* Astronomy

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RE: Andromeda galaxy (M31)

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Title: The kinematic footprints of five stellar streams in Andromeda's halo
Authors: S. C. Chapman, R. Ibata, M. Irwin, A. Koch, B. Letarte, N. Martin, M. Collins, G. F. Lewis, A. McConnachie, J. Penarrubia, R. M. Rich, D. Trethewey, A. M. N. Ferguson, A. Huxor, N. Tanvir
(Version v2)

We present a spectroscopic analysis of five stellar streams (`A', `B', `Cr', `Cp' and `D') as well as the extended star cluster, EC4, which lies within streamC, all discovered in the halo of M31 from our CFHT/MegaCam survey. These spectroscopic results were initially serendipitous, making use of our existing observations from the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, and thereby emphasizing the ubiquity of tidal streams that account for ~70% of the M31 halo stars in the targeted fields. Subsequent spectroscopy was then procured in streamCr/p and streamD to trace the velocity gradient along the streams. For the cluster EC4, candidate member stars with average [Fe/H]~-1.4 (Fe/H_spec=-1.6), are found at v_{hel}=-285 km/s suggesting it could be related to streamCp. No similarly obvious cold kinematic candidate is found for streamD, although candidates are proposed in both of two spectroscopic pointings along the stream (both at -400 km/s). Spectroscopy near the edge of streamB suggests a likely kinematic detection, while a candidate kinematic detection of streamA is found (plausibly associated to M33 rather than M31). The low dispersion of the streams in kinematics, physical thickness, and metallicity makes it hard to reconcile with a scenario whereby these stream structures as an ensemble are related to the giant southern stream. We conclude that the M31 stellar halo is largely made up of multiple kinematically cold streams.

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http://www.ast.cam.ac.uk/~schapman/streams.pdf

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Andromeda Stream

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RE: Andromeda galaxy (M31)

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Title: The Once and Future Andromeda Stream
Authors: Masao Mori (Senshu University), R. Michael Rich (UCLA)

The interaction between an accreting satellite and the Andromeda galaxy has been studied using an N-body simulation to investigate the self-gravitating response of the disk, the bulge, and the dark matter halo to an accreting satellite. Our simulation shows that the ``giant stream'' is the tidal debris of the infalling satellite. The debris also produces diffuse shells on the east and the west side of M31 in agreement with observations, but for an accreting satellite mass of M<5x10^9 Msun, the disk survives the collision in its present form and negligible disk stars are ejected into the halo. Following the evolution of the merger past the present day, these shells expand further and a multiple large scale-shell system is finally formed in the outer region and a dense core forms in the inner region. The outermost large-scale shells in our simulation have a radius of >50 kpc and these structures survive at least 4 Gyr from the present-day. We propose that recently discovered distant arc-like structures and metal rich stars at R>100 kpc may be the remnants of ancient radial infall collisions similar to the one responsible for the currently observed giant stream.
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Title: M31N-2007-06b: A Nova in the M31 Globular Cluster Bol 111
Authors: A. W. Shafter, R. M. Quimby

We report spectroscopic observations of the nova M31N-2007-06b, which was found to be spatially coincident with the M31 globular cluster Bol 111. This nova is the first out of more than 700 discovered in M31 over the past century to be associated with one of the galaxy's globular clusters. A total of three spectra of the nova were obtained 3, 6, and 36 days post discovery. The data reveal broad (FWHM ~3000 km/s) Balmer, NII, and NIII emission lines, and show that the nova belongs to the He/N spectroscopic class. The He/N class of novae are relatively rare, making up roughly 15% of the novae with measured spectra in M31, and roughly 20-25% of the Galactic novae for which spectroscopic data are available. The implications of a nova, particularly an He/N nova, occurring in a globular cluster are discussed.

