The obstructed jet in Mrk 231
Ailing Wang, Tao An, Sumit Jaiswal, Prashanth Mohan, Yuchan Wang, Willem A. Baan, Yingkang Zhang, Xiaolong Yang
MMNRAS , 1–8 (2015) Preprint 26 February 2021 Compiled using MNRAS L A TEX style file v3.0
The obstructed jet in Mrk 231
Ailing Wang, , Tao An, ★ Sumit Jaiswal , Prashanth Mohan , Yuchan Wang ,Willem A. Baan , , Yingkang Zhang , Xiaolong Yang Shanghai Astronomical Observatory, Key Laboratory of Radio Astronomy, CAS, 80 Nandan Road, Shanghai 200030, China University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, China Xinjiang Astronomical Observatory, Key Laboratory of Radio Astronomy, CAS, 150 Science 1-Street, Urumqi, Xinjiang 830011, China
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
Mrk 231 is the closest radio-quiet quasar known and one of the most luminous infrared galaxies in the local Universe. It ischaracterised by the co-existence of a radio jet and powerful multi-phase multi-scale outflows, making it an ideal laboratory tostudy active galactic nucleus (AGN) feedback. We analyse the multi-epoch very long baseline interferometry data of Mrk 231and estimate the jet head advance speed to be (cid:46) . 𝑐 , suggesting a sub-relativistic jet flow. The jet position angle changesfrom − ◦ in the inner parsec to − ◦ at a projected distance of 25 parsec. The jet structure change might result from either ajet bending following the rotation of the circum-nuclear disc or the projection of a helical jet on the plane of the sky. In the largeopening angle ( ∼ ◦ ) cone, the curved jet interacts with the interstellar medium and creates wide-aperture-angle shocks whichsubsequently dissipate a large portion of the jet power through radiation and contribute to powering the large-scale outflows. Thelow power and bent structure of the Mrk 231 jet, as well as extensive radiation dissipation, are consistent with the obstruction ofthe short-length jet by the host galaxy’s environment. Key words: galaxies: ISM – galaxies: jets – galaxies: kinematics and dynamics – quasars: individual (Mrk 231)
Mrk 231, also known as UGC 08058 and IRAS 12540+5708 at aredshift 𝑧 = .
042 (de Vaucouleurs et al. 1991), is a relatively nearbyultra-luminous infrared galaxy (ULIRG) (Sanders et al. 1988). Dis-covery of strong hydroxyl (OH) megamasers in this galaxy supportsan extreme starburst in the nuclear region (Baan 1985; Klöckner et al.2003), consistent with its high infrared luminosity of ≈ × 𝐿 (cid:12) (Soifer et al. 1989).Mrk 231 is characterised by prominent multi-phase outflows ex-tending over multiple physical scales (Leighly et al. 2014). Opticalbroad absorption lines (BALs) are detected in both low and highionisation species and indicate galactic-scale outflows arising at thepc-scales (Veilleux et al. 2013, 2016). The study of Feruglio et al.(2015) identifies a molecular gas outflow with a velocity ≥ − originating from the nuclear region and extending to ≈ ≈ − originating from the accretion disc; an analysis (basedon their energetics and momentum transfer) suggests that a bulk ofthe UFO kinetic energy is transferred to the mechanical energy ofthe kpc-scale molecular outflow. The study of Rupke & Veilleux(2011) reports on the detection of fast ( ≈ − ) outflowingneutral gas (as traced by the Na I D absorption) extending from thenuclear region up to 3 kpc; they suggest that this is driven by radiativepressure or mechanical energy from the central AGN. Neutral andionised gas outflows have also been observed with velocities up to ★ E-mail: [email protected] km s − (Rupke et al. 2005; Rupke & Veilleux 2011; Rupke et al.2017). Hi 21 cm absorption features are detected (Carilli et al. 1998;Morganti et al. 2016) on scales ranging from a few to a few hundredpc. The Hi line emission traces an east-west elongated disc possiblycoincident with the inner part of the molecular disc at an even largerscale.The multi-phase multi-scale outflows in active galactic nuclei(AGN) may be enabled by a mixture of driving mechanisms, includ-ing the central AGN (radiation pressure), a nuclear starburst (stellarwinds and radiation pressure), and the radio jet (kinetic energy).While the role of AGN-driven winds and outflows (non-relativistic)on feedback at large scales has been investigated in some detail (e.g.,Rupke et al. 2017; García-Bernete et al. 2021), the role of the radio jetin driving the outflow in the nuclear region requires clarification (e.g.Morganti et al. 2013). Mrk 231 is one of the few radio-quiet quasarscontaining a prominent jet. The source shows a triple (core and twolobes) radio morphology structure with an extent of ∼
