Tailed radio galaxies as tracers of galaxy clusters. Serendipitous discoveries with the GMRT
AAstronomy & Astrophysics manuscript no. Venturi˙accepted c (cid:13)
ESO 2018October 31, 2018
Tailed radio galaxies as tracers of galaxy clusters. Serendipitousdiscoveries with the GMRT
S. Giacintucci , and T. Venturi INAF – Istituto di Radioastronomia, via Gobetti 101, I-40129, Bologna, Italy Harvard–Smithsonian Centre for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USAReceived 00 - 00 - 0000; accepted 00 - 00 - 0000
ABSTRACT
Aims.
We report on the discovery of four new radio galaxies with tailed morphology. Tailed radio galaxies are generallyfound in rich environments, therefore their presence can be used as tracer of a cluster.
Methods.
The radio galaxies were found in the fields of Giant Metrewave Radio Telescope (GMRT) observations carriedout at 610 MHz and 327 MHz devoted to other studies. We inspected the literature and archives in the optical andX–ray bands to search for galaxy clusters or groups hosting them.
Results.
All the tailed radio galaxies serendipitously found in the GMRT fields are located in rich environments. Twoof them belong to the candidate cluster NCS J090232+204358, located at z phot =0.0746; one belongs to the clusterMaxBCGJ 223.97317+22.15620 at z phot =0.2619; finally we suggest that the fourth one is probing a galaxy clusterat z=0.1177, located behind A 262, and so far undetected in any band. Our results strenghten the relevance of highsensitivity and high resolution radio data in the detection of galaxy clusters at intermediate redshift.
Key words. radio continuum: galaxies - galaxies: clusters: general - galaxies: clusters: individual:NCSJ090232+204358,MaxBCGJ223.97317+22.15620, B20151+36
1. Distorted radio sources in clusters of galaxies
Radio galaxies located in dense environments, such asgalaxy clusters, often show complex and prominently dis-torted radio structures. A common morphology is repre-sented by tailed radio galaxies, i.e., double sources withFR–I or FR-I/FR–II morphology (Fanaroff & Riley 1974),whose jets and lobes are bent in U or C shapes. In wide–angle–tail radio galaxies (WAT, or C–shaped) the angleformed by the jets and lobes is usually very large (e.g.O’Donoghue et al. 1993; Feretti & Venturi 2002 for a re-view). The prototype of this class is 3C 465 in A 2634 (Eileket al. 1984). Sources whose jets and lobes form a small angleare referred to as narrow–angle–tail radio galaxies (NAT, orU–shaped; see Bliton et al. 1998, Feretti & Venturi 2002 andreferences therin). The prototype of this class is NGC 1265in the Perseus cluster (Wellington et al. 1973; O’Dea &Owen 1986).There is general consensus that the origin of the dis-torted radio morphologies lies in the interaction of the radiogalaxy with respect to the dense surrounding intraclustermedium (ICM), even though the details of such interactionare not fully disentangled. NAT sources are usually asso-ciated with galaxies moving at high velocity in the grav-itational potential of the cluster (Miley 1980). The rampressure exerted by the external medium is expected to belarge enough to curve the jets and sweep the radio emittingplasma behind the rapidly moving galaxy. In agreementwith this idea, the optical counterparts of narrow–angle–tails are not the bright dominant D or cD galaxies, usuallynearly at rest in the cluster potential well, but rather galax-ies with fainter optical magnitude and lower radio luminos-ity (Rudnick & Owen 1976; Valentijn 1979). However, the host galaxies of NATs have, on average, velocities similar tothose of typical cluster members, rather than moving withthe high peculiar velocity expected in the ram pressure sce-nario. Therefore, it has been suggested that the jet bendingmight be a by–product, at least partially, of bulk motionsin the ICM induced by cluster–subcluster mergers (Blitonet al. 1998).The physical mechanism responsible for the distorsion ofthe jets in WATs is not completely understood either. Bothram pressure (Owen & Rudnick 1976; Begelman et al. 1979)and buoyancy forces, active if the jet density is lower thanthe external gas density (Burns & Balonek 1982), may playa role, the latter being dominant at larger distances fromthe cluster centre (Sakelliou et al. 1996). However, the verylow peculiar velocities of their optical counterparts (Bird1994; Pinkney et al. 2000) – the cluster dominant D or cDellipticals – is not consistent with the observed curvatures.The presence of large scale bulk flows in the ICM, inducedby cluster or group mergers, is an appealing alternativefor the jet bending (Pinkney et al. 1994; Roettiger et al.1996; Gomez et al. 1997). Finally, a connection between theradio bending and gas sloshing in cluster cores has recentlybeen suggested (e.g. A 2029; Clarke et al. 2004; Ascasibar& Markevitch 2006).The association between tailed radio galaxies and clus-ters can be used to trace the high density structures in theUniverse. Indeed many previously unobserved clusters havebeen identified thanks to the detection of tailed sources (e.g.Fomalont & Bridle 1978; Burns & Owen 1979; and morerecently Blanton et al. 2000, 2001 and 2003; Smol˘ci´c et al.2007; Kantharia et al. 2009). In this paper we report onthe discovery with the Giant Metrewave Radio Telescope a r X i v : . [ a s t r o - ph . C O ] J u l Giacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters D ec li n a t i on J0902+2045J0902+2044 100 kpc
Fig. 1.
