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TOPIC: Virgo


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Posts: 131433
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Virgo Cluster
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Faint galaxies hiding in the Virgo Cluster.

A recent survey using the Canada-France-Hawaii Telescope has discovered hundreds of new galaxies in the Virgo Cluster, the nearest large cluster of galaxies. Most are extremely faint "dwarf" galaxies, objects hundreds of thousands of times less massive than our own galaxy, the Milky Way, and amongst the faintest galaxies known in the Universe. The Virgo cluster appears to be home to far more of such faint systems than the "Local Group" of galaxies to which the Milky Way belongs, suggesting that galaxy formation on small scales may be more complicated than previously thought, and that our Local Group may not be a typical corner of the universe.
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Posts: 131433
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RE: Virgo
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Title: The nature of filamentary cold gas in the core of the Virgo Cluster
Authors: N. Werner, J. B. R. Oonk, R. E. A. Canning, S. W. Allen, A. Simionescu, J. Kos, R. J. van Weeren, A. C. Edge, A. C. Fabian, A. von der Linden, P. E. J. Nulsen, C. S. Reynolds, M. Ruszkowski

We present a multi-wavelength study of the emission-line nebulae located southeast of the nucleus of M87, the central dominant galaxy of the Virgo Cluster. We report the detection of far-infrared (FIR) [CII] line emission from the nebulae using observations made with Herschel PACS. The infrared line emission is extended and cospatial with optical H{\alpha}+[NII], far-ultraviolet CIV lines, and soft X-ray emission. The filamentary nebulae evidently contain multi-phase material spanning a temperature range of at least 5 orders of magnitude, from ~100 K to ~10^7 K. This material has most likely been uplifted by the AGN from the center of M87. The thermal pressure of the 10^4 K phase appears to be significantly lower than that of the surrounding hot intra-cluster medium (ICM) indicating the presence of additional turbulent and magnetic pressure in the filaments. If the turbulence in the filaments is subsonic then the magnetic field strength required to balance the pressure of the surrounding ICM is B~30-70 G. The spectral properties of the soft X-ray emission from the filaments indicate that it is due to thermal plasma with kT~0.5-1 keV, which is cooling by mixing with the cold gas and/or radiatively. Charge exchange can be ruled out as a significant source of soft X-rays. Both cooling and mixing scenarios predict gas with a range of temperatures. This is at first glance inconsistent with the apparent lack of X-ray emitting gas with kT<0.5 keV. However, we show that the missing very soft X-ray emission could be absorbed by the cold gas in the filaments with an average absorption column density of ~10^21 cm^-2, providing a natural explanation for the apparent temperature floor to the X-ray emission at kT~0.5 keV. The FIR through ultra-violet line emission is most likely primarily powered by the ICM particles penetrating the cold gas following a shearing induced mixing process.

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Posts: 131433
Date:
The Virgo Cluster
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Title: The Virgo Cluster
Authors: Jeremy Mould

In the era of precision cosmology the Virgo cluster takes on a new role in the cosmic distance scale. Its traditional role of testing the consistency of secondary distance indicators is replaced by an ensemble of distance measurements within the Local Supercluster united by a velocity field model obtained from redshift survey based reconstruction. WMAP leads us to see the Hubble Constant as one of six parameters in a standard model of cosmology with considerable covariance between parameters. Independent experiments, such as WMAP and the HST Key Project (and their successors) constrain these parameters.

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Posts: 131433
Date:
Virgo Cluster of Galaxies
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Title: Hundred Thousand Degree Gas in the Virgo Cluster of Galaxies
Authors: W.B. Sparks (1), J.E. Pringle (1,2), R.F. Carswell (2), M. Donahue (3), R. Martin (1), M. Voit (3), M. Cracraft (1), N. Manset (4), J.H. Hough (5) (1. Space Telescope Science Institute, 2. Institute of Astronomy, University of Cambridge, 3. Dept. of Physics and Astronomy, Michigan State University, 4. CFHT, 5. University of Hertfordshire)

The physical relationship between low-excitation gas filaments at ~10^4 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~10^7 K in the centers of many galaxy clusters is not understood. It is unclear whether the ~10^4 K filaments have cooled and condensed from the ambient hot (~10^7 K) medium or have some other origin such as the infall of cold gas in a merger, or the disturbance of an internal cool reservoir of gas by nuclear activity. Observations of gas at intermediate temperatures (~10^5-10^6 K) can potentially reveal whether the central massive galaxies are gaining cool gas through condensation or losing it through conductive evaporation and hence identify plausible scenarios for transport processes in galaxy cluster gas. Here we present spectroscopic detection of ~10^5 K gas spatially associated with the H-alpha filaments in a central cluster galaxy, M87 in the Virgo Cluster. The measured emission-line fluxes from triply ionised carbon (CIV 1549 A) and singly ionised helium (HeII 1640 A) are consistent with a model in which thermal conduction determines the interaction between hot and cold phases.

