Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/people/Ray.Norris/papers/n227.pdf Äàòà èçìåíåíèÿ: Thu Sep 23 10:21:57 2010 Äàòà èíäåêñèðîâàíèÿ: Fri Feb 28 07:56:09 2014 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 63

Mon. Not. R. Astron. Soc. 000, 1­14 (200X)

Printed 24 May 2010

A (MN L TEX style file v2.2)

Wide-angle tail galaxies in ATLAS
Minnie Y. Mao1,2,3 , Rob Sharp2, D. J. Saikia3,4,5, Ray P. Norris3, Melanie Johnston-Hollitt6, Enno Middelberg7 and Jim E. J. Lovell 1
2 3 4 5 6 7

1

arXiv:1005.3649v2 [astro-ph.CO] 21 May 2010

School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart, 7001, Australia Anglo-Australian Observatory, PO Box 296, Epping, NSW, 1710, Australia CSIRO Australia Telescope National Facility, PO Box 76, Epping, NSW, 1710, Australia National Centre for Radio Astrophysics, Tata Insitute of Fundamental Research, Pune 411 007, India ICRAR, University of Western Australia, Craw ley, WA 6009, Australia School of Chemical and Physical Sciences, Victoria University of Wel lington, PO Box 600, Wel lington, New Zealand Astronomisches Institut, Ruhr-Universitat Bochum, Universitatsstr. 150, 44801 Bochum, Germany ¨ ¨

200X

ABSTRACT

We present radio images of a sample of six Wide-Angle Tail (WAT) radio sources identified in the ATLAS 1.4 GHz radio survey, and new spectroscopic redshifts for four of these sources. These WATs are in the redshift range of 0.1469-0.3762, and we find evidence of galaxy overdensities in the vicinity of four of the WATs from either spectroscopic or photometric redshifts. We also present follow-up spectroscopic observations of the area surrounding the largest WAT, S1189, which is at a redshift of 0.22. The spectroscopic observations, taken using the AAOmega spectrograph on the AAT, show an overdensity of galaxies at this redshift. The galaxies are spread over an unusually large area of 12 Mpc with a velocity spread of 4500 km s-1 . This large-scale structure includes a highly asymmetric FRI radio galaxy and also appears to host a radio relic. It may represent an unrelaxed system with different sub-structures interacting or merging with one another. We discuss the implications of these observations for future large-scale radio surveys. Key words: galaxies: clusters: general ­ galaxies: active ­ galaxies: general ­ radio continuum: galaxies ­ galaxies: distances and redshifts

1

INTRODUCTION

Wide-Angle Tail (WAT) galaxies are radio galaxies whose radio jets app ear to b end in a common direction. They are generally detected in dynamical, non-relaxed clusters of galaxies (e.g. Burns 1990) and may b e used as prob es or tracers for clusters (Blanton et al. 2000, 2001). Clusters of galaxies are the largest gravitationally b ound structures in the Universe and are p owerful testb eds of cosmological models (e.g. Borgani et al. 2004; Sahl´n et al. e 2009; Kravtsov et al. 2009). Clusters also host diffuse radio emission in the form of radio haloes and relics (Giovannini & Feretti 2000; Feretti 2005; Ferrari et al. 2008; Giovannini et al. 2009). The b ent nature of WATs has commonly b een

attributed to strong intra-cluster winds caused by dynamical interactions such as cluster-cluster mergers (Burns 1998). WATs are preferentially found in enhanced X-ray regions (Pinkney et al. 2000) and are usually associated with dominant cluster galaxies (Owen & Rudnick 1976). Mao et al. (2009a) found the tailed radio galaxies, including WATs, to b e located in the densest regions of clusters in the local Universe, consistent with earlier studies (e.g. Burns 1990; Blanton et al. 2000, 2001). Thus WATs represent valuable tracers of high density regions in the intracluster medium (ICM), and this approach has b een used in a numb er of recent studies (e.g. Blanton et al. 2000, 2003; Smolci´ et al. 2006; Giacintucci & Venturi c 2009; Kantharia, Das & Gopal-Krishna 2009; Oklop cic et al. 2010). Here we present the radio prop erties of six WATs

e-mail: mymao@utas.edu.au c 200X RAS


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that we have identified in ATLAS, the Australia Telescop e Large Area Survey, carried out with the Australia Telescop e Compact Array (ATCA) at 1.4 GHz (Norris et al. 2006; Middelb erg et al. 2008). ATLAS 1 will image seven square degrees of sky over two fields to an rms sensitivity of 10 µJy b eam-1 . The ATLAS fields have b een observed with a numb er of different ATCA configurations, and the typical resolution of the observations is 10 arcsec. The two ATLAS fields, Chandra Deep Field South (CDFS) and Europ ean Large Area ISO Survey-South 1 (ELAISS1), were chosen to coincide with the Spitzer WideArea InfraRed Extragalactic (SWIRE) survey program (Lonsdale et al. 2003) so that corresp onding optical and infrared photometric data are available. In addition to the radio prop erties we present new sp ectroscopic redshifts for four of the WATs and follow-up sp ectroscopic observations of galaxies in the vicinity of the largest WAT in order to prob e its surrounding structure. This WAT was first identified as radio source S1189 by Middelb erg et al. (2008), and is associated with the SWIRE source SWIRE4 J003427.54-430222.5 (Lonsdale et al. 2003). In this pap er we present a summary of the data in Section 2, while the WATs in ATLAS are presented in Section 3. Section 4 presents the results of sp ectroscopic observations of S1189 and its surrounding region, and discusses the large-scale structure in its vicinity. In Section 5 we discuss cosmological inverseCompton quenching and the implications for deep wide radio surveys and ATLAS. This pap er uses H0 = 71 km s-1 Mp c-1 , M = 0.27 and = 0.73 and the web-based calculator of Wright (2006) to estimate the physical parameters. Vega magnitudes are used throughout.