GMRT 610 MHz contours of the two tailed radio galaxies discovered at ∼ (cid:48) from the galaxy cluster Z 2089.The image is corrected for the primary beam. The radio contours are overlaid on the red optical image from the SDSS.The resolution of the radio image is 6.5 (cid:48)(cid:48) × . (cid:48)(cid:48) , p.a. 80 ◦ . The lowest contour is 0.5 mJy b − , and each contour increasesby a factor of two. The cross indicates the centre of the candidate galaxy cluster NSC J090232+204358. The linear scaleis 1 (cid:48)(cid:48) =1.56 kpc (see Sect. 3)(GMRT) of a number of distorted radio galaxies and onthe successful search for the galaxy clusters in which theyreside.The paper is organised as follows: in Section 2 wepresent the images of the new tailed radio sources; inSections 3 and 4 we describe the optical identification proce-dure and the association with candidate clusters with pho-tometric redshift; a brief summary and conclusions are re-ported in Section 5.We adopt the ΛCDM cosmology, with H =70 km s − Mpc − , Ω m = 0 . Λ = 0 .
7. The spectral index α is defined according to S ∝ ν − α .
2. Discovery of new radio galaxies
In Venturi et al. (2007 and 2008) we presented a large ra-dio survey of a sample of galaxy clusters carried out withthe GMRT at 610 MHz (the
GMRT Radio–Halo Survey ).During the analysis of those observations, we discoveredthree tailed radio galaxies in the wide–field images of twoclusters of the sample. In particular, two of them, GMRT–J 0902+2044 and GMRT–J 0902+2045, are located at ∼ (cid:48) from the phase centre of the observation of Z 2089; thethird tailed radio source, GMRT–J 1455+2209, is locatedat ∼ (cid:48) from the pointing of the cluster Z 7160.Similarly, we found a diffuse and amorphous radiosource, GMRT–J0154+3627, while imaging the 327 MHz emission of the cluster A 262 (archival GMRT observa-tions), as part of a large observational project devoted tothe study of the radio source feedback in groups and poorclusters of galaxies (Giacintucci et al. in prep.). The sourceis located at ∼ (cid:48) to the North–East of the antenna point-ing of A 262.Since the half power width of the primary beam of aGMRT antenna is ∼ (cid:48) at 610 MHz and ∼ (cid:48) at 327MHz, all the new radio sources lie well within the primarybeam of the Z 2089, Z 7160 and A 262 fields. The details on the GMRT observations are summarised inTable 1, where we provide the radio source name; pointingcoordinates of the observation; distance from the pointing;frequency, bandwidth and total lenght of the observations;half power beamwidth (HPBW) of the full array; rms level(1 σ ) measured in the region of the sources, prior to theprimary beam correction. All the GMRT observations inTab. 1 were obtained using both the upper and lower sidebands of 16 MHz each. The data were acquired in spectralline mode, with 128 channels/band and a spectral resolu-tion of 125 kHz/channel. The data reduction was carriedout using the standard procedure (calibration, Fourier in-version, clean and restore) with the NRAO AstronomicalImage Processing System (AIPS) package. In order to re- iacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters 3 D ec li n a t i on J1455+2209100 kpc
Fig. 2.
