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Deposited in DRO: 29 March 2018 Version of attached le: Accepted Version Peer-review status of attached le: Peer-reviewed Citation for published item: Hlavacek-Larrondo, J. and Gendron-Marsolais, M.-L. and Fecteau-Beaucage, D. and van Weeren, R. J. and Russell, H. R. and Edge, A. and Olamaie, M. and Rumsey, C. and King, L. and Fabian, A. C. and McNamara, B. and Hogan, M. and Mezcua, M. and Taylor, G. (2018) 'Mystery solved : discovery of extended radio emission in the merging galaxy cluster Abell 2146.', Monthly notices of the Royal Astronomical Society., 475 (2). pp. 2743-2753. Further information on publisher's website: https://doi.org/10.1093/mnras/stx3160

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Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk arXiv:1708.03641v2 [astro-ph.GA] 1 Dec 2017 o.Nt .Ato.Soc. Astron. R. Not. Mon. .Hlavacek-Larrondo J. 2146 Abell in cluster emission galaxy radio merging extended the of discovery solved: Mystery a Weeren van .C Fabian C. A. cetdXX eevdYY noiia omZZZ form original in YYY; Received XXX. Accepted

a ec eoiiso e huadk s km they thousand collide, few structures a of the velocities As reach groups. merge can and hierarchical the subclusters from of originate galaxies of Clusters INTRODUCTION 1 ⋆ c 1 10 9 8 7 6 5 4 3 2 ´ preetd hsqe nvri´ eMontréal, Montr Université de Physique, Département de eiee nttt o hoeia hsc,Wtro,ON Waterloo, Physics, Waterlo Theoretical of We for University 800 Institute Astronomy, Dallas, Perimeter and at Physics Texas of of Department University Imperi Physics, Cosmology(ICIC), of and Department Inference As for Experimental Centre Imperial for Centre Physics Battcock of Laboratory, Department Cavendish Astronomy, Madingle Extragalactic Cambridge, for of Centre University St Astronomy, Garden of 60 Institute Astrophysics, for Center Harvard-Smithsonian -al [email protected] E-mail: eateto hsc n srnm,Uiest fNwMexi New of University Astronomy, and Physics of Department 07RAS 2017 2 .R Russell R. H. , 3 .McNamara B. , 000 –3(07 rne eebr21 M L (MN 2017 December 5 Printed (2017) 1–13 , ytmi hc ohtebwadusra hc rnswr dete were was fronts It morphology. shock bullet-like upstream a and with bow sky the the (Mach both of wavelengths which plane in the system in merger ular elfrtefis ietepeec fa xedd( extended an of observation presence of the hours time 16 first totalling the 2146 for in veal Abell seen typically of 1 is observations multi-configuration as (VLA) cluster new, the present with We associated emission radio tended bl 16( 2146 Abell ABSTRACT r eetdtu a naycutr( cluster th any of r some in rad have radio a far structures a being thus detected as as newly detected consisted The classify ers is shock. we which bow which core the mu shock by subcluster harbour the upstream to with the appears associated with structure The associated 2146. one Abell with associated u eea ceeaino atce aito ehnss non mechanisms: radiation - particles of acceleration waves. - general uum P n ai ao nmrigcutr hudb eetdb uuerad future words: by Key detected (SKA). be Array should Kilometre clusters Square the merging that as in predict we halos observations, radio these extrem and cluste on detect to the Based VLA structures. of upgraded radio state the of dynamical capacity the the demonstrate with consistent passage), core sismrhlg,as ugs httehl a eetycreated recently was halo the that suggest also morphology, its as 1 ⋆ 1 . .L Gendron-Marsolais M.-L. , 4GHz , 3 relic .Edge A. , 8 − , 1 z 9 2 = .Hogan M. , n kinetic and aais lses bl 16-Xry:glxe:cutr ai c radio - clusters galaxies: X-rays: - 2146 Abell clusters: galaxies: 0 = . 2 . ± 3)i asv aaycutrcretyudron spectac- a undergoing currently cluster galaxy massive a is 232) ∼ e l CHC37 Canada 3J7, H3C éal, QC 0 2 Y,Canada 2Y5, N2L , od abig,C30A UK 0HA, CB3 Cambridge, Road, y . ) e epGR 2 H bevtosfie odtc ex- detect to failed observations MHz 325 GMRT deep yet 2), 4 uhmUiest,Dra,D13E UK 3LE, DH1 Durham, University, Durham , rpyis JTosnAeu,Cmrde B H,UK 0HE, CB3 Cambridge, Avenue, Thomson JJ trophysics, lClee rneCnotRa,Lno W A,U.K. 2AZ, SW7 London Road, Consort Prince College, al rs 2 et abig,M 23,USA 02138, MA Cambridge, reet, .Olamaie M. , ,Wtro,O,NL31 Canada 3G1, N2L ON, Waterloo, o, × tCmbl odRcado,Txs70032,USA 75080-3021, Texas Richardson, Road Campbell st 10 o luuru,N 73,USA 87131, NM Albuquerque, co, 23 8 inaogtepaeo h k snee,adhvn uha such having and needed, is sky the inclina of an plane detail, the in along phenomena these tion How- study turbulence. to and order 2004; shocks in pro- of ever, al. non-thermal generation and the et as 2006) Markevitch such al. 2004; cesses et al. Clowe 2006; et al. (Clowe Bradaˇc et eviden matter key dark including of processes, physical various studying nrisi xeso 10 of excess in energies .Mezcua M. , Hz W − P 1 1 − 1 .Teflxmaueeto h ao swell as halo, the of measurement flux The ). . .Fecteau-Beaucage D. , 4GHz H alG asyVr ag Array Large Very Jansky G. Karl GHz 2 5 A , T 6 E .Rumsey C. , , tl l v2.2) file style X halo 1 .Taylor G. , 64 2 = ≈ r.Te r da aoaoisfor laboratories ideal are They erg. 5 p) an ai structure radio faint kpc), 850 . 4 ± 0 . 2 aymr ai relics radio more many 5 l an n extended and faint ely .Teeobservations These r. × .King L. , 10 oetrdopow- radio lowest e 10 tpecomponents, ltiple .Teedt re- data These s. ( teml-shock - -thermal 23 ohl bounded halo io ≈ ofclte such facilities io tda X-ray at cted uhsystems. such 0 Hz W lcadone and elic 1 . y after Gyr 3 .J. R. , h first the 7 − , ontin- 1 and ce - 2 J. Hlavacek-Larrondo et al. favourable angle is rare. Until recently, only a handful of such this correlation. The cluster halos with weaker radio power clusters were known, including 1E 0657-55.8 (also known for a given X-ray luminosity are mostly thought to be the as the Bullet cluster; e.g. Markevitch et al. 2002), CIZA aged counterparts of those that follow the correlation (e.g. J2242.8+5301 (e.g. van Weeren et al. 2011), Abell 520 (e.g. Donnert et al. 2013). The non detection of a radio halo for Markevitch et al. 2005) and Abell 754 (e.g. Macario et al. A2146 by