Figure 1. SWIRE 3.6-µm image of the WAT, S1189, and the putative cD galaxy located south-west of the WAT. The 1.4 GHz radio contours which are overlaid start from 100 µJy beam-1 (3 â rms) and increase by factors of 2.

redshifts from the literature. We have obtained 395 new sp ectroscopic redshifts using AAOmega giving a total so far of 564 sp ectroscopic redshifts: 261 in CDFS and 303 in ELAIS-S1. All of the WATs presented in this pap er have sp ectroscopic data from either our AAT observations or 2dFGRS (Colless et al. 2001).

2.3

Follow-up Spectroscopy of the region around S1189

2 2.1

DATA Radio Data

ATLAS radio observations are currently partially complete with an rms noise of 20 - 30 µJy b eam-1 at 1.4 GHz. The data used in this pap er are taken from the first ATLAS catalogues (Norris et al. 2006; Middelb erg et al. 2008) which contain 2004 radio sources. We exp ect 16000 radio sources at the completion of the survey. 2.2 Spectroscopy

As part of ATLAS, we are undertaking a program of redshift determination and source classification of all ATLAS radio sources with AAOmega (Sharp et al. 2006) on the Anglo-Australian Telescop e (AAT). We are currently partway through our ATLAS sp ectroscopy campaign. A summary of these observations are presented by Mao et al. (2009b) while the detailed results will b e presented by Mao et al. (in preparation). 169 ATLAS sources already have sp ectroscopic
1

http://www.atnf.csiro.au/research/deep/index.html

There app ears to b e a cluster of galaxies within 2 arcmin of S1189 in the optical and infrared images (see Fig. 1). We obtained AAOmega observations for sources within a degree of S1189 in service mode during the night of 2008 Octob er 18. The AAOmega sp ectrograph was used in multi-ob ject mode (Saunders et al. 2004; Sharp et al. 2006) and centred on the WAT. We used the dual b eam system with the 580V and 385R Volume Phase Holographic (VPH) gratings centred at 4800 and 7150 covering the sp ectral range b etween 3700° and 8500° A A at central resolutions in each arm of R1300 p er 3.4 pixel sp ectral resolution element. The 5700° dichroic A b eam splitter was used. Observing conditions were good with clear skies and an average seeing of 1.6 arcsec. Two fibre configurations were observed with 3 â 1200 sec integrations and associated quartz-halogen flat fields and combined CuAr+FeAr, Helium and Neon arc lamp frames. Targets were identified from the SWIRE catalogues (Lonsdale et al. 2003). The target magnitude range was limited to 19 < R < 20.5. The bright limit was chosen to select against foreground galaxies based on the exp ected low numb er of galaxies brighter than L* in the p otential cluster. The faint limit was chosen due to the bright-of-moon service observations. The magnitude range yielded 7000 sources within a one degree radius (the field of view of the
c 200X RAS, MNRAS 000, 1­14


WATs in ATLAS
2dF/AAOmega fibre p ositioner (Lewis et al. 2002)) centered on S11892 . Targets were prioritized based on radial separation from the WAT S1189 with the exception of the putative cD galaxy which was assigned the highest priority to ensure that its redshift was obtained. Targets farther than 5 arcmin were randomly sampled using Fisher-Yates shuffles, to decrease the input catalogue to a practical working sample for the configure software and the Simulated Annealing fibre allocation algorithm (Miszalski et al. 2006), as given in Table 1. Regrettably no star-galaxy separation was p erformed resulting in the inclusion of stars in the input catalogue. Although 400 AAOmega science fibres are available, fibre allocation requires target separations in excess of 30 arcsec due to physical limitations. Consequently two indep endent fibre configurations were observed to secure as many high priority sources as p ossible. Data reduction followed the standard pattern for AAOmega sp ectroscopy using the 2dfdr software package. The red and blue arms were reduced indep endently and then spliced together so as to produce a continuous sp ectrum. The redshift was then determined from the sp ectra using runz.

3

in the SWIRE field by Rowan-Robinson et al. (2008). The 2dFGRS shows an overdensity of galaxies associated with S409, which is the nearest WAT in our sample, while the photometric redshifts indicate overdensities of galaxies associated with S483 and S1192. We have examined archival ROSAT All-Sky Survey (RASS) data for X-ray detections, and found no RASS detections towards these WATs. This implies an upp er limit to the X-ray luminosity of p otential host clusters of 2-11â1037 W s-1 which spans the upp er values typical for clusters of galaxies with known X-ray emission (B¨hringer et al. 2001). o This indicates upp er limits to the masses of 2-6â 1014 M (Pratt et al. 2009).