GMRT 610 MHz contours of the WAT discovered at ∼ (cid:48) from the galaxy cluster Z 7160. The image is correctedfor the primary beam. The radio contours are overlaid onthe red optical image from the SDSS. The resolution of theradio image is 6.1 (cid:48)(cid:48) × . (cid:48)(cid:48) , p.a. 39 ◦ . The lowest contour is 0.4mJy b − , and each contour increases by a factor of two. Thecentre of the galaxy cluster MaxBCG J223.97317+22.15620is coincident with the elliptical galaxy associated with theWAT. The linear scale is 1 (cid:48)(cid:48) =3.99 kpc (see Sect. 3).duce the size of the datasets and to minimize bandwidthsmearing effects within the primary beam, after bandpasscalibration the central channels of each observation wereaveraged to 6 channels of ∼ ≤ In Figure 1 we show the 610 MHz contours of the twotailed radio sources GMRT–J 0902+2044 and GMRT–J 0902+2045 (hereinafter J 0902+2044 and J 0902+2045 re-spectively), discovered in the field of Z 2089 (Tab. 1). Thetwo radio galaxies, both extending well beyond the opticalsize of the associated counterpart, are separated by ∼ . (cid:48) in the plane of the sky. The contours are superposed tothe red optical image from the Sloan Digital Sky Survey(SDSS; Data Release 7 ). The radio image has been cor-rected for the primary beam attenuation of the GMRT at610 MHz. In the region shown in Fig. 1 the sensitivity is 60 / dr7 / µ Jy b − , similar to the value achieved in the phase centreof the observations (45 µ Jy b − ; Venturi et al. 2008). Afterthe primary beam correction, the noise in the region of theradio galaxies increases to 150 µ Jy b − .J 0902+2044 has a symmetric WAT morphology, with acentral compact component and two bright jets marginallydeflected to form an angle of ∼ ◦ , at least out to ∼ (cid:48) from the central radio peak. Then the jets abruptly changedirection by ∼ ◦ , and merge into a single low surfacebrightness tail extending East of the source. A compressionof the radio isophotes is visible in the North–Western edgeof the jets, suggesting that the pressure necessary to bendthe radio emission is coming from that direction.The radio galaxy J 0902+2045 is characterized by twin–jets,smoothly curved in a C–shape. The jets appear rather sym-metric in morphology and extent, and can be traced out to ∼ (cid:48) from the central radio peak. The angle between thetails is ∼ ◦ out to approximately 30 (cid:48)(cid:48) from the centre.Then the jets undergo two more changes of direction, eachwith an angle of ∼ ◦ ; after the last bend, they lose col-limation and gently curve toward the outer regions. Froma morphological point of view, the very smooth and com-plete bending of J 0902+2045 resembles that of NAT ra-dio galaxies, and indeed the source is strongly reminiscentof the prototypical narrow–angle–tail source NGC 1265 inthe Perseus cluster (e.g. O’Dea & Owen, 1986; see theirFigs. 4 and 5); at the same time, radio sources similar toJ 0902+2045 have also been classified in the literature asWAT (Smol˘ci´c et al. 2007).The integrated flux density at 610 MHz isS
610 MHz = 258 . ± .
610 MHz = 145 . ± . ± . ± . α = 0 . ± .
11 for the J0902+2044 and α = 0 . +0 . − . for J0902+2045 (see Tab. 2).Fig. 2 shows the 610 MHz image of the WAT GMRT–J 1455+2209 (hereinafter J 1455+2209), discovered at ∼ (cid:48) from Z 7160 (Tab. 1), overlaid on the SDSS red opticalframe. The sensitivity in this region is 60 µ Jy b − , and thenoise level after the primary beam correction is 130 µ Jyb − . The source has a morphology similar to J 0902+2044(Fig. 1), however no clear compact component is visible atthe source centre, and some level of asymmetry is observedin the jet brightness. The flux density of J 1455+2209 at610 MHz and 1.4 GHz are S
610 MHz = 91 . ± . . = 49 . ± . α = 0 . +0 . − . (see Tab. 2).The spectral index α calculated for the three tailedsources is in good agreement with the typical values of ex-tended active radio galaxies. The radio information is sum-marised in Tab. 2. For the optical properties provided inthe Table, we refer to § Fig. 3 shows the 327 MHz images of the diffuse radio sourceGMRT–J 0154+3627 (hereinafter J 0154+3627), found inthe periphery of A 262. In the literature this source is knownas B2 0151+36, being part of the Second Bologna Catalog
Giacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters D ec li n a t i on
100 kpc D ec li n a t i on
100 kpc
Fig. 3.
