VLBI-Gaia offsets favor parsec-scale jet direction in Active Galactic Nuclei
AAstronomy & Astrophysics manuscript no. 30031_final c (cid:13)
ESO 2017January 3, 2017 L etter to the E ditor VLBI-
Gaia offsets favor parsec-scale jet direction inActive Galactic Nuclei
Y. Y. Kovalev , , L. Petrov , and A. V. Plavin , Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 86 /
32, 117997 Moscow, Russia; e-mail: [email protected] Astrogeo Center, 7312 Sportsman Dr., Falls Church, VA 22043, USA Moscow Institute of Physics and Technology, Institutsky per. 9, Dolgoprudny 141700, RussiaReceived 8 November 2016; accepted 8 December 2016
ABSTRACT
Context.
The data release 1 (DR1) of milliarcsecond-scale accurate optical positions of stars and galaxies was recently published bythe space mission
Gaia . Aims.
We study the o ff sets of highly accurate absolute radio (very long baseline interferometry, VLBI) and optical positions of activegalactic nuclei (AGN) to see whether or not a signature of wavelength-dependent parsec-scale structure can be seen. Methods.
We analyzed VLBI and
Gaia positions and determined the direction of jets in 2957 AGNs from their VLBI images.
Results.
We find that there is a statistically significant excess of sources with VLBI-to-
Gaia position o ff set in directions along andopposite to the jet. O ff sets along the jet vary from 0 to tens of mas. O ff sets in the opposite direction do not exceed 3 mas. Conclusions.
The presense of strong, extended parsec-scale optical jet structures in many AGNs is required to explain all observedVLBI-
Gaia o ff sets along the jet direction. The o ff sets in the opposite direction shorter than 1 mas can be explained either by anon-point-like VLBI jet structure or a “core-shift” e ff ect due to synchrotron opacity. Key words. galaxies: active – galaxies: jets – radio continuum: galaxies – astrometry – reference systems
1. Introduction
Gaia
Data Release 1 (DR1, Lindegren et al. 2016) provides acatalogue of highly accurate optical positions for many objects,including active galactic nuclei (AGNs) with milliarcsecond un-certainties. So far, only VLBI has been able to provide that levelof accuracy. A comparison of the
Gaia quasar auxiliary solutionwith the International Celestial Reference Frame 2 (ICRF2) cat-alog (Fey et al. 2015) demonstrated a good agreement in radioand optic positions, but singled out a fraction of 6% as outliers(Mignard et al. 2016). Petrov & Kovalev (2016) extended thiscomparison to the secondary
Gaia
DR1 catalogue of 1.14 bil-lion objects that have median position uncertainty 2.3 mas andmodern VLBI absolute astrometry catalogue RFC 2016c that,to date, is the most complete. They found 6055 firm matcheswith AGNs. Both ICRF2 and RFC 2016c catalogues have com-parable accuracy, but the latter utilized all VLBI observationsused for the ICRF2 and those that became available from Jan-uary 2008 through September 2016. This increased the totalnumber of VLBI sources by more than a factor of three withrespect to the ICRF2. Petrov & Kovalev (2016) revealed a popu-lation of approximately 400 objects with significant radio / opticalo ff sets after alignment of two catalogs that cannot be explainedby the random noise in the data. However, that study could notprovide information on the cause of these o ff sets.This motivated us to consider additional available informa-tion about AGN structure at milliarcsec scale that can shed lighton the cause of the o ff sets between the radio absolute referencepoints and the Gaia centroids in the optical band. The majority http://astrogeo.org/vlbi/solutions/rfc_2016c/ of radio-loud AGNs exhibit a typical core-jet morphology, thuspresenting a strong asymmetry in their structure. For many ofthem, the jet is resolved and strong enough for us to determineits direction from hybrid mapping results. Let us define the o ff -set vector of the Gaia position with respect to the VLBI position VG and the unit vector, defining the direction of a jet from the jetbase downstream as j (Fig. 1). The angular di ff erence betweenthese directions is denoted as ∆ P . A . VGjet . In the following, we an-alyze the distribution of ∆ P . A . VGjet for radio / optical AGN matchesand analyze its implications.