3.1

S132

3

WATS IN ATLAS

We have identified six WATs in ATLAS by visually examining the greyscale ATLAS images (Norris et al. 2006; Middelb erg et al. 2008). Fig. 2 shows the ATLAS greyscale radio images of the WATs in the left column, while images of the WATs sup erp osed on the Digitized Sky Survey (DSS) red and 3.6-µm Infrared Array Camera (IRAC) images are shown in the middle and right columns resp ectively. The WATs range in redshift from 0.1469 to 0.3762, and their prop erties are summarized in Table 2. The radio luminosities at 1.4 GHz range from 2-6â1024 W Hz-1 which places them in the FRI (Fanaroff & Riley 1974) category. For comparison the median luminosities of radio sources associated with cD galaxies in rich and p oor clusters studied by Giacintucci et al. (2007) are 0.7â1024 and 0.2â1024 W Hz-1 at 1.4 GHz. We have estimated the absolute R-band magnitudes of our ATLAS sources and find that these lie close to the transition region in the absolute red-magnitude-1.4 GHz radio luminosity plot of Owen & Ledlow (1994). The optical sp ectra of the five sources for which we have determined redshifts, of which four (S132, S483, S1189 and S1192) are new, are presented in Fig. 3. The redshift of the sixth WAT galaxy, S409, was determined by Colless et al. (2001). We have prob ed for overdensities of galaxies in the vicinity of the WATs. In addition to our observations of S1189 mentioned earlier, we have examined the 2dFGRS (Colless et al. 2001) sp ectroscopic survey, as well as the photometric redshifts of galaxies
2

The largest angular size of the source from end to end along the axis of the source is 0.96 arcmin, corresp onding to a physical size of 309 kp c. The p eak of emission to the southwest of the host galaxy has b een determined to b e an unrelated source, S131 (Middelb erg et al. 2008). There are several galaxies to the west of the southern tail which are seen more clearly in the 3.6-µm image, but at present no redshift information is available for these galaxies. The tails app ear to b end away from this overdensity of galaxies.

3.2

S483

This WAT, which has an overall linear size of 413 kp c, is highly asymmetric in the brightness of the two tails, with the p eak brightness in the northern tail b eing higher by a factor of 5. It would b e useful to image the source, esp ecially the southern tail, with higher surface brightness sensitivity to confirm the present classification. The photometric redshifts of the galaxies (Rowan-Robinson et al. 2008) within a radius of 2 arcmin, which corresp onds to 550 kp c at z=0.3164, show a concentration of galaxies at ab out the redshift of S483 (Fig. 4).

3.3

S1189

The SWIRE input catalogue of Lonsdale et al. (2003) excludes a number of small regions at the outer edge of the field. c 200X RAS, MNRAS 000, 1­14

S1189 is the largest WAT in our sample with an overall linear size of 1053 kp c. Its op ening angle, defined by the lines connecting the regions of highest surface brightness to the optical galaxy is 70 , which is slightly smaller than for the high-redshift WAT rep orted by Blanton et al. (2001) which has an op ening angle of 80 . Clearly these op ening angles would dep end on the resolution of the observations and pro jection effects. Rudnick & Owen (1977) distinguish b etween narrow-, intermediate- and wideangle tails by requiring that the op ening angle b e less than 20 for narrow-angle tailed sources and greater than 90 for WATs, based largely on tailed sources at smaller redshifts than our sources. Although it would b e relevant to examine the effects of resolution and surface brightness sensitivity as one


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S132

S483

S1189

S1192

S031

S409

Figure 2. The six WATs in ATLAS. From top to bottom the WATs are S132, S483, S1189 and S1192 in ELAIS-S1, and S031 and S409 in CDFS. The left column shows the 1.4 GHz radio continuum emission of the WATs in greyscale.The middle column shows the radio contours overlaid on DSS red images. The right column shows the radio contours overlaid on 3.6-µm IRAC images. The radio contours start from 100 µJy beam -1 (3 â rms) and increase by factors of 2. S409 is located at the edge of the 3.6-µm image.

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5

Table 1. Priority assignment for the target observations. AAOmega is configured based on source location and user-defined priority assignment with 9 being the highest priority and 1 being the lowest. Columns 1 and 2 list the priority assignment and the number of sources in each priority bin. Column 3 presents the number of sources for which we were able to obtain redshifts, while Column 4 gives the radii of the priority bin from S1189. Column 5 describes how many sources were selected randomly using Fisher-Yates shuffles. Priority 1 sources were not included in the target list.

Priority 9 8 7 6 5 4 3 2 1

No. sources 1 12 60 100 200 200 200 46 6197

Redshifts 1 0 7 8 15 28 19 8 0

Radii <2 2 to 5 5 to 10 10 to 15 1 5 to 30 30 to 1 deg >5

Comment putative cD

100/187 randomly selected 200/328 randomly selected 200/1956 randomly selected 200/4426 randomly selected sources with previously determined zsp sources not randomly selected

ec

Table 2. Column 4 COSMOS and 7 list the WAT Colless et

Sample of WATs in ATLAS. Columns 1 and 2 give the ATLAS and SWIRE names, Column 3 gives the redshift. lists the observed R-band magnitude from SWIRE, except for S132 where we have listed the value from supersince a value from SWIRE is not available, while Column 5 lists the absolute R-band magnitude. Columns 6 the flux density and luminosity respectively at 1.4 GHz. Columns 8 and 9 list the angular and physical size. All redshifts were obtained from our AAT observations with the exception of S409 whose redshift was determined by al. (2001).

ATLAS ELAIS-1 S132 S483 S1189 S1192 CDFS S031 S409

SWIRE Counterpart

z

Robs (mag) 3762 3164 2193 3690 18. 18. 17. 18. 1 28 12 92

Rabs (mag) - - - - 23. 22. 23. 22. 41 79 04 54

Flux1.4 (mJy) 5.35 6.72 45.03 10.46 42.29 42.35

Power1.4 (1024 W/Hz) 2. 2. 6. 4. 58 16 25 82

Sizeang (arcmin) 1. 1. 5. 0. 0 5 0 9

Sizephy s (kpc) 309 413 1053 274 567 396

SWIR SWIR SWIR SWIR

E E E E

4 4 4 4

J003236. J003311. J003427. J003320.

18215468-

442101. 435512. 430222. 430203.

1 3 5 6

0. 0. 0. 0.

