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Post Info TOPIC: Coma Cluster Galaxies

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Date:
 RE: Coma Cluster Galaxies Permalink Astronomers Discover More than 800 Dark Galaxies in the Famous Coma Cluster A group of researchers from the Stony Brook University (the State University of New York) and the National Astronomical Observatory of Japan has discovered 854 "ultra dark galaxies" in the Coma Cluster by analysing archival data from the Subaru Telescope. The discovery of 47 such mysterious dark galaxies was a surprising find in 2014, and the new discovery of more than 800 suggests galaxy clusters as the key environment for the evolution of these mysterious dark galaxies. "Not only these galaxies appear very diffuse," said Jin Koda, principal investigator of the study, "but they are very likely enveloped by something very massive." Read more __________________

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 Permalink Title: Forty-Seven Milky Way-Sized, Extremely Diffuse Galaxies in the Coma Cluster Author: Pieter van Dokkum, Roberto Abraham, Allison Merritt, Jielai Zhang, Marla Geha, Charlie Conroy We report the discovery of 47 low surface brightness objects in deep images of a 3 x 3 degree field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness mu(g,0) ranging from 24 - 26 mag/arcsec² and effective radii r_e = 3"-10", as measured from archival Canada France Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r_e = 1.5 - 4.6 kpc their sizes are similar to those of L* galaxies even though their median stellar mass is only ~6 x 107 Solar masses. The galaxies are relatively red and round, with = 0.8 and = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope ACS observation. The ACS imaging shows a large spheroidal object with a central surface brightness mu(g,0) = 25.8 mag/arcsec², a Sersic index n=0.6, and an effective radius of 7", corresponding to 3.4 kpc at the distance of Coma. The galaxy is unresolved, as expected for a Coma cluster object. To our knowledge such "ultra-diffuse galaxies" have not been predicted in any modern galaxy formation model. We speculate that UDGs may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.Read more (4263kb, PDF) __________________

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Date:
 Permalink Title: Dark energy and the structure of the Coma cluster of galaxies Authors: A. D. Chernin, G.S. Bisnovatyi-Kogan, P. Teerikorpi, M. J. Valtonen, G. G. Byrd, M. Merafina We consider the Coma cluster of galaxies as a gravitationally bound physical system embedded in the perfectly uniform static dark energy background as implied by the Lambda CDM cosmology. We ask if the density of dark energy is high enough to affect the structure of a large rich cluster of galaxies? We use recent observational data on the cluster together with our theory of local dynamical effects of dark energy, including the zero-gravity radius R_{ZG} of the local force field as the key parameter. {1) Three masses are defined which characterise the structure of a regular cluster: the matter mass M_{M}, the dark-energy effective mass M_{DE} (<0) and the gravitating mass M_{G} (= M_{M} + M_{DE}). 2) A new matter density profile is suggested which reproduces well the observational data for the Coma cluster in the radius range from 1.4 Mpc to 14 Mpc and takes into account the dark energy background. 3) Using this profile, we calculate upper limits for the total size of the Coma cluster, \$R \le R_{ZG} ~ 20 Mpc, and its total matter mass, M_{M} \la M_{M}(R_{ZG}) = 6.2 x 10^{15} solar masses. The dark energy antigravity affects strongly the structure of the Coma cluster at large radii R \ga 14 Mpc and should be taken into account when its total mass is derived. Read more (261kb, PDF) __________________

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 Coma I cloud Permalink Title: Fast motions of galaxies in the Coma I cloud: a case of Dark Attractor?Authors: Igor D. Karachentsev, Olga G. Nasonova, Helene M. CourtoisWe notice that nearby galaxies having high negative peculiar velocities are distributed over the sky very inhomogeneously. A part of this anisotropy is caused by the "Local Velocity Anomaly", i.e. by the bulk motion of nearby galaxies away from the Local Void. But a half of the fast-flying objects reside within a small region RA = [11.5h, 13.0h], Dec. = [+20°, +40°], known as the Coma I cloud. According to Makarov & Karachentsev (2011), this complex contains 8 groups, 5 triplets, 10 pairs and 83 single galaxies with the total mass of 4.7 x 10^13 solar masses. We use 122 galaxies in the Coma I region with known distances and radial velocities VLG < 3000 km/s to draw the Hubble relation for them. The Hubble diagram shows a Z-shape effect of infall with an amplitude of +200 km/s on the nearby side and -700 km/s on the back side. This phenomena can be understood as the galaxy infall towards a dark attractor with the mass of ~ 2 x 10^14 solar masses situated at a distance of 15 Mpc from us. The existence of large void between the Coma and Virgo clusters affects probably the Hubble flow around the Coma I also.Read more  (237kb, PDF) __________________

