Coma Cluster

The Coma cluster of galaxies is unique because of its high richness, low redshift (z=0.0232) and location at the North Galactic Pole. Therefore, over the past 50 years, it has been the focus of much observational and theoretical work, making it the best studied cluster in the night sky. For example, Colless & Dunn (1995) have recently published a list of 552 galaxy redshifts (243 new measurements), of which, 465 are in the Coma velocity field. Furthermore, the x--ray data amassed on Coma remains unparallelled (White, Briel \& Henry 1993, MNRAS, 261, L8).

Test Year

Coma will certainly be surveyed, in great detailed, as part of the full SDSS survey. However, I advocate here that this cluster be singled out, early on in the test year, for special attention in both spectroscopy and imaging mode. Coma can be observed, at reasonable airmasses, as late as June in the season. The motivations for this suggestion are summaried below.
  1. Godwin, Metcalfe & Peach (1983, MNRAS, 202, 113; GMP) have published accurate galaxy positions, colours (b&r), magnitudes and ellipticities for 6724 galaxies to bj<21.0 over 2.63 sq degrees centred on Coma. This data provides a solid base upon which to test both the SDSS spectroscopy and imaging. For example, the data can be used to construct and test spectroscopic plug-plates, while actual observations will assess the fibre throughput over a large range of known galaxy magnitudes (the positions can be checked using a COSMOS scan of the Coma plate which I already possess). In imaging mode, the data will test many aspects of the PHOTO software, especially at the fainter magnitude limits. It will be an excellent test of the deblending software.
  2. Colless & Dunn (1995) have published a list of 552 galaxy redshifts (and their confidences) in Coma. This database will be an excellent test for the SPECTRO software.
  3. Finally, if we published the data as soon as possible after our tests, the whole Coma exercise would be a strong public relations endeavour. The scientific merits of such a photometric & spectroscopic database on Coma would be immense. It would also hail a new era in survey astronomy. Scientific studies possible using this database include;

Suggestion

I suggest here that we aim to measure the spectrum of all 6745 galaxies in the GMP database. The limiting magnitude of this dataset is bj=21, or, r(photo)~20. This is below the nominal SDSS spectroscopic limit, but the extra magnitude would aid in assessing the limitations and throughput of the telescope. The alternative would be to complete the bj=20 sample which is close to the nominal SDSS limit and corresponds to the galaxy completeness limit in the GMP data (thus maximising the scientific returns for the smallest allocation of spectra). This latter sample is only 2510 galaxies over the same field. The number-magnitude plots for both the r and bj passbands are presented here . Both observations could be complete in a short period; a few nights of dark time. I present here an ascii list of the GMP data (no., ra(1950), dec, bj(26.5) mag, bj-r (equal area), bj(26.5)-(bj-r) and r(24.5)) and the Colless & Dunn list of 552 redshifts in Coma. I note here that Matthew Colless at Mount Stromlo is planning a similar project using the 2dF instrument on the AAT. Clearly, this southern telescope is not suited to doing such a project, but if he is successful there is an opportunity for greater spectroscopic checks using his data.
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nichol@oddjob.uchicago.edu
last modified 30th August 1995