The nature of the giant diffuse non-thermal source in the A3411-A3412 complex
G. Giovannini, V. Vacca, M. Girardi, L. Feretti, F. Govoni, M.Murgia
MMon. Not. R. Astron. Soc. , 1–7 (2013) Printed 12 November 2018 (MN L A TEX style file v2.2)
The nature of the giant di ff use non-thermal source in theA3411–A3412 complex G. Giovannini , (cid:63) , V. Vacca , , M. Girardi , L. Feretti , F. Govoni , M. Murgia Dipartimento di Fisica e Astronomia, via Ranzani 1, 40127 Bologna, I Istituto di Radioastronomia-INAF, via P.Gobetti 101, 40129 Bologna, Italy Dipartimento di Fisica-Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy Osservatorio Astronomico di Cagliari-INAF, Strada 54, Loc. Poggio dei Pini, 09012 Capoterra (Ca), Italy
ABSTRACT
VLA deep radio images at 1.4 GHz in total intensity and polarization reveal a di ff use non-thermal source in the interacting clusters A3411 – A3412. Moreover a small-size low powerradio halo at the center of the merging cluster A3411 is found. We present here new opticaland X-ray data and discuss the nature and properties of the di ff use non-thermal source. Wesuggest that the giant di ff use radio source is related to the presence of a large scale filamentarystructure and to multiple mergers in the A3411–A3412 complex. Key words:
Galaxies:cluster:non-thermal – Clusters: individual: Abell 3411, Abell 3412 –Cosmology: large-scale structure of the Universe
Clusters of galaxies are characterized by X-ray emission from a hotintra-cluster medium (ICM, T ∼ −
10 keV). Thermal emission is acommon property of all clusters of galaxies and has been detectedeven in poor groups as well as in optical filaments connecting richclusters.In some clusters the ICM is also characterized by non-thermaldi ff use emission: giant radio sources with a spatial extent similar tothat of the hot ICM, which are called radio halos or relics, accord-ing to their morphology and location in the cluster (see (Feretti etal. 2012), for a recent review). These sources are not directly as-sociated with the activity of individual galaxies and are related tophysical properties of the whole cluster. In a few cases a di ff use nonthermal emission is found also in filamentary structures (see e.g.,(Giovannini et al. 2010)), and in A399 & A401 we found two radiohalos in two interacting clusters ((Murgia et al. 2010)).In the last years, our knowledge of the non-thermal emissionin galaxy clusters and the properties of large scale magnetic fieldslargely increased thanks to the discoveries of new halo and relicsources and the large development in simulations and theoreticalmodels (see e.g., (Feretti et al. 2012), (Govoni et al. 2013), (Venturiet al. 2013), (Donnert et al. 2013), (Bonafede et al. 2012), (VanWeeren et al. 2012)).There is a substantial evidence that radio halos are foundin clusters showing significant substructures in X-ray images andcomplex gas temperature distributions, both signatures of clustermergers (e.g. (Feretti 1999), (Govoni et al. 2004), (Giovannini et al.2009), (Cassano et al. 2010), (Feretti et al. 2012)). The monochro-matic radio power of a halo at 1.4 GHz correlates with the clus- (cid:63) E-mail:[email protected] ter X-ray luminosity, mass and temperature (e.g. (Feretti 2000),(Brunetti et al. 2007), (Giovannini et al. 2009)), implying a directconnection between the radio and X-ray plasmas ((Feretti 2000);(Govoni et al. 2001)).In addition, di ff use radio sources classified as radio relics aredetected in cluster peripheral regions (see e.g. (Giovannini & Fer-etti 2004), (Van Weeren et al. 2011), (Feretti et al. 2012)). Ob-servations of relics provide the best indication for the presence ofmagnetic fields and relativistic particles in cluster outskirts. Relicsources are strongly polarized ( ∼ Tooth-brush cluster, found several di ffi culties in this interpretation. More-over we note the existence of relic sources with roundish structures,classified in (Feretti et al. 2012), whose morhology is quite di ff erentfrom that expected if they were originated by shock waves.In a few cases, non-thermal emissions has been detected instructures connecting merging clusters (filaments), see e.g., thefilament surrounding the cluster ZW2341.1 + +
275 in the ComaCluster ((Kim et al. 1989)). The orientation of this feature is, sug-gestively, aligned with the direction to A1367 (see also (Brown &Rudnick 2011)).Recently (Van Weeren et al. 2013) presented the detection of c (cid:13) a r X i v : . [ a s t r o - ph . C O ] J u l G. Giovannini et al.
