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Title: Does M31 result from an ancient major merger?
Authors: F. Hammer (1), Y. B. Yang (2), J. L. Wang (1,2), M. Puech (1), H. Flores (1), S. Fouquet (1) ((1) Laboratoire GEPI, Observatoire de Paris (2) National Astronomical Observatories, Chinese Academy of Sciences (NAOC))
(Version v2)

The numerous streams in the M31 halo are currently assumed to be due to multiple minor mergers. Here we use the GADGET2 simulation code to test whether M31 could have experienced a major merger in its past history. It results that a 3±0.5:1 gaseous rich merger with r(per)=25±5 kpc and a polar orbit can explain many properties of M31 and of its halo. The interaction and the fusion may have begun 8.75±0.35 Gyr and 5.5 ±0.5 Gyr ago, respectively. With an almost quiescent star formation history before the fusion we retrieve fractions of bulge, thin and thick disks as well as relative fractions of intermediate age and old stars in both the thick disk and the Giant Stream. The Giant Stream is caused by returning stars from a tidal tail previously stripped from the satellite prior to the fusion. These returning stars are trapped into elliptical orbits or loops for almost a Hubble time period. Large loops are also predicted and they scale rather well with the recently discovered features in the M31 outskirts. We demonstrate that a single merger could explain first-order (intensity and size), morphological and kinematical properties of the disk, thick disk, bulge and streams in the halo of M31, as well as the distribution of stellar ages, and perhaps metallicities. It challenges scenarios assuming one minor merger per feature in the disk (10 kpc ring) or at the outskirts (numerous streams & thick disk). Further constraints will help to properly evaluate the impact of such a major event to the Local Group.

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Nearby Black Hole is Feeble and Unpredictable

For over 10 years, NASA's Chandra X-ray Observatory has repeatedly observed the Andromeda Galaxy for a combined total of nearly one million seconds. This unique data set has given astronomers an unprecedented view of the nearest supermassive black hole outside our own Galaxy.
Astronomers think that most galaxies - including the Milky Way - contain giant black holes at their cores that are millions of times more massive than the Sun. At a distance of just under 3 million light years from Earth, Andromeda (also known as M31) is relatively close and provides an opportunity to study its black hole in great detail.
Just like the one in the centre of the Milky Way, the black hole in Andromeda is surprisingly quiet. In fact, Andromeda's black hole, known as M31*, is ten to one hundred thousand times fainter in X-ray light that astronomers might expect given the reservoir of gas around it.

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On clear nights, a faintly glowing smudge is visible without optical aid between Cassiopeia and the great square of Pegasus in the constellation Andromeda.
This patchy oval of light is the neighbouring Andromeda Galaxy which, at 2.5 million light years away, is actually the closest spiral galaxy to our own and the farthest object from Earth visible to the naked eye.
With an apparent magnitude of 3.4, the Andromeda Galaxy, or M31, is one of the brightest objects in the Messier catalogue, a list of nebulae and star clusters compiled by French astronomer Charles Messier. It is believed to span 220,000 light years and is tilted toward us at about a 15 degree angle from the edge-on position, allowing a decent glimpse of its structure.

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WISE gives Milky Way's closest sibling a new look

NASA launched the Wide-field Infrared Survey Explorer, or WISE, in December to map the sky in infrared wavelengths. The spacecraft has already discovered a new asteroid and comet near Earth, and on Wednesday the space agency released a series of images WISE has captured of known astronomical objects. In this infrared image of Andromeda, bluer light represents stars, which give off a great deal of heat, whereas redder hues mark cooler regions, such as dusty wisps within the galaxy. The blue lobe below the galactic disk is Messier 110, an elliptical galaxy that is one of Andromeda's many satellites.
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Andromeda Stella Streams
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androge1.gif
False-colour map of the density of red giant stars in Andromeda showing the Stella streams E, F, and SW.
Credit  M. Tanaka (Tohoku University)

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An international team of astronomers has identified two new tidal streams in the Andromeda Galaxy, the remnants of dwarf galaxies consumed by our large galactic neighbour.
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Astronomers discover new tidal streams in Andromeda Galaxy

An international team of astronomers has identified two new tidal streams in the Andromeda Galaxy, the remnants of dwarf galaxies consumed by our large galactic neighbour.
Analysis of the stars in Andromeda's tidal streams and other components of its extended halo is yielding new insights into the processes involved in the formation and evolution of massive galaxies, according to Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz.

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