Russell et al. (2011) might therefore indicate that 2011). this cluster fits well into this aged population. However, We have discovered another such cluster, Abell 2146 this is inconsistent with the X-ray observations since the (z = 0.232, hereafter A2146; Russell et al. 2010, 2012). It is estimated age from the shock fronts puts the cluster at an undergoing a spectacular merger in the plane of the sky with evolutionary stage of 0.2 0.3 Gyr after core passage (see − a similar structure to the Bullet cluster. The Chandra image, Russell et al. 2010, 2012; White et al. 2015), similar to the shown here in the top-left panel of Fig. 1, reveals the rem- Bullet cluster, yet the Bullet cluster clearly sits along the nants of a cool core (i.e. the bullet) being stripped of its gas correlation. and forming a tail of material leading back to a separate con- Radio halos can also be produced by inelastic hadronic centration of gas 400 kpc away. More importantly, the X-ray collisions between cosmic-ray protons and thermal protons observations show that there are unambiguous density and (see e.g. Dennison 1980). In this scenario, the radio lumi- temperature jumps associated with two X-ray shock fronts nosities of radio halos should be around 10 times less than (Mach 2): one preceding the cool core (the bow shock), expected from re-acceleration models, providing a possible ∼ and another propagating outwards from the primary cluster explanation for the weaker radio power sources in the LX P − (the upstream shock). Clear observations of shock fronts are correlation (see also Brown et al. 2011). These models also extremely rare since they require the merger axis to be close predict that magnetic fields should be different in clusters to the plane of the sky so that projection effects do not smear with or without halos and that radio halo hosting clusters out the surface brightness edges (see also Markevitch et al. should be gamma-ray luminous (e.g. Jeltema & Profumo 2002, 2005; Macario et al. 2011; Owers et al. 2011). 2011). Both of these predictions have not been observed (e.g. A2146 also hosts a brightest cluster galaxy (BCG; la- Bonafede et al. 2011; Ackermann et al. 2014). belled A2146-A in Fig. 1) which is located 10 arcsec ( 40 Prior to A2146, all merging clusters of galaxies with X- ≈ kpc) behind the X-ray surface brightness peak, placing it ray detected shock fronts had large diffuse radio emission as- behind the bow shock front. This is unexpected because sociated with them. The non-detection in A2146 was there- cluster galaxies should be collisionless, as opposed to the fore puzzling and deeper radio observations were needed to hot intracluster gas which is slowed down by friction (e.g. confirm the lack of extended radio emission. In this pa- Markevitch & Vikhlinin 2001). In a merger, the galaxies are per, we present new, multi-configuration Karl G. Jansky therefore expected to preceed the hot X-ray gas. This is not Very Large Array (VLA) observations of A2146 at 1 2 − the case in A2146, and the location of the BCG is unlikely GHz. In Section 2, we describe the observations, and then due to projection effects since the observations of shock we present our main result in Section 3, the detection of fronts require the merger axis to be close to the plane of extended radio emission in A2146. In Section 4, we dis- the sky (see also White et al. 2015). Instead, Canning et al. cuss the implications of these results. Finally, in Section 5 (2012) and White et al. (2015) argue that it may be due to we state our conclusions. Throughout this paper, we adopt −1 −1 another galaxy perturbing the system, or that the merger is H0 = 70 km s Mpc with Ωm = 0.3, ΩΛ = 0.7, result- slightly off-axis. ing in a scaling of 3.698 kpc per arcsec at the redshift of the Using 5.7 hours of observations from the Giant Metre- cluster (z = 0.232). All errors are 1σ unless otherwise noted. Wave Telescope (GMRT) at 325 MHz, Russell et al. (2011) found no evidence of extended radio emission in A2146 down to a rms of 250 µJy beam−1. The only two radio sources detected coincided with A2146-A and a second cluster galaxy to the north (labelled as radio source in Fig. 1, see bottom-right panel). Diffuse, cluster-wide ra- 2 OBSERVATIONS AND DATA REDUCTION dio emission is often seen in merging systems, classified as either radio halos or radio relics depending on their 2.1 VLA observations morphology and polarisation properties (see for a review Brunetti & Jones 2014). Radio relics are elongated, polar- Four observations of A2146 were carried out in L-band on ized structures that often trace shock fronts where particles the VLA between 16 March 2012 and 27 January 2013 (PI are being re-accelerated (e.g. Ensslin et al. 1998). Radio ha- Hlavacek-Larrondo). Two of these were in C-array, one in los on the other hand are thought to originate naturally from B-array and one in D-array. The L-band covers a frequency shocks and turbulence generated during a merger. These range from 1.0 to 2.0 GHz, with two 512 MHz IF pairs, amplify the magnetic fields and can re-accelerate existing each comprising 8 subbands of 64 MHz. We used the default populations of particles. This in return provides a source integration time for the L-band (3 seconds for B-array and of non-thermal emission that can be seen at radio wave- 5 seconds for C-array and D-array). The data were recorded lengths (e.g. Brunetti et al. 2001; Petrosian 2001). Previous in 16 spectral windows, each subdivided into 64 channels. observations of cluster mergers have suggested a strong cor- Full stokes correlations were recorded for a total acquisition relation between the radio power of halos and the X-ray time of 16 hours. Details of each dataset can be found in luminosity of the cluster, known as the LX P correlation Table 1. − (e.g. Liang et al. 2000; Cassano et al. 2006). However, ob- Data reduction was achieved using CASA (v.4.6.0), fol- servations have shown that there is a bimodal population in lowing the procedure described in the L-band tutorial found

c 2017 RAS, MNRAS 000, 1–13 VLA Observations of Abell 2146 3

Table 1. Radio observations of the merging galaxy cluster A2146.