Figure 1.
DSS Spatial distribution on the sky of likely cluster members –very small circles – and relative isodensity contour map are superimposedon the DSS2-red image. Levels are: 0.25 0.50 0.75 ... 3.00 gals / arcmin .The center and the size ( ∼ (cid:48) -radius regions around A3411(Northern structure) and A3412 (Southern structure) centered on the X-raypeaks as listed in the text. Crosses highlight the positions of the peaks inthe galaxy distribution as detected in our 2D-DEDICA analysis. The arrownear to the A3411 center indicates the galaxy with known redshift listed inthe 6dF Galaxy Survey, very close to a star. a new halo source located at the center of the cluster A3411, and acomplex region of di ff use, polarized emission found in the south-eastern outskirts of A3411, along the projected merger axis of thesystem. They classify this region as a peculiar 1.9 Mpc scale radiorelic.Here we present new optical results on this complex region,and the large scale X-ray image from archive ROSAT data, bothshowing that A3411 and A3412 are two interacting clusters in alarge scale filament. These data compared with high and low res-olution polarized VLA images, show that the giant di ff use radiosource is likely related to the filamentary structure and to the pres-ence of multiple mergers in the A3411–A3412 complex.The intrinsic parameters quoted in this paper are computed fora Λ CDM cosmology with H =
71 km s − Mpc − , Ω m = Ω Λ = = / arcsec. A3411 and A3412 are on the galactic plane therefore availableX-ray and optical information is poor. The cluster centers as re-ported in the Nasa / IPAC Extragalactic Database (NED) are: RA08 h : 41 m : 47 . s and DEC − ◦ : 28 (cid:48) : 46 (cid:48)(cid:48) for A3411, and RA08 h : 42 m : 05 . s and DEC − ◦ : 35 (cid:48) : 47 (cid:48)(cid:48) for A3412.For A3411 (see also (Van Weeren et al. 2013)) it is reporteda redshift = z = . Figure 2.
Radio X-ray overlay of the A3411 – A3412 region. Contoursshow the radio source at 1.4 GHz. The HPBW is 56.3” × ◦ ).Levels are: 0.12 0.24 0.48... mJy / beam. The noise level is 0.04 mJy / beam.In colour the X-ray image from archive ROSAT data is shown. The distance between the two cluster centers is ∼ (cid:48) corresponding to ∼ (cid:48) -radiusaround the A3411 center extracting the objects classified as “galax-ies” and having both B j and R magnitudes available and having R <
20 mag from the SSA - SuperCOSMOS Science Archive . Inthis photometric catalog, we selected “likely cluster members” onthe basis of the color–magnitude relation (hereafter CMR), whichindicates the early-type galaxy locus. We considered galaxies hav-ing observed colors within 0.4 mag from B j − R = . B − R values at z = .
17 in (L´opez-Cruz et al. 2004), opportunemagnitude conversions and Galactic absorption). Note that we pre-fer to rely on the expected CMR relation and a quite large colorrange since galaxy colors (and star / galaxy separation too) are ex-pected to have large uncertainties in the A3411 region.Figure 1 shows a zoomed region of the contour map for thelikely members (860 galaxies within the whole ∼ (cid:48) region) asobtained through to the 2D adaptive–kernel method (2D DEDICA,(Pisani 1996)). The two densest peaks in the galaxy distributioncorrespond to A3411 and A3412, nominally being A3412 evendenser than A3411. A third significant, but much less dense, peaklies at north at the limit of the plot we show. The three density peaksare significant at the > .
