Collimated synchrotron threads linking the radio lobes of ESO137-006
M. Ramatsoku, M. Murgia, V. Vacca, P. Serra, S. Makhathini, F. Govoni, O. Smirnov, L. A. L. Andati, E. de Blok, G. I. G. Józsa, P. Kamphuis, D. Kleiner, F. M. Maccagni, D. Cs. Molnár, A. J. T. Ramaila, K. Thorat, S.V. White
AAstronomy & Astrophysics manuscript no. AA_2020_37800 c (cid:13)
ESO 2020April 9, 2020 L etter to the E ditor Collimated synchrotron threads linking the radio lobes ofESO 137-006
M. Ramatsoku , (cid:63) , M. Murgia , V. Vacca , P. Serra , S. Makhathini , F. Govoni , O. Smirnov , , L. A. L. Andati , E.de Blok , , , G. I. G. Józsa , , , P. Kamphuis , D. Kleiner , F. M. Maccagni , D. Cs. Molnár , A. J. T. Ramaila , K.Thorat , and S.V. White . Department of Physics and Electronics, Rhodes University, PO Box 94, Makhanda, 6140, South Africa. INAF- Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy. South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, 7925, South Africa. Argelander-Institut fur Astronomie, Auf dem Hugel 71, D-53121 Bonn, Germany. ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands. Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa. Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands. Department of Physics, University of Pretoria, Hatfield, Pretoria, 0028, South Africa. Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany.Received 24 February 2020; Accepted 09 March 2020
ABSTRACT
We present MeerKAT 1000 MHz and 1400 MHz observations of a bright radio galaxy in the southern hemisphere, ESO 137-006. Thegalaxy lies at the centre of the massive and merging Norma galaxy cluster. The MeerKAT continuum images (rms ∼ / beamat ∼ (cid:48)(cid:48) resolution) reveal new features that have never been seen in a radio galaxy before: collimated synchrotron threads of yetunknown origin, which link the extended and bent radio lobes of ESO 137-006. The most prominent of these threads stretches inprojection for about 80 kpc and is about 1 kpc in width. The radio spectrum of the threads is steep, with a spectral index of up to α (cid:39) Key words. radio continuum: galaxies
1. Introduction
Radio galaxies residing in galaxy clusters exhibit a wide range ofdistorted morphologies (see Garon et al. 2019 for an overview).Some of these distortions are seen in the form of head tails,which are elongated radio sources with the galaxy at one end(Ryle & Windram 1968, Sebastian et al. 2017), and wide-angletails, where the jets form a ‘C’ shape (Owen & Rudnick 1976,Leahy 1993, Missaglia et al. 2019). Such radio morphologiesare due to the interaction between the radio lobes and / or jetsand the intra-cluster medium (ICM; Pinkney 1995, Sakelliou &Merrifield 2000). This makes radio galaxies important probes ofthe distribution of pressure, turbulence, and shocks within themagneto-ionic ICM (Owen et al. 2014, Feretti et al. 2012), andalso means that their detection is an e ffi cient way to identifyhigh-z clusters independent of dust extinction.In this letter we discuss the case of ESO 137-006, a radiogalaxy in the Norma galaxy cluster (Abell 3627; Abell et al.1989). Norma is located at a distance of ∼
70 Mpc ( z = ∼ M (cid:12) , Norma is characterised by numeroussubstructures and exhibits an elongated X-ray morphology, im-plying that it is not yet in dynamical equilibrium (Boehringeret al. 1996, Woudt et al. 2008). (cid:63) [email protected] ESO 137-006 (RA
J2000 = J2000 = − . = . × W Hz − ; Sun 2009). Observations at 408 MHz (Schilizzi &McAdam 1975), 843 MHz (Mauch et al. 2003), and 1400 MHz(Christiansen et al. 1977; Jones & McAdam 1996) show thatit has a wide-angle tail morphology. The bending of its radiolobes is thought to be caused by the galaxy infall towards themain cluster (Jones & McAdam 1996; Sakelliou & Merrifield2000). In this letter we present new radio continuum imagesof ESO 137-006 at ∼ ∼ Λ cold dark matter cos-mology with Ω M = . Λ Ω = . , and a Hubble constantof H =
70 km s − Mpc − . At the distance of ESO 137-006,1 (cid:48)(cid:48) corresponds to 0.33 kpc.