VLA B-array VLA C-array VLA D-array Combined VLA GMRT

Observing dates 2012 June 9 2012 March 16 2013 January 27 ... 2010 August 22 2012 April 22 On source time 2.4 hrs 7.4 hrs 1.3 hrs 11.1 hrs 5.7 hrs Correlations Full Stokes Full Stokes Full Stokes Full Stokes RR and LL Frequency 1 − 2 GHz 1 − 2 GHz 1 − 2 GHz 1 − 2 GHz 309 − 357 MHz Beam size 3.7” × 2.9” 9.9” × 7.7” 47.1” × 30.0” 3.9” × 3.2” 9.3” × 8.1” Beam PA −8.1o 48.5o 28.0o −3.7o 10.7o rms 11 µJy beam−1 11 µJy beam−1 64 µJy beam−1 11 µJy beam−1 250 µJy beam−1

Chandra 0.3-7.0 keV Subaru R-band

Upstream shock

Radio source

Subcluster core A2146-B

Plume

A2146-A A2146-A 1 arcmin 222 kpc Bow shock VLA 1-2 GHz image 325 MHz GMRT image

Radio source Radio source

A2146-B A2146-B

A2146-A Beam A2146-A Beam

Figure 1. All images are on the same scale. Top-left: Chandra image in the 0.3 − 7.0 keV band (418 ks) from Russell et al. (2012), smoothed with a gaussian function of 3 pixels. We highlight several features, including the two shock fronts and BCG A2146-A. Top- right: Subaru Suprime-Cam image in R band from White et al. (2015). Bottom-left: High-resolution 1 − 2 GHz VLA image, combining all datasets and conﬁgurations (see column 5 of Table 1 for details). The noise level in the vicinity of the cluster is σrms = 11µJy −1 beam , with a beam size of 3.9” × 3.2”. Contours levels are drawn at [1, 3, 9, 27,...] × 3σrms (8 levels in total). We highlight the location of the two BCGs (A2146-A and A2146-B), as well as the northern radio source which coincides with a member galaxy of the cluster. Bottom-right: GMRT 325 MHz radio image from Russell et al. (2011). Note the remaining artefacts surrounding the A2146-A and the northern radio source. The beams for the radio images are shown in red in the lower-left corners.

c 2017 RAS, MNRAS 000, 1–13 4 J. Hlavacek-Larrondo et al. on-line on the NRAO website1. Briefly, malfunctioning an- presented in detail in Russell et al. (2011). These observa- tennas as well as radio frequency interference (RFI) were tions were taken with the software backend correlator with flagged based on the observation logs and visual inspection a total bandwidth of 32 MHz divided between 512 channels. of the datasets. The data were then Hanning smoothed. Af- The data were reduced using AIPS and the deepest image ter a first bandpass correction, automative flagging routines constructed gave a rms noise far from any sources of 92µJy (TFCrop, RFlag) were applied. The data were calibrated beam−1, with beam size of 9.3′′ 8.1′′. However, the rms × − using the normal calibration tasks (setjy, delay and band- near the target goes up to 250 µJy beam 1 due to remain- pass calibrations, gain calibration and flux scaling the gain ing artefacts in the image. The bottom-right panel of Fig. solutions). The calibrations were then applied and the cor- 1 shows the resulting image at 325 MHz. The observations rected data were splitted. For all observations, 3C286 was are summarized in Table 1. Additional details regarding the used as the primary calibrator, while J1634+6245 was used data reduction can be found in Russell et al. (2011). as a phase calibrator. For each dataset, three rounds of phase-only self- calibration were performed in order to improve calibra- 2.3 Chandra X-ray observations tion accuracy. Due to the high number of radio point sources, the software PyBDSM (v.1.8.6 Mohan & Rafferty Throughout this paper, we also make use of Chandra ob- 2015) was used in order to create island-type masks, which servations of A2146. The details of these observations can were inspected and included in the clean process. For be found in Russell et al. (2012). Briefly, A2146 was ob- each of the imaging rounds during the self-calibration, we served with the ACIS-I detector for a total of 377 ks (Ob- used Briggs et al. (1996) weighting, multi-frequency syn- sIDs 10888, 12245, 13120, 12246, 13138, 12247, 13023, 13020 thesis (MS-MFS) with nterms=2 (Rau & Cornwell 2011), and 13021) and the ACIS-S detector for 39 ks (ObsID and 480 W-projection planes (Cornwell et al. 2005). We also 10464). The data were processed, cleaned and calibrated us- ciao ciaov4.3 caldb 4.4.0 used the robust factor set to 0.5 for these images. The re- ing ( , ), starting from the level 1 − sulting properties, for each configuration, are shown in Table event file. Charge transfer inefficiency and time-dependent 1. Note that the noise level (rms) shown for all datasets rep- gain corrections were applied and flares were removed using lc clean resents the rms located near the cluster. When considering the script. Datasets were reprojected to match a region located far from the cluster, the rms was typically a the ObsID 12245 dataset, combined and then exposure- factor 2 smaller and reached the thermal noise level. We also corrected to produce the image shown in the top-left panel note that the largest angular scales detectable are 8 ar- of Fig. 1. The net exposure time is 418 ks. ≈ cminutes for B-array, 70 arcminutes for C-array and 70 ≈ ≈ arcminutes for D-array. The final step consisted in merging the datasets to- 3 RESULTS gether, and producing a final combined (B-array, C-array, D-array) image of the cluster. First, we combined the two The bottom-left panel of Fig. 1 shows the deep 16-hour 1 2 − measurements sets in C-array to produce a final image in GHz VLA observations of A2146 at high-resolution. This im- that configuration. Here, the corrected column of each self- age was produced by oversampling the beam by a factor of at calibrated measurement set was split and then merged by least 3. It was therefore designed to maximise the resolution running a clean process. We then applied a long 10 minute and bring out point sources. This image reveals a multitude phase-only calibration and finally produced a combined im- of radio point sources in the vicinity of the cluster. As in age with clean. The extra calibration step allowed us to the GMRT 325 MHz image (lower-right panel of Fig. 1), we correct for mis-alignments between the datasets. A similar detect both A2146-A and the northern bright source, but we procedure was then applied to datasets from all 3 config- also detect A2146-B at a faint 3σrms level, where σrms is the urations (B-array, C-array and D-array), and a final, high- local noise level. In addition, both A2146-A and the north- resolution image was produced (shown in the lower-left panel ern bright source appear to be extended compared to the of Fig. 1). When producing this final image, we maximise the beam size (shown in the bottom corner of the bottom-left resolution (oversampling the beam by a factor of at least 3) panel of Fig. 1). and we use Briggs weighting, MS-MFS with nterms=2, In Fig. 2, we show the same VLA image, along with 480 W-projection planes and a robust factor of 0.5. The the Subaru R-band image, but we highlight in red and blue − resulting rms near the cluster of this combined image is 11 the 68 spectroscopically confirmed cluster members found by −1 −1 µJy beam , but goes down to 5 6µJy beam when mea- White et al. (2015), as well as those from the strong lens- − sured far from the source. Note that the final images were ing analysis by Coleman et al. (submitted). We identify all corrected for the primary beam. cluster members that have a 3σrms radio detection coincident with the galaxy center. Nine of the 68 spectroscopically confirmed cluster members are detected at this level (shown 2.2 GMRT observations with the blue regions), but we note that in addition to these 9 sources, several other galaxies are detected at a 2σrms level. GMRT continuum observations were obtained at 325 MHz In the discussion, we focuss only of the 3σrms detections. for A2146 on 22 August 2010 (5.7 hours on source), and are When producing the VLA images shown in Figs. 1 and 2, the parameters in the clean task were chosen such that it would maximise the resolution. However, when imaged indi- 1 https://casaguides.nrao.edu/index.php/Karl G. Jansky VLA vidually, both the VLA C-array and D-array datasets reveal Tutorials the presence of radio emission that extend well beyond the