99% c.l. and Table 1 lists relevant infor-mation: the number of assigned members, N S (Col. 2); the peak po-sition (Col. 3); the density (relative to the densest peak), ρ S (Col. 4);the value of χ for each peak, χ (Col. 5).To obtain more reliable positions and relative densities, westress the need of better quality images covering the whole clustercomplex in two magnitude bands in a homogeneus way, while to-date available, more recent images do not have both the advantages.The relatively small distance between A3411 and A3412 and http: // surveys.roe.ac.uk / ssa / c (cid:13) , 1–7 giant di ff use non-thermal source in A3411 and A3412 Table 1.
2D DEDICA optical structureSubclump N S α (J2000) , δ (J2000) ρ S χ h : m : s , ◦ : (cid:48) : (cid:48)(cid:48) A3412 49 08 42 08 . −
17 34 45 1 .
00 21A3411 34 08 41 51 . −
17 28 18 0 .
83 16Northernpeak 50 08 41 54 . −
17 23 05 0 .
55 12 the presence of galaxies in the connecting region suggests that theyare two interacting clusters. (Van Weeren et al. 2013) classify thecentral galaxy in A3411 as a cD galaxy from a VLT FORS1 imageof the central part of the cluster.
In Fig. 2 the ROSAT image obtained from archive data (observationrequest 801009) of this region is shown in color. The image showsan extended complex emission with three aligned major clumps.We estimated the X-ray luminosity of the whole region, fromROSAT All Sky Survey data, by considering a weighted aver-aged total Galactic HI column density 4.67 × cm − fromthe Leiden / Argentine / Bonn (LAB) Survey ((Kalberla et al. 2005)),an APEC model with kT = Z (cid:12) . We mea-sured in the 0 . − . ∼ . × erg / s, over a region of ∼ (cid:48) , centered at the location ofthe less bright X-ray peak and including all the three clumps. Thisvalue is slightly higher, but in agreement within the errors with thevalue published in (Ebeling et al. 2002).In more detail we note that the X-ray image shows an extendedemission with a peak near to the A3411 optical position: RA 08 h :41 m : 51 s and DEC − ◦ : 28 (cid:48) : 00 (cid:48)(cid:48) . We assume this as the A3411cluster center. The X-ray emission is extended in S direction anda secondary peak is present at RA 08 h : 42 m : 00 s DEC − ◦ :30 (cid:48) : 30 (cid:48)(cid:48) . For the A3411 structure, a total X-ray luminosity of ∼ . × erg / s is measured over a region of ∼ . (cid:48) . From X-raydata we conclude that A3411 is a bright X-ray cluster in a mergingphase.At RA 08 h : 42 m : 06 s DEC − ◦ : 34 (cid:48) : 00 (cid:48)(cid:48) one more brightX-ray emission is present, slightly extended to SW. This compactstructure has a X-ray luminosity of ∼ . × erg / s over a regionof ∼ (cid:48) in size. Because of its position we identify this X-ray emis-sion with A3412, even if its compactness could suggest the identi-fication of the X-ray structure with a discrete source (e.g., a radiogalaxy; see Sect. 4.1). We note that A3412 is aligned with the veryelongated X-ray structure of A3411 and that the X-ray emission isin good agreement with the optical galaxy distribution.We compared ROSAT X-ray data with the Chandra X-ray im-age of A3411 published by (Van Weeren et al. 2013) (their Fig. 1).Both images confirm the evidence that A3411 is a cluster in a merg-ing phase. The high resolution Chandra image by (Van Weeren etal. 2013) (their Fig. 1-right) shows the inner disturbed region ofA3411. The secondary peak in between A3411 and A3412, presentat ∼ σ level in the ROSAT All Sky Survey data, is marginallyvisible being at the bottom left corner.In the ROSAT image one more peak of emission is present atthe NE of A3411 at ∼ σ level. No radio emission is present fromthis region and no optical condensation was found. With presentdata we cannot confirm if it is one more substructure related to Figure 3.