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Title: Chandra detection of diffuse hot gas in and around the M31 bulge
Authors: Z. Li, Q. D. Wang (UMASS)

We report the detection of diffuse hot gas in M31, using archival Chandra observations which allow us to map out a 30' by 30' field (covering a galactocentric radius up to 4.5 kpc) and to detect sources in the galaxy down to a 0.5-8 keV luminosity limit of ~10^35 ergs/s. We estimate the remaining stellar contribution from fainter X-ray sources (primarily cataclysmic variables and coronally active binaries), assuming that they spatially follow the stellar distribution. Indeed, the near-IR K-band light of the galaxy closely traces the 2-8 keV unresolved X-rays, indicating a collective stellar X-ray emissivity consistent with those determined for the Galactic ridge and M32, whereas the amount of the 0.5-2 keV unresolved emission is significantly greater than the expected stellar contribution, especially within a galactocentric radius of ~2 kpc. Morphologically, this soft X-ray excess appears substantially rounder than the bulge as seen in K-band and is elongated approximately along the minor-axis at large radii. The excess thus most likely represents the emission of diffuse hot gas in and around the galactic bulge. Furthermore, the near side of the M31 disk casts an apparent shadow against the soft X-ray excess, indicating that the hot gas extends to at least 2.5 kpc from the galactic plane. We briefly discuss the implications of these results on the energy balance in the M31 bulge and on understanding the large-scale soft X-ray enhancement observed toward the inner region of our own Galaxy.

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OLYMPIAN GALAXY
A newly discovered dwarf galaxy in the Local Group has been found to have formed in a region of space far from our own and is falling into our system for the first time in its history, according to new data obtained at the W. M. Keck Observatory. An international team of astronomers report that the dwarf galaxy, Andromeda XII, marks the best piece of evidence for small galaxies which are just now arriving in our Local Group. The finding provides an important test for simulations of galaxy formation.

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Star Cluster Holds Midweight Black Hole
Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope have greatly strengthened the case that supermassive black holes at the cores of galaxies may have formed through mergers of smaller black holes. Their VLA studies showed that a globular star cluster in the galaxy M31 probably has a black hole with 20,000 times the mass of the Sun at its core.
Based on observations with optical telescopes, Karl Gebhardt of the University of Texas at Austin and his colleagues, which include Ho, suggested in 2002 that the globular cluster G1 in the Andromeda Galaxy (M31) contains a compact concentration of mass that is intermediate in mass between stellar and supermassive black holes. Other researchers disputed that conclusion.

According to Ho, "In 2005, we obtained better data that clinched the case that the cluster really does contain a dark object with 20,000 times the mass of the Sun. What we can't be sure of, however, is whether the dark mass is a single object --- that is, an intermediate-mass black hole --- or a cluster of smaller dark objects such as neutron stars or stellar-sized black holes."

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Andromeda, the nearest major galaxy to the Milky Way, is shown here in this wide-field optical image from Kitt Peak. The central region of Andromeda is shown in a composite image, with X-rays from NASA's Chandra X-ray Observatory (blue) combined with the optical image. Astronomers believe that Andromeda, also known as Andromeda Galaxy (M31), and the Milky Way will merge in a few billion years.

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Credit: X-ray: NASA/CXC/MPE/W.Pietsch et al; Optical: NOAO/AURA/NSF/T.Rector & B.A.Wolpa

In the composite image, hot, X-ray bright gas is seen to envelop the central region of Andromeda. Point sources are also prominent, mostly from pairs of stars that are interacting with each other. Many of these double stars are thought to include white dwarfs that are pulling large amounts of material away from a companion star. When the amount of gas being dumped onto the white dwarf gets too high a thermonuclear explosion occurs on the surface of the white dwarf, emitting bright X-rays.
By taking multiple observations of these so-called novae with Chandra and ESA's XMM-Newton observatories, a team of astronomers studied how long the burst of X-ray emission lasts. They found that several novae are bright in X-rays for surprisingly short periods of time, suggesting that the corresponding nova explosions were missed in earlier observations. Such short periods of bright X-ray emission, according to theoretical calculations, indicate that the white dwarfs have relatively high masses. This makes them good candidates for progenitors of Type Ia supernovas, where a white dwarf reaches a mass limit and undergoes a thermonuclear explosion and is completely destroyed. The high masses suggested by the short X-ray outbursts suggests that the white dwarfs do not have to gain very much mass before reaching their limit and being destroyed. A long-running goal in stellar astrophysics has been to identify the elusive stars that explode as Type Ia supernovas.

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