c 2017 RAS, MNRAS 000, 1–13 VLA Observations of Abell 2146 5

VLA 1-2 GHz image Subaru R-band

S9 S9 S8 S8 S7 S7

S5 S6 S5 S6

S4 S4 S3 S3 S2 S2 S1 S1 1 arcmin Beam 222 kpc

Figure 2. Left: High-resolution 1 − 2 GHz VLA image (same as in Fig. 1). In blue and red, we highlight the 68 spectroscopically conﬁrmed cluster members found by White et al. (2015) and Coleman et al. (submitted), The blue regions indicate those that coincide −1 with a radio source detected above a 3σrms level (labelled S1 to S9 in the right panel), where σrms = 11µJy beam . Right: Subaru Suprime-Cam R-band image (same as in Fig. 1).

28:00.0 Low-resolution VLA 1-2 GHz image

26:00.0

24:00.0 Extended radio emission

22:00.0

66:20:00.0

18:00.0

16:00.0 4 arcmin Beam

14:00.0 888 kpc

30.0 57:00.0 30.0 15:56:00.0 30.0 55:00.0 54:30.0

Figure 3. Large-scale low-resolution 1 − 2 GHz image obtained with the VLA observations, combining all datasets and revealing the −1 presence of extended radio emission centered on A2146. The noise level is σrms = 12µJy beam . beam in these images, indicating the presence of an addi- array and D-rray), but at low-resolution with a beam size tional faint and extended radio component associated with of 14.1 13.5 arcsec2 and position angle (PA) of 27.5o. × − ′′ the cluster. This image was created by applying an outertaper of 15 , corresponding to a scale of 55 kpc, a robust factor set to To demonstrate the existence of this radio component, 0.25 and a cell size of 4′′. It highlights the presence of a in Fig. 3, we show the large-scale VLA image (B-array, C- −

c 2017 RAS, MNRAS 000, 1–13 6 J. Hlavacek-Larrondo et al.

Low-resolution VLA 1-2 GHz image Chandra temperature map Upstream shock

Extended radio emission

Beam 1 arcmin Plume Bow shock 222 kpc

2 4 6 8 10 12 14 Figure 4. Left: Same low-resolution 1 − 2 GHz image as in Fig. 3, but zoomed-in onto the cluster on the same scale as Fig. 2. The five −1 2 contours levels are drawn at [1, 2, 4,...] × 3σrms, where σrms ∼ 12µJy beam . The beam size is 14.1 × 13.5 arcsec . Right: Chandra projected temperature map from Russell et al. (2012). Same contours shown as in the left panel. The colour bar reflects the temperature in keV units. faint radio structure that extends on the scale of the cluster, the subcluster core (A2146-A) and the northern bright radio −1 detected above a 3σrms level where σrms 12µJy beam . source labelled as radio source in Fig. 1. At 325 MHz, the ∼ In Fig. 4, we show the same low-resolution VLA image, first has an integrated flux of S325MHz = 47 5 mJy, while ± but zoomed-in onto the cluster on the scale of Fig. 2. We the latter has an integrated flux of S325MHz = 93 9 mJy ± also show the projected temperature map from Russell et al. (Russell et al. 2011). (2012), created by extracting the temperature in binned re- The high-resolution 1 2 GHz image shown in Fig. 2 also − gions reaching a signal-to-noise ratio of 32 ( 1000 X-ray reveals the presence of A2146-A (S1−2GHz = 15.6 3.5 mJy) ≈ ± counts). and the northern bright radio source (S1−2GHz = 36.6 4.0 ± Finally, in Fig. 5 we show a point source subtracted mJy), in agreement with the values reported by the 1.4 GHz VLA image. Here, we first create a point source only im- NRAO VLA Sky Survey catalogue (NVSS; Condon et al. age by running a clean task and applying a uvrange cutoff 1998). However, our new image shows that both of these (>1kλ) while using a normal briggs robust factor set to sources are extended on scales of 37 kpc (10 arcsec; A2146- 0. We then subtract this measurement set from the orig- A) and 63 kpc (17 arcsec; northern radio source). inal dataset using the task uvsub. Finally, we image the Both of these sources are not detected in the VLA point source subtracted dataset while using a robust factor Low Frequency Sky Survey (VLSS, rms 0.1 Jy beam−1; ′′ ∼ set to 0, no inner uvrange cutoff and a uvtaper of 30 . In Cohen et al. 2007). However, the reprocessed 150 MHz im- all cases clean boxes were used. This image reveals that the ages obtained with the alternative TIFR GMRT Sky Sur- extended radio emission is actually composed of two compo- vey (TGSS2; Intema et al. 2017) detects both A2146-A nents, a first extended component associated with the sub- (S150MHz = 29 8 mJy) and the northern bright radio source ± cluster core and a second, narrower component associated (S150MHz = 183 21 mJy), although the first falls below the ± with the upstream shock. We also show the 0.3 7.0 keV catalogue detection limit. Interestingly, the TGSS 150 MHz − unsharp-masked Chandra image, created by subtracting the image also reveals that the northern radio source is extended X-ray images smoothed with a Gaussian σ of 5 arcsec and and surrounded by faint 2σ radio emission. We include the 20 arcsec, and then dividing the result by the sum of the values for the TGSS in Table 2. Finally, we include values at two. This X-ray image shows nicely different features, in 5 GHz in Table 2 for A2146-A and the northern source, based particular the two shock fronts as well as the plume to the on archival VLA observations (private communication, see south West. It also shows that the diffuse radio structures also Hogan et al. 2015). We note that the 408 MHz images are strongly bounded by the shock fronts. We discuss these taken with the Westerbork Northern Sky Survey (WENSS; features further in Section 4. Haslam et al. 1982) detects the two radio sources but barely resolves them. In Fig. 6, we show the spectral energy distribution of 4 DISCUSSION A2146-A (red points) and the northern bright source (blue points). The dashed lines show the expected flux density val- 4.1 Compact radio sources The original GMRT image at 325 MHz revealed the presence of two unresolved point sources: the BCG belonging to 2 http://tgssadr.strw.leidenuniv.nl/doku.php

c 2017 RAS, MNRAS 000, 1–13 VLA Observations of Abell 2146 7

Point source subtracted VLA 1-2 GHz image Chandra unsharp-masked

Radio relic Upstream shock

Radio halo

Beam

Plume 1 arcmin Bow shock 222 kpc Radio relic?