40 pc (50 mas)in the north-south direction, revealed by the European VLBI Network(EVN) observation at 1.6 GHz (Neff & Ulvestad 1988) and the VeryLong Baseline Array (VLBA) observations at multiple frequenciesfrom 1.4 to 8.4 GHz (Ulvestad et al. 1999a). The higher-resolutionVLBA images at 15 GHz resolve the central component into twocompact components separated by ∼ ∼ ◦ . Reynolds et al. (2009, 2017, © a r X i v : . [ a s t r o - ph . GA ] F e b Wang et al. 𝑇 B > K, are2–3 orders of magnitude lower than that of typical blazars (Kovalevet al. 2005), but are consistent with radio-quiet quasars (Blundellet al. 1996; Ulvestad et al. 2005). The kinematic property of theVLBI jet is still under debate. Ulvestad et al. (1999b) determined ajet proper motion speed of 0 . ± . 𝑐 in the period from 1996.8 to1998.7, while Reynolds et al. (2017) measured a higher jet apparentspeed > . 𝑐 during a flaring state in 2015. This can be explainedif Mrk 231 contains episodic fast-moving ( 𝑣 ≈ − 𝑐 ) knots and acontinuous slower background jet body (Reynolds et al. 2020).In this paper, we use archival VLBI data of Mrk 231 to study thejet kinematics. We discuss the complex nature of the pc-scale jet andpossible connection between the wide-angle outflows and the radiojet. Throughout, we adopt the following cosmological parameters: H = 71 km s − Mpc − , Ω Λ = .
73 and Ω 𝑚 = .
27. At 𝑧 = . − jet proper motion is convertedto 2 . 𝑐 . We obtained the 8-GHz VLBA data at nine epochs from the publicarchive of the National Radio Astronomy Observatory (NRAO) of theUS and Astrogeo Center . The data covers a 25-yr long interval from1994 to 2019. The observational setup is presented in Table 1. TheNRAO VLBA data was calibrated and analysed in Astronomical Im-age Processing System (AIPS, Greisen 2003) following the standardprocedure described in the cookbook. The Astrogeo data has beencalibrated using the VLBI processing software PIMA (Petrov et al.2011). The calibrated visibility data sets were further analysed in theCaltech DIFMAP software package (Shepherd et al. 1995) to handlethe residual phase errors and imaging. Data with significant scatterwere averaged over a time span of 30 s. Bad data points appearing asoutliers were mostly caused by the radio frequency interference andthus flagged. The edited data was then self-calibrated and mappedfollowing an automated pipeline and cross-verified through manualinteraction. Natural weighting was utilised to create the final mapsto obtain higher sensitivity. The images were de-convolved with therestoring beam using the CLEAN algorithm (Högbom 1974). TheCLEAN-ing was done up to three times the rms noise level of theresidual map.We used circular Gaussian models to fit the self-calibrated visibil-ity data and estimated the integrated flux densities and angular size ofthe core and jet components. First, a single Gaussian component wasfitted on the source. Next, we extracted the model component fromthe data and created the residual map, from which we continued to fita second component with a circular Gaussian model. Then, we tookthese two circular Gaussian components together to fit the originalvisibility data to ensure that the model best matches the data. Themodel fitting stopped when there was no visible feature brighter than5 𝜎 appearing in the residual map. The archive data was mostly fromastrometric or geodetic observations carried out in snapshot mode.The on-source time ranged from 3.2 to 37.7 minutes, and the acquiredimages were of varying qualities (see Table 1). Therefore, we onlyfocused on these two bright components, and did not attempt to fit https://science.nrao.edu/observing/data-archive http://astrogeo.org/ maintained by L. Petrov. other weaker emission structures. We also used two elliptical Gaus-sian models to fit the visibility data and found consistency with thecircular Gaussian models. Based on previous VLBI observations (Ul-vestad et al. 1999b; Reynolds et al. 2009, 2017, 2020), the northeastcomponent is identified as the core, and the southwest component isa jet knot. The model-fitting results, including the flux density, size,radial separation with respect to the core and brightness temperatureare presented in Table 2.The uncertainties in the model fitting parameters were estimatedusing the relations given in Schinzel et al. (2012). The calculationswere based on the approximations introduced by Fomalont (1999)and were modified for the strong sidelobes induced by incompleteVLBI ( 𝑢, 𝑣 ) coverage. This modified error estimate (Schinzel et al.2012) was suitable for the current data obtained from snapshot ob-servations and thus had large sidelobes. When the fitted Gaussiansize 𝑑 of a component was smaller than the minimum resolvable size 𝑑 min of a given interferometer, 𝑑 min was used in place of 𝑑 . 