Keck Telescopes
Located in Hawaii near the summit of Mauna Kea, these twin telescopes are providing optical spectroscopy of the faint Coma cluster galaxies.
Web Site: https://www.keckobservatory.org
(Image credit: Rick Peterson.)
Lead Investigator: R. Brent Tully
The goals of the Keck campaign are threefold.
(1) Using the DEIMOS spectrograph on Keck II, we have obtained spectra of 51 galaxies in the faint magnitude range 16 < r < 19 mag. This has enabled us to measure line strength indices, which are used to estimate the ages and metallicities of the stars in these dwarf galaxies. From the same galaxy spectra we are able to accurately measure the random motions of these stars (a quantity referred to as the "velocity dispersion"), thereby extending our analysis of "scaling laws" into the dwarf galaxy regime.
(2) Targetting even fainter galaxies, we have been using the LRIS spectrograph on Keck I to measure redshifts for low surface brightness dwarf galaxies in the apparent magnitude range 20 < r < 22 mag. This is complementary to the work done with Hectospec in that we are pushing to fainter magnitudes and surface brightnesses. These redshifts will unambiguously establish which galaxies are true members of the cluster, as opposed to background/foreground galaxies, enabling an accurate determination of the faint end of the Coma cluster Luminosity Function. This work will also provide a larger and deeper sample of known dwarf galaxy cluster members, permitting one to better study the properties of the low mass dwarf galaxy population. In addition, we are using these redshifts to test our success rate of determining cluster membership via surface brightness and morphology criteria as measured from the HST/ACS images. This will allow us to obtain a firm measurement of the cluster Luminosity Function down to an absolute magnitude M,sub>r ~ -9 mag.
(3) A third goal is the search for ultra compact dwarf galaxies (UCDs) down to an absolute magnitude limit of Mr = -11.5 mag. Such galaxies have been found in other nearby clusters and appear to be strongly concentrated in dense, dynamically evolved regions. The core of the Coma cluster is therefore a likely place for them to appear in large numbers. With half-light radii smaller than 100 pc (about 0.2 arcseconds), at the distance of the Coma cluster these objects look similar to stars - even in HST/ACS images. However, in addition to being slightly resolved, the colors of the these galaxies can be used to help distinguish them from stars, although redshifts and thus distances are required to confirm their true nature.