof the optical luminosity of the host galaxy (Ledlow & Owen 1996). In the so-calledOwen-Ledlow plot, there is clearly a region in the radio luminosity space where FRIs andFRIIs can co-exist (see also Baum et al. 1995): this is typically between Log L . of ≈ . − . − . The relevance of this will be discussed ahead in Section 2.1.It was soon realised that many different types of AGN were the same phenomena1 P. Kharb et al.modified in appearance due to orientation-effects of relativistic jets and the presence ofdusty tori which shielded the central regions containing the black hole, accretion diskand the broad line region, from certain lines of sight. This gave rise to the radio-loudUnified Scheme which postulated that BL Lac objects were the pole-on counterparts ofFRI radio galaxies and radio-loud quasars were the pole-on counterparts of FRII radiogalaxies (Urry & Padovani 1995). BL Lac objects and radio-loud quasars are collectivelyreferred to as blazars.However, several burning questions remain. Why do only ∼ −
20% of AGN(e.g., Kellermann et al. 1989) produce the large radio jets observed in radio galaxiesand blazars ? Why do they exhibit distinctly different radio morphologies and possesscharacteristic radio powers ? How do these radio jets interact with the surrounding matterin the interstellar/intergalactic medium ?It was noticed early on, however, that not all radio galaxies fit neatly into the FRI/FRIIcategories (see examples of sources with complex radio morphologies in Bridle et al.1994). Some sources seem to have one jet that terminates in a terminal hot spot likean FRII, while the other jet fades in surface brightness with distance from the radio corewithout forming a terminal hotspot, like an FRI (e.g., Morganti et al. 1993, Figure 1).These Intermediate or Hybrid sources straddling the Fanaroff-Riley divide are the focusof the present work. 1.1.
The Importance of being Intermediate
Several suggestions have been put forward to explain the FR dichotomy. These includesuggested differences in black hole masses with FRIs possessing higher mass black holesthan FRIIs (e.g., Ghisellini & Celotti 2001); differences in the spin rates of black holeswith FRIIs having faster spinning black holes (e.g., Meier 1999); differences in the ac-cretion rates with FRIIs typically having higher accretion rates (e.g., Marchesini et al.2004), and differences in jet composition with FRI jets comprising electrons and positronsand FRII jets comprising electrons and protons (e.g., Celotti & Fabian 1993). Such dif-ferences cannot account for the existence of Hybrid or Intermediate sources. Althoughfor the last suggestion of jet composition, a variant of an electron-positron “spine” sur-rounded by an electron-proton “sheath” could still work for jets in hybrid sources (e.g.,Pelletier & Roland 1989). As such, the study of hybrid sources could go a long way inhelping us resolve the Fanaroff-Riley dichotomy.1.2.
How Rare are Hybrid Sources ?
After parsing through the radio images of nearly a thousand sources in the litera-ture, Gopal-Krishna & Wiita (2000) identified only six sources exhibiting a hybrid ra-dio morphology. The detection rate for such sources was therefore quoted at < S . >
20 mJy and radio extents > ′′ , identified five hybrid sources. The detection rate therefore remained at < † sample of blazars (Lister et al. 2009). The parsec-scale jets in the MOJAVEblazars have been regularly monitored with Very Long Baseline Interferometry (VLBI)at 15 GHz for nearly two decades now. This has aided in the determination of jet speeds onparsec-scales for individual jet components/knots over a long time baseline. Kharb et al.(2010) observed the MOJAVE-1 sample ‡ with the VLA A-array configuration at 1.4 GHz.Through this study, they identified a much larger fraction of hybrid sources at ≈ † Monitoring Of Jets in AGN with VLBA Experiments ‡ ets in Sources Straddling the Fanaroff-Riley Divide D e c li na t i on ( J ) Right ascension (J2000)22 31 26 24 22 20 18 16 1439 23 300022 300021 300020 300019 30 0 20 40 D e c li na t i on ( J ) Right ascension (J2000)03 59 00 58 58 56 54 52 50 4810 282726252423 D e c li na t i on ( J ) Right ascension (J2000)20 05 38 36 34 32 30 28 26 2477 53 0052 55504540353025
Figure 1.
