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The Coma cluster (Abell 1656) is one of the best studied nearby clusters of galaxies. Among nearby clusters, it is the richest and the most compact one showing a reasonable degree of spherical symmetry. It is classified as type III in Bautz & Morgan classification and as type B ("Binary") in Rood & Sastry classification because two cD galaxies (NGC 4874 and NGC 4889) are located at the center.

Since the Coma cluster is more than 5 times as distant as the Virgo cluster, its dwarf galaxy population has not been studied well. If we assume that Coma dwarfs are similar to Virgo and Fornax dwarfs, they have scale length of about 1" and are fainter than Rsim 16. The image area of a single CCD  is quite small and inefficient to cover the whole extent of the Coma cluster with sufficient angular resolution. 

The technique of CCD mosaicking was a breakthrough to this dilemma. Mosaic CCD cameras made observations of dwarf galaxies feasible beyond a few very nearby clusters such as the Virgo and the Fornax clusters.

A team of astronomers has carried out a deep photometric and spectroscopic survey of wide areas in the Coma cluster, aiming to investigate the properties of galaxy population in different environments within the cluster, using a wide-field mosaic CCD camera (MCCDII) for photometric survey and the AF2/WYFFOS multiobject spectrograph at prime focus of the WHT to increase spectra of both giant and dwarf galaxies available to the study.

Imaging data covered a large field of view (2.22 deg2) from the cluster center to the outskirts, and the photometry was complete to a limiting magnitude of R 23 mag.  This photometric survey covered a wide area down to a considerably deep limiting magnitude compared to previous studies. In particular, the surveyed area is the largest among the recent CCD surveys of the Coma cluster. It covers the area in the Johnson B and Cousins R bands for which only photographic data had been available before. 

The Mosaic CCD Camera II (MCCDII) was developed in a collaborative program between the University of Tokyo and National Astronomical Observatory of Japan. MCCDII has 40 1k × 1k CCDs produced by TI/Japan. Since the CCDs are not buttable, they are aligned in the 5 × 8 array with gaps (hence one contiguous image consists of 160 frames). The camera is designed so that four different exposures provide one contiguous image without any gap. The gap between each CCD is about 900 pixels, which is optimized to give both as large field of view as possible and an appropriate overlap (100 pixels) between adjacent images. MCCDII was attached to the prime focus of the William Herschel Telescope. The scale at the prime focus of WHT is 17.75" mm-1, which results in 0.21" per pixel sampling and 50 × 32 arcmin2 field of view is attained with four exposures.

Mosaic CCD Camera II (MCCDII). Forty CCDs are seen aligned in 5 × 8 array. A liquid nitrogen tank is attached backside of the Dewar to cool the CCDs down to appropriate temperature.  [ JPEG | TIFF ]

To study the spectral properties of galaxies as a function of their local environment, two fields were selected for spectroscopic observations to cover both the core and outskirts of the cluster. Medium resolution spectroscopy was then carried out for a total of 490 galaxies in both fields, using the AF2/WYFFOS multifiber spectrograph. The limiting magnitude was R=19.75 and a total of 279 galaxies were identified as members of the Coma Cluster.

The mean metallicity decreases with galaxy magnitude and, at a given luminosity, appears to be generally lower for galaxies in the southwest region of Coma as compared to the center of the cluster. A broad range of ages, from younger than 3 Gyr to older than 9 Gyr, is found in galaxies of any magnitude. However, systematic trends of age with luminosity are present among galaxies in the central field, including a slight decrease of the mean age for fainter galaxies. Furthermore, in the central Mpc of Coma, a large fraction of galaxies at any luminosity show no evidence in their central regions of star formation occured at redshift z<2, while the proportion of galaxies with significant star formation occuring at intermediate (0.35<z<2) and low (z<0.35) redshifts is found to depend on galaxy luminosity. An additional surprising result is that the faint galaxies with young luminosity-weighted ages appear to have a bimodal metallicity distribution that would point to a composite formation scenario involving different physical processes.

The R-band luminosity function is found to be the same between the inner and outer regions and close to those from measurements for field galaxies. This is remarkable given the variation in the spectral types of galaxies between field and cluster environments. The total B-band luminosity function shows a dip at MB = -18 mag, in agreement with previous studies. The luminosity function is studied in B-R color intervals and shows a steep faint-end slope for red (B-R > 1.35) galaxies, at both the core and outskirts of the cluster. This population of low-luminosity red galaxies has a higher surface density than the blue (B-R < 1.35) star-forming population and dominates the faint end of the Coma Cluster luminosity function. It is found that the relative number of high surface brightness galaxies is larger at the cluster core, implying the destruction of low surface brightness galaxies in the dense core environment.

A significant gradient in Mg2, in the sense that galaxies in the core of the cluster have stronger Mg2 is found in a sample of galaxies spanning a wide range of absolute luminosity in the Coma Cluster. The astronomers attribute the Mg2 gradient to changes in metal abundance. One possible mechanism to create this abundance gradient is pressure confinement by the intracluster medium of material from supernova-driven winds early in the history of the galaxies.

The ages of stellar populations in 52 elliptical and S0 galaxies in the Coma Cluster were also investigated. More than 40% of the S0's are found to have undergone star formation in their central regions during the last sim 5 Gyr, while such activity is absent in the ellipticals. Galaxies in this sample have absolute magnitudes in the range -20.5 < MB < -17.5, and the fraction of S0 galaxies with recent star formation is higher at fainter luminosities. The observed luminosity range of S0 galaxies with signs of recent star formation activity is consistent with them being the descendants of typical star-forming spirals at intermediate redshift whose star formation has been halted as a consequence of the dense environment.

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Last modified: 13 December 2010