Contour image of the central di ff use radio source identified as aradio halo at the center of A3411. Discrete sources in the halo region havebeen subtracted. The HPBW is 35”. Contours are 0.06 0.08 0.10 0.15 0.300.50 mJy / beam. The noise level is 0.03 mJy / beam. A3411 or if it is an unrelated discrete source. This structure is out-side the Fig. 1-right in (Van Weeren et al. 2013). We note that thewhole X-ray structure (including secondary peaks) is present bothin the X-ray image derived from the ROSAT All Sky Survey dataand in the image from the ROSAT archive pointed observation: thetwo independent images are in very good agreement.
A field centered on A3411 was observed at 1.4 GHz in full po-larization mode in October 2003 in A / B configuration and in June2004 in C / D configuration (project AC696). Because of the smallangular distance between A3411 and A3412 ( ∼ = = = -5). Final images were corrected for the primary beam attenua-tion, to properly measure the flux density. Polarized images wereobtained combining Q and U images and correcting for the positivebias, using standard AIPS tasks.In Fig. 2, we show the radio image at the angular resolutionof 56.3” × ◦ , overimposed onto a ROSAT X-ray im-age (colour) obtained from archive data (Observing request number801009).At the position of A3411, coincident with the main peak ofthe X-ray emission we confirm ((Van Weeren et al. 2013)) the pres- c (cid:13)000
A field centered on A3411 was observed at 1.4 GHz in full po-larization mode in October 2003 in A / B configuration and in June2004 in C / D configuration (project AC696). Because of the smallangular distance between A3411 and A3412 ( ∼ = = = -5). Final images were corrected for the primary beam attenua-tion, to properly measure the flux density. Polarized images wereobtained combining Q and U images and correcting for the positivebias, using standard AIPS tasks.In Fig. 2, we show the radio image at the angular resolutionof 56.3” × ◦ , overimposed onto a ROSAT X-ray im-age (colour) obtained from archive data (Observing request number801009).At the position of A3411, coincident with the main peak ofthe X-ray emission we confirm ((Van Weeren et al. 2013)) the pres- c (cid:13)000 , 1–7 G. Giovannini et al.
Figure 4.
Contour image of the A3411–A3412 region. The HPBW is 15arcsec. Contours are: 0.04 0.07 0.15 0.3 0.5 1 1.5 2 3 4 5 mJy / beam; thenoise level is 0.02 mJy / beam. R1, R1B, and R2 refer to radio structuresdiscussed in the text. Figure 5.
Contour map of the di ff use emission regions R1 and R2 (see text).The HPBW is 5”. Levels are: 0.04 0.07 0.1 0.15 0.3 0.5 0.7 mJy / beam; thenoise level is 0.016 mJy / beam. ence of a di ff use faint radio source that we interpret as a possiblesmall size central halo (see Fig. 2, and Fig. 3). The morphology ofthis extended source is rather irregular in agreement with the recentfinding of irregular low power radio halos, discussed in (Giovan-nini et al. 2009). At high resolution a few contaminating discretesources are present, subtracted in Fig. 3 (see Sect. 5.2).Moreover as discussed by (Van Weeren et al. 2013) a remark-able extended di ff use radio source is detected at the peripheral re-gion of A3411. We confirm the presence of this structure crossingthe A3412 cluster center. For a more clear discussion, we separatethis extended di ff use, non-thermal source in two sub-structures: R1elongated perpendicularly to the A3411 and A3412 filament, andR2 a N-S more elongated structure aligned with the A3411 andA3412 filament, southern to A3412 (see Fig. 4). To the West sideof R1 another di ff use, non-thermal source is present with a lowersurface brightness (named R1B in Fig. 4), aligned with the brighterstructure. Low resolution images show that these structures are con-nected and that the di ff use, non-thermal source R1 extends up to ∼ RA 08 h : 41 m : 40 s DEC − ◦ : 37 (cid:48) : 30 (cid:48)(cid:48) with a total size ∼ ff use structure starts from the A3412 region, andextends outwards, in the direction of the A3411-A3412 connectingline (Fig. 4). Its size is ∼
