Coma

The HST/ACS Coma Cluster Survey

The HST/ACS Coma Cluster Survey is the only European led HST Treasury project. It consists of a two passband survey in F814W (equivalent to Cousins I band) and F475W (equivalent to Sloan g band) filters. The area surveyed by HST to date is 230 square arcminutes, out of the 740 sq arcminutes originally proposed. The Advanced Camera for Surveys (ACS), the instrument used by the survey, was repaired by the Space Shuttle crew in May 2009, and is ready for use. We will be applying for more time to complete and extend the survey.

Colour image of a region of the ACS survey of the Coma Cluster, generated from the ACS images by the Hubble Heritage Team.

Coma provides the opportunity to study galaxies in a very different environment to anything previously part of a large HST programme. The Coma core is the densest galaxy environment in the local universe. As such, it provides a key local, high-density benchmark for comparison to surveys of less dense and relaxed environments (Virgo, Fornax, Perseus), high-redshift HST cluster surveys, and field surveys such as HUDF, GOODS, and GEMS. Key science goals of the ACS project are:

Central region of the Coma cluster, showing the distribution of globular clusters and low surface brightness dwarf galaxies around NGC4874.

• To measure the luminosity function down to M_V = −9 for comparison with other, lower density, nearby clusters studied by HST (Virgo, Fornax). This is critical for detecting dwarf galaxies at luminosities that overlap with the Local-group dwarf spheroidals, and testing whether the faint-end LF slope in clusters is universal.
• To measure detailed morphologies using bulge-disk decomposition techniques for a wide range of galaxy luminosities, including those at the faint end of the cluster LF. HST images will also reveal compact nuclei, nuclear bars, disks, and dust rings. Comparison with current results in Fornax, Perseus, and Virgo will outline their dependence upon galaxy density within the cluster.
• To derive global colors and color gradients at a range of luminosity and environment to investigate the origin of cluster galaxies, and to understand the environmental process acting upon them.
• To investigate the bright and faint ends of the global scaling laws among structural parameters (e.g., luminosity-radius, luminosity-velocity dispersion, fundamental plane) and assess any environmental dependence.
• To provide the structural information with which to define and classify large samples of Coma dwarfs for complementary spectroscopic observations on the latest, and future, generations of high-resolution, multi-object spectrographs on the largest ground-based telescopes.

Galex image of the central region of Coma, false colour image generated from Galex FUV and NUV channels. PI of Galex programme: Dr. R.J. Smith, Durham.
• Comparison of the luminosity functions, colour distributions, and colour magnitude diagrams of Globular Clusters between galaxies of a range of luminosities.
• Study of mergers in a high density environment, and their effect on star formation activity, for instance the morphology of "E+A" galaxies detected in spectroscopic surveys.

In addition to the two-passband HST imaging survey, the project also encompasses near infra-red imaging data from the UKIRT) and CFHT, spectroscopy from Keck, MMT and Gemini telescopes, infra-red and ultraviolet surveys from Spitzer and Galex space telescopes respectively, H alpha imaging from Subaru and the INT, optical intermediate-band imaging from KPNO. Many more observations are planned, including completion of the survey and extension to the infrared with the new WFC3 on the HST.

ARI staff involved in the survey

• Professor David Carter - PI of the HST survey and co-ordinator of the wider project.
• Dr. Mustapha Mouhcine - PI of the UKIRT wide field K-band survey and the INT H alpha survey of the Coma cluster.
• Dr. Arna Karick - Postdoctoral research assistant working on the HST survey and on spectroscopic followup observations (particularly with the Keck 10 metre telescope).
• Dr. Phil James - Working on the H alpha survey and on-going star formation in Coma and other clusters.
• Dr. Sue Percival - Working on modelling of stellar populations of galaxies, and applying these to the Coma data.

Papers from the Survey

• The Hubble Space Telescope Advanced Camera for Surveys Coma Cluster Survey. I. Survey Objectives and Design (D. Carter et al.) ApJS 176 424.
• The HST/ACS Coma Cluster Survey - V. Compact stellar systems in the Coma Cluster (J. Price et al.) MNRAS 397 1816

Observing proposals for 2010

The following observing proposals have been or are about to be submitted, decision dates are given where known:

• HST Multi-Cycle Treasury Programme applicaion - 642 orbits of HST time. Proposal deadline November 18th 2009, decisions by January 2010
  • Completion of the central mosaic from the Cycle 15 ACS Treasury programme, and extension to east and west by ~6 arcminutes.
  • Smaller (12 x 12 arcmin) mosaics in the infall region at around 45 arcminutes radius, two in the well-studied region around NGC 4839 to the SW, and at further two to the NE, to establish environmental dependences.
  • WFC3/IR F160W (H band) observations covering most of the central mosaic area and part of the outer mosaics, achieved largely through WFC parallels, with dedicated WFC3 orbits to infill gaps. Aim to measure metallicity distributions in galaxies and associated globular cluster populations.
  • Deep observations (8 orbits per passband) of a sample of ~20 galaxies in F814W and F160W passbands. Observed in pairs where possible to allow ACS and WFC3/IR observations in parallel. Science aims: deep Surface Brightness Fluctuation and globular cluster observations.
  • Deep UV (F225W and F275W, 4 orbits per passband per field) observations of samples of specific galaxies, to look at ongoing star formation, and the UV upturn due to Horizontal Branch and Post-Asymptotic Giant Branch stars.
• HST Cycle 18 Guest Observer applications. Proposal Deadline February 2010
• Keck Observatory DEIMOS application to measure internal velocity dispersions, for Ultra-Compact Dwarf galaxies in Coma. Led from the University of California. Deadline September 11th 2009.
• Keck Observatory LRIS application to establish membership of faint compact and low surface brightness dwarf galaxies in Coma. Led from University of Hawaii. Deadline October 2010.