𝑑 min canbe estimated using the equation given by Lobanov (2005): 𝑑 min = − 𝛽 𝜋 (cid:34) 𝜋𝑎𝑏 ln 2 ln (cid:32) SNRSNR − (cid:33)(cid:35) / (1)where 𝑎 , 𝑏 are the major and minor axes sizes of the point spreadfunction (the synthesised beam), SNR is the signal-to-noise ratio ofthe component’s peak brightness to the image noise, 𝛽 is the power-law index describing the visibility data weighting, i.e., 𝛽 = 𝛽 = Figure 1 shows the 8-GHz VLBI images of Mrk 231 observed on1999 March 8 and 2018 December 4. The images of other epochsshow a similar morphology but a lower quality and thus are not shownhere. The image parameters are presented in Table 1. The emissionstructure has a largest extent of about two mas (corresponding to aprojected size of about 1.6 pc) along the northeast–southwest direc-tion, consistent with the previous VLBI observational results (e.g.,Ulvestad et al. 1999b; Reynolds et al. 2017). The northeastern com-ponent was identified as the core in previous studies (Ulvestad et al.1999b; Reynolds et al. 2009). We use the same nomenclature in thispaper. In the 8-GHz images, the core is weaker but more compactthan the jet component (see Table 2); however, the core is brighterat higher frequencies ( ≥
15 GHz), especially during flaring periods(e.g., Reynolds et al. 2017)).The jet component’s peak emission in the 8 GHz images is about1.2 mas ( ∼ MNRAS000
15 GHz), especially during flaring periods(e.g., Reynolds et al. 2017)).The jet component’s peak emission in the 8 GHz images is about1.2 mas ( ∼ MNRAS000 , 1–8 (2015) he obstructed jet in Mrk 231 Table 1.
Logs of the 8.4 GHz VLBI observations.Date Project Code Bandwidth On-source time Beam 𝜎 rms 𝑆 peak (yyyy-mm-dd) (MHz) (min) (maj, min, PA) (mJy beam − ) (mJy beam − )1994-08-12 BB023 16 3.9 1 . × . , 15 . ◦ . × . , 14 . ◦ . × . , − . ◦ . × . , 32 . ◦ . × . , 57 . ◦ . × . , 49 . ◦ . × . , − . ◦ . × . , 4 . ◦ . × . , 15 . ◦ rms noise and peak intensity in the image, respectively. Table 2.
Model fitting results. Time Comp. 𝑆 int R PA 𝜃 FWHM 𝑇 B (yyyy-mm-dd) (mJy) (mas) (deg) (mas) (10 K)(1) (2) (3) (4) (5) (6) (7)1994-08-12 J1 92 ±
14 1.086 ± − . ± . ± ± ± ± ± ± ± − . ± . ± ± ± ± > . ± ± − . ± . ± ± ± ± > . ± ± − . ± . ± ± ± ± > . ± ± − . ± . ± ± ± ± > . ±
12 1.154 ± − . ± . ± ± ± ± > . ± ± − . ± . ± ± ± ± > . ±
12 1.006 ± − . ± . ± ± ± ± > . ±
17 0.982 ± − . ± . ± ± ± ± > . and the core–jet separation (R) are plotted as a function of timein Figure 2. We find that the jet features are not easily describedwith a single component from the image. Instead, two groups of jetcomponents may be inferred: the components between 1994 and 2007have similar separation ( ∼ .
15 mas) and position angle ( ∼ − ◦ ),and are labelled as J1; other components between 2014 and 2019are labelled as J2 (see Table 2). Reynolds et al. (2017, 2020) alsodetected a jet component K1 at a similar distance and position anglewith J2 during the 2015 and 2017 flares. The authors assumed K1 is astationary component and determined the relative motion of the coretoward K1, 0 . 𝑐 . This motion is attributed to a newly discrete jetejection that also dominates the total nuclear flux density at 43 GHz.Determining this small displacement ( ∼ ∼ ∼
30 mas) away from the core at a position angle of ∼ − ◦ .Figure 1 displays the inner-parsec jet extending to the southwest di-rection at a position angle ∼ − ◦ . The misalignment between theinner 1-pc and the outer 25-pc southern jet is about 60 ◦ , and themean position angle of the VLBI jets is − ◦ . The jet geometryis consistent with that of the molecular outflow. The molecular CO(2–1) outflow shows a wide-angle biconical geometry with a sizeof ∼ − ◦ , in a good alignment with the central axis of the cone MNRAS , 1–8 (2015)
Wang et al. -5-4-3-2-1012345
Relative RA (mas) -5-4-3-2-1012345 R e l a t i v e D ec ( m a s ) Date: 1999-03-08rms = 0.26 mJy/beampeak = 86 mJy/beam
J1C J y / b e a m -5-4-3-2-1012345 Relative RA (mas) -5-4-3-2-1012345 R e l a t i v e D ec ( m a s ) Date: 2018-12-04rms = 0.15 mJy/beampeak = 288 mJy/beam
J2C J y / b e a m Figure 1.