860 kpc S to A3412.Both radio structures if observed at high resolution (Fig. 5)show a uniform brightness distribution with no filamentary sub-structure typical of relic sources. At this high angular resolutionthe low brightness structure R1B to the west side of R1 is com-pletely resolved; for this reason in Fig. 5 we show a smaller field ofview.The discrete sources imbedded in the extended radio sourceare only a few and not relevant: a discrete source is present at theA3412 cluster center (see Fig. 5). This discrete radio source is lo-cated at the X-ray peak emission and we identify it as the bright-est cluster galaxy of A3412. This source could be the origin of thecompact X-ray emission, we note however that the radio power (seeSect. 4.3.3) is significantly lower than 3C radio sources with a sim-ilar X-ray luminosity (see e.g. (Massaro et al. 2013)), and signifi-cantly lower than expected from the correlation between X-ray andradio luminosity discussed by (Balmaverde et al. 2006). Thereforewe tentatively identify the X-ray emission present here, with thecompact cluster A3412. One more extended discrete radio sourceis at the periphery of A3412 East-side, just outside the di ff use, non-thermal source (Fig. 5). It could be a head-tail or double sourceaccording to the optical identification.A few additional sub-mJy, slightly extended discrete sourcesare visible. All discrete sources have been subtracted before mea-suring the radio parameters of the di ff use, non-thermal source.(Van Weeren et al. 2013) distinguish in the di ff use emissionfive elongated components (see their Fig. 3.Left), even if they clas-sify this complex as a radio relic. From the comparison of ourFig. 4 and Fig. 5 we do not find a clear evidence of di ff erent sub-components, but connected regions with a slightly higher and lowerbrightness, as expected (and found) in many di ff use sources (halosand relics) correlated to the turbulence and / or shock waves in thehot ISM. The extended di ff use, non thermal source is strongly polarized. InFig. 6 we show the total intensity image with superimposed polar-ization vectors (no correction was applied to their orientation). In c (cid:13) , 1–7 giant di ff use non-thermal source in A3411 and A3412 Figure 6.
Contour image of the di ff use radio source (total intensity). TheHPBW is 15”; levels are: 0.03 0.05 0.1 0.2 0.4 1 2 3 4 5 mJy / beam. Over-imposed are vectors oriented as the polarization angle, with a length pro-portional to the polarized intensity: 1” = × − Jy / beam. this figure the extreme region at SW (R1B) does not appear sinceno polarization is detected here.The average polarization percentage in R1 is = < ∼ ◦ in the SW region and at ∼ -25 ◦ in the NE region. In R2 vector orientation moves from ∼ ◦ to -45 ◦ .The polarization vectors show no preferential orientation.There are several changes of direction on scales of 1-2 arcmin.This could suggest the presence of significant Rotation Measure(RM) e ff ects, due to a magnetized ICM. Being very distant from theA3411 center this ICM has to be related to the presence of A3412. ff use and discrete source parameters & R2 structures
The total flux density of the R1 + R2 structure is 67 ± = / Hz), after the subtraction of discretesources. We note that: 1) the flux density uncertainty is not due tothe noise or zero level in the image, but to the problem to include ornot some peripheral regions; 2) the only relevant discrete sourcesare the one at the center of A3412, and the peripheral extendedsource just outside the region R2. Other discrete sources are sub-mJy sources. We have measured the flux density in images withdi ff erent angular resolutions and results are consistent. Most of theflux density is in the R1 structure (considering also the R1B struc-ture): 38 mJy (log P = / Hz), for R2 we measured 28 mJy(log P = / Hz). The total extension of R1 is ∼
11’ corre-sponding to ∼ ∼
5’ for R2 corresponding to ∼ In low resolution images a di ff use, non-thermal source is presentcoincident with the central region of A3411. We identify this emis-sion as a di ff use small-size halo source. The total flux density fromthis region is ∼ = ± = / Hz). No polarized flux has been detected in this region.We estimate a size of ∼ ∼
430 kpc (see Fig.3). This measure is very uncertain being the halo brightness at afew sigma level, and a ff ected by confusion. The estimated radiopower, halo linear size and X-ray luminosity are in agreement withthe X-ray luminosity-radio power and halo linear size-radio powercorrelations (see (Feretti et al. 2012)). The discrete source present at the center of A3412, shows a doublemorphology with a total flux density of 8.5 mJy (log P = / Hz), and a size ∼ ∼
