An important result of the RASS-VLA cross-correlation is the detection of a number of likely galaxy groups and clusters where the X-ray emission may derive from a hot intracluster medium. While most clusters reported from X-ray surveys depend on X-ray extent and optical galaxy counts, we include radio extent as an alternative criterion, that is, the presence of extended or complex radio emission from a radio galaxy within the cluster. Studies with earlier X-ray satellites have established that a statistical link between X-ray and radio emission is present in clusters (Sarazin 1986 and references therein; but see e.g. Burns et al.1994), perhaps the consequence of lobe confinement or feeding of the central AGN by the hot medium.
Ten sources in our study are definite or likely galaxy groups, six of which are known from previous optical or X-ray surveys and four are proposed new identifications. Candidates were chosen by examining the morphology of RASS-VLA sources for which either the X-ray or radio emission is extended. We eliminated from consideration sources likely to be AGN, where an unresolved X-ray source coincides with the core of the radio morphology. Optical images were then examined for evidence of a group or cluster of galaxies. No formal galaxy count criterion was used. The ten group candidates exhibit a mix of morphological characteristics. In eight cases, the X-ray emission is extended in the RASS data. Conversely, all but one of the sources with extended X-ray emission satisfied our criteria for a candidate group. (The exception has only a marginally significant ML value, see notes to Table 2.) At least three cases have compact radio emission rather than resolved lobes. In four cases, both the X-ray and radio emission are extended or have multiple components.
While our use of radio morphology finds new group candidates, we recovered some but not all of the groups found in previous X-ray studies of the NEP region. The Einstein Extended Medium Sensitivity Survey (EMSS) serendipitously discovered one cluster in our survey region (Henry et al.1992). One of our group candidates, 1755.4+6803, lies near the EMSS cluster's position, but the VLA source lies several arcminutes east of the main cluster emission and may not be related. Burg et al. (1992) analyzed a 50 ksec PSPC pointing and all-sky scans, and identified a region of extended, low X-ray surface brightness (first reported by Hasinger et al.1991) as a new cluster with a richness class of and redshift . This cluster, labeled NEPX1, along with six Abell clusters and other likely groups of similar redshift within 50 Mpc, suggest a supercluster is present. We find a RASS-VLA source (VLA 1801.5+6645) at NEPX1 which marginally fits our criteria for a group candidate (see notes to the figures, below); however, the X-ray source is not extended in the RASS data. Henry et al. (1995) used RASS observations of a large region around the NEP with an optical criterion to select galaxy groups. They identified four groups within the boundaries of our survey, two of which (1756.5+6512 and 1751.1+6531) we also select as groups. Both have unresolved radio sources and optical images suggesting the groups are cD-dominated. Of the other two Henry et al.groups, one has a number of VLA sources within the apparent group boundaries, while the other may have one or two associated VLA sources. Bower et al.(1996) analyzed 80 ksec of PSPC pointings and did not detect NEPX1, but noted that their detection algorithm would miss a cluster of such low X-ray surface brightness. They did detect a new, high-redshift (z=0.57) cluster, RX J1801.7+6637, with an X-ray flux of erg cm-2 s-1, near the limit of our survey and not detected by us in the X-ray or radio.
Of the four Abell clusters (Abell et al.1989) which lie within the boundaries of our survey, two are associated with RASS-VLA sources: 1743.4+6342 lies within the region of Abell 2280, and the triple VLA source 1819.6+6857, 1819.6+6856, and 1819.7+6856 is a member of Abell 2304. X-ray emission for these sources has been previously reported from ROSAT data (Gioia et al.1995; Gómez et al. 1997).
Due to the diversity of morphologies, we provide figures for each of the group candidates and discuss them individually. Figure 5 gives grey-scale optical images of each candidate group, produced with the Skyview utility provided by the HEASARC, overlaid with the radio contours of the VLA sources (Hertz et al.1994). Contours levels are percentages of the radio peak chosen so that the least significant contour is approximately twice the noise and subsequent contours increase by factors of two; the highest contour is 99% of the radio peak. Except for 1801.5+6645, which has a slightly larger field, the images are 44 in size. The lowest contour level for each of the images is given in notes to the figures. Radio spectral indices are given in Kollgaard et al.(1994). Additional source information is given in the notes to Tables 2 and 3.