VLBI images of Mrk 231 observed at 8 GHz on the epoch 1999March 8 and 2018 December 4. The images are created with natural weighting.The negative red coloured contour is at − 𝜎 level and the positive black-coloured contour levels are in the series of 3 𝜎 × (1, 2, 4, 8, 16, 32, 64), where 𝜎 is the rms noise in each image. The colour scale shows the intensity in thelogarithmic scale. cleaned by the jet. The radio jet seems to follow a twisted path withinthe conical cone and is prominent at the boundaries of the cone.Both the core and jet components show a compact morphology.The brightness temperatures (T B ) of the VLBI components are calcu-lated using the equation 2 of Condon et al. (1982). The core brightnesstemperature is remarkably higher than the jet brightness temperature(Table 2), reinforcing the core identification. We should note that, ex-cept for the first epoch, the core in the other epochs is unresolved; thesize ( 𝜃 FWHM ) is given as an upper limit, and thus the T B representsa lower limit. The core brightness temperature of Mrk 231 exceeds10 K, substantially higher than typical values for radio-quiet AGN(Ulvestad et al. 2005; Blundell et al. 1996). In the flaring state, thecore T B considerably increases, as also found in previous observa- tions. The highest T B > . × K is detected on 2018 October 4when the flux density is at the maximum among these datasets. Aneven higher value ∼ K was obtained in Reynolds et al. (2009)during the 2006 flare. These values approach the equipartition tem-perature limit for flat-spectrum radio-loud quasars (e.g., Homanet al. 2006), suggesting a blazar-like flare in Mrk 231 (Reynoldset al. 2013). The peculiar radio core properties (variability, bright-ness temperature) place Mrk 231 at a position between the radio-loudand radio-quiet AGN populations.
The bottom panel of Figure 2 shows the radial distance of the jetcomponent as a function of time. In addition to the archival AstrogeoVLBI data, we also added the published 8-GHz data points fromReynolds et al. (2009, 2017, 2020). We did not include the higherfrequency ( 𝜈 ≥
15 GHz) data since the frequency-dependent opacityis likely to induce systematic errors to the core positions. Comparedto the previous proper motions made by Reynolds et al. (2017, 2020),the 8-GHz data used here cover a more extended time baseline ofover 14 yr, allowing to determine (or constrain) the kinematics ofthe underlying jet flow. As we discussed early in Section 3.1, J1 andJ2 have distinctively different position angles, i.e., PA J1 = − ◦ ± ◦ , and PA J2 = − ◦ ± ◦ . If they are associated with the samecomponent, the resultant jet proper motion speed would be − . ± .
002 mas yr − ( − . ± . 𝑐 ). Negative proper motion lacks aphysical ground; therefore J1 and J2 must be different jet components.The historical light curve of Mrk 231 shows that the source is in anactive state after 2010 (Reynolds et al. 2020). Several prominent flaresare found, peaking in 2011.7, 2013.1, 2015.2 and 2017.9, roughlyin a two-year time interval. In contrast, it remained at a relativelylower level of activity from 1994 to 2010. The Mrk 231 jet consistsof a slowly moving, continuous background flow and discrete, fast-moving knots which are associated with large episodic flares. Thelatter might be short-lived and fade rapidly.The relative positions of K1 and J2 shows a large scattering (Fig-ure 2 bottom panel). The possible reason is that the newly-formedflaring jet component in epochs 2015 and 2017 changed the emis-sion structure in the core region. Since the new jet component is stillblended with the core, using the core peak position as the referencepoint would lead to a systematic error at an uncertain level. For thisreason, we do not attempt to calculate the proper motion of J2.We use the data points of J1 from 1994 to 2007 to estimate theproper motion. The linear regression fitting gives the proper motionspeed, 𝜇 J1 = . ± .