2. Observational data and basic analysis
Let us consider a simplified AGN diagram in Fig. 1. It was shownthat the apparent base of the jet in radio band ‘C r ’ typically asso-ciated with the brightest and most compact region in AGN jets atparsec scales changes its position with frequency due to the syn-chrotron self-absorption (Marcaide & Shapiro 1984; Lobanov1998). Observations demonstrated that the core-shift is typicallyat a sub-mas level at centimeter wavelengths (e.g., Sokolovskyet al. 2011; Pushkarev et al. 2012; Fromm et al. 2013; Kutkinet al. 2014). Kovalev et al. (2008) predicted that the apparent jetbase in the optical band ‘C o ’ will be shifted at 0.1 mas level withrespect to the jet base ‘C r ’ at 8 GHz in the direction to ‘BH’.However, if the core-shift depends on frequency as ν − , it hasno contribution to group delay that is used for absolute VLBIastrometry (Porcas 2009) and thus, does not a ff ect the absoluteVLBI positions. In that particular case ‘C r ’ and ‘C o ’ coincide.The presence of the asymmetric radio structure causes an ad-ditional term in group delay (Charlot 1990). This term is ignored Article number, page 1 of 4 a r X i v : . [ a s t r o - ph . GA ] J a n & A proofs: manuscript no. 30031_final
Relative R.A. (mas) R e l a t i v e D e c ( m a s ) J0005+382015 GHz Relative R.A. (mas) R e l a t i v e D e c ( m a s ) J0433+05218 GHz
Relative R.A. (mas) R e l a t i v e D e c ( m a s ) J1229+02038 GHz
Relative R.A. (mas) R e l a t i v e D e c ( m a s ) J1800+782815 GHz
Fig. 2.
VLBI images of brightness distributions at 8 and 15 GHz are shown together with estimated median direction of their jets (green arrow,the j vector) for the four targets out of the total of 2957 used in the analysis. Low-to-high surface brightness is shown by color from dark blue todark red. The VLBI beam is shown by the light green ellipse at the half power level in the bottom left corner. The relative position of the peakintensity pixel on the map is formally chosen as the VLBI astrometry reference point of the target to illustrate the e ff ect. The vector VG is shownin light blue relative to this location from VLBI-to- Gaia position and black ellipses represent 1 σ errors. ∆ P . A . VGjet is measured to be 4 ◦ , 73 ◦ , 164 ◦ ,and 1 ◦ for J0005 + + + + + in both ICRF2 and RFC 2016c data analyses. Neglecting thisterm results in a shift of the VLBI reference point ‘P V ’ with re-spect to the jet base, predominantly in the direction along the jet(Fig. 1). Charlot (2002); Sovers et al. (2002) investigated this ef-fect in detail and found that an unaccounted-for structure termcauses source position jitter with the rms that, on average, doesnot exceed 0.11 mas, but for extreme cases can cause peak-to-peak variations of up to 2 mas. It should be noted that AGNsare very active, sometimes flaring objects with physical con-ditions changing dramatically in regions close to the nucleus.Thus, the distance between the apparent jet base ‘C r ’ and thereference point ‘P V ’ may change. Analysis of source positiontime series (Feissel et al. 2000) found that variations in sourcepositions caused by unaccounted-for changes in source structurerarely exceed 1 mas. Strong scattering of radio emission coulda ff ect positions (e.g., Pushkarev et al. 2013) but for most targetscan be neglected (Pushkarev & Kovalev 2015). A counter-jet isobserved in a small fraction of AGNs (e.g., Liodakis et al. 2016)in addition to the main jet, although usually it is weak due tode-boosting.Similarly, milliarcsecond-scale structure of AGNs a ff ects theposition of the centroid in the optical band. Some active galax-ies are known to have extended and bright jets (e.g., Prieto et al. Fig. 1.