Radio images of the FRI galaxy 3C 449 (Top left) and the FRII galaxy 3C 98 (Topright). The brightest radio emission, seen in red, is closest to the radio core in FRIs and furthestaway from the core, at the termination points of the jets, in FRIIs. (Bottom) The radio imageof the hybrid morphology blazar, 2007+777, clearly shows an FRI-like jet on the right side andan FRII-like jet (actually only the terminal hot spot) on the left side of the radio core. In thissource the one-sided VLBI jet is pointing along the FRI (i.e., the right) side.
This larger fraction is likely to be the result of the MOJAVE selection criteria, whichare based on relativistically boosted parsec-scale jet emission, rather than low frequencylobe emission like the 3C sample. Therefore, the MOJAVE survey may be picking upsources that span a larger range in intrinsic radio powers and morphologies. As discussedin Kharb et al. (2010), there may also be a greater tendency to pick up jets that arehighly bent. P. Kharb et al.1.3.
Previous Chandra − HST − VLA Observations of FRIs & FRIIs
The
Chandra X-ray Observatory has demonstrated clearly that X-ray jets are prevalentin blazars (Sambruna et al. 2004; Marshall et al. 2005; Hogan et al. 2011). A generaltrend has been observed in the X-ray emission mechanisms of FRI and FRII sources,with the former typically emitting via the synchrotron mechanism (e.g., Sambruna et al.2007; Kharb et al. 2012b) and the latter emitting via the inverse-Compton scatteringof CMB photons (IC/CMB, e.g., Tavecchio et al. 2000; Kharb et al. 2012a). However, ashighlighted in the review by Harris & Krawczynski (2006), each of the suggested emissionmechanisms are fraught with difficulties. While it is impossible to get synchrotron X-rayemission from 100 kpc scale jets without invoking in situ particle acceleration throughoutthe extent of the jets on account of the short lifetimes of X-ray emitting electrons ( ∼ fewyears), the requirement that the kpc-scale jets stay highly relativistic with bulk Lorentzfactors, Γ ∼ −
10, is contradicted by the Γ ∼ Previous Chandra − HST − VLA Observations of Hybrid Sources
Ten hybrid morphology sources have so far been observed with the
Chandra and insome cases also the
Hubble Space Telescope (HST) . The results from the multi-bandSED modelling of these sources are mixed. (Note that we have not considered bona-fide
X-shaped radio sources that consist of two pairs of radio lobes, one pair possessingterminal hot spots and the other pair not, into the hybrid category. However, a couple ofsources with ambiguous morphologies that have been classified as X-shaped sources, like3C 433 or NGC 6251, are included in our analysis.) The multi-wavelength observations ofhybrid sources have shown that the X-ray jets are sometimes synchrotron in origin (e.g.,Sambruna et al. 2007), but more often from the IC/CMB mechanism (e.g., Miller et al.2006; Miller & Brandt 2009).1.5.
New Multi wavelength Observations of Three MOJAVE Hybrid Blazars
In order to augment the multi-wavelength data on hybrid morphology sources in theliterature, as well as to make greater sense of the results obtained from only a handfulof sources so far, we observed three MOJAVE blazars exhibiting a hybrid morphologywith the
Chandra ( ∼ −
80 ks using the ACIS) and
HST (2 orbits each using theWFC3/F160W and F475W filters). The first results from this study will be presentedin E. Stanley et al. 2015 (in preparation). The three blazars viz., − − >
100 mJy and radio jet extents > ′′ . Thesethree have been observed previously for ∼
10 ks each with the
Chandra as part of theMOJAVE-Chandra Sample (MCS) pilot study (Hogan et al. 2011).
2. Results and Comparison with Sources in the Literature
Multi-waveband SED modelling indicates that the X-ray emission is from the IC/CMBmechanism in all three MOJAVE hybrid blazars that we studied. In addition, the SEDbest-fit parameters are similar to those observed in regular (non-hybrid) blazar jets:Doppler factors of ∼ −
4, magnetic fields of ∼ µ G, minimum electron Lorentz factorsof ∼ −
20, maximum electron Lorentz factors of ∼ − and electron powerlaw indices of ∼ − ets in Sources Straddling the Fanaroff-Riley Divide Figure 2.
