* Astronomy

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Andromeda galaxy

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Messier 31

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RE: Andromeda galaxy (M31)

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Hubble Finds Giant Halo Around the Andromeda Galaxy

The Andromeda galaxy is our Milky Way's nearest neighbor in space. The majestic spiral of over 100 billion stars is comparable in size to our home galaxy. At a distance of 2.5 million light-years, it is so close to us the galaxy can be seen as a cigar-shaped smudge of light high in the autumn sky. But if you could see the huge bubble of hot, diffuse plasma surrounding it, it would appear 100 times the angular diameter of the full Moon! The gargantuan halo is estimated to contain half the mass of the stars in the Andromeda galaxy itself. It can be thought of as the "atmosphere" of a galaxy. Astronomers using Hubble identified the gas in Andromeda's halo by measuring how it filtered the light of distant bright background objects called quasars. It is akin to seeing the glow of a flashlight shining through a fog. This finding promises to tell astronomers more about the evolution and structure of one of the most common types of galaxies in the universe.
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Title: Dense gas tracing the collisional past of Andromeda. An atypical inner region?
Author: A.-L. Melchior, F. Combes

The central kiloparsec region of the Andromeda galaxy is relatively gas poor, while the interstellar medium appears to be concentrated in a ring-like structure at about 10 kpc radius. The central gas depletion has been attributed to a possible head-on collision 200 Myr ago, supported by the existence of an offset inner ring of warm dust. We present new IRAM-30m observations of the molecular gas in the central region, and the detection of CO and its isotopes ¹³CO(2-1) and C¹⁸O(2-1), together with the dense gas tracers, HCN(1-0) and HCO+(1-0). A systematic study of the observed peak temperatures with non-LTE equilibrium simulations shows that the detected lines trace dense regions with nH2 in the range 2.5 x 10⁴-5.6 x 10⁵ cm^-3, while the gas is very clumpy with a beam filling factor of 0.5-2 10^-2. We also show that the gas is optically thin in all lines, except the ¹²CO(1-0) and ¹²CO(2-1) lines, that the CO lines are close to the thermal equilibrium at 17.5-20 K and a molecular hydrogen density larger than critical and that the HCN and HCO+ lines have a subthermal excitation temperature of 8-10 K with a density smaller than critical. The molecular mass we derive is compatible with the dust mass derived from the far-infrared emission, assuming a dust-to-gas mass ratio of 0.01. In one of the regions, the ¹²CO/¹³CO line ratio is quite high (~20), and equals to the ¹²CO/C¹⁸O ratio. The fact that the optically thin ¹³CO and C¹⁸O lines have comparable intensities means that the secondary element ¹³C is depleted with respect to the primary ¹²C, as is expected just after a recent star formation. This suggests that there has been a recent starburst in the central region, supporting the head-on collision scenario.

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Title: Evidence for a Massive, Extended Circumgalactic Medium Around the Andromeda Galaxy
Author: Nicolas Lehner, Chris Howk, Bart Wakker

We demonstrate the presence of an extended and massive circumgalactic medium (CGM) around Messier 31 using archival HST COS ultraviolet spectroscopy of 5 QSOs projected within the virial radius (Rvir = 300 kpc). We detect Si III absorption associated with the M31 CGM toward all the 5 sightlines, including the most distant at impact parameter r=245 and 294 kpc, C IV and Si IV in 4 sightlines, and singly-ionized species toward only the three targets with r<50 kpc. Two targets at r = 26 and 294 kpc were observed by FUSE and show O VI M31 CGM absorption. The M31 CGM gas is therefore multiphase, mostly ionized, and becomes more highly ionized gas at larger r. The detection of C IV and O VI absorption at 25<r< 300 kpc strongly suggests an extended hot corona around M31. We estimate using Si II, Si III, and Si IV a CGM metal mass of 2.9x10^6 Msun and gas mass of 4x10^9 (Zsun/Z) Msun within 1/6Rvir, implying nearly as much metal and gas mass within 1/6Rvir of the CGM of M31 as in the disk of M31. We estimate the total metal mass and baryon mass of the cool M31 CGM gas at Rvir to be ~2-10x10^7 Msun and ~3-10x10^10 Msun, respectively. Compared with galaxies in the larger COS-Halos survey, the CGM of M31 appears to be quite typical for a L* galaxy.

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NASA'S Chandra Turns up Black Hole Bonanza in Andromeda Galaxy

Using data from NASA's Chandra X-ray Observatory, astronomers have discovered an unprecedented bonanza of black holes in the Andromeda Galaxy, one of the nearest galaxies to the Milky Way.
Using more than 150 Chandra observations, spread over 13 years, researchers identified 26 black hole candidates, the largest number to date, in a galaxy outside our own. Many consider Andromeda to be a sister galaxy to the Milky Way. The two ultimately will collide, several billion years from now.
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New Views of Andromeda Galaxy

Two new eye-catching views from the Herschel space observatory are fit for a princess. They show the elegant spiral galaxy Andromeda, named after the mythical Greek princess known for her beauty.
The Andromeda galaxy, also known as Messier 31, lies 2 million light-years away, and is the closest large galaxy to our own Milky Way. It is estimated to have up to one trillion stars, whereas the Milky Way contains hundreds of billions. Recent evidence suggests Andromeda's overall mass may in fact be less than the mass of the Milky Way, when dark matter is included.
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Title: The Bulge of M31
Authors: Jeremy Mould (Swinburne University)

Bulges are not just elliptical subgalaxies situated in the centers of large spirals. It might seem that way from their ages and chemistry, but bulge kinematics have been known to be different since the first long slit spectra were obtained. M31 presents the best opportunity to investigate all the issues of the stellar populations of bulges. This review collects the array of probing data that has been accumulated in the last decade. But the intriguing question 'how did it form like this ?' remains.

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Title: A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy
Authors: Rodrigo A. Ibata, Geraint F. Lewis, Anthony R. Conn, Michael J. Irwin, Alan W. McConnachie, Scott C. Chapman, Michelle L. Collins, Mark Fardal, Annette M. N. Ferguson, Neil G. Ibata, A. Dougal Mackey, Nicolas F. Martin, Julio Navarro, R. Michael Rich, David Valls-Gabaud, Lawrence M. Widrow

Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. An early analysis noted that dwarf galaxies may not be isotropically distributed around our Galaxy, as several are correlated with streams of HI emission, and possibly form co-planar groups. These suspicions are supported by recent analyses, and it has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence (99.998% significance) of a planar sub-group of satellites in the Andromeda galaxy, comprising approximately 50% of the population. The structure is vast: at least 400 kpc in diameter, but also extremely thin, with a perpendicular scatter <14.1 kpc (99% confidence). Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This finding shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum, a new insight for our understanding of the origin of these most dark matter dominated of galaxies. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way's disk and is co-planar with the Milky Way to Andromeda position vector. The existence of such extensive coherent kinematic structures within the halos of massive galaxies is a fact that must be explained within the framework of galaxy formation and cosmology.