SWIRE3 J032639.11-280801.5 SWIRE3 J033210.74-272635.5

0.2183 0.1469

16.63 16.35

-23.52 -22.84

5.81 2.41

2.7 2.6

finds more tailed sources at moderate and high redshifts, the op ening angle of S1189 is close to that of a WAT. Although WATs do tend to b e associated with the dominant galaxy, it could b e associated with a bright galaxy close to the brightest galaxy in a cluster or group (see Rudnick & Owen 1977; Blanton et al. 2001). The associated galaxy of S1189 is the next brightest galaxy, only 0.75 mag fainter than the cD galaxy. Rudnick & Owen (1977) also suggested that WATs tend to have larger sizes than the narrow-angle tailed sources. With a total size of over a Mp c, it would b e more consistent with the sizes of WATs. Considering all the asp ects, we presently classify it as a WAT. We discuss the results of our AAOmega observations and the environment of this source in Section 4.

ture along with the host galaxy of the WAT. The photometric redshifts (Rowan-Robinson et al. 2008) within a radius of 2 arcmin, which corresp onds to 500 kp c at z=0.3690, show a concentration of galaxies at ab out the redshift of S1192 (Fig. 5). This overdensity is largely due to the galaxies in the filamentary-like structure.

3.5

S031

3.4

S1192

S1192 is similar to S132 in b oth shap e and extent, but the two tails in S1192 are more symmetric in brightness. Both the DSS red and 3.6-µm images show a numb er of galaxies forming a filamentary-like strucc 200X RAS, MNRAS 000, 1­14

Although S031 exhibits distinct gaps of emission b etween the radio core and the two tails of emission, the identification process describ ed by Norris et al. (2006) unambiguously classifies these three comp onents as a triple radio source. The p eaks of emission in the tails are towards the radio core as exp ected in FRI radio sources. We do not have redshift information at present to determine which of the galaxies seen in Fig. 2 may b e a part of the group or cluster associated with S031. The gaps of emission b etween the central source and the lob es are reminiscent of the large radio galaxy in Ab ell 2372 (Owen & Ledlow


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S132 z=0.37620
300 250 200 200 f 100 50 0 4000 0 4000 f 150 300

S483 z=0.31635

100

5000

6000 Wavelength (A)

7000

8000

5000

6000 Wavelength (A)

7000

8000

S1189 z=0.21932
1000 800 600 f 400 200 0 4000 100 0 4000 f 200 400 300

S1192 z=0.36898

5000

6000 Wavelength (A)

7000

8000

5000

6000 Wavelength (A)

7000

8000

S031 z=0.21832
600 500 400 300 200 100 0 4000 5000 6000 Wavelength (A) 7000 8000 f

Figure 3. Spectra of the host galaxies of the five WATs for which we have measured redshifts using the AAOmega spectrograph on the AAT. The spectra are typical of early-type galaxies that host luminous radio sources. The red dotdashed lines indicate the prominent stellar absorption features typical of an early-type galaxy spectrum (Ca H+K, G-band, H-beta and Mg-b) from which the redshift has been derived via template cross correlation. The green dash-dot-dot-dot lines indicate the Fraunhofer A+B atmospheric absorption bands from O2 and the blue long dashed lines indicate the atmospheric water absorption band, neither of which have been corrected due to the absence of appropriate telluric standards in the redshift survey data.

1997; Giacintucci et al. 2007), which has b een suggested by Giacintucci et al. (2007) to b e due to recurrent radio activity (see Saikia & Jamrozy 2009 for a review). Although such a p ossibility cannot b e ruled out, more detailed sp ectral and structural information are required to clarify whether this is indeed the case.

4

LARGE-SCALE STRUCTURE AROUND S1189

There are a total of 309 galaxies with sp ectroscopic redshifts within a radius of one degree of S1189, including 94 galaxies whose redshifts we have measured from our service mode observations with the AAOmega sp ectrograph. The other redshifts are obtained from sp ectroscopic observations of ATLAS sources (Section 2.2). The redshifts of these 94 new galaxies are listed in App endix A.

3.6

S409 4.1 Redshift distribution The redshifts of the 309 galaxies within a radius of one degree (12.6 Mp c at z 0.22) from the WAT source extend to 1.95. The distribution for the subset of 299 galaxies with z0.8 is shown in Fig. 7. The data are binned in intervals of z = 0.005 which corresp onds to 1500 km s-1 . There is a clear excess of galaxies at the redshift of the WAT source, with a distinct p eak at the redshift bin 0.22 z < 0.225. 20 galaxies lie in the p eak-redshift bin, and a further 22 galaxies lie in the two neighb ouring bins resulting in 42 galaxies over three redshift bins, the concentration b eing significant
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S409 is the closest of the WATs in ATLAS with a redshift of 0.1469 (Colless et al. 2001) and a size of 396 kp c. It has an interesting radio structure with the western lob e exhibiting two sharp b ends and forming a long narrow tail of emission. A deep X-ray image would b e useful to understand how the gas distribution may have shap ed the unusual radio structure. The 2dFGRS data (Colless et al. 2001) within a radius of 1 Mp c show a clear excess of galaxies in the same redshift bin as S409 (Fig. 6). Within a radius of 500 kp c (3.5 arcmin at z = 0.1469) we find 3 galaxies at ab out the redshift of S409.


WATs in ATLAS

7

Figure 4. Photometric redshift distribution of galaxies within 2 arcmin of S483 (550 kpc at z = 0.3164). The data is binned in intervals of z = 0.03. The vertical dotted line indicates the redshift of the host galaxy.

Figure 6. 2dFGRS spectroscopic redshifts (Colless et al. 2001) of sources within 7 arcmin of S409 (1 Mpc at z = 0.1469). The data is binned in intervals of z = 0.005. The vertical dotted line indicates the redshift of the host galaxy.