Left:
GMRT 327 MHz full resolution contours of the diffuse radio source J 0154+3627, discovered at ∼ (cid:48) fromA 262. The image is corrected for the primary beam attenuation. The radio contours are overlaid on the red opticalimage from the POSS–2. The resolution of the radio image is 12.2 (cid:48)(cid:48) × . (cid:48)(cid:48) , p.a. 87 ◦ . The lowest contour is 1.6 mJy b − ,and each contour increases by a factor of two. Right : GMRT 327 MHz low resolution image (contours and grey scale) ofJ 0154+3627. The resolution is 28.8 (cid:48)(cid:48) × (cid:48)(cid:48) , p.a. −
88. Contours are spaced by a factor 2, starting from ± σ ( 1 σ = 600mJy b − . For this source the scale is 2.127 kpc/ (cid:48)(cid:48) (see Sect. 4).(B2) of radio sources (Colla et al. 1973), obtained using theBologna Northern Cross telescope at 408 MHz. However,given the low resolution of that instrument (3 (cid:48) in RA, and10 (cid:48) in DEC), no morphological classification was providedat that time. The 327 MHz full resolution image, shown inthe left panel of Fig. 3 overlaid on the POSS–2 red opticalplate, clearly reveals the presence of a bright and compactcomponent at the source centre, two short symmetric jets(extended ∼ . (cid:48) ), and a region of low brightness radioemission, with a rather amorphous morphology and a sizeof ∼ . (cid:48) × (cid:48) . To better highlight the distribution of the lowsurface brightness emission, we produced a low resolutionimage at 327 MHz, shown in the right panel of Fig. 3. Atthis lower resolution (28.8 (cid:48)(cid:48) × . (cid:48)(cid:48) ) the source reveals twofaint radio tails, bent toward the same south–western direc-tion. These tails can be well traced out to a distance of ∼ (cid:48) from the compact component. However, positive residualsof emission suggest that they might be more even extendedthan what is detected at the sensitivity level of the image.We searched the radio archives for available observa-tions of this source at different frequencies. We found twoVery Large Array (VLA) observations at 1.4 GHz and 4.9GHz (both in the C–array configuration; project AC 483),that we re–analysed. In Fig. 4 we show the 4.9 GHz con-tours ( ∼ (cid:48)(cid:48) resolution) superposed to the grey scale imageat 1.4 GHz ( ∼ (cid:48)(cid:48) resolution). At 4.9 GHz the extendedemission is mostly elongated along the North–South axis,consistent with the inner part of the 327 MHz image. Theobserved morphology at 1.4 GHz is very similar to the im-ages at 327 MHz (Fig. 3). An elongated region of very lowbrightness emission is detected on the western side of thesource, where the low resolution image at 327 MHz revealsthe existence of two radio tails.The total flux density at 327 MHz, measured on the lowresolution image in Fig. 3, is 620.5 ± . ± . ± . α = 0 . +0 . − . between 327 MHz and 1.4 GHz, and α = 0 . +0 . − . between 327 MHz and 4.9 GHz. The radioinformation is summarised in Tab. 3.The compact component accounts for ∼
3% only of thetotal flux at 327 MHz (full resolution), while at higher fre-quency its contribution increases to ∼
6% at 1.4 GHz and ∼
7% at 4.9 GHz. Its spectral index is flat ( α =0.2) overthe whole frequency range, indicating that this componenthosts the radio core of J 0154+3627. The spectral index ofthe diffuse emission is α =0.6 in the 327 MHz–4.9 GHz in-terval.In Fig. 5 we show the integrated radio spectrum of thesource between 327 MHz and 4.9 GHz (empty circles). Thespectrum was obtained by combining the fluxes in Tab. 3,with the literature data provided by the NASA/IPACExtragalactic Database (NED) for B2 0151+36. The spreadin the flux values at 1.4 and 4.9 GHz is most likely due tothe different angular resolution of the observations. In thefigure we also show the spectrum of the core (filled trian-gles) and of the diffuse component (filled circles).The morphological classification of this radio galaxy isnot straightforward. The image shown in the left panel ofFig. 3 is reminiscent of 3C 317, classified as a core–halo ra-dio galaxy (Zhao et al. 1993), a class of sources typicallyfound at the centres of cool core clusters (e.g. Baum &O’Dea 1991; Sarazin et al. 1995; Mazzotta & Giacintucci2008). However, the diffuse emission in core–halo radiogalaxies usually has a steep spectrum ( α ≥ iacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters 5