Figure 5. Left: Point source subtracted 1 − 2 GHz image on the same scale as Figs. 2 and 4. The 6 contours levels are drawn at −1 2 [3, 5, 7,...] × σrms, where σrms ∼ 23µJy beam . The beam size is 27.7 × 29.0 arcsec . Right: Unsharp-masked Chandra image in the 0.3 − 7.0 keV band.

Table 2. Radio emission from A2146 cluster members. The type refers to compact (C) or extended (E). We also indicate in column 6 the ﬂux density when detected at 325 MHz with the GMRT (Russell et al. 2011) and at 150 MHz with the TGSS.

Source RA DEC Type S1−2GHz (mJy) Notes S1 15:56:19.157 +66:20:25.860 C 0.26±0.05 S2 15:56:16.549 +66:20:33.260 C 0.18±0.03 S3 15:56:14.418 +66:20:40.350 C 0.05±0.01 S4 (A2146-A; BCG) 15:56:14.002 +66:20:53.220 E 15.6±3.5 S325MHz = 47 ± 5 mJy; S150MHz = 29 ± 8 mJy; S5GHz = 6.6 ± 0.3 mJy S5 15:56:02.166 +66:21:47.770 C 0.08±0.02 S6 15:55:55.224 +66:21:48.819 C 0.06±0.02 S7 (northern source) 15:56:03.933 +66:22:14.410 E 36.6±4.0 S325MHz = 93 ± 9 mJy; S150MHz = 183 ± 21 mJy; S5GHz = 8.9 ± 0.5 mJy S8 (A2146-B; BCG) 15:56:00.149 +66:22:23.940 C 0.08±0.02 S9 15:56:11.734 +66:22:37.690 C 0.6±0.1 ues for spectral indexes calculated between 325 MHz and 1.4 Larrondo et al. in prep). However, we note that A2146 ap- GHz for A2146-A (α = 0.7 0.2) and between 150 MHz and pears to harbour an unusually large population of radio ± 325 MHz values for for the northern source (α = 0.9 0.1). point sources (see King et al. in prep), which may indi- ± This figure highlights two important results, that the spec- cate that cluster mergers strongly affect their population trum of the northern source rolls over at high frequencies, of galaxies either through star formation or AGN emission indicative of synchrotron spectral ageing, and that the spec- that shines at radio wavelengths (e.g Miller & Owen 2003; trum of A2146-A turns over at 200 300 MHz. Such Poggianti et al. 2004). ∼ − a turnover is characteristic of a Compton Steep Spectrum (CSS) source, which are usually associated with powerful compact radio sources in their early stages of formation. We 4.2 Detection of extended radio emission note that O’Dea (1998) report a correlation between fre- The original GMRT image at 325 MHz did not reveal quency turnover and linear projected size for CSS and gi- the presence of a radio halo. This is not surprising con- gahertz peaked-spectrum (GPS). For a turnover frequency sidering that the GMRT is known to have problems in of 200 300 MHz, the correlation predicts that A2146-A ∼ − achieving high dynamic range images. Indeed, remaining should extend no more than 30 40 kpc, which is just ≈ − artefacts in the form of ripples surrounding the two point within the linear sizes we find for A2146-A. sources could be seen in the original image (see lower-right The VLA image shown in Fig. 2 also reveals the pres- panel of Fig. 1), which increased the noise level to 250 ence of several other radio point sources not previously µJy per beam in the inner regions. For a 1 Mpc source, known. Nine of these radio sources coincide with known clus- Russell et al. (2011) found that the 3σ upper limit at 325 ter members (labelled S1 to S9 in Fig. 2), as identified by MHz for extended radio emission was 13 mJy determined White et al. (2015) and Coleman et al. (submitted). The ra- as 3 rms source area/beam area. Assuming a spectral × × p −α dio fluxes of these galaxies are shown in Table 2. The only ra- index of α = 1 defined such that Sν ν , Russell et al. ∝ dio source that is also clearly detected at X-ray wavelengths (2011) further estimated that the 3σ upper limit was 3 mJy is A2146-A (see Fig. 1). ( 5.0 1023 W Hz−1) at 1.4 GHz. ∼ × A detailed analysis of the population of radio sources Our new radio image reaches a noise level of 10µJy −1 ∼ in A2146 will be conducted in a future paper (Hlavacek- beam , which is an order of magnitude deeper than the

c 2017 RAS, MNRAS 000, 1–13 8 J. Hlavacek-Larrondo et al.

