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Post Info TOPIC: Virgo HI Cloud (NGC 4388)


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Markarian's Chain
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Markarian's Chain is a stretch of galaxies that forms part of the Virgo Cluster. It's called a "chain" because, when viewed from Earth, the galaxies lie along a smoothly curved line. It was named after the Armenian astrophysicist, B. E. Markarian, who discovered their common motion in the early 1960s. Member galaxies include M84 (NGC 4374), M86 (NGC 4406), NGC 4477, NGC 4473, NGC 4461, NGC 4458, NGC 4438 and NGC 4435.
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Intergalactic stars in the Virgo cluster
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Most of the intergalactic stars in the Virgo cluster are old and surprisingly metal-poor, say astronomers who have used the Hubble Space Telescope to discover thousands of such stars. The low metallicity suggests that the stars were born both in small Virgo galaxies which disintegrated and at the edges of the large galaxies which tore the small ones apart.
Although most stars, including the Sun, reside in a galaxy, collisions between galaxies can catapult stars out of their homes. The danger is especially great in galaxy clusters. During the 1990s, astronomers discovered intergalactic stars in the Virgo and Fornax clusters, the two nearest galaxy clusters to Earth.
Now Benjamin Williams of Pennsylvania State University and his colleagues have used the Hubble Space Telescope's Advanced Camera for Surveys to discover 5,300 intergalactic stars in the Virgo cluster. Most of the stars are red giants about 52 million light-years from Earth.
Williams and his colleagues chose to observe an apparently empty field located between the two giant elliptical galaxies M86 and M87, both Virgo cluster members. The field was 37 arcminutes, or 550,000 light-years, southeast of M86 and 40 arcminutes, or 620,000 light-years, west-northwest of M87. The field's epoch 2000 coordinates were right ascension 12 hours, 28 minutes, and 10.8 seconds; declination +12 degrees, 33 minutes, and 20 seconds.

Virgostars
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The image here shows a small portion of this field that measures 1.0 arcminute by 0.8 arcminutes, or 15,000 by 12,000 light-years. The many galaxies in the image lie far beyond the Virgo cluster, but the white circles indicate probable intergalactic stars that belong to the cluster.

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Markarian's Chain of Galaxies
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Across the heart of the Virgo Cluster of Galaxies lies a striking string of galaxies known as Markarian's Chain. The chain, pictured above, is highlighted with two large but featureless lenticular galaxies, M84 and M86, and connects through several large spiral , including M88.

The home Virgo Cluster is the nearest cluster of galaxies, contains over 2,000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The centre of the Virgo Cluster is located about 70 million light years away toward the constellation of Virgo. At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance. The above image is just a small part of a mosaic dubbed the Big Picture taken by the Samuel Oschin Telescope at Palomar Observatory, in California, USA. A mural of the Big Picture will be displayed at the newly renovated Griffith Observatory near Los Angeles, California.

-- Edited by Blobrana at 13:16, 2008-07-23

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RE: Virgo HI Cloud (NGC 4388)
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Title: The Distance to the Virgo Cluster from a Recalibrated Tully-Fisher Relation Based on HST Cepheids and a Demonstrated Teerikorpi Cluster Incompleteness Bias
Authors: Allan Sandage (Obs. Carnegie Inst. Washington), G.A. Tammann (Astro. Inst. Univ. Basel)