30 kpc). It is not polarized, at a level of < = = / Hz), and a size ∼
45” ( ∼
120 kpc). Theaverage polarized flux percentage is ∼ ff use non-thermal source The non-thermal properties of this region are very complex. (VanWeeren et al. 2013) identify the complex di ff use radio emission asa radio relic broken into five fragments suggesting that the shockresponsible for the radio emission has been broken up due to in-teraction with a large-scale galaxy filament connected to A3411 orother substructures.Observational data presented here, suggest that the elongatedstructure R1 is a di ff use non-thermal source associated to A3411,with many properties similar to relic sources: elongated shape andpolarization properties. This interpretation is supported by the pres-ence of a secondary peak in the X-Ray emission of A3411 confirm-ing that R1 is located outward of a merging structure. However, theorigin of the extended feature R2 is puzzling. Although the radioproperties (elongated shape and polarization) suggest its identifi-cation as a relic source too, its extension aligned with the A3411– A3412 structure as visible in the X-ray image, requires a morecomplex dynamical scenario. We remember that most (all) knownelongated relics show the major axis roughly perpendicular to thedirection of the cluster centers (see e.g. (Br¨uggen et al. 2011)).Some radio properties of these sources are consistent withproperties of relics (see e.g. (Feretti et al. 2012)) and in agreementwith known correlations (e.g., radio power versus linear size, andradio power versus X-ray Luminosity). R1 would be one of thelargest relic sources. However we note that R1 and R2 are di ff usecomplex radio sources which are clearly connected and thus of sim-ilar origin and nature. We note also that, when observed at highresolution, R1 and R2 are di ff use, homogeneous regions with noevidence of substructures. There is no sharp edge in the brightnessdistribution, expected if the radio emission is the tracer of shocks in c (cid:13)000
120 kpc). Theaverage polarized flux percentage is ∼ ff use non-thermal source The non-thermal properties of this region are very complex. (VanWeeren et al. 2013) identify the complex di ff use radio emission asa radio relic broken into five fragments suggesting that the shockresponsible for the radio emission has been broken up due to in-teraction with a large-scale galaxy filament connected to A3411 orother substructures.Observational data presented here, suggest that the elongatedstructure R1 is a di ff use non-thermal source associated to A3411,with many properties similar to relic sources: elongated shape andpolarization properties. This interpretation is supported by the pres-ence of a secondary peak in the X-Ray emission of A3411 confirm-ing that R1 is located outward of a merging structure. However, theorigin of the extended feature R2 is puzzling. Although the radioproperties (elongated shape and polarization) suggest its identifi-cation as a relic source too, its extension aligned with the A3411– A3412 structure as visible in the X-ray image, requires a morecomplex dynamical scenario. We remember that most (all) knownelongated relics show the major axis roughly perpendicular to thedirection of the cluster centers (see e.g. (Br¨uggen et al. 2011)).Some radio properties of these sources are consistent withproperties of relics (see e.g. (Feretti et al. 2012)) and in agreementwith known correlations (e.g., radio power versus linear size, andradio power versus X-ray Luminosity). R1 would be one of thelargest relic sources. However we note that R1 and R2 are di ff usecomplex radio sources which are clearly connected and thus of sim-ilar origin and nature. We note also that, when observed at highresolution, R1 and R2 are di ff use, homogeneous regions with noevidence of substructures. There is no sharp edge in the brightnessdistribution, expected if the radio emission is the tracer of shocks in c (cid:13)000 , 1–7 G. Giovannini et al. merger events. Shock waves are expected to accelerate radio emit-ting electrons, and to amplify and order local magnetic fields. InR1 and R2 we have a highly polarized emission with vectors lo-cally ordered but changing orientation on scales of 1-2 arcmin.The morphology and properties of the di ff use non-thermalsource can be explained by the presence of a complex conditionwith accretion shocks able to accelerate