A simplified diagram of an AGN at milliarcsecond scales in theplane of the sky. The ‘BH’ cross marks a position of the supermassiveblack hole as well as the accretion disk. The arrow represents the jet ( j vector) while the counter-jet goes in the opposite direction. The appar-ent base of the jet in radio is shown by ‘C r ’, in the optical band it isexpected to be closer to the central engine ‘BH’ (Kovalev et al. 2008)and is shown as ‘C o ’. The absolute radio VLBI ‘P V ’ and optical Gaia ‘P G ’ reference points are shown by blue and green dots, respectively.The red o ff set vector VG connects these points. The host galaxy canbe relatively bright in optical band and shift the optical centroid in anydirection, the galaxy center is marked by a star. o ’ to ‘P G ’ (Fig. 1) by values that could exceed the 1-mas scale.It is important to note that VLBI positions are determined onthe basis of VLBI visibility measurements, not sensitive to ex-tended structures, while Gaia detects total power. For this rea-son, the extended optical emission a ff ects the source positiondi ff erently than extended radio emission. Additionally, an accre-tion disk may have the optical emission centered at the super-massive black hole ‘BH’, which may be shifted at a level of afraction of a milliarcsecond with respect to the apparent jet base‘C o ’ in the direction opposite to the jet. The optical ‘host galaxy’center of mass might be shifted from ‘BH’ in any direction onthe milliarcsecond scale.Petrov & Kovalev (2016) have associated the Gaia
DR1 sec-ondary dataset and the VLBI RFC 2016c catalogs. For the fol-lowing analysis, we selected 6054 matches with the probabil-ity of false association PFA < · − . PFA was calculatedusing Gaia source density averaged within a cell of a regular0 ◦ . × ◦ .
25 grid (Petrov & Kovalev 2016). The 50th percentileof
Gaia – RFC o ff set lengths is 2.2 mas and the 99th is 76 mas.The position angle P . A . jet of the parsec-scale radio jet, vec-tor j , is determined using VLBI images from the Astrogeo VLBIFITS image database . The images that we used come mostlyfrom the analysis of the VLBA Calibrator Survey (VCS; Beasleyet al. 2002; Fomalont et al. 2003; Petrov et al. 2005, 2006; Ko-valev et al. 2007; Petrov et al. 2008) and regular geodesy VLBIprogram (Petrov et al. 2009; Pushkarev & Kovalev 2012; Pineret al. 2012) at 2 and 8 GHz. Additionally, we made some useof images from the VLBI Imaging and Polarimetry Survey at5 GHz (Helmboldt et al. 2007; Petrov & Taylor 2011), the VCSreleases 7, 8, and 9 (Petrov 2016) at 7.4 GHz; VLBI observ-ing programs for Fermi -AGN associations (e.g., Schinzel et al.2015) at 8 GHz; the 15 GHz Monitoring Of Jets in Active Galac-tic Nuclei with VLBA Experiments program (MOJAVE, Listeret al. 2009), 24 and 43 GHz images from the K / Q survey (Char-lot et al. 2010) and the VLBA-BU Blazar Monitoring Program(Jorstad & Marscher 2016). The jet direction j is determinedfrom the inner direction of the jet ridge line calculated directlyfrom the VLBI images. If more than one image was available fora given target, a median P . A . jet was used. For 90% of cases weestimated jet direction with accuracy better than 10 ◦ . Variabledirection of ejections (Lister et al. 2013) or apparent jet curva- http://astrogeo.org/vlbi_images/ Article number, page 2 of 4ovalev, Petrov, & Plavin: VLBI-
Gaia o ff sets favor AGN jet direction Full sample0 180 P . A . VGjet (deg) N | VG | > 2 max( G , V )| VG | < 3 mas0 180 P . A . VGjet (deg) N
334 sources | VG | > 2 max( G , V )3 | VG | < 10 mas0 180 P . A . VGjet (deg) N
183 sources | VG | > 2 max( G , V )| VG | 10 mas0 180 P . A . VGjet (deg) N
117 sources
Fig. 3.
Distribution of ∆ P . A . VGjet for di ff erent sub-samples of VLBI- Gaia associations with probability of false association less than 2 · − . Thevertical dashed lines are shown for ∆ P . A . VGjet = ◦ (VLBI to Gaia VG reference point o ff set vector along the jet) and 180 ◦ ( VG o ff set vectoropposite to the jet direction) values. ture, enhanced by the projection e ff ect (e.g., Agudo et al. 2007),results in a larger error for remaining objects. We succeeded indetermining the jet orientation for 2957 matched AGNs; approx-imately half of the total number of matches. A significant frac-tion of images did not have high-enough dynamic range to allowa robust jet direction determination.Examples of VLBI images for four AGN targets overlayedwith the VG and j vectors are shown in Fig 2. Since the VLBIimages do not contain information on their absolute positions,for illustration purposes, the VLBI- Gaia o ff set vector VG isshown relative to the peak intensity pixel on the maps. Wechecked whether or not the VLBI- Gaia o ff set position angleP . A . VG or parsec-scale radio jet P . A . jet have preferred directionson the sky and found that their distributions are flat over 360 ◦ .