The importance of the distance of the Virgo cluster in the ongoing debate on the value of the Hubble constant is reviewed. A new calibration of the Tully-Fisher 21-cm line width-absolute magnitude relation is made using Cepheid distances to 25 galaxies determined in various HST programs and reduced with the new Cepheid P-L relations that vary from galaxy-to galaxy. The calibration is applied to a complete sample of Virgo cluster spirals for the purpose of demonstrating the Teerikorpi cluster incompleteness bias. A diagnostic test is shown that should be useful in identifying the presence of bias in incompletely sampled data for distant clusters. The bias-free TF distance modulus for the Virgo cluster is m - M = 31.67 (D = 21.6 Mpc). A systematic correction of 0.07 mag is made because the cluster members are redder in B I on average than the calibrators at a given line width, giving a final adopted modulus for the Virgo cluster core of 31.60 + 0.09. If we assign a generous range of systematic error of ~ 0.3 mag, the distance D = 20.9 Mpc (m - M = 31.60) has a range from 24.0 Mpc to 18.2 Mpc (m - M between 31.9 and 31.3), and a Hubble constant of Ho = 56 between the limits of 49 and 65 when used with a cosmic expansion velocity of 1175 km s-1 determined by the method of distance ratios of remote clusters to Virgo. This range overlaps our preferred value of Ho = 62 from the HST Cepheid calibration of type Ia supernovae recently determined. The TF modulus of Virgo determined here cannot be reconciled with the recent high value of Ho = 72 from Freedman et al.

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NGC4647
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Title: The Evolution of the ISM in the Mildly Disturbed Spiral Galaxy NGC 4647
Authors: L. M. Young (New Mexico Tech), E. Rosolowsky (CfA), J. H. van Gorkom (Columbia), S. A. Lamb (U. Illinois)

Researchers present matched-resolution maps of HI and CO emission in the Virgo Cluster spiral NGC 4647. The galaxy shows a mild kinematic disturbance in which one side of the rotation curve flattens but the other side continues to rise. This kinematic asymmetry is coupled with a dramatic asymmetry in the molecular gas distribution but not in the atomic gas.
An analysis of the gas column densities and the interstellar pressure suggests that the H2/HI surface density ratio on the east side of the galaxy is three times higher than expected from the hydrostatic pressure contributed by the mass of the stellar disk. They discuss the probable effects of ram pressure, gravitational interactions, and asymmetric potentials on the interstellar medium and suggest it is likely that a m=1 perturbation in the gravitational potential could be responsible for all of the galaxy's features. Kinematic disturbances of the type seen here are common, but the curious thing about NGC 4647 is that the molecular distribution appears more disturbed than the HI distribution. Thus it is the combination of the two gas phases that provides such interesting insight into the galaxy's history and into models of the interstellar medium.

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Virgo Cluster
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The wide-angle Burrell-Schmidt telescope on Kitt Peak National Observatory in Arizona, USA took hundreds of 15-minute exposures over two months in early 2004 of the nearby Virgo Cluster of Galaxies.
The result is a dramatically deep and wide angle image of Virgo, the closest cluster of galaxies to our Milky Way Galaxy.


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Bright foreground stars have been digitally removed from the image but are still represented by numerous unusual dark spots. Inspection of the above image shows unusually large halos for the brightest galaxies as well as unusual faint streams of stars connecting Virgo galaxies that previously appeared unrelated.
The image allows a better reconstruction of the past few billion years of the gigantic Virgo cluster and illuminates the dynamics of clusters of galaxies in general.

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The Virgo Cluster
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CLEVELAND Case Western Reserve University astronomers have captured the deepest wide-field image ever of the nearby Virgo cluster of galaxies, directly revealing for the first time a vast, complex web of "intracluster starlight" -- nearly 1,000 times fainter than the dark night sky -- filling the space between the galaxies within the cluster.
The streamers, plumes and cocoons that make up this extremely faint starlight are made of stars ripped out of galaxies as they collide with one another inside the cluster, and act as a sort of "archaeological record" of the violent lives of cluster galaxies.

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The Virgo image was captured through Case's newly refurbished 24-inch Burrell Schmidt telescope, built in the 1930s and located at the Kitt Peak National Observatory in Arizona. Over the course of 14 dark moonless nights, the researchers took more than 70 images of the Virgo Cluster, then used advanced image processing techniques to combine the individual images into a single image capable of showing the faint intracluster light.

"When we saw all this very faint starlight in the image, my first reaction was WOW! Then I began to worry about all the things we could have done wrong" - Chris Mihos, project leader.

Many effects, such as stray light from nearby stars, from instruments in the observatory and even from the changing brightness of the night sky could all contaminate the image and lead to inaccurate results.