3. Tailed radio galaxies as probes of the clusterenvironment
We searched the literature to collect information about theenvironment of the new tailed radio galaxies presented inSect. 2.2.We found that J 0902+2044 and J 0902+2045 (Fig. 1)are located in the region of NSC J090232+204358 (here-inafter NSC J090+2043), a candidate galaxy cluster, on thebasis of photometric redshifts, listed in the catalog of theNorthern Sky Optical Cluster Survey (Gal et al. 2003). Itsphotometric redshift is z phot =0.075 ± gal =39.2 (Tab. 4).The WAT J 1455+2209 (Fig.2) is associated with the cen-tral galaxy of MaxBCG J223.97317+22.15620, classified asgalaxy cluster by Koester et al. (2007) using the photo-metric information in the Sloan Digital Sky Survey. Forthis cluster Koester et al. provide z phot =0.262 ± gal =18. Table 4 summarizes the general properties of thetwo candidate galaxy clusters.We inspected the optical images from the SDSS to iden-tify the host galaxies of the three radio sources. The opticalcounterparts of J 0902+2044 and J 0902+2045 are two el-liptical galaxies with spectroscopic redshift z spec =0.083 andz spec =0.082, respectively . The agreement between theseredshifts and the photometric redishift of the cluster, cou-pled with the tailed morphology of the two radio galaxies,confirms that NSC J090+2043 is the cluster hosting them.The WAT J 1455+2209 is associated with an elliptical atz spec =0.257 . Also in this case the galaxy redshift is con-sistent, within the errors, with the photometric redshift ofthe cluster MaxBCG J223.97317+22.15620 (Tab. 4).The optical properties of the three tailed radio galaxiesare summarised in Tab. 2, where we provide the opticalname and coordinates, spectroscopic redshift, and absolutered magnitude M R . All the optical information is takenfrom the latest release of the SDSS (Data Release 7).Using the redshift of the optical counterparts and theNVSS flux density in Tab. 2, we calculated the radio powerat 1.4 GHz. The WAT and the NAT in NSC J090+2043have P . = 2 . × W Hz − and 1 . × W Hz − ,respectively. The radio power of the WAT J 1455+2209 isP . = 9 . × W Hz − (Tab. 2). The combination ofthese radio powers and the optical magnitude of the asso-ciated galaxies, in the range M R = –22.21 ÷ –22.61 (seeTable 2) places them in the transition region between FRIand FRII radio galaxies in the M R – logP . diagram(Owen & Ledlow, 1994).The projected linear sizes of these three radio galaxies, inthe range 100 – 300 kpc (see Table 2), are consistent withwhat is typically found for tailed radio galaxies in clusters,whose sizes range over a wide interval, from tens to hun-dreds of kpc.We inspected the X–ray images from the ROSATAll Sky Survey (RASS) both for NCS J 0902+2043 andMaxBCG J 223.97317+22.15620. No X–ray counterpart isdetected in both cases. Such lack of detection may be dueto a number of reason. On one side, the number of op-tical galaxies provided in the literature (Tab. 4) suggeststhat they are poor clusters, and the lack of X–ray infor-mation might be due to the faint X–ray emission typical At these redshifts the linear scale is 1.56 kpc/ (cid:48)(cid:48) At this redshift the linear scale is 3.99 kpc/ (cid:48)(cid:48) D ec li n a t i on
100 kpc
Fig. 4.
VLA 4.7 GHz contours on the 1.4 GHz grey scaleimage of J 0154+3627. The resolution is 4.9 (cid:48)(cid:48) × (cid:48)(cid:48) , p.a. −
80 and 15.8 (cid:48)(cid:48) × . (cid:48)(cid:48) , p.a. −
74. Contours are spaced by afactor 2, starting from ± σ ( 1 σ = 45 µ Jy b − and 1 σ = 50 µ Jy b − ). For this source the scale is 2.127 kpc/ (cid:48)(cid:48) . Fig. 5.
