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RE: Hercules Dwarf Galaxy
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Hercules dwarf galaxy explodes after diving into the Milky Way

A dim galaxy in the constellation Hercules has learned this lesson the hard way after diving into our galaxy and being torn asunder by its gravitational pull.
At least 50 galaxies orbit our own. The Hercules dwarf is currently 460,000 light years from Earth, nearly three times farther than the Milky Ways brightest satellite galaxy, the Large Magellanic Cloud.
But Herculess stars spread over a great expanse of space, which suggests the Milky Ways gravitational pull has yanked them away from one another.

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Title: Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy
Authors: Andreas Koch, Sofia Feltzing, Daniel Aden, Francesca Matteucci

We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 AA absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon (Ba) < 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 AA-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.

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Hercules stellar stream
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Title: The Hercules satellite: a stellar stream in the Milky Way halo?
Authors: Nicolas F. Martin (MPIA), Shoko Jin (ARI/ZAH, University of Heidelberg)

We investigate the possibility that the recently discovered Hercules Milky Way satellite is in fact a stellar stream in formation, thereby explaining its very elongated shape with an axis ratio of 3 to 1. Under the assumption that Hercules is a stellar stream and that its stars are flowing along the orbit of its progenitor, we find an orbit that would have recently brought the system close enough to the Milky Way to induce its disruption and transformation from a bound dwarf galaxy into a stellar stream. The application of simple analytical techniques to the tentative radial velocity gradient observed in the satellite provides tight constraints on the tangential velocity of the system (v_t = -16^{+6}_{-22} km/s in the Galactic Standard of Rest). Combined with its large receding velocity, the determined tangential velocity yields an orbit with a small pericentric distance (R_peri = 6^{+9}_{-2} kpc). Tidal disruption is therefore a valid scenario for explaining the extreme shape of Hercules. The increase in the mean flattening of dwarf galaxies as one considers fainter systems could therefore be the impact of a few of these satellites not being bound stellar systems dominated by dark matter but, in fact, stellar streams in formation, shedding their stars in the Milky Way's stellar halo.

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Hercules Dwarf Galaxy.kmz
Google Sky file

-- Edited by Blobrana at 15:18, 2008-01-12

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An international team of astronomers using the Large Binocular Telescope (LBT) in Arizona has discovered that the Hercules Dwarf Galaxy is shaped like a cigar. That makes it an oddball among millions of its peers.
The Hercules Dwarf is a companion galaxy to our own Milky Way, but has only one ten-millionth as many stars. It was among several dwarf galaxies discovered earlier this year by the Sloan Digital Sky Survey.
Now astronomers using the LBT have discovered that this tiny galaxy, which is 430,000 light years away, is flat rather than round. Spiral galaxies, including our Milky Way, are flat because they rotate around their galactic centres. But dwarf galaxies don't rotate this way. They are more spherical. The Hercules Dwarf Galaxy is the first flat dwarf galaxy astronomers have found among millions of well-studied dwarf galaxies.

"The Hercules Dwarf Galaxy is either unlike any of the millions of dwarf galaxies studied so far, or (else) it circles our Galaxy on an extremely plunging orbit" - Matthew Coleman of the Max Planck Institute for Astronomy, Germany.

If the Hercules Dwarf revolves in some wildly eccentric orbit with the Milky Way, our big galaxy's gravity may have pulled Hercules' stars into a squashed, cigar-shaped disk when they were near the Milky Way's galactic core, Coleman said. The Milky Way has "tidally stretched" the Sagittarius Dwarf Galaxy this way. The Milky Way's gravity pulls more strongly on one side of Sagittarius than the other, stretching it out into a stream of stars. But Sagittarius is 10 times closer to the Milky Way's centre than the Hercules Dwarf Galaxy is.

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Title: The Elongated Structure of the Hercules dSph from Deep LBT Imaging
Authors: Matthew G. Coleman, Jelte T. A. De Jong, Nicolas F. Martin, Hans-Walter Rix, David J. Sand, Eric F. Bell, Richard W. Pogge, David J. Thompson, H. Hippelein, E. Giallongo, R. Ragazzoni, Andrea DiPaola, Jacopo Farinato, Riccardo Smareglia, Vincenzo Testa, Jill Bechtold, John M. Hill, Peter M. Garnavich, Richard F. Green

We present a deep, wide-field photometric survey of the newly-discovered Hercules dwarf spheroidal galaxy, based on data from the Large Binocular Telescope. Images in B, V and r were obtained with the Large Binocular Camera covering a 23' times 23' field of view to a magnitude of ~25.5 (5 sigma). This permitted the construction of colour-magnitude diagrams that reach approximately 1.5 magnitudes below the Hercules main sequence turnoff. Three-filter photometry allowed us to preferentially select probable Hercules member stars, and examine the structure of this system at a previously unattained level. We find that the Hercules dwarf is highly elongated (3:1), considerably more so than any other dSph satellite of the Milky Way except the disrupting Sagittarius dwarf. While we cannot rule out that the unusual structure is intrinsic to Hercules as an equilibrium system, our results suggest tidal disruption as a likely cause of this highly elliptical structure. Given the relatively large Galactocentric distance of this system (132 12 kpc), signs of tidal disruption would require the Hercules dwarf to be on a highly eccentric orbit around the Milky Way.

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