Notes to the figures:
1743.4+6342 Fig. 5a: 8C 1743+637. The radio position is in the outer regions of extended X-ray structure located predominantly to the south and associated with Abell 2280 (see Gioia et al.1995). However, Lacy et al.(1993) suggest that the optical counterpart to the radio galaxy is at a much higher redshift. (See Table 4 for X-ray spectral fit.) The lowest radio contour level is at 2% of the peak value.
1749.8+6824, 1749.8+6823 Fig. 5b: The centroid of the X-ray emission lies approximately midway between the VLA sources 1749.8+6824 (MCG+11-22-005) and 1749.8+6823 (KUG1750+683A). The extended X-ray structure is complex with a pronounced "emission hole" on the single radio galaxy, 1749.9+6824 (CGCG1750.1+6825). Lowest radio contour level at 5% of the peak value.
1751.1+6531 Fig. 5c: NGC 6505 (CGCG1751.1+6533). The radio source is centered on the maximum of the X-ray emission. Strong extended X-ray structures lie mainly in the north-western part of the image. The optical galaxy is an E/S0 with readily apparent halo, possibly a cD galaxy in the group or cluster. (See Henry et al.1995). Lowest radio contour level at 30% of the peak value.
1751.4+6719, 1751.6+6719 Fig. 5d: The X-ray centroid is between these two VLA sources, closest to 1751.4+6719 which appears to be the core of a radio galaxy; 1751.6+6719 and 1751.2+6719 are the apparent lobes. The X-ray emission is circularly extended with 1751.2 just outside the main emission. (See Table 4 for X-ray spectral fit.) All three radio sources are shown in Fig. 5d with the lowest radio contour at 8% of the peak.
1755.4+6803 Fig. 5e: We select a group candidate near the position of the EMSS cluster MS1754.9+6803. The RASS source spatially correlated with the VLA source is embedded in a relatively faint chain of emission. The main, very strong cluster emission lies several arcminutes west and has been identified with ZwC1754.5+6807 (Gioia & Luppino 1994 and Henry, private communication). Thus our selection of this cluster may be coincidental. Lowest radio contour at 16% of peak value.
1755.7+6754 Fig. 5f: The radio source is located in the outer part of relatively weak extended emission of size, elongated towards the southeast. The optical image shows a possible elliptical with readily apparent halo, perhaps a cD galaxy in a group or cluster. The lowest radio contour is at 12% of the peak value.
1756.5+6512 Fig. 5g: CGCG1756.3+6513. The radio source coincides with one of the peaks of very extended strong X-ray emission, mainly in a south-westerly direction. The optical galaxy is an E/S0 with readily apparent halo, possibly a cD galaxy in the group or cluster. (See Henry et al.1995). (See Table 4 for X-ray spectral fit.) Lowest radio contour at 20% of peak value.
1801.5+6645 Fig. 5h: Two VLA sources, 1801.5+6645 (unresolved, weak) and 1801.4+6643 (possibly resolved, strong) lie near the unresolved RASS source. In the south the maximum of the X-ray emission coincides with the compressed contour lines to the south-east of 1801.4+6643. The chain of optical objects north of 1801.5+6645 follows a narrow structure of reduced X-ray emission. Burg et al.(1992) detected NEPX1, a region of low surface brightness X-ray emission in the vicinity of the VLA and RASS sources. They give optical identification of several associated galaxies. Note: the RASS-VLA source is listed in Table 3 because the X-ray-radio offsets are greater than our criteria for unresolved sources (but within our criteria for extended or multiple-component sources). Lowest radio contour at 8% of the peak value.
1806.8+6536 Fig. 5i: The radio source coincides with a local maximum of a cross-like structure in X-rays. The strongest X-ray emission is found to the north-east of 1806.8+6536, near the brighter, extended optical object. Lowest radio contour is at 8% of the peak value.
1819.6+6857, 1819.6+6856, 1819.7+6856 Fig. 5j: 4C 68.21 in Abell 2304. 1819.6+6857 is one of a sample of wide-angle tailed radio galaxies in Abell clusters analyzed by Gómez et al. (1997) as part of a study of substructure in cluster X-ray emission. (See also Owen et al.1996 and references therein.) While the bulk of the cluster's X-ray emission is a few arcminutes to the east, local X-ray maxima trace closely the peaks of the radio contours. An 15.0 mag optical galaxy coincides with the apparent radio core, 1819.7+6856. (See Table 4 for X-ray spectral fit.) Lowest radio contour at 0.5% of the peak value.
|Figure 6: Flux distribution of the NEP sources (shaded) compared to that of the ROSAT - 87 GB survey (open histogram)|
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