Radio spectrum of the candidate tailed radio galaxyJ0154+3627 between 327 MHz and 4.9 GHz (empty circles).The spectrum of the core and extended emission are shownas filled triangles and filled circles, respectively.of these environments; on the other side, at the redshiftof MaxBCG J 223.97317+22.15620 the cluster X–ray emis-sion might be below the sensitivity limit of the RASS.We note, for instance, that the rich and X–ray luminous(L X = 9 . × erg s − ) merging cluster RXCJ 2003.5–2323, located at z=0.3171, was barely detected on theRASS, and deep pointed Chandra observations were neces-sary for a radio/X–ray analysis (Giacintucci et al. 2009). Giacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters D ec li n a t i on J0154+3627 A 262
Fig. 6.
GMRT 327 MHz contours, overlaid on the RASS image in the 0.5–2.1 keV band of the A 262 region. The resolutionof the radio image is 19.5 (cid:48)(cid:48) × (cid:48)(cid:48) , p.a. − ◦ . The lowest contour is at the 5 σ level=2.5 mJy b − , and each contoursincreases by a factor of two. The RASS image has been smoothed with a gaussian with σ =6 (cid:48) .
4. A galaxy cluster behind A 262?
As clear from Fig. 3 (left), the radio core of the candidatetailed source J 0154+3627 is coincident with a bright opticalgalaxy on the POSS–2 image. Its projected distance fromthe centre of the galaxy cluster A 262 is ∼ (cid:48) , which cor-responds to ∼
600 kpc at the redshift of A 262 (z=0.0163).However, its spectroscopic redshift, z spec = 0 . (Smithet al. 2004), rules out the membership to A 262.Using the flux density measured on the NVSS, and as-suming a redshift of 0.1177, we calculated the radio powerof the source at 1.4 GHz. We obtained P . = 8 . × W Hz − . Also this radio galaxy is located in the FRI/FRIItransition region in the M R – logP . plot. Note thatthis galaxy is the most luminous among those presented inthis paper, even though it is not as bright as the brightestcentral dominant galaxies in clusters (M R ∼ −
23, Owen &Ledlow 1994).It is difficult to provide an unambiguous morphological clas-sification for J 0154+3627, since projection effects are likelyto play a role. The two short jets visible in Fig. 3, extendingbeyond the optical galaxy, and the asymmetry of the diffuseemission (extended westwards) lead us to think that we aredealing with a tailed source rather than with a core–halo ra-dio galaxy. The bright optical magnitude of the associated At this redshift the linear scale is 2.13 kpc/ (cid:48)(cid:48) galaxy and the radio power would be consistent with a cen-trally located WAT, but the possibility that we are dealingwith a NAT radio galaxy cannot be ruled out. In any case,we can safely conclude that J 0154+3627 is a tailed radiogalaxy, most likely associated with a cluster at z=0.1177located behind A 262.We checked on the X–ray archives in search for informationin this band. J 0154+3627 is located outside the field of viewof all the pointed observations of A 262, and the only avail-able image for this source is provided by the RASS. Fig. 6shows the smoothed RASS image in the 0.5–2.1 keV bandof A 262 and the region of J 0154+3627, with the GMRTcontours at 327 MHz overlaid. The image reveals a region ofdiffuse emission spatially coincident with the radio galaxy.Its projected distance from the centre of A 262 is ∼
600 kpc,and thus we cannot rule out the possibility that such emis-sion is a clump of gas or an infalling group at the outskirts ofthe cluster, rather than emission coming from a backgroundcluster (or group). Pointed X–ray observations would beuseful to investigate the morphology of this emission andits possible connection to the radio galaxy J 0154+3627.