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 RE: Coma Cluster Galaxies Permalink Title: Ultraviolet tails and trails in cluster galaxies: A sample of candidate gaseous stripping events in ComaAuthors: Russell J. Smith, John R. Lucey, Derek Hammer, Ann E. Hornschemeier, David Carter, Michael J. Hudson, Ronald O. Marzke, Mustapha Mouhcine, Sareh Eftekharzadeh, Phil James, Habib Khosroshahi, Ehsan Kourkchi, Arna KarickWe have used new deep observations of the Coma cluster from GALEX to identify 13 star-forming galaxies with asymmetric morphologies in the ultraviolet. Aided by optical broad-band and H-alpha imaging, we interpret the asymmetric features as being due to star formation within gas stripped from the galaxies by interaction with the cluster environment. The selected objects display a range of structures from broad fan-shaped systems of filaments and knots (jellyfish') to narrower and smoother tails extending up to 100 kpc in length. Some of the features have been discussed previously in the literature, while others are newly identified here. As an ensemble, the candidate stripping events are located closer to the cluster centre than other star-forming galaxies; their radial distribution is similar to that of all cluster members, dominated by passive galaxies. The fraction of blue galaxies which are undergoing stripping falls from 40% in the central 500 kpc, to less than 5% beyond 1 Mpc. We find that tails pointing away from (i.e. galaxies moving towards) the cluster centre are strongly favoured (11/13 cases). From the small number of outgoing' galaxies with stripping signatures we conclude that the stripping events occur primarily on first passage towards the cluster centre, and are short-lived compared to the cluster crossing time. Using infall trajectories from simulations, the observed fraction of blue galaxies undergoing stripping can be reproduced if the events are triggered at a threshold radius of ~1 Mpc and detectable for ~500 Myr. HST images are available for two galaxies from our sample and reveal compact blue knots coincident with UV and H-alpha emission, apparently forming stars within the stripped material. Our results confirm that stripping of gas from infalling galaxies, and associated star formation in the stripped material, is a widespread phenomenon in rich clusters. Read more (2675kb, PDF) __________________

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 HST/ACS Coma Cluster Survey Permalink Title: The HST/ACS Coma Cluster Survey: V - Compact Stellar Systems in the Coma ClusterAuthors: J. Price, S. Phillipps, A. Huxor, N. Trentham, H.C. Ferguson, R.O. Marzke, A. Hornschemeier, P. Goudfrooij, D. Hammer, R.B. Tully, K. Chiboucas, R.J. Smith, D. Carter, D. Merritt, M. Balcells, P. Erwin, T.H. PuziaThe HST ACS Coma Cluster Treasury Survey is a deep two passband imaging survey of the nearest very rich cluster of galaxies, covering a range of galaxy density environments. The imaging is complemented by a recent wide field redshift survey of the cluster conducted with Hectospec on the 6.5m MMT. Among the many scientific applications for this data are the search for compact galaxies. In this paper, we present the discovery of seven compact (but quite luminous) stellar systems, ranging from M32-like galaxies down to ultra-compact dwarfs (UCDs)/dwarf to globular transition objects (DGTOs). We find that all seven compact galaxies require a two-component fit to their light profile and have measured velocity dispersions that exceed those expected for typical early-type galaxies at their luminosity. From our structural parameter analysis we conclude that three of the sample should be classified as compact ellipticals or M32-like galaxies, the remaining four being less extreme systems. The three compact ellipticals are all found to have old luminosity weighted ages (> 12 Gyr), intermediate metallicities (-0.6 < [Fe/H] < -0.1) and high [Mg/Fe] (> 0.25). Our findings support a tidal stripping scenario as the formation mode of compact galaxies covering the luminosity range studied here. We speculate that at least two early-type morphologies may serve as the progenitor of compact galaxies in clusters. Read more  (4989kb, PDF) __________________

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 RB199 Permalink Title: Strange filamentary structures ("fireballs") around a merger galaxy in the Coma cluster of galaxiesAuthors: M. Yoshida, M. Yagi, Y. Komiyama, H. Furusawa, N. Kashikawa, Y. Koyama, H. Yamanoi, T. Hattori, S. OkamuraWe found an unusual complex of narrow blue filaments, bright blue knots, and H-alpha emitting filaments and clouds, which morphologically resembled a complex of "fireballs", extending up to 80 kpc south from an E+A galaxy RB199 in the Coma cluster. The galaxy has a highly disturbed morphology indicative of a galaxy--galaxy merger remnant. The narrow blue filaments extend in straight shapes toward the south from the galaxy, and several bright blue knots are located at the southern ends of the filaments. The Rc band absolute magnitudes, half light radii and estimated masses of the bright knots are -12 - -13 mag, 200 - 300 pc and 10^6-7 Msolar, respectively. Long, narrow H-alpha emitting filaments are connected at the south edge of the knots. The average colour of the fireballs is B - Rc = 0.5, which is bluer than RB199 (B - R = 0.99), suggesting that most of the stars in the fireballs were formed within several times 10^8 yr. The narrow blue filaments exhibit almost no H-alpha emission. Strong H-alpha and UV emission appear in the bright knots. These characteristics indicate that star formation recently ceased in the blue filaments and now continues in the bright knots. The gas stripped by some mechanism from the disk of RB199 may be travelling in the intergalactic space, forming stars left along its trajectory. The most plausible fireball formation mechanism is ram pressure stripping by high-speed collision between the galaxy and the hot intra-cluster medium. The fireballs may be a snapshot of diffuse intra-cluster population formation, or halo star population formation in a cluster galaxy. Read more (289kb, PDF) __________________

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Date:
 RE: Coma Cluster Galaxies Permalink NASA's Hubble Space Telescope captures the magnificent starry population of the Coma Cluster of galaxies, one of the densest known galaxy collections in the universe.Expand (893kb, 1280 x 909)Credit NASAPosition (2000):      R.A. 12h 59m 48s.70, Dec. +27° 58' 50".00The Hubble's Advanced Camera for Surveys viewed a large portion of the cluster,spanning several million light-years across. The entire cluster contains thousands of galaxies in a spherical shape more than 20 million light-years in diameter.Also known as Abell 1656, the Coma Cluster is over 300 million light-years away. The cluster, named after its parent constellation Coma Berenices, is near the Milky Way's north pole. This places the Coma Cluster in an area unobscured by dust and gas from the plane of the Milky Way, and easily visible by Earth viewers.Read more __________________
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