WAT, 60 p er cent of the galaxies listed in Table 3 are within 6 Mp c of the WAT (30 arcmin). We also note that the larger numb er of sources in the southern part of Fig. 8 is due to the uneven coverage of the one-degree-radius field surrounding S1189. 4.3 cD Galaxy

Figure 5. Photometric redshift distribution within 2 arcmin of S1192 (500 kpc at z = 0.3690). The data is binned in intervals of z = 0.03. The vertical dotted line indicates the redshift of the host galaxy.

at 7 . Prop erties of the galaxies in the p eak histogram bin and the two adjacent bins, which includes the putative cD galaxy at a redshift of 0.2204, are listed in Table 3. The total spread in velocity of the 42 galaxies is 4500 km s-1 , and the velocity disp ersion is 870 km s-1 . This is similar to the spread for typical rich clusters in the local Universe undergoing mergers such as A3667 and A3376 which b oth show radio relic emission and have a velocity spread of 4200 km s-1 (Johnston-Hollitt, Hunstead & Corb ett 2008; Johnston-Hollitt et al. 2010; Owers, Couch & Nulsen 2009). The redshift distribution of the 42 galaxies is shown in greater detail as an inset in Fig. 7. The distribution is not a smooth Gaussian and shows substructure, consistent with dynamic, merging systems. 4.2 Spatial Distribution

The bright galaxy, SWIRE3 J003419.26-430334.0, located southwest of the WAT source, has a redshift of 0.2204, implying a velocity difference b etween the two galaxies of 320 km s-1 . Their pro jected separation is 2 arcmin, corresp onding to 420 kp c at a redshift of 0.22. SWIRE3 J003419.26-430334.0 is the brightest galaxy in the cluster and has a diffuse envelop e, therefore we classify it as a p ossible cD galaxy. There is a marginal detection of associated radio emission with a flux density of 140 µJy at 1.4 GHz which corresp onds to a radio luminosity of 1.96 â 1022 W Hz-1 . Centrally dominant cD galaxies are usually giant ellipticals residing in the centres of clusters of galaxies. These are much larger and brighter than other galaxies in the cluster and are often surrounded by a diffuse envelop e (Matthews, Morgan & Schmidt 1964). Their large size is usually attributed to mergers and galaxy cannibalism (e.g. De Lucia & Blaizot 2007). 4.4 Extended radio sources in the vicinity of the WAT

In Fig. 8 we plot the p ositions of the 42 galaxies listed in Table 3, with the galaxies in the three redshift bins (0.215 z < 0.22; 0.22 z < 0.225; 0.225 z < 0.23) indicated by circles of varying size. Despite considerable overlap, there is a suggestion of a velocity gradient with the galaxies in the lowest redshift bin (largest circles) extending towards the south-west and those in the highest redshift bin (smallest circles) extending towards the south-east. Although galaxy redshifts were measured within a radius of 12 Mp c from the
c 200X RAS, MNRAS 000, 1­14

In addition to double-lob ed radio sources, radio haloes, relics and core haloes or mini haloes may also b e associated with clusters of galaxies. Corehaloes are usually less than 500 kp c in extent and associated with the dominant galaxy in cooling core clusters. Haloes and relics are not associated with any particular galaxy, and are often larger in size. Radio haloes are usually pro jected towards the cluster centre, while relics are seen towards the p eriphery (e.g. Giovannini & Feretti 2004). There are 30 radio haloes in nearby (z<0.4) clusters of galaxies (e.g. Giovannini et al. 2009), and there are 30 clusters of galaxies with at least one radio relic (Giovannini & Feretti 2004). While models for haloes


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Figure 7. Histogram of redshifts of 299 galaxies located redshifts less than or equal to 0.8. The redshift bin size redshift distribution of the galaxies at the peak and two which corresponds to 375 km s-1 . The vertical dotted li source.

within a one degree radius is 0.005 which corresponds adjacent bins (0.215 z < nes in both histograms are

centred on the WAT, S1189, and with to 1500 km s-1 . The inset shows the 0.23). The redshift bin size is 0.00125 at z=0.2193, the redshift of the WAT

range from re-acceleration of particles by turbulence to production of relativistic electrons by hadronic collisons, relics are b elieved to arise due to cluster mergers and/or matter accretion (Sarazin 1999; Ryu et al. 2003; Pfrommer et al. 2006; Giacintucci et al. 2008; Johnston-Hollitt, Hunstead & Corb ett 2008; Brown & Rudnick 2009). Recent work suggests that halos are found in massive, unrelaxed clusters, with the radio and Xray luminosity b eing strongly correlated, consistent with the re-acceleration scenario (Brunetti et al. 2007; Venturi et al. 2008; Cassano 2009). However, the present studies have b een based on X-ray selected clusters of galaxies, and p ossible biases arising from it should b e b orne in mind. For example, the limited sensitivity of the radio observations would make it easier to detect halos in only the more X-ray luminous clusters of galaxies. Radio relics on the other hand are b elieved to arise due to mergers accompanied by shocks and/or matter accretion (e.g. Bagchi et al. 2006, and references therein). These shocks are capable of accelerating particles to high energies, giving rise to the observed synchrotron radio emission. Harris, Kapahi & Ekers (1980) and Tribble (1993) were amongst the early ones to suggest and explore the p ossibility of acceleration of particles due to shock fronts on a large scale caused by mergers. These ideas were expanded up on by Enúlin et al. (1998), Roettiger, Burns & Stone (1999) Enúlin & Gopal-Krishna (2001) and Ricker & Sarazin (2001), producing more sophisticated models.