007 mas yr − (or 0 . ± . 𝑐 ). This smallproper motion value is consistent with those of many other radio-quiet AGN jets in magnitude and indicates a subluminal motion.We mention that J1’s apparent speed is substantially lower than theprevious measurements for the discrete jet knot (Reynolds et al. 2017,2020). Our proper motion measurement reinforces the idea that thejet components J1 and J2 detected in the low radio frequency imagesare associated with the background jet flow. They are distinctive fromthe superluminal jet knot ejected from the core in the flaring state. The complex jet structure from the inner pc to outer 25-pc scales in-dicates morphological and kinematic evolution. The sub-relativisticspeed of the background jet flow is suggestive of outflows drivenfrom the inner region, powered by the accretion disc winds (e.g.,Yang et al. 2021). This is especially the case for highly luminous
MNRAS000
MNRAS000 , 1–8 (2015) he obstructed jet in Mrk 231 Observing epoch (year) -120-118-116-114-112-110 P o s i t i o n A n g l e ( ° ) J1 J2K1
This workReynolds et al.(2017)Reynolds et al.(2020)
Observing epoch (year) R e l a t i v e p o s i t i o n ( m a s ) r = ( . ± . ) c J1 J2K1
This workReynolds et al.(2017)Reynolds et al.(2020)
Figure 2.
The jet position angle change with time (upper panel) and therelative jet distance with respect to the core versus the observing epoch(bottom panel). Symbols are as follows: blue circles – Astrogeo database(e.g., Petrov et al. 2008), light blue triangles – Ulvestad et al. (1999a), pinkhexagon – Reynolds et al. (2017), and green triangle – Reynolds et al. (2020).
AGN where the bolometric luminosity can be super-Eddington, in-dicative of a high accretion rate and hence a prominent accretion disc(e.g., Yang et al. 2020, 2021). The specific scenarios are elaboratedbelow.
As shown in Section 3.1, the position angle of the southern advancingjet has a difference of 60 ◦ from the inner parsec to the outer 25 par-secs. This large change may be enabled by a variety of mechanisms. • deflection of the jet flow caused by a collision between the jet anda massive cloud in the ISM. Such deflections may cause enhancedbrightness and polarisation at the sharp bending, especially if the density contrast between the jet and ISM is large (e.g. Jaiswal et al.2019; An et al. 2020). Besides the jet structure change, jet-cloudcollisions may also result in deceleration and distortion of the jetstructure, as has been seen in the radio-loud quasars 3C 43 (Cottonet al. 2003) and 3C 48 (Wilkinson et al. 1991; An et al. 2010). Suchdisruption effects should be even prominent in low-power radio jets(e.g., NGC 1068 (Muxlow et al. 1996) and NGC 4258 (Plante et al.1991)). However, the southern jet/lobe in Mrk 231 at 25 pc shows awell-defined shape after the presumed collision without any signatureof distortion (Ulvestad et al. 1999a). This scenario may thus not beoperational at these scales. • random wandering of the jet head over a large range of positionangles. It is possible that the jet impacts on a dense ISM producingan advancing shock which is observed as a radio lobe. The active jetcan be deflected by this action and encounter the external medium atdifferent sites until it finds an eventual lower-density location to breakthrough and proceed (e.g. Morganti et al. 2016). This wanderingbehaviour of the jet head is usually used to explain the observedappearance of multiple hotspots in the lobes of the Fanaroff-Rileytype II galaxies (e.g., Cygnus A, Williams & Gull 1985). However,the position angle difference ∼ ◦ is too large for the opening angleof a lobe, making this scenario less likely. • rotation of the jet body as influenced by the circum-nuclear disc.This may happen when the jet is not aligned with the rotation axisof the circum-nuclear disc. When the (low-power) jet advances intothis disc, a portion of the gas in the disc is entrained into the jetand increases the angular momentum in the perpendicular directionof the jet. The jet body is subject to the differential rotation of thedisc through the entrained external gas. This scenario seems possiblesince the tilted nuclear disc is indeed observed in Mrk 231; theOH megamaser emission (Klöckner et al. 2003) reveals a clockwiserotating torus or thick disc located between 30 and 100 pc from thenucleus. The disc axis points at a position angle of − ◦ , in a goodalignment with the axis of the cone traversed by the curved jet. Theobserved inner 1-pc and outer 25-pc radio jets coincidentally alignwith the boundaries between the cone cleaned by the jet and the disc;the mixture and entrainment of external gas into the jet flow at theboundary results in enhanced emission and bending of the jet bodyfollowing the rotation of the disc. Moreover, the Figure 2 upper panelhighlights the change of the inner pc-scale jet position angle from − ◦ ± ◦ to − ◦ ± ◦ in a time interval of ∼
