CLASH

Marc Postman, postman@stsci.edu

The composition of the universe has proven far more intriguing than we thought it to be, even only 15 years ago. It is a “dark” universe, where 23% of the mass-energy is made up of as yet undetected, weakly interacting, non-baryonic particles (a.k.a. dark matter, DM), and 73% is associated with as yet unknown physics that drives accelerated cosmic expansion (a.k.a. dark energy, DE). The Multi-Cycle Treasury (MCT) program presents an opportunity to make major progress on understanding the physics behind these phenomena. The MCT time-allocation committee awarded 524 orbits of Hubble time to the Cluster Lensing and Supernova Survey with Hubble (CLASH). This survey uses panchromatic imaging from the new Wide Field Camera 3 (WFC3) and the restored Advanced Camera for Surveys (ACS). The goal is to harness the power of strong gravitational lensing to test models of the formation of cosmic structure with unprecedented precision.

When combined with existing wide-field optical and X-ray imagery, CLASH observations will be a giant advance in the quality and quantity of information from strong lensing. The strong lenses in the CLASH sample—25 massive, intermediate redshift galaxy clusters—will allow us to identify hundreds of multiply imaged sources, which in turn, will allow us to challenge theoretical scenarios for the distribution of DM in clusters with ~10% precision. The CLASH images will also yield a tenfold advantage for identifying galaxies with z > 7, compared with any field survey of comparable area. The high magnification provided by the cluster lenses presents a unique opportunity to obtain spectra for very young galaxies that would otherwise be beyond the reach of large ground-based telescopes.

In parallel with the lensing survey, we will use both ACS and the WFC3 infrared (IR) channel to detect type-Ia supernovae (SNe Ia) in the redshift range 1 < z < 2.5, which is uniquely accessible from space. Because the SNe Ia will be detected when these cameras are operating in parallel, they will be located far from the cluster core, where the effects of lensing are small (and correctable). Therefore, any SNe that are detected can be used to improve  the limits on the redshift variation of the DE equation of state.

The CLASH MCT program combines five features that make its resulting data exceptionally well-suited to studying DM, DE, and the formation and evolution of galaxies:

  • An X-ray–selected sample of 25 dynamically relaxed, massive galaxy clusters. This sample is large enough that observed deviations as small as 15% from the expected mean predicted DM distribution will allow us to reject models at the 99% confidence level.
  • The availability of wide-field optical and X-ray imaging for all of the clusters. We will have full coverage of the distribution of DM and baryons.
  • The use of 16 Hubble broadband filters from the UV through the near infrared (NIR). We will obtain photometric redshifts with an accuracy of Dz ~ 0.02(1 + z) for all objects with AB magnitude brighter than 26.
  • A total exposure allocation of 20 orbits for each cluster. The depth of the CLASH images is designed to achieve a five-sigma detection limit of at least 26.5 AB mag in all 16 passbands (some of the ACS and WFC3/IR images will be slightly deeper). This ensures we will be able to produce robust photo-z measurements to 26 AB mag.
  • Hubble observations spread over eight epochs for each cluster. We will be able to search the parallel fields for SNe Ia at z > 1.