from van Weeren et al. (2017), the fossil electrons from this relic may have originated from the bright northern radio source in A2146 (labelled as S7 in Fig. 2), but higher resolution images are needed to confirm this association. The origin of the south-east radio component is however more uncertain. Figs. 1 and 4 show that A2146 harbours a cool core that has survived the merger. This cool core also appears to coincide with the south-east radio component, implying that the radio emission seen to the south-east could originate from a classical mini-halo associated with the cool core. However, given the clear nature of the merger in A2146, the extended morphology of the south-east radio component ( 300 kpc) and the fact that many halos (as opposed to ∼ mini-halos) appear to align with their subcluster cores, we conclude that the south-east radio component is most likely not associated with a mini-halo. Instead, we argue that it is most likely associated with a radio halo either bounded Figure 6. Left: Shown are the flux densities of the two bright by the bow shock (as often seen, e.g Shimwell et al. 2014) or radio sources in A2146: A2146-A (red points) and the northern composed of a halo and a second relic that coincides with the bright source (blue points). The dashed lines highlight the ex- bow shock (e.g van Weeren et al. 2010). It could also con- pected fluxes when extrapolating from the measurements at 325 sist of an unusual large box-like relic, such as the one seen MHz and 1.4 GHz for A2146-A and from 150 MHz and 325 MHz in A2256 (Owen et al. 2014), but this would imply that the for the northern bright source. second relic extends significantly in the downstream region of the shock front given that A2146 is undergoing a merger in the plane of the sky. We therefore favour the interpreta- GMRT image. This has allowed us to discover that A2146 tion in which the south-east radio structure is composed of a hosts a large extended radio structure that may harbour halo or a halo and relic. Since relics are known to be highly- multiple components, one associated with the subcluster polarized compared to halos, resolved polarization measure- core to the south-east and another associated with the up- ments could provide valuable information, but we note that stream shock to the north-west. As shown in Figs. 4 and the intrumental set-up of the VLA observations were not 5, the radio emission is centrally located within the cluster, designed to do these measurements. The resolution of the has a morphology that extends along the merger axis of the current VLA images are also not sufficient to determine if merger (north-west to south-east) with no clear association the radio structure seen to the south-east actually habours to individual galaxies. Considering both components, it has multiple components. In summary, future observations will a largest linear size of 850 kpc and sharp edges that co- be required to determine the precise nature of the south- ∼ incide with both the bow and upstream X-ray shock fronts. eastern component. For now, given its morphology and as- Such edges have also been observed in other merging clusters sociation with the subcluster core, we consider this structure including the Bullet cluster (see Shimwell et al. 2014). to be a radio halo. We note that the literature has reported the existence of As shown in Fig. 5, the radio halo (south-eastern com- several elongated, highly-polarized arc-like radio structures ponent) and relic (north-western component) are very faint known as radio relics (see for a review Brunetti & Jones and were only detectable through the very deep VLA im- 2014). These relics are thought to trace the outward go- ages. To estimate the radio powers, we measure the to- ing shock fronts produced by the cluster mergers (e.g. tal emission detected above 3σrms in Fig. 5. We find that Ensslin et al. 1998, and references therein). One possible the halo has a flux density of S1.4GHz = 1.5 mJy, whereas solution involves Diffusive Shock Acceleration (DSA) in the relic has a flux density of S1.4GHz = 1.1 mJy. We which electrons from the thermal pool of particles go then compute the k corrected radio power using P1.4GHz = 2 −−(−α+1) through several collisions across the shock fronts (e.g. Drury 4πS1.4GHzDL(1 + z) (see van Weeren et al. 2014). 1983). However, DSA is known to have several issues and For the halo, we use the derived spectral index of α 1 −α ≈ where S ν , see below, but stress that even if we used van Weeren et al. (2017) recently argued that relics origi- ∝ a value of α 2, the flux would only change at the 20 per nate from the re-acceleration of fossil relativistic electrons ≈ from past active galactic nuclei outflows, based on the dis- cent level. For the relic, we use the derived spectral index covery of a direct connection between a radio relic and a of α 2. Following Cassano et al. (2013), we estimate the ≈ radio galaxy in the merging galaxy cluster Abell 3411-3412. error on the flux density, and thus the radio power, as Several clusters with X-ray detected shock fronts host such σ = (σ S )2 + (rms√N )2 + σ2 (1) relics coincident with the shocks (e.g. Macario et al. 2011; SHalo q cal halo beam sub Ogrean & Bruggen¨ 2013). which considers the uncertainties on the flux density scale In the case of A2146, the elongated morphology of the (σcal), typically on the order of 5 per cent, and the noise level north-west radio component, as well as its strong association of the image (rms= 23µJy beam−1) weighed by the number with the upstream shock as shown in Figs. 4 and 5, leads us of beams within the structures. We do not consider σsub, to surmise that this structure is most likely a radio relic the uncertainty when removing the point sources, since the (Markevitch 2010; Macario et al. 2011; van Weeren et al. point sources have already been removed in Fig. 5. Overall, 2016). Based on our current understanding of radio relics we find that the point-source subtracted k corrected radio − c 2017 RAS, MNRAS 000, 1–13 VLA Observations of Abell 2146 9 power of the halo (south-eastern component) is P1.4GHz = 2.4 0.2 1023 W Hz−1 and for the relic (north-western ± × 23 −1 component) its P1.4GHz = 2.2 0.2 10 W Hz . ± × A precise calculation of the spectral indexes can be determined when radio structures have been detected at multiple frequencies (see Thierbach et al. 2003 for the Coma cluster; Komissarov & Gubanov for A1914; Bacchi et al. for A754; Feretti et al. for A2319). In the case of A2146, we have only detected the extended radio emission at 1 2 GHz and − can therefore only obtain a rough estimate of the spectral indexes based on this image. In this case, we use the point source subtracted image of Fig. 5 and we only consider data points detected above 3σrms, where σrms is the local noise level. We estimate α by dividing the total integrated flux of the second Taylor term image (.tt1 image produced with the clean task) with the the total integrated flux of the total-intensity image (.tt0 image). This allows us to obtain Figure 7. Spectral index measurements (α) of radio halos, taken an estimate of the average spectral index integrated over the from Feretti et al. (2012); Venturi et al. (2013); Bonafede et al. entire radio structure. We find a value of α = 1.2 0.1 for (2014a, 2014b); Knowles et al. (2016). The dark grey area shows ± the halo (south-east component) and α = 2.3 0.3 for the the USSRH sources with α> 1.6. Also highlighted is the value of ± relic (north-west component), as well as an average value of α in A2146, estimated to be roughly 1.2 ± 0.1 based on the VLA α = 1.7 0.1 when considering both components. We note 1 − 2 GHz image. ± that the uncertainty quoted on these values reflects only the uncertainty of the fluxes measured via the .tt1 and .tt0 images. Considering that we are estimating the spectral index We also note that the non-detection of the halo at 325 MHz based only on the detection of the halo in a very narrow with the GMRT also implies that the halo must have a low frequency range 1 2 GHz, the true uncertainty is likely value of the spectral index, roughly α 1.0. − ≈ significantly higher. We therefore stress that these are only Donnert et al. (2013) argue that the properties of ha- rough estimates and that a detection at other frequencies is los in clusters of galaxies vary depending on the stage of needed to further constrain the spectral index measurements the merger. Based on high-resolution magnetohydrodynamic 15 of the radio emission in A2146. simulations of a 1.5 10 M⊙ cluster with a mass ratio of × Nonetheless, the derived spectral index of the relic from 1 : 8, the authors found that both X-ray and radio lumi- the VLA 1 2 GHz image (α 2.3) is roughly consistent nosities increase during the infall phase (< 0.1 Gyr after − ≈ with the expected injection spectral index (αinj) predicted core passage), and that radio emission is localized within the by DSA, where shock fronts. Roughly 0.1 0.5 Gyr after core passage, the ≈ − 2 + 1 1 X-ray emission declines due to the decrease in density, while αinj = M . (2) the radio emission continues to increase as turbulence drives 2 1 − 2 M − the re-acceleration of particules throughout the cluster vol- Indeed, based on deep Chandra observations, Russell et al. ume. Finally, > 0.5 Gyr after core passage, the radio emis- (2012) estimate that the Mach number for the