25 year, providing ahint of the rotation. Continuous monitoring of the VLBI jet structureover a longer time scale would be essential to confirm a smoothand continuous change of the jet direction, as is expected from therotating jet model. • projection effect of a helical jet. The helical jet flow produces anS-shape in projection on the plane of the sky. The S-shaped structureis seen in nearby Seyfert galaxies (e.g., NGC 4258, Giroletti et al.2005) and in small-sized GHz-peaked spectrum (GPS) galaxies (e.g.,B0500+019, Stanghellini et al. 2001). The driving mechanism ofhelical jets may include a regular precession of the jet axis (e.g.,Lobanov & Roland 2005; Valtonen & Wiik 2012) near the baseenabled by a sub–pc-scale SMBH binary system (e.g., Begelman et al.1980; Mohan et al. 2016), a tilted or warped disc caused by a spinningblack hole (e.g., Bardeen & Petterson 1975; Pringle 1992; King et al.2005; Lu & Zhou 2005), magneto-hydrodynamic instabilities (e.g.,Hardee 1987), or due to developing disc-jet instabilities (e.g., Listeret al. 2003; An et al. 2010, 2013). For a simple ballistic motion ofthe precessing jet, the twisted trajectory is a projection of the jetknots ejected at various position angles (Linfield 1981). It is likelythat the component J1 (1994 – 2007), the component J2 (2014 –2019), and the 25-pc southern lobe, which are at different position MNRAS , 1–8 (2015)
Wang et al. angles, are indicative of such episodic behaviour. A SMBH binarymodel was proposed by Yan et al. (2015) to account for the deficitof near-UV continuum flux in Mrk 231. If the helical jet is driven bythe tidal interaction between the primary and secondary black holes,the inferred precession period is on the order of 10 yr and the massratio is less than 0.03, i.e. , an intermediate-mass black hole orbits theprimary 10 𝑀 (cid:12) one. However, the presence of binary black holeslacks convincing observational evidence (e.g., Veilleux et al. 2016;Leighly et al. 2016). Owing to the data used (PA variation) beingsparse and spanning a very short time span, a precession of the jetcan not be confirmed from the currently available VLBI data. .In summary, the first two models ( i.e. , deflection due to jet-cloudcollision, and random wandering of jet head) lack strong observa-tional support from the existing data. High sensitivity VLBI polari-metric observations of Mrk 231 are necessary to explore the locationof the jet bending and the polarisation structure to confirm or excludethese scenarios. Other mechanisms (jet rotation and helical jet) seempossible but warrant further observations and detailed studies. In ei-ther scenario, the interactions between the bending jet and the ISMresult in large-aperture-angle shocks that sweep the ISM within acone and expel the gas to move outward. This dynamic process hap-pens at the onset of the large-scale ionised and molecular outflowsand may participate in powering the outflows (see more discussionbelow). Mrk 231 lacks a large-scale radio jet structure beyond a few kpc(Carilli et al. 1998; Ulvestad et al. 1999a; Taylor et al. 1999; Mor-ganti et al. 2016). Although there is some extended emission a fewhundred mas south of the nucleus, those diffuse features might notbe directly related to the jet (Morganti et al. 2016). The compacttriple structure has a projected extent of ∼
50 mas ( ∼
40 pc) in thenorth-south direction. The southern lobe is brighter and longer (25pc in projection), suggesting it is associated with the advancing jet.The sub-kpc source size and symmetry morphology is reminiscentof a compact symmetric object (CSO). The radio power of Mrk 231is log 𝑃 . = .
15 W Hz − (Morganti et al. 2016), placing it inthe low-power CSO population (An & Baan 2012). The subluminaljet speed measured for the background jet in Mrk 231 is consistentwith its low-power nature. The proper motion vector component ofthe jet head in the south direction is 0 . 𝑐 . If we take this value asthe upper limit of the lobe expansion speed, we can estimate the ageof the radio structure to be (cid:38) 𝐿 j ) is composed of radiative power ( 𝐿 j , rad ) andkinetic power ( 𝐿 j , kin ). The study of (Foschini 2014) employs a sta-tistically viable sample of jetted AGN for a resulting empirical re-lationship between the constituent jet luminosities 𝐿 j , rad and 𝐿 j , kin and the core radio luminosity 𝐿 radio ,log 𝐿 j , rad = ( ± ) + ( . ± . ) log 𝐿 radio (2)log 𝐿 j , kin = ( ± ) + ( . ± . ) log 𝐿 radio . Owing to the small physical separation between the jet and core com-ponents, and the unresolved nature of the latter, the putative radiocore may be brighter. We then take the combined flux density of ≈
400 mJy during the 2018 epoch, an upper limit as representativeof that from the radio core. The corresponding radio luminosity isabout 1 . × erg s − , which converts to log 𝐿 j , rad = .
89 andlog 𝐿 j , kin = .