5. Summary and conclusions
In this paper we report on the serendipitous discovery offour radio galaxies, whose tailed radio morphology is sug- iacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters 7 gestive of a cluster environment. Such discovery was possi-ble thanks to the high sensitivity and wide field of view ofthe GMRT at 610 MHz (Venturi et al. 2007, 2008) and at327 MHz.We searched for the optical counterparts of the newtailed radio galaxies, and found that J 0902+2044 andJ 0902+2045, respectively a WAT and a NAT, are locatedin the candidate galaxy cluster NCS J0902+2043 with pho-tometric redshift 0.075. Our radio images thus confirm thatNCS J0902+2043 is indeed a galaxy cluster, whose redshiftis consistent with the radio power derived for the two radiogalaxies.Similarly, we confirm thatMaxBCG J223.97317+22.15620 (z phot =0.262) is a galaxycluster, which hosts the central WAT J 1455+2209.The optical counterpart of the candidate tailed sourceJ 0154+3627 is a galaxy at redshift z=0.118. This rules outthe possibility that the radio galaxy is associated with A 262(z=0.016). The observational properties of the radio source,i.e., morphology, spectrum, radio power and size, suggestthat it is member of a galaxy cluster, which is so far unde-tected in the optical band. Inspection of the X–ray RASSimage reveals a clump of diffuse emission coincident withthe radio galaxy. However, the available X–ray data do notallow us to tell whether such emission is related to the en-vironment of J 0154+3627 (z=0.118), or to the closer A 262(z=0.016).Our findings confirm that distorted radio galaxies canbe used as tracers of galaxy clusters. The low observingfrequencies available with the GMRT (few hundred ofMHz), coupled with its large field of view (primary beam ∼ µ Jygoing from 610 MHz to 240 MHz) and angular resolution(of the order of 5 (cid:48)(cid:48) and 15 (cid:48)(cid:48) respectively at 610 MHz and240 MHz) are an ideal combination for the discovery ofgalaxy clusters through tailed radio galaxies, not onlyin the Local Universe but also at intermediate redshifts.We point out that the sensitivity of the REFLEX survey(B¨ohringer et al. 2004) does not allow to detect anycluster with X–ray luminosity L X ≤ × erg s − beyond z ∼ Acknowledgements.
We thank Prof. D. Dallacasa forinsightful discussion and careful reading of the manuscript.We thank the staff of the GMRT for their help during theobservations. GMRT is run by the National Centre forRadio Astrophysics of the Tata Institute of FundamentalResearch. We acknowledge financial contribution from theItalian Ministry of Foreign Affairs, from MIUR grantsPRIN2004, PRIN2005 and 2006, from PRIN–INAF2005and from contract ASI–INAF I/023/05/01.
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Giacintucci & Venturi: Tailed radio galaxies as tracers of galaxy clusters
Table 1.
Details of the GMRT observations.
Source name Pointing RA & DEC d ν ∆ ν Obs. time HPBW, PA rms(GMRT –) (h,m,s & ◦ , (cid:48) , (cid:48)(cid:48) ) ( (cid:48) ) (MHz) (MHz) (min) ( (cid:48)(cid:48) × (cid:48)(cid:48) , ◦ ) (mJy b − )J0902+2044, J0902+2045 09 00 45.9 +20 55 13 27 610 32 80 6.5 × × ∗
270 12.2 × ∗ The observations were carried out using a total bandwidth of 32 MHz (USB+LSB), but only the USB dataset was used for theanalysis.
Table 2.
Radio and optical data of the tailed radio galaxies.
Radio name RA radio
DEC radio S
610 MHz S . α . . Radio(GMRT–) (h,m,s) ( ◦ , (cid:48) , (cid:48)(cid:48) ) (mJy) (mJy) morphologyOptical name RA opt DEC opt z spec M R logP . GHz
LLS(SDSS–) (h,m,s) ( ◦ , (cid:48) , (cid:48)(cid:48) ) (W Hz − ) (kpc)J 0902+2044 09 02 34.93 +20 44 17.9 258.2 ± ± ± ± − ± ± +0 . − . NATJ 090225.86+204546.5 09 02 25.87 +20 45 46.5 0.0820 ± − ± ± +0 . − . WATJ 145553.56+220922.3 14 55 53.56 +22 09 22.3 0.2566 ± − Table 3.
Radio and optical data of the candidate NAT J 0154+3627.
Radio name RA radio
DEC radio S
327 MHz S . S . α . . logP . GHz (GMRT–) (h,m,s) ( ◦ , (cid:48) , (cid:48)(cid:48) ) (mJy) (mJy) (mJy) (W Hz − )Optical name RA opt DEC opt z spec r M R α . . LLS(NFP–) (h,m,s) ( ◦ , (cid:48) , (cid:48)(cid:48) ) kpcJ 0154+3627 01 54 51.53 +36 27 46.2 620.5 ± ± ± +0 . − . ± − +0 . − . Table 4.
General properties of the candidate galaxy clusters.
Cluster name RA DEC z phot N gal (h,m,s) ( ◦ , (cid:48) , (cid:48)(cid:48) )NSC J090232+204358 09 02 32.7 +20 43 57 0.075 ± ±±