Total radio luminosity at 1.4 GHz (left) and extended radio luminosity at 74 MHz(right) w.r.t. redshift. The red symbols denote sources with X-ray jets from the synchrotronmechanism ( z < . only two of the 13 sources, viz., 3C 371, 1045 − > “Hybrid” in Radio Morphology But Not in Radio Power We looked at the total and extended (=total − core) emission on kpc-scales at 1.4 GHzand 74 MHz respectively for the 13 hybrid sources. The total 1.4 GHz flux densitieswere derived from the total 74 MHz flux densities assuming a spectral index α of 0.7( S ν ∝ ν − α ). The 74 MHz flux density values were obtained from the VLA Low FrequencySurvey (VLSS; Cohen et al. 2007). For estimating the extended flux densities we obtainedthe core flux densities from high resolution 8 GHz images in the NRAO VLA Imagearchive † and the Gaussian-fitting AIPS task JMFIT , and subtracted them from the total74 MHz flux densities. We note that if our assumption that the core spectral indices areflat ( α = 0) all the way down to 74 MHz is incorrect, and the spectrum turns over at lowfrequencies, then our estimated extended flux densities are in fact lower limits. The totaland extended luminosities for the hybrid sources are plotted against redshift in Figure 2.Blue dashed lines indicate the FRI, FRII and intermediate FRI/II categories (see Section1). In the right panel of Figure 2, we have shifted the FR dividing lines assuming α = 0 . < . † https://archive.nrao.edu/archive/archiveimage.html P. Kharb et al.
Figure 3.
Total (left) and extended (right) radio power versus the maximum apparent jetspeeds on parsec-scales. The red symbols denote sources with X-ray jets from the synchrotronmechanism ( z < . determinant for the X-rays being IC/CMB (in high power sources) or synchrotron inorigin (in low power sources).2.2. Information from Parsec-scales
Ten of the 13 hybrid sources have so far been imaged with VLBI. However, only seven ofthem have been imaged at multiple epochs with VLBI to yield apparent jet speeds. Wefind that the one-sided VLBI jets are always on the same side as X-ray jets except appar-ently in the blazar 1510 − − L . ) and extended luminosities ( L ext ) versus themaximum jet component speeds β max for seven hybrid sources in Figure 3. Kharb et al.(2010) had found a significant correlation between β max , and extended radio luminosity.On average, the radio-loud quasars had faster jets than BL Lac objects. However, no cleardivision in β max or L ext was observed between quasars and BL Lacs. The β max − L . and β max − L ext correlation is also observed for the seven hybrid sources with measuredVLBI jet speeds (Figure 3). We see that the two sources with synchrotron X-ray emission(red symbols) are at the lower end of the β max − luminosity correlations, while the X-rayjets from the IC/CMB process are towards the higher end. Also for the few sourcesconsidered here the BL Lac objects (NGC 6251 is an FRI radio galaxy) and quasarsare clearly divided below and above a β max of around 3. This study will benefit frommulti-epoch VLBI observations of a greater number of hybrid sources.