06 using the above equations and hence, a luminosityratio 𝐿 j , kin / 𝐿 j = 𝐿 j , kin /( 𝐿 j , kin + 𝐿 j , rad ) of 0.6. The balance of mo-mentum flux between the jet and external ISM on interaction and thedensity contrast can be cast in terms of the kinematic, geometric, andenergetic parameters governing the jet and ISM (e.g., Ferrari 1998). 𝐿 j ( 𝛽 𝑗 − 𝛽 ℎ ) 𝑐 = 𝜅𝜌 𝑗 𝛽 ℎ 𝑐 𝐴 ℎ (3) 𝜅 = (cid:18) 𝛽 𝑗 𝛽 ℎ − (cid:19) , (4)where 𝛽 𝑗 is the speed of the jet flow, 𝛽 ℎ is the speed of the advancingjet head at the working surface, and 𝐴 ℎ = 𝑟 ℎ sin 𝜃 is the surfacearea in contact at the working surface of size 𝑟 ℎ and for a jet halfopening angle 𝜃 , and 𝜅 is the density contrast between the externalISM 𝜌 ISM and the jet flow 𝜌 𝑗 (An et al. 2020). The density of thebaryonic jet is 𝜌 𝑗 = 𝐿 j , kin ( 𝜋𝑟 / ) 𝛽 𝑗 𝑐 Γ 𝑗 , (5)where 𝑟 is the jet extent and Lorentz factor Γ 𝑗 = ( − 𝛽 𝑗 ) − / . Theequations (3), (4) and (5) are used along with sin 𝜃 ≈ 𝑟 ℎ / 𝑟 to castthe density contrast as 𝜅 = (cid:169)(cid:173)(cid:171) − 𝜋 (cid:18) 𝐿 j , kin 𝐿 j (cid:19) / sin 𝜃 Γ / 𝑗 (cid:170)(cid:174)(cid:172) − , (6)which after some algebraic manipulation can be numerically solvedfor the density contrast 𝜅 as 𝐶 𝛽 ℎ 𝑥 + 𝐶 𝛽 ℎ 𝑥 + ( − 𝐶 + 𝐶 𝛽 ℎ ) 𝑥 + 𝑥 + 𝑥 + = , (7)where 𝐶 = (cid:16) 𝜋 𝐿 j , kin 𝐿 j (cid:17) sin 𝜃 and 𝑥 = 𝜅 / . The contrast ratio 𝜅 isnow a function of jet properties that can be directly inferred from theVLBI observations or constrained. The measured 𝛽 ℎ = . 𝑐 isused in the above equation to solve for 𝜅 = 𝜅 (cid:16) 𝐿 j , kin / 𝐿 j , 𝜃 (cid:17) and thecontours are plotted in Figure 3. The luminosity ratio is taken in therange 0 . − . . ◦ − ◦ for the numerical solution. We obtain 𝜅 in the range 1 . − . 𝜅 is larger for wider openingangles, consistent with the expectation for a weaker, relatively un-beamed jet/outflow.Comparing the jet radiative power and kinetic power shows that asubstantial fraction of the jet power is in the form of radiation. The jet-ISM interactions lead to a bulk of jet mechanical energy dissipationto the shocks, which in turn power and expel the large-scale nuclearoutflow winds/outflows. As a result, the advancing motion of the jetflow is likely to inhibit or slow down. Thus, in a rotating, clumpy,dense ISM environment such as in Mrk 231, the growth of the low-power radio source could be stopped at some distance, as predictedby the frustrated model for compact radio sources (van Breugel et al.1984; Bicknell et al. 1997; Snellen et al. 2000). Wide-angle outflows on sub-kpc to kpc scales are ubiquitous in radio-quiet quasars (Veilleux et al. 2005). Sustenance of the wide-angled
MNRAS000
MNRAS000 , 1–8 (2015) he obstructed jet in Mrk 231
24 6810 15 H a l f op e n i ng a ng l e ( d e g r ees ) Contours of density ratio
Figure 3.