2. MeerKAT observations and data reduction
We observed the Norma cluster at radio frequencies withMeerKAT (Jonas & MeerKAT Team 2016; Mauch et al. 2019)in May 2019 (project ID SCI-20190418-SM-01). The observa-tions were conducted with all 64 MeerKAT antennas in L-band(856 – 1712 MHz) using the 4k mode of the SKARAB correla-
Article number, page 1 of 5 a r X i v : . [ a s t r o - ph . GA ] A p r & A proofs: manuscript no. AA_2020_37800 tor, which samples the observed band with 4096 channels, eachof 209 kHz in width, in full polarisation. The total integrationtime is 14 hours on target.We reduced the data in two frequency intervals largely freeof radio-frequency interference: 980-1080 MHz and 1356-1440MHz (hereafter referred to as 1030 MHz and 1398 MHz, respec-tively). The data reduction was conducted independently in thetwo intervals using the caracal pipeline currently under devel-opment . The pipeline is built using stimela (Makhathini 2018),a radio interferometry scripting framework based on Python andcontainer technologies. stimela allows users to run several open-source radio interferometry software packages in the same script.Using this pipeline, we flagged the calibrator data based on theStokes Q visibilities with AOF lagger (O ff ringa et al. 2010). Wedetermined the complex flux, bandpass, and gains using the casa (McMullin et al. 2007) tasks bandpass and gaincal , and ap-plied the calibration to the visibilities of the target with the casa task mstransform . The calibrated target visibilities were flaggedwith AOF lagger , again based on Stokes Q. We then iterativelyimaged the radio continuum emission with WS clean (O ff ringaet al. 2014) in Stokes I using multi-scale cleaning (O ff ringa &Smirnov 2017), and self-calibrated the gain phase with cubical (Kenyon et al. 2018) with a solution interval of 128 seconds.The imaging was done using Briggs robust value 0 and cleaningdown to 0.5 σ within a clean mask made with sofia (Serra et al.2015). Finally, we generated MeerKAT primary beam imagesat the mean frequency of the two processed bands using eidos (Asad et al. 2019), and created primary beam-corrected contin-uum images of the target.The resulting 1030 MHz image has a restoring beam of10 . (cid:48)(cid:48) × . (cid:48)(cid:48) FWHM with PA = ◦ , and rms noise level30.8 µ Jy beam − . The 1398 MHz image has a restoring beam of7 . (cid:48)(cid:48) × . (cid:48)(cid:48) FWHM with PA = ◦ , and rms noise level 20.8 µ Jybeam − .
3. ESO 137-006 as seen by MeerKAT
Figure 1 shows the 1030 MHz MeerKAT image of ESO 137-006.Emission shown in red to yellow is known from previous, shal-lower observations obtained with other telescopes (see Sec. 1).Emission shown in grey scale is revealed here for the first timedue to the increased sensitivity and resolution of the MeerKATdata. A number of new features are now apparent.Similar to earlier observations, we detect a point sourcecorresponding to the galaxy ESO 137-006 (Jones & McAdam1996). At the resolution of our images this source has a fluxdensity of 140 mJy at 1030 MHz and 167 mJy at 1398 MHz.The source is also seen in X-rays (see Fig. 3) and is thought tobe the core of ESO 137-006 (e.g., Jones & McAdam 1996). Nar-row jets start from the core along an east–west axis and formtwo broad, bright spots ∼ (cid:48) from it. Their peak flux densitiesare on average ∼ − at 1030 MHz and ∼
18 Jy beam − at1398 MHz. Jones & McAdam (1996) suggested that the bright-ening and expansion of the jets into these bright spots is due tothe decreased ISM pressure. The general structure of these brightspots (see also Fig. 2) is similar to that seen in the hydrodynam-ical simulations of jets in the FR I radio galaxy 3C 31, where thejets recollimated at ∼ https: // caracal.readthedocs.io https: // github.com / SpheMakh / Stimela
Further out, the jets flare into the lobes and then fade intodi ff use emission. On both sides, the radio emission bends south-wards; in the western lobe the bending occurs right after thebright spot while in the eastern lobe it occurs further out.Besides the previously known features described immedi-ately above, Fig. 1 shows additional, low-surface-brightness ra-dio emission at the edge of both lobes and, most strikingly, anumber of CSTs in the region south of the core. These extendfrom the lobes southward of the core. Figure 2 shows the gradi-ent image obtained with a Sobel convolution kernel that high-lights the presence of three threads: CST1, CST2, and CST3.These threads are reminiscent of twin jets originating from twonuclei associated with radio source 3C 75 (Owen et al. 1985), ex-cept for the fact that in the case of ESO 137-006 only one nucleusis seen. Figure 1 also shows a di ff use loop of emission whichstarts at the base of CST3, extends around the eastern lobe onthe south side, and connects back to the lobe on its easternmostside where it fades into di ff use emission. The origin of these fea-tures is not known. It is possible that these loops and CSTs arethe result of some electromagnetic e ff ect like parallel magneticfield lines within the ICM dragging particles between the lobesas the galaxy moves north, perhaps similar to the description byHeyvaerts & Norman (1989).The spectral index image ( S ν ∝ ν − α ) of ESO 137-006 be-tween 1030 and 1398 MHz is presented in the top panel of Fig. 3.We find a spectral index distribution typical of an active wide-angle tail radio source. The radio core has a flat spectral in-dex α (cid:39) α (cid:39) . − .
