Clusters of Galaxies as Nature's Giant Telescopes

Shude Mao

Jodrell Bank Center for Astrophysics, University of Manchester, Manchester M13 9PL, UK;
shude.mao@manchester.ac.uk

Clusters of galaxies are the largest gravitational bound structures in the universe. Their number density depends sensitively on the cosmological model, in particular the amplitude of the initial fluctuations of the cosmological density field. Clusters of galaxies also provide a more direct test of the predicted density profiles of dark matter haloes than galaxies since the effect of baryons is less important. Clusters are ideal laboratories to study the dynamical and gas processes such as tidal stripping and ram-pressure stripping as galaxies merge and evolve within the cluster potential. Thus, the understanding of the population of clusters of galaxies is an important task in astrophysics.

The intense gravitational field of a galaxy cluster distorts, deflects and magnifies background galaxies along the line of sight (gravitational lensing). The most spectacular strong gravitational lensing occurs when a background galaxy is sufficiently well aligned with a cluster. In such cases, a source can be multiply-imaged or even distorted into long, thin giant arcs (see the beautiful Hubble Space Telescope images of Abell clusters A1689 and A2218). Clusters are undoubtedly Nature's giant (gravitational) telescope. As such, they allow us to study the lensed background source at great magnificaton, for example, to probe the highest-redshift galaxies (e.g. Richard et al. 2008). More subtle shape distortions of background galaxies (``weak lensing'') occur in the outer regions of clusters of galaxies. It is clear that strong lensing by clusters of galaxies is quite rare (e.g. Li et al. 2005; Wambsganss, Ostriker, & Bode 2008; Dalal, Holder, & Hennawi 2004; Puchwein et al. 2005), so systematic large-area surveys of the sky are necessary to identify these systems. Several previous surveys have been performed in the X-ray and optical (e.g. Luppino et al. 1999; Gladders et al. 2003; Sand et al. 2005). The large-area, uniform Sloan Digital Sky Survey (SDSS) provides an ideal database to search for strong lenses since tens of thousands of new clusters of galaxies have been uncovered (e.g. Koester et al. 2007).

The paper by Wen et al. (2009) conducted just such a search using their own catalog of $\sim 40,000$ clusters identified with Data Release 6 of SDSS. They identified 4 almost certain strong lensing systems and 9 other possible systems in addition to the 6 known cases. Two of the four definite cases are Einstein rings while the other two are giant arcs. Five of these (including the two most secure candidates) overlap with strong lens candidates discovered by Belokurov et al. (2008) in the CASSOWARY survey1. The latter survey was triggered when the author gave a talk at Cambridge about cluster strong lensing. It is interesting to see that none of the two groups identified all the candidates. This arises partly because they are selected differently, but it is also because visual searches such as the one conducted by Wen et al. are subjective. An objective systematic search of strong lensing clusters is desirable. Several automated methods have been proposed to search for giant arcs by clusters (see Alard 2006; Seidel & Bartelmann 2007); for galaxy-scale lenses, see Marshall et al. (2008). A neural network approach may be a competitive way to discover lenses in huge databases. In any case, the secure confirmation of these candidates will require followup spectroscopy and ideally high-resolution images from space. Deep, high-resolution images of these candidates will also allow a combined study of these lensing clusters, probing both the central and outskirts of clusters simultaneously. In fact, the field is advancing so rapidly that two of the candidates discovered in this paper have already been spectroscopically confirmed by Kubo et al. (2008) - one almost certain candidate SDSS J111310.6+235639, and one probable candidate SDSS J113740.0+493635; the latter was also discovered by Belokurov et al. (2008).

The lens candidates with separations of a few arcseconds from the cluster center may be particularly interesting since they provide an important probe for the central cluster galaxy. A long slit suitably oriented (or better, an integrated field unit) will not only provide the redshift for the background galaxy but also dynamical information about the central galaxy. The combination of lensing and dynamics may provide unique insights into whether their kinematics are significantly different from intermediate mass ellipticals that usually reside in groups (or in the field). This powerful combination has already been demonstrated with the SLACS survey for galaxies 2. In the singular isothermal sphere model for the lens, the angular Einstein radius is given by

\begin{displaymath} \theta_E \approx 4 \pi \left(\frac{\sigma}{c}\right)^2 \frac{D_{\rm ls}}{D_{\rm s}}, \end{displaymath}

where $\sigma$ is the velocity dispersion; $D_{\rm s}$, $D_{\rm ls}$ are the angular diameter distances from the observer to the source and between the lens and the source, respectively. Since the velocity dispersion and the lens (cluster) redshift are known, and $\theta_E$ is roughly equal to the distance of the arc from the cluster center, one may hope to derive approximate constraints on the source redshift. One uncertainty in this exercise is the unknown extra boost provided by the cluster itself. The fact that the velocity dispersions inferred using this model by Kubo et al. (2008) can reach as high as $878 {\rm km s^{-1}}$ implies that the clusters (groups) have almost certainly boosted the image separation, similar to that for the first gravitational lens ever discovered Q 0957+561, also residing in a cluster (Walsh, Carswell & Weymann 1979).

The study by Wen et al. underscores an important opportunity for astronomers without easy access to telescopes - with the exploding observational data created by multi-wavelength surveys, astronomical research with archive data will become more and more important: potential scientific results will only be limited by our imagination to come up with different ways of exploiting observational data. With a large number of ongoing and planned large surveys, many discoveries (including strong lenses by clusters) from archive data are surely in store for us. The work by Wen et al. demonstrates that, with modest resources, new exciting discoveries can be made anywhere in the world.

References

Alard, C. 2006, arXiv e-prints (arXiv:astro-ph/0606757)
Belokurov, V., Evans, N. W., Hewett, P. C., et al. 2008, ArXiv e-prints (arXiv:0806.4188)
Dalal, N., Holder, G., Hennawi, J. F., 2004, ApJ, 609, 50
Gladders, M. D., Hoekstra, H., Yee, H. K. C., et al. 2003, 593, 48
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Kubo, J. M. Allam, S. S., Annis, J., Buckley-Geer, E. J., Diehl, H. T., Kubik, D., Lin, H., Tucker, D. 2008, arXiv e-prints (arXiv:0812.3934)
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Footnotes

... survey1
http://www.ast.cam.ac.uk/research/cassowary/
... galaxies2
http://www.slacs.org/, and references therein