3. Parsec-scale jet direction is preferred by theVLBI-
Gaia offset
The distribution of the di ff erences ∆ P . A . VGjet between the VLBI-
Gaia o ff set position angle P . A . VG and the VLBI jet directionP . A . jet is presented in Fig. 3. It is evident that ∆ P . A . VGjet prefersvalues 0 ◦ and 180 ◦ even for the full sample (top left histogram)with significance < − and 0.007, respectively. To furtherdemonstrate this e ff ect, we plot the other histograms in Fig. 3,only for those associations that have the o ff set | GV | at least twotimes larger than the errors of their VLBI, σ V , and Gaia , σ G , po-sitions. For this sub-sample, the excess of targets that prefer theVLBI jet direction is estimated as 34 % and 13 % along and op-posite to the jet direction, respectively. The excess may rise withimproving Gaia position accuracy in future data releases. Thehistograms cover di ff erent typical intervals of the o ff set lengths | VG | , and clearly demonstrate di ff erent e ff ects that are presentat di ff erent o ff set scales. To present this in even more details, | VG | ( mas ) P e a k s t r e n g t h Fig. 4.
A measure of the peak strength in ∆ P . A . VGjet distributions cal-culated as discussed in § 3. The horizontal dashed red line presents theflat distribution case. The solid blue curve describes the 0 ◦ peak, greencolor – the peak around 180 ◦ . Results for | VG | >
10 mas present anintegration over all targets with | VG | >
10 mas. The shading shows the90 % confidence interval.
Fig. 4 shows a measure of the peak significance for the peakat ∆ P . A . VGjet = ◦ and ∆ P . A . VGjet = ◦ for associations withPFA < · − , σ G < . | VG | , and σ V < . | VG | . The measureis calculated as the number of targets within ± ◦ from the peakdivided by the total number of targets within ± ◦ of the corre-sponding peak for distributions with o ff set values ± | VG | . Binomial proportion intervals are calculatedand shown for the 90 % confidence level.We performed a Monte Carlo simulation in order to providea quantitaive measure of the systematic VLBI-Gaia o ff set in thepresence of the random noise. The simulation aims to reproducethe peak at ∆ P . A . VGjet = ◦ in the histogram of the sub-samplewith o ff set | VG | < σ G < . | VG | , and σ V < . | VG | (Fig. 3, top right). In our model, the Gaia positions were sub-ject to two factors: systematic shift s with respect to VLBI po-sition that obeys the Gaussian distribution with the standard de-viation σ, and random noise. The vector of random noise hasuniform angular distribution and length that obeys the power-law transformed Rayleighian distribution with parameters foundby Petrov & Kovalev (2016). We ran one million trials at a gridover s and σ . The probability of getting or exceeding the peakat histogram at ∆ P . A . VGjet = ◦ at s = , σ =
0, that is, whenthe o ff sets have isotropic distribution, is 3 · − . The probabilityexceeds the confidence level 0.05 either when s > .
06 mas inthe opposite jet direction or when σ > .
26 mas.We ran a similar simulation to reproduce the peak at ∆ P . A . VGjet = ◦ for the sub-sample with o ff set lengths | VG | > σ G < . | VG | , and σ V < . | VG | (Fig. 4). The prob-ability of getting or exceeding the peak at ∆ P . A . VGjet = ◦ for s = , σ =
0, is less than 10 − . The probability exceeds theconfidence level 0.05 either when s > . σ > . r ’-‘P V ’ distance at the diagram in Fig. 1), which is morethan one order of magnitude less and, moreover, is expected for ∆ P . A . VGjet = ◦ . Article number, page 3 of 4 & A proofs: manuscript no. 30031_final
Results presented here are confirmed on a lower significancelevel if the ICRF2 catalog (Fey et al. 2015) or the
Gaia
DR1quasar auxiliary solution (Mignard et al. 2016) are used.
