6 Arclet and candidate lensing clusters

We believe that all 8 of the clusters in the previous section show clear examples of lensing. In this section, the situation is not so obvious. We present 8 additional clusters which contain structures we classify as arclets or candidate arcs. Of these 8 clusters, we are confident that at least 2 of them (MS1008-12 and MS1231+15) are true lens systems. The properties of the arclets and candidate arcs are listed in Table 3.

As mentioned earlier in Sect. 4, we define an arclet simply as a short arc with l$\,<\,$8'' and with an axis ratio 3$\,<\,$l/w$\,<\,$10. Although, as with the giant arcs, we do not discriminate between high and low surface brightness arclets (the former sometimes called "mini-arcs''; e.g. see Lavery et al.1993) for the most part our data are not deep enough (see limits in Table 1) for us to have detected faint arclet populations similar to the ones in A1689 (Tyson et al. 1990; Tyson & Fischer 1995) or A2218 (Pello et al.1992; Kneib et al.1995; Kneib et al.1996).

Most of the arclets we find do not display any obvious curvature, partly because they are short, but also possibly because of the presence of a secondary deflector along the line of sight. It is difficult, therefore, to distinguish these putative arclets from blue, edge-on disk galaxies, whether foreground, background, or cluster members, or possibly interacting or morphologically-disturbed systems. Although edge-on spirals often have well-defined nuclei, there are examples where no nucleus can be seen, and without additional information (e.g. spectra or infrared colors) these objects will be classified as arclets. However, we expect the number of cluster and projected edge-on spirals to be small (see discussion in LeFèvre et al.1994). Furthermore, we expect the real arclets to be oriented more-or-less perpendicular to the cluster radius vector. In the following subsections, we describe each arclet and candidate lensing cluster in detail.

  

Table 3: Candidate arcs and arclets

6.1 MS0451.5+0250

  

Figure 7: R-band images of EMSS clusters with arclets and candidate arcs

This cluster lacks a well defined optical center. The X-ray emission is extended and may be associated with both the primary SW clump of galaxies and a secondary clump located to the NE where we find the candidate arclet. The two clumps are separated by $\,\sim\,$6'. The image of the candidate arclet shown in Fig. 7 is the sum of the B and R frames. The arclet is located 21'' to the NE of the large elliptical galaxy. We cannot rule out that this is simply an edge-on cluster spiral.

  

Figure 8: R-band images of EMSS clusters with arclets and candidate arcs

6.2 MS0839.8+2938

The core of this rich, z=0.194 cluster contains a number of short, blue, linear structures that may be arclets or edge-on spirals. The longest of these structures extends radially $3\hbox{$.\!\!^{\prime\prime}$}8$ (16$\,h_{50}^{-1}$ kpc) from the center of the cluster to the SE of the central cluster galaxy and may be another example of a radial arc (see Fig. 7). We have also considered whether this structure could be a cooling flow filament, since the brightest cluster galaxy has strong [OII], H$_\alpha$, and [NII] emission, that Nesci et al.(1989) interpreted as the signature of a large cooling flow. Recent ROSAT observations have confirmed this cooling flow exists (Nesci et al. 1995). When the central particle density in a cooling flow exceeds $\,\sim\,$$5-6\ 10^{-3}$ cm^-3, we expect to see strong optical emission lines (hydrogen recombination lines and collisionally excited forbidden lines) from a $\sim$10 kpc asymmetrically-extended region around the cluster BCG (Hu et al. 1985; Cowie et al.1983). Such filaments have been seen in the narrow band images of EMSS cluster BCG's taken by DSG, so it is possible that the radial structure in MS0839+2938 is such a filament. For now, we tentatively classify this radial structure as a candidate radial arc, and we intend to obtain a spectrum which will allow us to distinguish between the lensing and cooling flow interpretations.

6.3 MS0906.5+1110

MS0906+1110 is a rich cluster with a large cD galaxy at z=0.180. The X-ray luminosity is $L_{\rm x}=5.77$ 10⁴⁴ ergs^-1. The cluster displays obvious optical substructure with a bright elliptical galaxy $\,\sim\,$95'' to the SW and another bright elliptical nearly an equal distance to the NE, both along the major axis of the cD. These two galaxies are not visible in Fig. 7, but can be seen in the wide field image in Gioia & Luppino (1994). This cluster has no obvious giant arcs or bright arclets, but does contain a number of faint arclets arranged tangentially around the cD. The largest of these is labeled in Fig. 7 and is listed in Table 3.