3. Are Hybrid Sources FRIIs in Asymmetric Environments ?
Miller & Brandt (2009) suggested that the hybrid morphology sources were likely to beFRIIs in asymmetric environments, consistent with the suggestions of Gopal-Krishna & Wiita(2000): the jets that traversed the denser media eventually became de-collimated andFRI-like. However, Miller & Brandt (2009) concluded this on the basis of the bent radio ets in Sources Straddling the Fanaroff-Riley Divide < −
50 kpc. A few of these sources are well-documented galaxypairs and some show signatures of galaxy-galaxy interactions. Therefore, &
60% of thehybrid sources considered here may indeed have asymmetric galactic environments onaccount of galaxy interactions. However, to confirm their group or cluster membership,we need the as yet unavailable redshifts of the surrounding galaxies. Results from amore detailed analysis including a comparison sample of regular active galaxies will bepresented in Stanley et al. (2015).However, there is some circumstantial evidence that does not favour the asymmetricenvironment argument. We know that the X-ray jets are always on the same side asthe one-sided VLBI jets. This implies that the X-ray jets are also the approaching jets.Secondly, the majority of the jets that emit in X-rays are on the side without a terminalhot spot, or the FRI side. The asymmetric environment argument predicts that this isthe side with the denser medium. However, it would be peculiar if the denser mediumwas always on the same side as the approaching jet.Another question then arises: why is the X-ray emission typically on the side withoutthe terminal hot spot ? This could happen due to unfavourable Doppler boosting effects.For example, for an approaching jet, the backflow from the hot spot could be movingaway from us resulting in net surface brightness dimming. This result could also implythat even on the FRI side there is a fast collimated inner spine, just like an FRII jet, butwhich dissipates before a terminal hot spot is formed.
4. Summary
Studying sources with morphologies that are intermediate between FRIs and FRIIscould be the key to understanding the Fanaroff-Riley dichotomy. Intrinsic differences inthe central engine (e.g., black hole mass, spin, accretion rate, jet composition) can beruled out for hybrid sources. These studies can in turn provide invaluable clues about jetformation and propagation. While most theories of hybrid sources have invoked asymme-try in the external environment, Wang et al. (1992) have proposed that if the magneticfield structure in the accretion disk is asymmetric with respect to the disk mid-plane, thenintrinsically asymmetric jets could be launched. Such a model could explain the presenceof hybrid morphology sources. We need to further explore this and similar models.We summarise below the results from the
Chandra-HST-VLA observations of 13 hybridsources. Data from ten sources in the literature are analysed along with new data fromthree hybrid blazars belonging to the MOJAVE sample.( a ) A majority of the 13 hybrid sources considered in this study show FRII-like totalradio powers. Therefore, these hybrid sources are “hybrid” only in radio morphology butnot in total radio power.( b ) For ten of the 13 hybrid sources with parsec-scale data from VLBI, the one-sidedVLBI jets which are the relativistically-boosted approaching jets, are on the same side P. Kharb et al.as the X-ray emission. Therefore, the X-rays are emitted from approaching jets. This isconsistent with the X-ray emission being IC/CMB in origin, in the majority of sources.( c ) It appears that the total radio power, rather than the FR radio morphology, is themain determinant for the X-rays being IC/CMB (in high power sources) or synchrotronin origin (in low power sources).( d ) There is a tendency for the X-ray jets to lie on the side without the terminal hotspots, i.e., the FRI side. This could occur due to unfavourable Doppler boosting effects:for an approaching jet, the hot spot backflow could be moving away from us resulting insurface brightness dimming.( e ) It is not completely clear that asymmetries in the surrounding medium can createhybrid sources from regular FRII sources. It is expected in this scenario that the jettraversing the denser medium would get de-collimated and become FRI-like. While theSDSS images do reveal the presence of nearby ( < −
50 kpc) galaxies for &
60% ofthe hybrid sources, confirmation requires the yet unavailable redshifts of the apparentlynearby galaxies. However, following points ( b ) and ( d ) above, it would be peculiar tohave the denser environment always on the same side as the approaching jets. We needto further explore the realm of intrinsically asymmetric jets to produce hybrid radiomorphologies. Acknowledgements
PK would like to thank the organisers for the invitation to speak at this excellentconference. This work was supported by the National Aeronautics and Space Adminis-tration (NASA) through Chandra Award Number (GO3-14120A) issued by the ChandraX-ray Observatory Center (CXC), which is operated by the Smithsonian AstrophysicalObservatory (SAO) for and on behalf of NASA under contract NAS8-03060. Support forprogram number 13116 was provided by NASA through a grant from the Space TelescopeScience Institute, which is operated by the Association of Universities for Research in As-tronomy, Inc., under NASA contract NAS5-26555. The MOJAVE program is supportedunder NASA-Fermi grant NNX12A087G. The National Radio Astronomy Observatoryis a facility of the National Science Foundation operated under cooperative agreementby Associated Universities, Inc.
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