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Posts: 131433
Date:
Virgo Cluster
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Title: The Arecibo Galaxy Environment Survey V : The Virgo Cluster (I)
Authors: R. Taylor, J. I. Davies, R. Auld, R. F. Minchin

We present 21 cm observations of a 10 x 2 degree region in the Virgo cluster, obtained as part of the Arecibo Galaxy Environment Survey. 289 sources are detected over the full redshift range (-2,000 < v_{hel} < + 20,000 km/s) with 95 belonging to the cluster (v_{hel} < 3,000 km/s). We combine our observations with data from the optically selected Virgo Cluster Catalogue (VCC) and the Sloan Digital Sky Survey (SDSS). Most of our detections can be clearly associated with a unique optical counterpart, and 30% of the cluster detections are new objects fainter than the VCC optical completeness limit. 7 detections may have no optical counterpart and we discuss the possible origins of these objects. 7 detections appear associated with early-type galaxies. We perform HI stacking on the HI-undetected galaxies listed in the VCC in this region and show that they must have significantly less gas than those actually detected in HI. Galaxies undetected in HI in the cluster appear to be really devoid of gas, in contrast to a sample of field galaxies from ALFALFA.

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Posts: 131433
Date:
Virgo cluster core
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Title: Spitzer IRAC Low Surface Brightness Observations of the Virgo Cluster
Authors: J. E. Krick, C. Bridge, V. Desai, J. C. Mihos, E. Murphy, C. Rudick, J. Surace, J. Neill

We present 3.6 and 4.5 micron Spitzer IRAC imaging over 0.77 square degrees at the Virgo cluster core for the purpose of understanding the formation mechanisms of the low surface brightness intracluster light features. Instrumental and astrophysical backgrounds that are hundreds of times higher than the signal were carefully characterized and removed. We examine both intracluster light plumes as well as the outer halo of the giant elliptical M87. For two intracluster light plumes, we use optical colours to constrain their ages to be greater than 3 & 5 Gyr, respectively. Upper limits on the IRAC fluxes constrain the upper limits to the masses, and optical detections constrain the lower limits to the masses. In this first measurement of mass of intracluster light plumes we find masses in the range of 5.5 x 10^8 - 4.5 x 10^9 and 2.1 x 10^8 - 1.5 x 10^9 solar masses for the two plumes for which we have coverage. Given their expected short lifetimes, and a constant production rate for these types of streams, integrated over Virgo's lifetime, they can account for the total ICL content of the cluster implying that we do not need to invoke ICL formation mechanisms other than gravitational mechanisms leading to bright plumes. We also examined the outer halo of the giant elliptical M87. The colour profile from the inner to outer halo of M87 (160 Kpc) is consistent with either a flat or optically blue gradient, where a blue gradient could be due to younger or lower metallicity stars at larger radii. The similarity of the age predicted by both the infrared and optical colours (> few Gyr) indicates that the optical measurements are not strongly affected by dust extinction.

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Posts: 131433
Date:
Virgo Cluster
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Fifty million light years from Earth - practically next door by celestial standards - the equivalent of a massive highway pileup is under way.
Tugged inexorably together by gravity, whole galaxies and the billions of suns they contain are careening into one another like cars at the bottom of an icy hill. The smashups fling out long whip-tails of "orphaned" stars, the debris from a cosmic crash site.
This vast, slow-motion pinball game has been playing out for several billion years in the Virgo Cluster, a seething clump of more than 2,000 galaxies.

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Posts: 131433
Date:
NGC 4522 and NGC 4402
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Stripped down: Hubble highlights 2 galaxies that are losing it
A newly released set of images, taken by the NASA/ESA Hubble Space Telescope before the recent Servicing Mission, highlight the ongoing drama in two galaxies in the Virgo Cluster affected by a process known as "ram pressure stripping", which can result in peculiar-looking galaxies. An extremely hot X-ray emitting gas known as the intra-cluster medium lurks between galaxies within clusters. As galaxies move through this intra-cluster medium, strong winds rip through galaxies distorting their shape and even halting star formation.
Both images were taken by the Advanced Camera for Surveys on Hubble before it suffered from a power failure in 2007. Astronauts on Servicing Mission 4 in May 2009 were able to restore ACS during their 13-day mission.

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Posts: 131433
Date:
RE: Virgo
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One of the hallmark regions of the early summer Colorado sky is a seemingly barren section of the heavens know as the "Realm of the Galaxies.
Situated in the "bowl" of the Virgo "champagne glass," roughly equidistant from the bright summertime stars of Arcturus, Spica, and Regulus, the Realm of the Galaxies is devoid of even moderately bright stars, and thus at first glance, appears to be an rather uninteresting area of the night sky.
A quick sweep with binoculars or a small telescope a clear dark night, however, reveals it to be anything but uninteresting.

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Posts: 131433
Date:
Virgo galaxy cluster
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Our own Milky Way and the Andromeda galaxy are the largest of the forty members of the Local Group of galaxies, all held together by mutual gravitational attraction. But the Local Group is puny compared to the massive Virgo Cluster, home to as many as 2,000 galaxies spread across our sky between the stars Denebola in Leo and Vindemiatrix in Virgo. Dozens of these galaxies are visible from your backyard.

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