6.4 MS1008.1-1224

MS1008-1224 is a very rich cluster at z=0.301 with $L_{\rm x}=4.49\ 10^{44}$ ergs^-1. Yee et al. (1998), measured 112 cluster member redshifts and give an updated z=0.306. This cluster contains at least one obvious example of a lensed arclet, but no giant arcs (LeFèvre et al.1994). When displayed at high contrast, the cluster image exhibits a clear patterned circularity centered on the brightest cluster galaxy. The longest arclet located 51'' to the North of the BCG is obviously curved. There is another bright, blue candidate arclet to the East of the BCG (see Fig. 8).

6.5 MS1231.3+1542

This z=0.238 cluster contains a faint arc $4\hbox{$.\!\!^{\prime\prime}$}7$ long located midway between the two brightest cluster galaxies, a bright arclet $25\hbox{$.\!\!^{\prime\prime}$}7$ to the south of the BCG, and a faint arclet to the North of the second ranked galaxy. The faint arc and arclet are difficult to see in Fig. 8 because they are embedded in the common halo of the cluster dumbbell galaxies. This cluster and lens system is similar to MS0302+1658. Both clusters have a lensed arc or arclet midway between two nearly-equal-brightness dumbbell galaxies in the cluster core. With $L_{\rm x}=2.88$ 10⁴⁴ ergs^-1, this cluster is one of the lowest $L_{\rm x}$ EMSS clusters to display obvious lensing.

6.6 MS1455.0+2232

This cluster was first discovered by Schild et al.(1980) and later observed by Mason et al.(1981) who measured the redshift of z=0.259. MS1455+2232 is among the most X-ray luminous clusters in the EMSS with $L_{\rm x}=1.6$ 10⁴⁵ ergs^-1. It also has, by far, the largest narrow-band line emission luminosity in the cooling flow survey of DSG (see Table 1).

MS1455+22 has been the focus of a weak lensing study by Smail et al.(1994) and Smail et al. (1995) using the weak lensing technique of Kaiser & Squires (1993). Smail et al.(1994) detect a clear lensing signal from the cluster and estimate a lower limit to the projected mass within the central $\,\sim\,0.9$ Mpc of $\,\sim\,2.2$ 10¹⁴ $M_{\hbox{$\odot$}}$.However, their images only had a field of view of $5'\times5'$and so only cover the core of the cluster and do not provide any information about the mass distribution at large radii. Smail et al.(1995) have also presented ROSAT HRI data that detect extended emission out to $\sim$1 Mpc and resolve the central 50kpc. Their data show the X-ray emission is highly peaked on the cD galaxy and confirm the cluster has a massive cooling flow of 630$^{+257}_{-178}\; M_{\hbox{$\odot$}}$ yr^-1.

This cluster contains a $\,\sim\,7\hbox{$.\!\!^{\prime\prime}$}5$ long arc-like structure located 20'' NE of the BCG (see Fig. 7). This putative arc is also visible in Smail et al.images, which are considerably deeper than ours (for this particular cluster) and have comparable seeing. Upon close inspection, we notice what may be a galaxy nucleus to one side and the arc appears slightly curved, but in the direction away from the central galaxy. Although we include this arclet in our list of candidates, we consider this a questionable case.

6.7 MS1546.8+1132

MS1546+11 is a rich, elongated cluster with a large, double-nucleus cD galaxy. The ellipticity of the cD halo is oriented in the same direction (EW) as the elliptical distribution of the optical galaxies, and orthogonal to the 2 arc-like structures to the East of the cD. The largest candidate arc is $4\hbox{$.\!\!^{\prime\prime}$}2$ long, is bright, and appears to be marginally resolved. Again, this could simply be an edge-on cluster galaxy since it has a somewhat lenticular shape, but we see no obvious bright nucleus.

6.8 MS1910.5+6736

This cluster at z=0.246 has a double nucleus BCG and is rather poor with an irregular morphology. There are 2 elongated, linear structures located $\,\sim\,$1' to the SE of the BCG. The northernmost object has an obvious core and looks like an edge-on spiral. The other object, however, is $\,\sim\,$6'' long, has no apparent core, and appears unresolved. LeFèvre et al.(1994) classified this object as a giant luminous arc since it had an axis ratio l/w>10 and was oriented perpendicular to the radius vector to the BCG. Our criteria in this paper, however, require that an object have $l\geq8''$to be called a giant arc. Furthermore, we believe the lensing explanation for this object is less convincing than in some of the other cases, particularly since the object has no apparent curvature.