6. There is a steepening in the radio spectrum go-ing from the radio lobes down to the tails where we measurea spectral index as high as α (cid:39)
4. The spatial variation of thespectral index in the eastern lobe is more gradual than in thewestern lobe, where a sharp transition, likely due to projectione ff ects, is observed between the edge of the lobe and the underly-ing tail emission. The newly discovered CSTs are characterisedby a steep radio spectrum with α (cid:39)
4. Basic properties of CST1
Figure 4 shows the deconvolved full width half maximum(FWHM dec ) and the peak brightness (I peak ) as a function of po-sition along CST1 at 1030 and 1398 MHz, as well as the spec-tral index ( α ). We measure these quantities at the posi-tions shown by the red bars in the inset. The cross in the in-set corresponds to the origin of the plot’s horizontal axis. Er-rors were computed using a Monte Carlo simulation with in-put parameters sampled and noised in the same way as for thereal data. The errors represent the scatter of the input versusoutput best-fit parameters. The mean values of the peak bright-ness are respectively I peak , = (8 ±
1) mJy / beam andI peak , = (4 . ± .
6) mJy / beam, and the average spectralindex α = . ± .
3, consistent with the values shown inFig. 3.The peak brightness shows a similar profile at both frequen-cies, with a valley at ∼ (cid:48)(cid:48) at 1030 MHz only, which cor- Article number, page 2 of 5. Ramatsoku et al.: Collimated synchrotron threads linking the radio lobes of ESO 137-006
Fig. 1: Radio continuum emission from ESO 137-006 detected by MeerKAT at 1030 MHz. The top colour bar (yellow and redtones) represents the brightness of the brighter regions of the radio source in the range from 10 to 860 mJy beam − . The bottomcolour bar (grey tones) represents the brightness of the fainter plumes and filaments in the range from 10 down to -1.6 mJy beam − .The circular ∼ (cid:48)(cid:48) synthesised beam of the image is shown in the bottom left corner.responds to a depression in the spectral index profile. At thesame location, a rapid local increase of the deconvolved FWHMcan be observed at both frequencies. The mean width of CST1is FWHM dec , = (3 . ± . (cid:48)(cid:48) and FWHM dec , = (3 . ± . (cid:48)(cid:48) . The two profiles do not show any dependence onfrequency but are rather characterised by a similar average trendalong the full extension of the filament, consistent within thescatter. This projected width translates into 1 . ± . ∼
5. Discussion and summary
In this letter we present new MeerKAT images of the radiosource ESO 137-006 at 1000 and 1400 MHz. The galaxy lies atthe centre of the merging Norma cluster near the Great Attractor.Here we summarise our main findings: – With these sensitive MeerKAT observations, new featureshave been revealed in the form of multiple collimated syn- chrotron threads (CSTs) connecting the lobes of the radiogalaxy. It is worth noting that examples of filamentary struc-tures associated with radio galaxies are well known in theliterature. However, these filaments are usually observed in-side the radio lobes (see e.g. the notable cases of FornaxA and Cygnus A; Maccagni et al. 2020, Perley et al. 1984)and the tails of radio galaxies (e.g., NGC 1265, 3C 129, andNGC 326; Sijbring & de Bruyn 1998, Lane et al. 2002, Hard-castle et al. 2019). The CSTs detected in ESO 137-006 aredi ff erent in that they are observed outside the main body ofthe radio galaxy and connecting (at least in projection) thetwo radio lobes. The radio galaxy 3C 338 (Burns et al. 1983)at the centre of Abell 2199 presents a single filament that isreminiscent of one of the CSTs observed in ESO 137-006.However, while the filament in 3C 338 could be a relic jetfrom a past epoch of activity, this same interpretation doesnot hold for ESO 137-006 where we observe multiple close- Article number, page 3 of 5 & A proofs: manuscript no. AA_2020_37800