The ATLAS radio image at 1.4 GHz (Fig. 9) shows two more extended sources within 20 arcmin of the WAT source, one of which (S1081) app ears to b e a radio relic (Middelb erg et al. 2008), while the other (S1110) is an FRI radio galaxy. Sup erp ositions of the radio image of the relic on an optical DSS red image as well as an infrared 3.6-µm image are shown in Fig. 10. While no optical ob ject is visible within the radio contours, there is an infrared object towards the central region of the source. This ob ject has b een classified as an Sb c galaxy (optical template typ e 5) using a total of 6 photometric bands by Rowan-Robinson et al. (2008). Its photometric redshift has b een estimated to b e 1.18. Given the properties of the ob ject, it is likely to b e unrelated. The radio prop erties of the WAT and these two sources are summarised in Table 4. At a redshift of 0.22, the relic would have a physical size of 274 kp c and a luminosity of 3.3â1023 W Hz-1 , which would make it similar to the relics found in the p eriphery of clusters of galaxies in the local Universe (Ferrari et al. 2008). The relic is at a pro jected distance of 2 Mp c from the cD galaxy. Typically relics have b een observed at distances of ab out a Mp c from the cluster centre, although some systems are known to have relics up to distances of 4 Mp c (e.g. Giovannini & Feretti 2004). Some of the known examples of relics which lie at distances b eyond 2 Mp c from the nearest cluster core, such as B0917+75 (Harris et al. 1993; JohnstonHollitt 2003), are typically associated with structure larger than a single cluster. In the case of B0917+75 it is the Rood 27 cluster group. This is similar to
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9

Table 3. Optical and infrared properties of the putative cluster members. Column 1 gives the SWIRE identification. Column 2 lists the R-band magnitude from SWIRE while Column 3 lists the redshifts. Column 4 lists the radio flux density at 1.4 GHz while Column 5 provides comments.

SWIRE ID SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E 3 4 4 4 4 4 3 3 3 4 4 3 4 4 4 3 4 3 3 3 4 3 3 4 3 4 3 4 4 3 3 4 4 4 3 4 4 4 4 3 4 3 J003236. J003411. J003107. J003559. J003109. J003512. J003203. J003500. J003355. J003123. J003427. J003422. J003432. J003748. J003525. J003419. J003713. J003339. J003711. J003415. J003714. J003526. J003503. J003645. J003443. J003344. J003707. J003242. J003326. J003229. J003242. J003721. J003306. J003609. J003322. J003604. J003340. J003640. J003734. J003659. J003502. J003300. 915743438531059292875408807213265484928711709881667912011891010530950009234209305209432040. 425952. 434037. 430324. 435010. 425437. 434121. 430309. 424153. 430940. 430222. 430623. 424555. 430211. 432941. 430334. 431342. 430908. 430711. 430840. 430833. 430418. 425710. 432016. 424544. 431627. 430302. 432630. 434051. 425457. 432630. 434240. 431029. 435002. 430419. 435802. 432542. 430000. 433339. 431824. 432410. 432819. 8 0 5 8 9 5 6 5 9 5 5 7 1 9 4 0 8 8 4 9 3 7 2 0 6 8 7 6 0 7 5 0 8 2 5 3 2 1 3 1 9 9

R mag 19.44 18.92 18.74 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

z 2169 2171 2176 2180 2181 2181 2184 2187 2190 2191 2193 2201 2202 2203 2204 2204 2214 2215 2215 2220 2221 2222 2222 2222 2225 2228 2229 2230 2232 2233 2233 2251 2252 2252 2253 2255 2258 2263 2263 2263 2265 2267

Radio flux (mJy) 0.53 0.24 0.21 3.93 14.79

comment

18.07 20.06

17.12 19.53 18. 18. 16. 17. 18. 18. 18. 17. 18. 65 93 37 53 07 32 97 59 45

0.34 45.03 1.08 0.23 0.15

WAT

cD galaxy 2.39

0.53

17.73 17. 19. 18. 18. 18. 17. 17. 19. 20. 18. 17. 17. 18. 18. 18. 87 18 78 76 78 40 71 84 12 24 32 92 20 02 26

0.49 0.91 0.32 0.27

1.33 12.33 0.32 0. 0. 0. 1. 18 34 75 20

double radio

0.19

our situation. It is also relevant to note that the minor axis of the relic does not p oint towards either the WAT source or the cD galaxy, suggesting substructure in this large-scale structure. Simulations of shock generation during hierarchical mass assembly suggest relics can b e produced over 8 Mp c from the cluster centre (Miniati et al. 2000; Pfrommer et al. 2006; Pfrommer, Enúlin & Springel 2008; Hoeft et al. 2008; Vazza, Brunetti & Gheller 2009). These asp ects along with its radio structure and lack of an obvious optical identification make it very likely to b e a radio relic. One could enquire whether this ob ject might b e a dying radio galaxy. The non-detection of an earlytyp e galaxy associated with it suggests that this is unc 200X RAS, MNRAS 000, 1­14

likely to b e the case. There are very few relics known b eyond a redshift of 0.2 (e.g. Giovannini & Feretti 2004), which makes this finding a significant one.

The other interesting source in the field is the FRI radio source S1110. The radio emission from S1110 is symmetric within 80 kp c from the host galaxy, SWIRE4 J003306.30-431029.8, reminiscent of the large-scale jets in FRI radio sources. However, the extended lob es are highly asymmetric, the p eak brightness in the outer extremities differing by a factor of 4. This may b e due to density asymmetries on opp osite sides of the source.


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Mao et al.

