* Astronomy

Astronomers identify thick disc of older stars in nearby Andromeda galaxy

An international team of astronomers has identified for the first time a thick stellar disc in the Andromeda galaxy, the nearest large spiral galaxy to our own Milky Way.
The discovery of the thick disc, a major result from a five-year investigation, will help astronomers better understand the processes involved in the formation and evolution of large spiral galaxies like ours, according to the team, which includes UCLA research astronomer Michael Rich and colleagues from Europe and Australia.
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Title: Star Clusters in M31: II. Old Cluster Metallicities and Ages from Hectospec Data
Authors: Nelson Caldwell (CfA), Ricardo Schiavon (Gemini), Heather Morrison (Case), James A. Rose (UNC), Paul Harding (Case)

We present new high signal-to-noise spectroscopic data on the M31 globular cluster system, obtained with the Hectospec multifibre spectrograph on the 6.5m MMT. More than 300 clusters have been observed at a resolution of 5A. The primary focus of this paper is the determination of mean cluster metallicities, ages and reddenings. Metallicities were estimated using a calibration of Lick indices with [Fe/H] provided by Galactic GCs. The metallicity distribution is not generally bimodal, in strong distinction with the bimodal Galactic globular distribution. Rather, the M31 distribution shows a broad peak, centered at [Fe/H]=-1, suggesting that the cluster systems of M31 and the Milky Way had different formation histories. Ages for clusters with [Fe/H] > -1 were determined using the automatic stellar population analysis program EZ_Ages. We find no evidence for massive clusters in M31 with intermediate ages. Moreover, we find that the mean ages of the old GCs are remarkably constant over about a decade in metallicity (-0.95 < [Fe/H] < 0.0).

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Title: The Murmur of The Hidden Monster: Chandra's Decadal View of The Super-massive Black Hole in M31
Authors: Zhiyuan Li, Michael R. Garcia, William R. Forman, Christine Jones, Ralph P. Kraft, Dharam V. Lal (CfA), Stephen S. Murray (CfA/JHU), Q. Daniel Wang (UMass)
(Version v2)

The Andromeda galaxy (M31) hosts a central super-massive black hole (SMBH), known as M31*, which is remarkable for its mass (~10^8 solar masses) and extreme radiative quiescence. Over the past decade, the Chandra X-ray observatory has pointed to the center of M31 ~100 times and accumulated a total exposure of ~900 ks. Based on these observations, we present an X-ray study of a highly variable source that we associate with M31* based on positional coincidence. We find that M31* remained in a quiescent state from late 1999 to 2005, exhibiting an average 0.5-8 keV luminosity \lesssim 10^{36}ergs~s^{-1}, or only ~10^{-10} of its Eddington luminosity. We report the discovery of an outburst that occurred on January 6, 2006, during which M31* radiated at ~4.3 x 10^{37}ergs~s^{-1}. After the outburst, M31* entered a more active state that apparently lasts to the present, which is characterised by frequent flux variability around an average luminosity of ~4.8 x 10^{36}ergs~s^{-1}. These flux variations are similar to the X-ray flares found in the SMBH of our Galaxy (Sgr A*), making M31* the second SMBH known to exhibit recurrent flares. Future coordinated X-ray/radio observations will provide useful constraints on the physical origin of the flaring emission and help rule out a possible stellar origin of the X-ray source.

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Andromeda 'born in a collision'

The nearest spiral galaxy to the Milky Way - Andromeda - was born when two smaller galaxies collided, say astronomers.
An international team conducted a computer simulation of how Andromeda evolved over time.
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