B.11. DOES STRONG LENSING HAVE A FUTURE?
Well, you can hardly expect an answer of "No!" at this point, can you? Since we have just spent nearly 170 pages on the astrophysical uses of lenses, there is no point in reviewing all the results again here. Instead I suggest some goals for the future.
Our first goal is to expand the sample of lenses from ~ 100 to ~ 1000. While 80 lenses seems like a great many compared to even a few years ago, it is still too few to pursue many interesting questions. The problem worsens if the analysis must be limited to lenses meeting other criteria (radio lenses, lenses found in a well-defined survey, lenses outside the cores of clusters ... ) or if the sample must be subdivided into bins (redshift, separation, luminosity ... ). For example, one of the most interesting applications of lenses will be to map out the halo mass function. This is difficult to do with any other approach because no other selection method works homogeneously on dark low-mass halos, galaxies of different types, groups and clusters. Unlike any other sample in astronomy, gravitational lenses are selected based on mass rather than luminosity, so the same search method works for all halos - the separation distribution of lenses is a direct mapping of the halo mass function. It is not a trivial mapping because the structure of halos changes with mass, but the systematics are far better than those of any other approach. The fact that lenses are mass-selected also gives them an enormous advantage in studying the evolution of galaxies with redshift over optically-selected samples where it will be virtually impossible to select galaxies in the same manner at both low and high redshift. There is no shortage of detectable lenses in the universe - it is simply a question of imaging enough of the sky at high angular resolution. The upgraded VLA and Merlin radio arrays are the most promising tools for this objective.
Our second goal is to systematically monitor the variability of as many lenses as possible. Time delays, if measured in large numbers and measured accurately, can resolve most of the remaining issues about the mass distributions of lenses. This is true even if you regard the H0 as unmeasured or uncertain - the Hubble constant is the same number for all lenses, so as the number of time delay systems increases, the contribution of the actual value of the Hubble constant to constraining the mass distribution diminishes. At the present time, we are certain that the typical early-type galaxy has a substantial dark matter halo, but we are uncertain how it merges with the luminous galaxy. Steady monitoring of microlensing of the source quasars by the stars in the lens galaxy will also help to resolve this problem because the patterns of the microlensing variability constrain both the stellar surface density near the lensed images and the total density (Part 4, Schechter & Wambsganss ). The constraints from time delays and microlensing will be complemented by the continued measurement of central velocity dispersions.
Our third goal should be to obtain ultra-deep, high resolution radio maps of the lenses to search for central images in order to measure the central surface densities of galaxies and to search for supermassive black holes. Keeton ([2003a]) showed that the dynamic ranges of the existing radio maps of lenses are 1-2 orders of magnitude too small to routinely detect central images given the expected central surface densities of galaxies. Only very asymmetric doubles like PMN1632-0033, where Winn et al. () have detected a central image, are likely to show central images with the present data. Once we reach the sensitivity needed to detect central images, we will also either find central black holes or set strict limits on their existence (Mao, Witt & Koopmans ). This is the only approach that can directly detect even quiescent black holes and determine their masses at cosmological distances. The existing limits could be considerably improved simply by co-adding the existing radio maps either for individual lenses or even for multiple lenses in order to obtain statistical limits.
Our fourth goal should be to unambiguously identify a "dark" satellite of a lens galaxy. For starters we need to conduct complete statistical analyses of lens galaxy satellites in general, by determining their mass functions and radial distributions. As part of such an analysis we can obtain upper bounds on the number of dark satellites. Then, with luck, we will find an example of a lens that requires a satellite at a specific location for which there is no optical counterpart. This may be too conservative a condition. For example, Peng () argues that much of the flux of Object X in MG0414+0534 (Fig. B.6) is actually coming from lensed images of the quasar host galaxy rather than the satellite.
Finally, lens magnification already means that it is far easier to do photometry of a lensed quasar host galaxy than an unlensed galaxy. The next frontier is to measure the kinematics of cosmologically distant host galaxies. This might already be doable for the host galaxy of Q0957+561 at zs = 1.41, but will generally require either JWST or the next generation of ground based telescopes. With larger lens samples we may also find more cases like SDSS0924+0219 where gravitational lensing provides a natural coronograph for the quasar.
Acknowledgments: These lectures are dedicated to Bohdan Paczynski at a very difficult time for a man who made enormous contributions to this field both through his own work and his support for the work of others. I would like to thank E.E. Falco, C.R. Keeton, L.V.E. Koopmans, D. Maoz, J. Muñoz, D. Rusin, D.H. Weinberg and J.N. Winn for commenting on this manuscript, and S. Dye, C.D. Fassnacht, D.R. Marlow, J.L. Mitchell, C.Y. Peng, D. Rusin and J.N Winn for supplying figures. G. Meylan showed considerable patience with the author while waiting for these lectures to be delivered. This research has been supported by the NASA ATP grant NAG5-9265, and by grants HST-GO-7495, 7887, 8175, 8804, and 9133 from the Space Telescope Science Institute. The continuing HST observations of gravitational lenses are an absolutely essential part of converting gravitational lenses from curiosities into astrophysical tools.