Figure 8. Spatial distribution of sources are at 0.215 z < 0.23. show the sources that are at 0.22 location of the WAT is indicated

all galaxies in the field surrounding S1189 that have spectroscopic redshifts. The circled The largest circles show the sources that are at 0.215 z < 0.22. The medium circles z < 0.225 and the smallest circles show the sources that are at 0.225 z < 0.23. The by the large "X".

Table 4. Radio properties of S1189 and extended radio sources in its vicinity. The size of the WAT (S1189) was measured from the outer edge of one lobe to the core and out to the outer edge of the other lobe. The relic is assumed to be at a redshift of 0.22.

ATLAS ID WAT Double radio Relic S1189 S1110 S1081

RA (J2000) 00 34 27.6 00 33 06.3 00 34 11.7

Dec (J2000) -43 02 22.5 -43 10 29.8 -43 12 39.4

Redshift 0.2193 0.2252 (0.22)

S1.4 (mJy) 45.03 12.33 2.35

(10

Power1.4 24 W Hz-1 ) 6.25 1.82 (0.33)

sizeang (arcmin) 5.0 2.6 1.25

sizephy (kpc) 1053 559 (274)

5

IMPLICATIONS FOR DEEP WIDE RADIO SURVEYS AND ATLAS

In this pap er we have rep orted the detection of six WATs from a sample of 2004 radio sources. Extrap olating this to future deep wide surveys, we might exp ect to detect ab out 200,000 WATs from the catalogue of 70 million radio sources that will b e generated by the ASKAP-EMU (Australia SKA Pathfinder - Evolutionary Map of the Universe) pro ject (Norris et al.

2009). Since each of these WATs is likely to b e associated with a cluster, such surveys will b e p owerful tools for detecting clusters and exploring their prop erties, particularly since the radio luminosity of WATs makes them detectable and capable of b eing studied up to high redshifts. Such surveys are therefore likely to contribute significantly to areas such as the formation and evolution of clusters, the formation of massive ellipticals,
c 200X RAS, MNRAS 000, 1­14


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11

Figure 9. 1.4 GHz radio image showing the WAT (S1189), the double-lobed radio galaxy (S1110) and the radio relic (S1081). The WAT S1192 , at z = 0.3690, is also seen in the image.

Figure 10. 1.4 GHz radio contours of the relic (S1081) overlaid on the DSS red optical image (left panel) and the 3.6 µm IRAC image (right panel) The contours start at 100 µJy beam-1 and increase by factors of 2.

c 200X RAS, MNRAS 000, 1­14


12

Mao et al.
(2009) find that most of the sources are compact. Blanton et al. (2003) have identified a WAT galaxy at z=0.96, while Saikia, Wiita & Muxlow (1993) and Saikia et al. (1987) explored the p ossibility that B1222+216 (4C21.35) and B2 1419+315 might b e WAT quasars at redshifts of 0.435 and 1.547 resp ectively. To explore and understand these asp ects will require more detailed modelling and significantly deep er large-scale radio surveys, which is the primary goal of ASKAP-EMU.

and the relationship b etween giant ellipticals and sup ermassive black holes, or SMBHs (e.g. Blanton et al. 2003; Chiab erge et al. 2009). Furthermore, while optical and X-ray surveys tend to select the optically-rich or most X-ray-luminous clusters of galaxies at moderate and high redshifts, sensitive radio observations could help identify clusters with a wide range of optical and X-ray prop erties. However, WATs are characterized by diffuse lob es of emission extending to hundreds of kp c, and two effects p otentially make such structures difficult to observe at high redshift. First, the radiating electrons of a synchrotron source lose energy by inverse-Compton (iC) scattering of the cosmic microwave background radiation (CMBR), whose energy density increases as (1+z)4 . This effect is supp orted by evidence that the X-ray emission from the lob es of large radio galaxies is due to iC scattering of the radiating electrons with the CMBR, which has b een used to make an indep endent estimate of the magnetic field strength of the radio lob es (e.g. Croston et al. 2004, 2005; Konar et al. 2009). Furthermore, Konar et al. (2004) have found that the bridge emission in giant radio sources is less prominent at higher redshifts, which they interpret as b eing caused by iC scattering with the CMBR. Loss of electron energy by iC scattering from the CMB overtakes synchrotron cooling at a redshift z R 0.556 B - 1 where B is the synchrotron magnetic flux density in µGauss (Schwartz et al. 2006). So, for a constant B, one might exp ect synchrotron emission to fall sharply ab ove that redshift. However, if a low-luminosity radio source is modelled as two cones of expanding plasma on either side of the central SMBH, then the magnetic field would b e exp ected to fall as the square of the distance r from the SMBH, resulting in a transition radius rcrit at which the dominant electron cooling mechanism switches from synchrotron to iC, where rcrit (1 + z )-1 . Thus, rather than synchrotron emission falling sharply ab ove some redshift, the size of the synchrotron-emitting region shrinks linearly with redshift. We conclude that, while iC cooling reduces the apparent size of the emitting region, it does not imp ose a fundamental redshift limit ab ove which WATs will b e invisible. Second, high-redshift galaxies are sub ject to cosmological surface brightness dimming, (e.g. Lanzetta et al. 2002, and references therein) which causes the observed surface brightness p er unit frequency interval of a resolved source to decrease as (1+z)3 . Thus, nearby radio galaxies are detectable to much lower intrinsic surface brightness thresholds than high-redshift sources. While b oth these effects are going to present challenges to the identification of WATs at high redshifts, they accentuate the normal challenges of resolution and sensitivity, rather than presenting fundamental limits of observability. In their search for FRI radio sources in the redshift range 1
6