Contours of the density contrast as a function of the luminosityratio 𝐿 j , kin / 𝐿 j and jet half opening angle 𝜃 . The vertical dashed line marks 𝐿 j , kin / 𝐿 j = . 𝜅 (cid:38) . outflows at large scales requires the AGN energy and momentum to betransferred into the ISM over a substantial volume through radiativeor mechanical processes (Rupke & Veilleux 2011; Sharma & Nath2012). Radio jets can enable this feedback and facilitate the expulsionof neutral and ionised gas from the nuclear region (Morganti et al.2005; Holt et al. 2009; Fu & Stockton 2009; Morganti 2020).Mrk 231 hosts powerful multi-phase outflows extending at multi-ple scales, including the nuclear ultra-fast X-ray winds, ionised gasoutflows arising from the broad-line region, and the galactic-scalemolecular and atomic gas outflows (Feruglio et al. 2015; Morgantiet al. 2016). The inferred jet flow speed in this work is slightly lowerthan the X-ray ultra-fast winds (up to 20000 km s − ). The inversecorrelation between the radio and far-UV luminosity in Mrk 231indicates a direct connection between the jet and the BAL winds(Reynolds et al. 2017).Unlike the radio-loud quasars which usually have collimated andhighly relativistic jet, the jets in radio-quiet quasars are less powerfuland uncollimated. The subrelativistic jets are often regarded as windsor outflows (Stocke et al. 1992). Large-scale (up to ∼
10 kpc) looselycollimated outflows are found to be ubiquitous in radio-quiet Type1 quasars, mostly along the minor axes of the host galaxies, andare thought to be quasar-driven (e.g., Rupke et al. 2017). Mrk 231sits between the radio-loud and radio-quiet quasars. It is one of thefew radio-quiet Type 1 quasars having prominent jets; another suchexample is PG 1700+518 which shows strong evidence of a jet-drivenoutflow on kpc scales (Yang et al. 2012; Runnoe et al. 2018). The PG1700+518 jet lies along with the outflows. Similarly, the Mrk 231 jetand the BAL ionised outflows exist on the same scale. The changeof the pc-scale jet structure over a broad range of directions resultsin advancing shocks that can participate in ionising and acceleratingthe BAL winds, and in the expulsion of gas at larger scales. Thetime-averaged kinetic energy flux of the jet is ∼ − erg s − in the quiescent state, and increases to ∼ erg s − during theflaring state (Reynolds et al. 2009), which accounts for (1–35)% ofthe energy rate of the molecular outflow (Feruglio et al. 2015). Theseclues suggest that the contribution of the jet to powering the outflowsis not negligible when Mrk 231 is in its flaring state. We analysed archive VLBI data of Mrk 231 and presented the milli-arcsecond resolution images that reveal the inner parsec jet structure.Comparing the published data with ours, we found a large misalign-ment ( ∼ ◦ ) between the inner pc jet and the outer 25-pc jet. A hintof jet position angle change is seen in the inner pc scale and warrantsnew observations to confirm. The jet structure can be explained by ascenario in which the jet lies in a large opening angle cone and ex-pands in various directions. The cone swept by the jet is likely to bephysically associated with the dusty disc revealed from the previousOH megamaser observations. The jet knots revealed in the 8-GHzVLBI images represent the active working surface of the jet headhitting on the external ISM. The entrainment of the gas from therotating circum-nuclear disc in the jet may result in the rotation ofthe jet body, or the jet itself follows a helical trajectory driven by jetprecession or other mechanisms. The expansion of the jet within thecone creates large-aperture-angle shocks that accelerate and expelmaterial from the nuclear region. The propagation of the jet energyand momentum outwards further powers the molecular and atomicgas outflows. Although the quasar radiation might be the dominantpower source of the AGN winds/outflows, the jet power accounts fora fraction of the energy rate of the molecular outflow in Mrk 231.Thus, the Mrk 231 jet’s contribution to the feedback to its nuclearenvironment, especially in the flaring state, may not be negligible.The jet direction change within the large-opening-angle cone causesthe time-averaged kinetic energy and momentum in a specific direc-tion to be significantly lower than a collimated jet flow. Moreover, asubstantial fraction of the jet power is dissipated in radiation. Con-sequently, the Mrk 231 jet is obstructed by the dense ISM within afew tens pc scale and fails to grow into a large-sized radio source. ACKNOWLEDGEMENTS
We thank the anonymous referee for her/his constructive commentswhich greatly improved the manuscript. This work is supported by theNational Key R&D Programme of China (2018YFA0404603) and theChinese Academy of Sciences (CAS, 114231KYSB20170003). S.J.is supported by the CAS-PIFI (grant No. 2020PM0057) postdoctoralfellowship. Y.C.W. thanks the hospitality of Shanghai AstronomicalObservatory during her summer internship. The authors acknowl-edge the use of Astrogeo Center database maintained by L. Petrov.The National Radio Astronomy Observatory are facilities of the Na-tional Science Foundation operated under cooperative agreement byAssociated Universities, Inc.
DATA AVAILABILITY
The datasets underlying this article were derived from the publicdomain in NRAO archive (project codes: BU013, BA080; https://science.nrao.edu/observing/data-archive ) and Astro-geo archive (project codes: BB023, RDV13, BG219D, UF001B,UG002U; http://astrogeo.org/ ). The calibrated visibility datacan be shared on reasonable request to the corresponding author.
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