4. Discussion O ff sets in the jet direction ( ∆ P . A . VGjet ≈ ◦ ) are observed formany matches for a wide range of VLBI- Gaia distances | VG | :from less than 1 mas to more than 10 mas (Figures 3, 4). SeeJ0005 + + ∆ P . A . VGjet contradicts the assertionthat the radio-optical o ff sets can be caused by the presenceof extended-frequency-dependent parsec-scale radio structurealone. We must assume the presence of extended optical struc-ture at milliarcsecond scales in order to explain the distributionsin Figures 3 and 4. Thus, we find direct massive observationalevidence of the existence of elongated, milliarsecond-scale opti-cal jet structures. We note that observational evidence of a directrelation between optical and radio jet properties at parsec scaleswas discussed previously by, for example, Marscher et al. (2008,2010). The optical parsec-scale synchrotron AGN jets should beobserved as extended and bright enough for a significant frac-tion of targets shifting the centroid of optical positions by morethan 1 mas. We note that some optical jets possess bright featureseven at a 1 (cid:48)(cid:48) distance from the nucleus (see, e.g., the jet in M87in Biretta et al. 1999; Prieto et al. 2016) which results in the ob-served significant shift in the optical centroid position relative tothe VLBI-compact radio emission. No clear distinction betweenparsec-scale structure of sources showing di ff erent typical | VG | and ∆ P . A . VGjet values was found. Net rotation of
Gaia referenceframe with respect to VLBI for the found AGN matches changesby less than 0.05 mas when the 384 AGNs with significant o ff -sets (Petrov & Kovalev 2016) are excluded.O ff sets opposite to the jet direction ( ∆ P . A . VGjet ≈ ◦ ) aremeasured mainly at small VLBI- Gaia length | VG | < < | VG | <
10 mas.An example of this case is presented in Fig. 2 by J1229 + ff ect, (ii) the e ff ect of resolved VLBI jet structure, or (iii)the potential e ff ect of optical emission from the accretion disk.With the current accuracy of Gaia positions and the lack of in-formation of the
Gaia positional variability, we are unable todistinguish between these three scenarios. For future releases of
Gaia positions one could limit the analysis to the most compactVLBI targets to partially mitigate the e ff ect of the parsec-scaleVLBI structure of AGN jets. Kovalev et al. (2008); O’Sullivan& Gabuzda (2009); Pushkarev et al. (2012); Sokolovsky et al.(2011); Martí-Vidal et al. (2016) have demonstrated that thecore-shift can be precisely measured. Charlot (2002) has shownhow radio source structure can be accurately taken into accountusing their images. After calibrating the radio position for core-shift and radio structure, the contribution of emission from theaccretion disk and optical jet will be seen much more clearly. Weanticipate observing programs targeting massive core-shift mea-surements and processing radio astrometry surveys with appliedsource structure term in the future, though this will require re-construction of high-quality images within every VLBI astrom-etry session.A weaker peak in the distribution is observed for VLBI- Gaia shifts along the jet direction ( ∆ P . A . VGjet ≈ ◦ ) for | VG | >
10 mas (Figs. 3, 4). This might result from the influence of brightradio features far away from the central engine.
5. Summary
We find that the VLBI-
Gaia positional o ff sets prefer to be paral-lel to parsec-scale radio jet direction.The o ff set from VLBI to Gaia positions happens along thejet in a range from a fraction of a mas to more than 10 mas.In this case, optical centroids are farther away from the centralnucleus. This can only be explained if elongated bright opticaljets exist at parsec scales in many AGNs and significantly shiftthe reference point from their apparent optical base.Position o ff sets opposite to the jet direction do not exceed3 mas and are as common as shifts along the jet. For the oppositeo ff sets, radio reference points are farther away from the centralnucleus. This could be due to several factors, including the ap-parent core-shift e ff ect due to the synchrotron self-absorption, orthe extended VLBI structure of radio jets. Acknowledgements.
We thank A. B. Pushkarev, A. P. Lobanov, E. Ros,A.V. Moiseev, G.V. Lipunova, and the anonymous referee for useful discussionsand suggestions. This project is supported by the Russian Science Foundationgrant 16-12-10481. We deeply thank the teams referred to in § 2 for makingtheir fully calibrated VLBI FITS data publicly available. This research has madeuse of data from the MOJAVE database that is maintained by the MOJAVE team(Lister et al. 2009). This study makes use of 43 GHz VLBA data from the VLBA-BU Blazar Monitoring Program, funded by NASA through the Fermi Guest In-vestigator Program. This research has made use of NASA’s Astrophysics DataSystem.
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