electrons, combined withturbulence in the thermal gas connected to the presence of A3412.The multiple mergers of the A3411 – A3412 structure could bethe origin of the R1 + R2 di ff use source, with R2 oriented along thegiant filament merging into A3411. The R2 structure could be pow-ered by accretion shocks as material falls onto the filament, as e.g.,suggested by (Brown & Rudnick 2011) for the relic present in theComa cluster. We remember that in the Coma cluster hosting a ra-dio halo and a relic source, also a Mpc scale radio bridge connect-ing the halo to the relic and elongated in the same direction of thegiant Coma-A1367 supercluster ((Kim et al. 1989)) is present.Alternatively (Van Weeren et al. 2013) suggest that the com-plex morphology reflects the presence of electrons in fossil radiobubbles that are re-accelerated by a shock. In their Fig. 3 five re-gions are individuated. We think unlikely this hipothesis becausethe presence of so many bubbles implies a strong past activityof some radio galaxies that at present should be completely radioquiet. No connection with the present 2 radio galaxies in A3412 isvisible. Moreover as already discussed, at high resolution the mor-phology of the giant non-thermal source is very homogeneous withno evidence of shocks and at high and low resolution the wholestructure is connected and likey to have a common origin.New multifrequency observations to study the spectral indexdistribution and polarization properties are necessary to properlydiscuss these structures. The presence of one more optical conden-sation to the North of A3411 (see Table 1), suggests a possible morecomplex structure even if present data do not show radio or X-rayemission from this region. A di ff use, non-thermal source at the center of A3411 is presentin our images. Despite of the large uncertainties previously dis-cussed, we note that in comparison with other radio halos, the haloin A3411 is one with the lowest radio power. Only the A3562 halo((Venturi et al. 2003)) is known so far to show a lower radio power((Feretti et al. 2012) and references therein).These faint radio halos both follow the general correlationsbetween X-ray luminosity, radio power and linear size for radiohalos (see e.g (Feretti et al. 2012)). It is important to increase thestatistical information on these low power radio halos to discuss ifall merging clusters host a central di ff use source (in most cases notvisible with present radio telescopes because of sensitivity, see e.g.(Xu et al. 2012)) or if an unknown critical value of the halo radiopower is present. We have presented here the detection of a complex non-thermalsource in the A3411-A3412 structure. This source has been foundat the periphery of A3411, where the cluster A3412 is located,aligned with the A3411 merging structure. We suggest that thisgiant, di ff use, and peculiar non-thermal source is related to a fil-amentary structure and to the presence of multiple mergers. Bet-ter optical, radio and X-ray data are necessary to investigate this complex region and to derive its physical properties. A few casesare present in literature where intergalactic filaments have beenfirmly detected and only in the Coma cluster ((Kim et al. 1989)),and in ZW2341.1 + ff use sources allowing to extend ourknowledge of non-thermal emission from clusters to filaments.Moreover, we have found that the radio halo at the center ofA3411 has a low power, despite of the large uncertainties related toits low brightness. It is important to increase the number of knownlow power radio halos to confirm or not that they follow the gen-eral correlation between X-ray luminosity and radio power of Mpc–scale radio halos.The presence of this source is related to an active merger wellevident in the X-ray image. Data from the new generation of radiotelescopes are needed to better study low power radio halos. ACKNOWLEDGEMENTS
The National Radio Astronomy Observatory is operated by Ass.Univ., Inc., under cooperative agreement with the National ScienceFoundation. This research has made use of the NASA / IPAC Extra-galactic Database (NED) which is operated by the Jet PropulsionLaboratory, California Institute of Technology, under contract withthe National Aeronautics and Space Administration. We thank theReferee and the Scientific Editor for useful suggestions and com-ments.
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