upstream sion fades significantly and becomes offset from the primary shock is = 1.6 0.1, which would imply that αinj 1.8. M ± ≈ core of the merger, essentially entering a radio-off state. In However, we stress once more that a better constraint on addition to variations of the radio intensity, Donnert et al. the spectral index by detecting the relic at other frequencies (2013) predict that the spectral index of the halo will vary beyond 1 2 GHz is needed to test this further. with time, from flat spectra early on (α 1.0), down to very − ≈ steep spectra at late times (α> 1.5). In the case of A2146, Fig. 5 shows that the halo is 4.3 Properties of the radio halo strongly confined within the bow shock front such that the In Fig. 7, we show a compilation of radio halo spectral in- merger-induced turbulence has not yet reached the entire dex measurements from the literature (Feretti et al. 2012; cluster. We note in particular that the halo appears to be Venturi et al. 2013; Bonafede et al. 2014a, 2014b; Knowles more extended along the eastern direction, even though the et al. 2016). The values are total spectral index values for bow shock extends to the west, see Fig. 5. This is intriguing straight spectra, or low frequency spectral index values for given that the temperature map in Fig. 4 shows that there curved spectra. As shown, most halos have spectral index is more mixing (i.e. turbulence) on the eastern side of the measurements of α 1 1.5, but some, known as ultrasteep bow shock, as opposed to the western side. The halo may ≈ − spectrum radio halos (USSRHs) and shown in dark grey in also extend to the southern X-ray plume. Fig. 7 have α 1.6 1.9. These are thought to be the In addition, the radio power of the halo is among the ≈ − 23 −1 older counterparts of halos, in which the population of elec- lowest detected thus far (P1.4GHz = 2.4 0.2 10 W Hz ; ± × trons has aged, causing the spectra to steepen even further e.g. Knowles et al. 2016), and our rough estimate of the (e.g. Venturi et al. 2013). The value derived for the spec- spectral index based on the VLA image is consistent with the tral index of the halo (south-eastern component) in A2146 younger population of halos (α< 1.5). All of these observa- (roughly α1−2GHz 1.2; shown in red in Fig. 7) does not tions are consistent with the halo being in a very early stage ≈ appear to be consistent with the aged population of halos. of formation, roughly 0.3 Gyr after core passage according ≈ c 2017 RAS, MNRAS 000, 1–13 10 J. Hlavacek-Larrondo et al. to the simulations of Donnert et al. (2013), although A2146 plex dynamical state, varies significantly from one study 14 is thought to be a merger with a much smaller mass ratio of to the next: from M500 3.8 10 M⊙ for an SZ esti- ≈ × 4 3 : 1 (Russell et al. 2010; White et al. 2015) compared to mate with the Planck satellite (Planck Collaboration et al. − +4.3 14 the cluster in the simulations. Interestingly, this falls within 2014), to M200 = 8.5 10 M⊙ based on a dynami- −4.7 × the age estimate obtained by White et al. (2015), who used cal analysis of the cluster members (White et al. 2015), to +0.3 15 spectroscopic measurements of cluster members to establish M200 = 1.4 10 M⊙ for a Hubble Space Telescope −0.4 × the dynamical state of A2146. Based on a two-body dynam- weak lensing analysis (King et al. 2016). Instead, we focus ical model, the authors estimated that the time-scale after on the X-ray luminosity and integrated SZ measurement ob- first core passage must be on the order of 0.24 0.28 Gyr. tained with the Planck satellite (Y500 = 0.000032 0.000006 ≈ − ± We therefore conclude that the radio halo in A2146 is not Mpc2), which is consistent with the value obtained with the only among the faintest ever detected, but that its proper- Arcminute Microkelvin Imager (AMI; Y500 = 0.000029 2 ± ties are also consistent with the halo being in its early stages 0.000014 Mpc , private communication; see also Zwart et of formation. This makes the halo in A2146 extremely inter- al. 2008, Rodriguez-Gonzalvez et al. 2011). The left panel of esting to study, since it allows us to probe the complete Fig. 8 essentially shows that the halo of A2146 is indeed one evolutionary stages of halos. of the faintest detected thus far. It also appears to be consistent with the P1.4GHz Y500 correlation, although offset − from the P1.4GHz L500 correlation (however, we note that − 4.4 Implications for radio halos the X-ray luminosity of the cluster might be boosted during Radio halos are not ubiquitous in galaxy clusters. Cur- the merger phase). We also note that, as argued in Section rently, the literature reports the existence of 40 halos 4.2, if the halo (south-east component seen in Fig. 5) were ≈ (Giacintucci et al. 2017), all of which are identified in mas- actually composed of a halo and a relic, the derived radio 14 sive clusters (M >> 10 M⊙). Halos are thought to orig- power of the halo would be even smaller and A2146 would inate from the re-acceleration of in situ relativistic parti- sit even further below the correlations, still making it one of cles by turbulence generated from the cluster merger (e.g. the faintest ever detected. Liang et al. 2000; Cassano et al. 2006). Radio halos may also More recently, Eckert et al. (2017) derived the veloc- be produced by inelastic hadronic collisions between cosmic- ity dispersion of 51 clusters based on the amplitude of den- ray protons and thermal protons (see e.g. Dennison 1980), sity fluctuations, and found that the turbulent velocity of but this scenario is less favoured because of the lack of pre- the cluster with radio halos was on average a factor of two dicted gamma-ray emission and the existence of USSRHs. higher than those without halos. This result supports the For the latter, the hadronic models predict that the rela- scenario in which halos are driven by turbulent motions. tivistic energy would exceed the thermal energy, which is They also find a strong correlation between radio halo power implausible. at 1.4 GHz (P1.4GHz) and the turbulent velocity (σv). These Radio haloes are known to exhibit empirical correlations authors include A2146 in their study, for which they find −1 between radio halo power and various thermal cluster prop- that σv = 319 5 km s . The detection of the candidate ± 23 −1 halo in A2146 with P1.4GHz = 2.4 0.2 10 W Hz erties, including X-ray luminosity, gas temperature, cluster ± × mass and the cluster integrated Sunyaev-Zel’dovich (SZ) sig- places A2146 within the scatter of the correlation observed nal (see Cassano et al. 2013, for a recent re-visitation of the by Eckert et al. (2017), see right panel of Fig. 8. relations). Several studies confirm that the correlations exhibit a bimodal distribution, with the X-ray luminous or massive clusters either falling on the correlation or a factor 5 CONCLUDING REMARKS 2 3 below, known as the radio-off state (Venturi et al. 2007, − 2008). In the case of A2146, because of the non-detection of We have detected a diffuse, radio structure associated with the halo in A2146 with the GMRT, Russell et al. (2011) ar- the merging cluster A2146 using deep, multi-configuration gued that A2146 may not be massive enough to generate a VLA observations at 1 2 GHz (16 hours). The radio struc- − 23 −1 luminous halo, explaining why the cluster falls well below ture is faint (P1.4GHz,total = 4.6 0.3 10 W Hz ), elon- ± × the correlation even if strongly merging. Low-mass systems gated along the merger axis and strongly confined between are expected to generate less turbulence, producing less lu- both the bow and upstream X-ray shock fronts. This struc- minous halos. If this is the case, radio halos might therefore ture extends over 850 kpc in length and appears to har- be significantly more numerous than thought, but our cur- bour multiple components, one associated with the upstream rent view is limited by the sensitivity of radio telescopes (e.g. shock which we classify as a radio relic, and another associ- GMRT, VLA). ated with the subcluster core which is consistent as being a In the left panel of Fig. 8, we illustrate the loca- radio halo bounded by the bow shock. The latter is among tion of the halo in A2146 compared to other clusters for the faintest radio halos detected thus far in the bimodal the 1.4 GHz radio halo power versus integrated SZ signal distributions of radio halo power (P1.4GHz) versus thermal (Y500) within a radius at which the average density is 500 cluster properties (Y500,L500, M500). We note that the halo times the critical density at the cluster redshift (R500) from could actually be composed of a halo and a second relic that Cassano et al. (2013). Although the X-ray luminosity of the coincides with the bow shock, but the current observations cluster might be boosted during the merger phase, we also do not allow us to distinguish between possibilities. Future show in Fig. 8 (middle-panel) the P1.4GHz L500 diagram. observations, in particular via polarization, could provide a − Here, we use the X-ray luminosity within R500 derived by definitive answer. However, if this were the case, than the Cassano et al. (2013). We do not show the P1.4GHz M500 radio halo would be even fainter. − correlation, since the mass estimate for A2146, given its com- The spectral index of the halo, derived from the 1 2 − c 2017 RAS, MNRAS 000, 1–13 VLA Observations of Abell 2146 11

