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Post Info TOPIC: Coma Cluster Galaxies
Blobrana


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RE: Coma Cluster Galaxies
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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."
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Blobrana


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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 <g-i> = 0.8 and <b/a> = 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.

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Blobrana


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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.

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Blobrana


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Title: Suzaku X-Ray Observations of the Accreting NGC 4839 Group of Galaxies and the Radio Relic in the Coma Cluster
Authors: Hiroki Akamatsu, Susumu Inoue, Takuya Sato, Kyoko Matsushíta, Yoshítaka Ishisaki, Craig L. Sarazin

Based on Suzaku X-ray observations, we study the hot gas in regions around the NGC 4839 group of galaxies and the radio relic in the outskirts of the Coma cluster. From spectral analysis, the temperature of the gas shows a gradual decline from 5 keV around NGC4839 to about 3.6 keV at the radio relic. Across the relic, the temperature drops steeply by approximately a factor of 2 from 3.6 to 1.5 keV. This temperature drop can be interpreted as a shock with Mach number M = 2.2 pm 0.5. The existence of a shock front suggests that it may be responsible for accelerating the non-thermal electrons. However, if they are accelerated according to the simplest theory of diffusive shock acceleration (DSA) in which test particles are injected from a thermal distribution, the electron spectrum expected from the measured Mach number and shock compression would be steeper than that inferred from the observed, spatially integrated spectrum of the radio relic with index alpha = 1.18, taking into account radiative loss effects. The expected electron acceleration efficiency may also be too low to be compatible with the observed radio flux. The radio relic may nevertheless result from DSA if it is initiated by the injection of a pre- existing population of non-thermal electrons. Based on the correspondence between the temperature jump and the radio relic as well as the metal distribution, these facts can be interpreted in terms of a picture in which NGC 4839 is falling into the Coma cluster and the accompanying, metal-enriched gas is being stripped because of ram pressure. We suggest that the shock front associated with the radio relic is generated by the combined interaction of the pre-existing intracluster medium with gas conveyed by the infalling NGC 4839 group, together with the inflow of cooler gas from the large-scale structure filament that connects Coma with Abell 1367.

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Blobrana


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Title: Constraints on Cosmic Rays, Magnetic Fields, and Dark Matter from Gamma-Ray Observations of the Coma Cluster of Galaxies with VERITAS and Fermi
Authors: The Veritas Collaboration with C. Pfrommer, A. Pinzke: T. Arlen, T. Aune, M. Beilicke, W. Benbow, A. Bouvier, J. H. Buckley, V. Bugaev, K. Byrum, A. Cannon, A. Cesarini, L. Ciupik, E. Collins-Hughes, M. P. Connolly, W. Cui, R. Dickherber, J. Dumm, A. Falcone, S. Federici, Q. Feng, J. P. Finley, G. Finnegan, L. Fortson, A. Furniss, N. Galante, D. Gall, S. Godambe, S. Griffin, J. Grube, G. Gyuk, J. Holder, H. Huan, G. Hughes, T. B. Humensky, A. Imran, P. Kaaret, N. Karlsson, M. Kertzman, Y. Khassen, D. Kieda, H. Krawczynski, F. Krennrich, K. Lee, A. S Madhavan, G. Maier, P. Majumdar, S. McArthur, A. McCann, P. Moriarty, R. Mukherjee, T. Nelson, A. O'Faoláin de Bhróithe, R. A. Ong, M. Orr, A. N. Otte, N. Park, J. S. Perkins, M. Pohl, H. Prokoph, J. Quinn, K. Ragan, L. C. Reyes, et al. (21 additional authors not shown)

Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. Protons should likewise be accelerated, suggesting that clusters may also be sources of very high-energy (VHE; E>100 GeV) gamma-ray emission. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of imaging Cherenkov telescopes, with complementing Fermi-LAT observations at GeV energies. No significant gamma-ray emission from the Coma cluster was detected. Integral flux upper limits at the 99% confidence level were measured to be on the order of (2-5)*10^-8\ ph. m^-2 s^-1 (VERITAS, >220 GeV} and ~2*10^-6 ph. m^-2 s^-1 (Fermi, 1-3 GeV), respectively. We use the gamma-ray upper limits to constrain CRs and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be < 16% from VERITAS data and < 1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Assuming that the radio-emitting electrons of the Coma halo result from hadronic CR interactions, the observations imply a lower limit on the central magnetic field in Coma of (2 - 5.5) muG, depending on the radial magnetic-field profile and on the gamma-ray spectral index. Since these values are below those inferred by Faraday rotation measurements in Coma (for most of the parameter space), this {renders} the hadronic model a very plausible explanation of the Coma radio halo. Finally, since galaxy clusters are dark-matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally-averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, <\sigma v>.

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Blobrana


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Coma I cloud
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Title: Fast motions of galaxies in the Coma I cloud: a case of Dark Attractor?
Authors: Igor D. Karachentsev, Olga G. Nasonova, Helene M. Courtois

We 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.

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Blobrana


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RE: Coma Cluster Galaxies
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Title: Ultraviolet tails and trails in cluster galaxies: A sample of candidate gaseous stripping events in Coma
Authors: 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 Karick

We 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.

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Blobrana


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HST/ACS Coma Cluster Survey
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Title: The HST/ACS Coma Cluster Survey: V - Compact Stellar Systems in the Coma Cluster
Authors: 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. Puzia

The 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.

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RB199
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Title: Strange filamentary structures ("fireballs") around a merger galaxy in the Coma cluster of galaxies
Authors: M. Yoshida, M. Yagi, Y. Komiyama, H. Furusawa, N. Kashikawa, Y. Koyama, H. Yamanoi, T. Hattori, S. Okamura

We 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.

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Blobrana


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RE: Coma Cluster Galaxies
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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.

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Credit NASA

Position (2000):      R.A. 12h 59m 48s.70, Dec. +27° 58' 50".00

The 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.

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