Fig. 2: Gradient image (Sobel convolution kernel) with the threemost prominent CSTs labelled.Fig. 3: Top panel: Spectral index image between 1030 and 1398MHz. Bottom panel: Radio continuum emission from ESO 137-006 detected by MeerKAT at 1030 MHz (green tones and con-tours) superimposed on the X-ray emission map from XMM-Newton at 0.5 - 2 keV. In both panels, radio contours refer tothe 1030 MHz image; they start at 0.5 mJy / beam and scale by afactor of two. Fig. 4: Top panel: Spectral index profile between 1030 and1398 MHz computed along the filament at the locations shownby the slices in the inset (see text for details). Middle panel: Peakbrightness at 1030 MHz (blue triangles) and at 1398 MHz (redtriangles) along the filament at the location of the slices. Bot-tom panel: Deconvolved FWHM at 1030 MHz (blue dots) andat 1398 MHz (red dots) along the filament at the location of theslices. The plotted error bars are comparable to the size of thedots.by threads formed at the same time (as suggested by theirsimilar spectral-index distributions). – The most prominent and straight of the CST in ESO 137-006 (CST 1) has a characteristic width of ∼ / beam at the 10 (cid:48)(cid:48) resolution of our images. The othertwo CSTs originate from the same point in the eastern lobe.CST 2 starts straight and then fades rapidly after ∼
25 kpcfrom the lobe. CST 3 seems to follow a faint, closed loopwith a radius of ∼
64 kpc, which reconnects with the lobe atits far end. The nature of these unusual features is unclear.We speculate that they could be due to the interaction of themagnetic fields of the radio lobes with the magneto-ionicICM, or caused by some sort of re-connection of filamentsassociated with the tails back into the radio lobes. Furtherobservations and theoretical e ff orts are required to clarify thenature of these newly discovered features. – The spectral index distribution observed across the jets,lobes, and tails is typical of an active radio source. The ra-dio spectrum of the CST is steep with α (cid:39)
2. Due to thissteep spectrum, deep low-frequency observations at high res-olution with instruments such as the LOw Frequency ARraycould play a role in the study of CSTs. – Whatever their origin, our findings pose the following ques-tions: How common are these features? Are CSTs specificto the case of ESO 137-006 and its environment, perhapsdue to the kinematics and pressure gradient of this ICM andrelative motion of the galaxy in the cluster? Or, on the con-trary, are CSTs common in radio galaxies but have so far not
Article number, page 4 of 5. Ramatsoku et al.: Collimated synchrotron threads linking the radio lobes of ESO 137-006 been detected due to sensitivity and resolution limits? If fu-ture observations confirm the latter hypothesis, understand-ing the nature and the physics of these features could opena new science case for the next generation of sensitive radiointerferometers like the Square Kilometre Array.
Acknowledgements.
We thank the referee, Manel Perucho for the useful com-ments and suggestions. The authors also wish to thank Ming Sun for helpingwith the access to some of the X-ray data. This project has received funding fromthe European Research Council (ERC) under the European Union’s Horizon2020 research and innovation programme grant agreement no. 679627 projectname FORNAX. MR acknowledges support from the Italian Ministry of For-eign A ff airs and International Cooperation (MAECI Grant Number ZA18GR02)and the South African Department of Science and Technology’s National Re-search Foundation (DST-NRF Grant Number 113121) as part of the ISARP RA-DIOSKY2020 Joint Research Scheme. This paper makes use of the MeerKATdata (Project ID: SCI-20190418-SM-01). The MeerKAT telescope is operatedby the South African Radio Astronomy Observatory, which is a facility of theNational Research Foundation, an agency of the Department of Science and In-novation. MR and SM’s research is supported by the SARAO HCD programmevia the "New Scientific Frontiers with Precision Radio Interferometry" researchgroup grant. This work is based upon research supported by the South AfricanResearch Chairs Initiative of the Department of Science and Technology and Na-tional Research Foundation. (Part of) the data published here have been reducedusing the CARAcal pipeline, partially supported by BMBF project 05A17PC2for D-MeerKAT. Information about CARAcal can be obtained online under the https://caracal.readthedocs.io/en/latest/ . References
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