CONCLUSIONS

We have identified a sample of six Wide-Angle Tail (WAT) radio sources. We present new sp ectroscopic redshifts for four of these sources, and find that these WATs lie in the redshift range 0.1469-0.3762. We have examined the fields using b oth sp ectroscopic and photometric redshifts of galaxies in the vicinity of the WATs and find evidence of an overdensity of galaxies in four of these WATs. From a more detailed study of the field around S1189 we find an overdensity of galaxies which is spread over 12 Mp c and has a velocity spread of 4500 km s-1 , and a velocity disp ersion of 870 km s-1 . This large-scale structure hosts a putative cD galaxy with, at b est, weak radio emission, a radio relic which has a size of 274 kp c, and an asymmetric FRI radio galaxy with an extent of 559 kp c. The p eak brightness at the extremities of the outer lob es of the FRI source differ by a factor of 4, p ossibly due to differences in the environment on opp osite sides. The minor axis of the relic is not directed towards either the host galaxy of the WAT or the putative cD galaxy. This large-scale structure may represent an unrelaxed system with different sub-structures interacting or merging with one another. Therefore, deep X-ray observations of the field would b e very valuable to further understand this interesting large-scale structure. WATs are known to occur in clusters of galaxies, and could in principle b e useful tracers of clusters at moderate and high redshifts. IC cooling of electrons by interaction with CMBR increases rapidly with z. However, this does not imply a sharp drop in the numb er of WATs at high z. Deep and wide-field surveys, such as the Evolutionary Map of the Universe (EMU) (Norris et al. 2009), should provide additional information and insights on the range of structures at moderate and high redshifts. We exp ect these to b e invaluable prob es of large-scale structure.

ACKNOWLEDGEMENTS We thank Emil Lenc and Jamie Stevens for their help, the ATLAS team and Mark Birkinshaw for many fruitful discussions, and an anonymous referee whose comments help ed improve the manuscript. MYM acknowledges the supp ort of an Australian Postgraduc 200X RAS, MNRAS 000, 1­14


WATs in ATLAS
ate Award as well as Postgraduate Scholarships from AAO and ATNF. We thank the staff at AAO and ATCA for making these observations p ossible. The ATCA is part of the Australia Telescop e, which is funded by the Commonwealth of Australia for op eration as a National Facility managed by CSIRO. This research has also made use of NASA's Astrophysics Data System.

13

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APPENDIX A: NEW REDSHIFTS

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Table A1. New redshifts of galaxies near S1189.

15

SWIRE ID SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 J003134. J003147. J003152. J003203. J003205. J003209. J003223. J003229. J003229. J003236. J003242. J003243. J003243. J003248. J003249. J003251. J003300. J003308. J003309. J003310. J003312. J003313. J003317. J003322. J003322. J003335. J003339. J003343. J003348. J003351. J003355. J003400. J003403. J003404. J003404. J003408. J003410. J003410. J003413. J003415. J003415. J003419. J003421. J003422. J003426. J003428. J003430. 0297980598954813919101839195799209149594889194007904849184109208958190198196832287269908018297425148. 432431. 431701. 434121. 432339. 432147. 432147. 434406. 425457. 432040. 432630. 430936. 425533. 425132. 423818. 432910. 432819. 430217. 430020. 424121. 431547. 432722. 432925. 430419. 431047. 425458. 430908. 432149. 430904. 424258. 424153. 430537. 425805. 431335. 430945. 431736. 424105. 430444. 425647. 430234. 430840. 430334. 425817. 430623. 434349. 425203. 425901. 8 8 7 6 9 1 1 2 7 8 5 9 9 5 9 4 9 3 2 5 4 2 6 5 0 6 8 5 4 2 9 8 9 8 6 3 3 7 6 2 9 0 0 7 1 7 6

z 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 1. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 21061 44433 24509 21837 39604 20451 27936 35209 22330 21686 22334 20711 14944 21149 30177 28767 22674 18372 37209 24725 27951 33233 19152 22525 07282 21277 22149 40215 20033 26489 21902 20725 32737 18986 27894 14801 27872 42139 32816 18821 22201 22040 41943 22014 20469 12161 14762

SWIRE ID SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR SWIR E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 3 3 3 3 3 J003443. J003445. J003446. J003446. J003448. J003452. J003455. J003458. J003458. J003459. J003500. J003501. J003503. J003506. J003509. J003513. J003519. J003526. J003526. J003527. J003530. J003535. J003537. J003538. J003542. J003551. J003552. J003552. J003556. J003556. J003600. J003609. J003611. J003619. J003630. J003636. J003645. J003645. J003647. J003655. J003659. J003706. J003706. J003707. J003711. J003728. J003748. 6606929284689293950392049823898109707525921870087483049802950600101677800688940730387012920209424544. 425832. 431108. 431221. 424223. 430124. 433249. 430150. 425637. 425642. 430309. 424205. 425710. 425900. 430642. 430046. 431158. 430418. 435641. 425327. 424426. 430900. 422625. 425640. 425959. 424442. 430205. 432142. 421810. 433947. 424555. 424433. 425004. 424839. 423814. 432152. 423419. 431028. 431037. 425404. 431824. 431442. 431836. 430302. 430711. 434143. 433353. 6 2 0 4 5 9 9 5 6 3 5 0 2 7 4 2 8 7 2 0 3 6 0 0 5 1 0 4 9 0 6 8 1 2 6 6 9 3 1 1 1 3 0 7 4 2 8

z 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 22247 42247 32148 02507 38740 18383 18833 31710 32933 33043 21871 41316 22218 12141 20673 32167 17832 22216 32339 04506 53011 32241 03611 26549 05296 07070 18486 39342 24246 42403 54820 18637 33055 20139 05462 15536 32288 30023 29938 27827 22634 66842 01959 22293 22153 20727 30986

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