"But as we corrected for each of these contaminants, not only did the faint starlight not disappear, it became even more apparent. That's when we knew we had something big"

The new image gives dramatic evidence of the violent life and death of cluster galaxies. Drawn together into giant clusters over the course of cosmic time by their mutual gravity, galaxies careen around in the cluster, smashing into other galaxies, being stripped apart by gravitational forces and even being cannibalised by the massive galaxies which sit at the cluster's heart. The force of these encounters literally pulls many galaxies apart, leaving behind ghostly streams of stars adrift in the cluster, a faint tribute to the violence of cluster life.

"From computer simulations, we've long suspected this web of intracluster starlight should be there; but it's been extremely hard to map it out because it's so faint" - Chris Mihos, associate professor of astronomy at Case.

Mihos and graduate students Craig Rudick (Case) and Cameron McBride (University of Pittsburgh, and former Case undergraduate) have developed computer simulations that track how clusters of galaxies evolve over time, to study exactly how this intracluster starlight is created.

"With the data from the telescope, we see how a cluster looks today. But with computer simulations, we can watch how a cluster evolves over 10 billion years of time. By comparing the simulation to the real features we now see in Virgo, we can learn how the cluster formed and what happened to its many galaxies" - Chris Mihos.

For example, the fact that the intracluster light in Virgo is so complex and irregular lends credence to the theory of "hierarchical assembly," where clusters grow sporadically when groups of galaxies fall into the cluster, rather than through the smooth, slow addition of galaxies one by one.
To detect the faint intracluster light, upgrades were needed to Case's Burrell Schmidt telescope, originally part of the original Warner and Swasey Observatory in Cleveland until its move to Kitt Peak in 1979.
The improvements included the installation of a new camera system and upgrades to the telescope to make it more structurally stable and reduce unwanted scattered light.

"It's like 'The Little Engine that Could'. It's the smallest telescope on the mountain, but with these upgrades it's capable of some pretty incredible science" - Paul Harding, Case astronomer, who directed the refurbishment of the telescope.

The telescope's wide field of view -- enough to fit three full moons across the image -- proved crucial to the project, allowing the team to map out the intracluster light over a much larger part of the Virgo Cluster than would be possible using larger telescopes with their much smaller fields of view.
The Virgo Cluster of galaxies -- so named because it appears in the constellation of Virgo -- is the nearest galaxy cluster to the Earth, at a distance of approximately 50 million light years. The cluster contains more than 2,000 galaxies, the brightest of which can be seen with the aide of a small telescope.
The Case findings are reported in the paper "Diffuse Light in the Virgo Cluster" to be published in the September 20th issue of The Astrophysical Journal Letters. Along with Mihos team researchers included Case astronomers Heather Morrison and Paul Harding, and John Feldmeier, a National Science Foundation Fellow at the National Optical Astronomy Observatory in Tucson, Arizona, US, (and formerly of Case).

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Posts: 131433
Date:
Virgo HI Cloud (NGC 4388)
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Researchers are reporting the discovery of a large hydrogen (HI) cloud in the central regions of the Virgo cluster. It is 110 x 25 kpc in size and contains 3.4 x 10^8 solar masses of hydrogen gas. The shape and movement of this cloud strongly suggest that it consists of gas removed from the galaxy NGC 4388 by ram-pressure stripping.

Probably the result of an interaction of the interstellar gas of NGC 4388 with the hot halo of the M86 group and not with the intra-cluster medium (ICM) centred on M87. The large extent of the plume shaped gas suggests that hydrogen stripped from cluster galaxies can remain neutral for at least 10^8 yr.
Locally, the column density is well above 10^20 cm^-2, suggesting that the intra-cluster HII regions known to exist in Virgo may have formed from gas stripped from cluster galaxies.
The existence of the HI plume suggests that stripping of in falling spirals contributes to the enrichment of the intra-cluster medium. The HI object in the Virgo cluster recently reported may have a similar origin and may therefore not be a "dark galaxy".



NGC 4388 is classified as an active galaxy. The spiral, relatively close by at 60 million light years, is a member of the nearest major cluster of galaxies: the Virgo Cluster.


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-- Edited by Blobrana at 08:21, 2006-08-31

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