Figure 8. Left: Correlation of the radio halo power at 1.4 GHz (P1.4GHz) as a function of the integrated SZ signal (Y500) of the cluster within R500 (Cassano et al. 2013). USSRHs lie to the right of the correlation. The shaded region shows the 95 per cent confidence level. Middle: Correlation of the radio halo power at 1.4 GHz (P1.4GHz) as a function of the X-ray luminosity (L500) of the cluster within R500, also from (Cassano et al. 2013). The shaded region also shows the 95 per cent confidence level. Right: Correlation of the radio halo power at 1.4 GHz (P1.4GHz) as a function of the turbulent velocity measured from the amplitude of density fluctuations (σv) for 51 clusters (Eckert et al. 2017). The shaded region shows the 68 per cent confidence region. The halo in A2146 is highlighted with the red star in all three panels.

GHz VLA image, as well as its non-detection at 325 MHz ACKNOWLEDGMENTS with the GMRT imply that the structure must have a low JHL is supported by NSERC through the discovery grant value of the spectral index, with α 1. The confinement ≈ and Canada Research Chair programs, as well as FRQNT of the halo within the X-ray shock fronts indicates that the through the new university researchers start up program. merger-induced turbulence has not yet reached the cluster MLGM is supported by NSERC through the NSERC Post- scale and that the halo is most likely in its early stages of graduate Scholarship Doctoral Program (PGSD). DFB is formation. The low spectral index and the location of A2146 supported by NSERC, through the summer internship pro- in the Y500 P1.4GHz correlation also suggest that this halo − gram. We thank Becky Canning for helpful discussions about is in its early stages of formation, roughly 0.3 Gyr after ≈ the point sources in the cluster, as well as the cluster mem- first core passage, consistent with the dynamical analysis of bers. the cluster members. Such low radio power halos provide a unique opportunity to study the physical processes that govern the initial stages of halo formation, making the study of faint radio halos just as important as the study of the REFERENCES luminous radio halos. Ackermann M., Ajello M., Albert A., Allafort A., et al. Given that the detection of the extended radio emission 2014, ApJ, 787, 18 in A2146 was only possible through deep VLA observations Bonafede A., Govoni F., Feretti L., Murgia M., Giovannini reaching a noise level of 10µJy beam−1, i.e. more than an G., Bruggen¨ M., 2011, A&A, 530, A24 order of magnitude deeper than previous observations, sev- Bonafede A., Intema H. T., Bruggen¨ M., Girardi M., Non- eral other radio halos are bound to be discovered with the ino M., Kantharia N., van Weeren R. J., Röttgering next generation of radio telescopes such as the Long Wave- H. J. A., 2014, ApJ, 785, 1 length Array (LWA; Taylor et al. 2012), the MWA (Lonsdale Bonafede A., Intema H. T., Bruggen¨ M., Russell H. R., et al. 2009; Bowman et al. 2013; Tingay et al. 2013), LO- Ogrean G., Basu K., Sommer M., van Weeren R. J., Cas- FAR (LOw Frequency ARray, van Haarlem et al. 2013) and sano R., Fabian A. C., Röttgering H. J. A., 2014, MNRAS, the upcoming SKA (Square Kilometer Array, Dewdney et 444, L44 al. 2013). Indeed, these telescopes will not only reach even BradaˇcM., Clowe D., Gonzalez A. H., Marshall P., Forman lower noise levels (e.g. SKA is predicted to reach flux den- W., Jones C., Markevitch M., Randall S., Schrabback T., sities as least 10 times fainter), but they will also provide Zaritsky D., 2006, ApJ, 652, 937 the good uv-coverage at short baselines needed to detect ex- Briggs H., Voges W., Bohringer H., Edge A. C., Huchra tended diffuse emission in clusters. This will be extremely J. P., Briel U. G., 1996, MNRAS, 281, 799 important in light of the faint halo and relic discovered in Brown S., Emerick A., Rudnick L., Brunetti G., 2011, A2146, and for any similar structures caught in their early ApJL, 740, L28 stages of formation. Brunetti G., Jones T. W., 2014, International Journal of In summary, our results highlight the need for addi- Modern Physics D, 23, 1430007 tional deep observations of clusters to better understand the Brunetti G., Setti G., Feretti L., Giovannini G., 2001, MN- faint end of the bimodal P1.4GHz versus thermal property RAS, 320, 365 correlations, and that many of the clusters in the upper- Canning R. E. A., Russell H. R., Hatch N. A., Fabian A. C., limit region may indeed host faint radio halos. Zabludoff A. I., Crawford C. S., King L. J., McNamara

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c 2017 RAS, MNRAS 000, 1–13 VLA Observations of Abell 2146 13

RAS, 453, 2718 Zwart J. T. L., Barker R. W., Biddulph P., Bly D., Boysen R. C., Brown A. R., Clementson C., Crofts M., et al. 2008, MNRAS, 391, 1545

This paper has been typeset from a TEX/ LATEX ﬁle prepared by the author.

c 2017 RAS, MNRAS 000, 1–13