Andromeda Galaxy

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See Andromeda (disambiguation) for other uses of "Andromeda".
Andromeda Galaxy

A visible light image of the Andromeda Galaxy.
Credit: John Lanoue.
Observation data (J2000 epoch)
Constellation Andromeda
Right ascension 00h 42m 44.3s[1]
Declination +41° 16′ 9″[1]
Redshift −301 ± 1 km/s[2]
Distance 2.54 ± 0.06 Mly
(778 ± 17 kpc)[3][2][4][5][6][a]
Type SA(s)b[1]
Apparent dimensions (V) 190′ × 60′[1]
Apparent magnitude (V) 4.4[1]
Other designations
M31, NGC 224, UGC 454, PGC 2557[1]
See also: Galaxy, List of galaxies
This box: view • talk • edit

The Andromeda Galaxy (IPA: /ænˈdrɒmədə/, also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2.5 million light-years away[4] in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way.

Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.[7] However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy.[8] 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1×1011 solar masses.[2]

At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[9] making it easily visible to the naked eye even when viewed from areas with moderate light pollution. It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon.

Contents

[edit] Observation history

The earliest recorded observation of the Andromeda Galaxy was in 964 CE by the Persian astronomer 'Abd Al-Rahman Al Sufi,[10] who described it as a "small cloud". Star charts of that period have it labeled as the Little Cloud.[10] The first description of the object based on telescopic observation was given by Simon Marius[10] in 1612. Charles Messier catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi's earlier work. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the "great nebulae" and, based on the color and magnitude of the nebula, he estimated (incorrectly) that it was no more than 2,000 times the distance of Sirius.[11]

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula.[12] The spectra of M31 displayed a continuum of frequencies, superimposed with dark lines. This was very similar to the spectra of individual stars. From this it was deduced that M31 had a stellar nature.

In 1885, a supernova (known as "S Andromedae") was seen in M31, the first and so far only one observed in that galaxy. At the time, since M31 was considered to be a "near by" object, it was thought to be a much less luminous and unrelated event called a nova, and was named accordingly Nova 1885.

Great Andromeda Nebula by Isaac Roberts.
Great Andromeda Nebula by Isaac Roberts.

The first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.[13] However, at the time this object was commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets.

The radial velocity of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the Lowell Observatory, using spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (186 miles/sec.), moving in the direction of the Sun.[14]

[edit] Island universe

In 1917, Heber Curtis had observed a nova within M31. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our Galaxy. As a result he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.[15]

In 1920 the Great Debate between Harlow Shapley and Heber Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that "Great Andromeda Nebula" (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant Doppler shift. Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31. These were made using a 2.5 metre (100 in) reflecting telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own.[16]

This galaxy plays an important role in galactic studies, since it is the nearest giant spiral (although not the nearest galaxy). In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Based on his observations of this galaxy, he was able to discern two distinct populations of stars, naming the young, high velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort.)[17] Dr. Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to M31, as well as the remainder of the Universe.

The first radio maps of the Andromeda Galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group. The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalogue.

[edit] General information

The Andromeda Galaxy is approaching the Sun at about 300 kilometers per second (186 miles/sec.), so it is one of the few blue shifted galaxies. Given the motion of the Solar System inside the Milky Way, one finds that the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometres per second (62–87 miles/sec.).[18] The impact is predicted to occur in about 2.5 billion years. In that case the two galaxies will likely merge to form a giant elliptical galaxy.[19] However, Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of when the collision will take place and how it will proceed.[20] Such events are frequent among the galaxies in galaxy groups.

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. In the 1990s, Hipparcos satellite measurements were used to calibrate the Cepheid distances. The corrected value gives the Andromeda Galaxy a distance of 2.9 million light-years. Unfortunately, all Cepheids lie further than Hipparcos could measure accurately, and it became clear that Hipparcos-calibrated values for Cepheids were not reliable.

The Andromeda Galaxy pictured in ultraviolet light by GALEX
The Andromeda Galaxy pictured in ultraviolet light by GALEX
See also: Andromeda-Milky Way collision

[edit] Recent distance estimates

At least four distinct techniques have been used to recently measure distances to M31.

In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of -0.2 mag dex-1 in (O/H), an estimate of 2.57 ± 0.06 Mly (787 ± 18 kpc) was derived.

Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.[3][2]

In 2005, a group of astronomers consisting of Ignasi Ribas (CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236,[c] has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 Mly (770 ± 40 kpc) and the whole Andromeda Galaxy at about 2.5 Mly.[4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Andromeda is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (785 ± 25 kpc).[5]

Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly (778 ± 17 kpc).[a] Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly.[d]

[edit] Recent mass estimates

Current mass estimates for the Andromeda halo (including dark matter) give a value of approximately 1.23×1012 M☉[21] (or 1.2 million million solar masses) compared to 1.9×1012 M☉ for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. M31 does contain many more stars than our own galaxy and has a much larger size.

In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity of M31 is double that of our own galaxy.[22] However the rate of star formation in the Milky Way is much higher, with M31 only producing about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of novae in the Milky Way is also double that of M31.[23] This suggests that M31 has experienced a great star formation phase in its past, while the Milky Way is in the middle of a current star formation phase. This could mean that in the future, the number of stars in the Milky Way will match the number observed in M31.

[edit] Structure

Based on its appearance in visible light, the Andromeda galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs-Sandage extended classification system of spiral galaxies.[1] However, data from the 2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred galaxy with the bar viewed almost directly along its long axis.[24]

The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories
The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories

In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself.[25] This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, estimates of Andromeda's size ranged from 70,000 to 120,000 light-years across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.[26] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. It also should be noted that the galaxy M33 could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of this galaxy at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 miles/sec.) at a radius of 1,300 light-years, then descends to a minimum at 7,000 light-years where the rotation velocity may be as low as 50 kilometres per second (31 miles/sec.). Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years, where it reaches a peak of 250 kilometres per second (155 miles/sec.). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (124 miles/sec.) at 80,000 light-years. These velocity measurements imply a concentrated mass of about 6×109 M☉ in the nucleus. The total mass of the galaxy increases linearly out to 45,000 light-years, then more slowly beyond that radius.[27]

The spiral arms of Andromeda are outlined by a series of H II regions that Baade described as resembling "beads on a string". They appear to be tightly wound, although they are more widely spaced than in our galaxy.[28] Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core. The most likely cause of the spiral pattern is thought to be interaction with M32. This can be seen by the displacement of the neutral hydrogen clouds from the stars.[29]

Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPL-Caltech/K. Gordon (University of Arizona)
Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPL-Caltech/K. Gordon (University of Arizona)

In 1998, images from the European Space Agency's Infrared Space Observatory demonstrated that the overall form of the Andromeda galaxy may be transitioning into a ring galaxy. The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years from the core.[30] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.

Close examination of the inner region of Andromeda showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of Andromeda along the latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.[31]

Studies of the extended halo of M31 show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal"-poor, and increasingly so with greater distance.[32] This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years.[33] The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.

[edit] Nucleus

HST image of Andromeda galaxy core showing possible double structure. NASA/ESA photo.
HST image of Andromeda galaxy core showing possible double structure. NASA/ESA photo.
Artist's concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. NASA/ESA photo.
Artist's concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. NASA/ESA photo.

M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.

In 1991 Tod R. Lauer used WFPC, then on board the Hubble Space Telescope, to image Andromeda's inner nucleus. The nucleus is double, consisting of two concentrations separated by 1.5 parsecs. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a 108 M☉ black hole.

Scott Tremaine has proposed the following explanation of the double nucleus: P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. The eccentricity is such that stars "linger" longer at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A-stars. The A-stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.[34]

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy 'cannibalized' by Andromeda,[35] this is no longer considered to be a viable explanation. Such a nucleus would have an exceedingly short lifetime to tidal disruption by the central black hole, nor does it have its own black hole to stabilize it. Further the clump does not resemble a galactic nucleus, nor is there any evidence of a merger at larger radii in the bulge.

[edit] Discrete sources

Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory. Dr. Robin Barnard et al hypothesized that these are candidate black holes or neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.[36]

There are approximately 460 globular clusters associated with the Andromeda galaxy.[37] The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the local group of galaxies.[38] It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by M31 in the distant past.[39] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.[10]

In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.[40]

[edit] Satellites

Main article: Andromeda's satellite galaxies

Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110.

Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, which lasted until the relative recent past.[41]

M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies.[42] M110 does contain a dusty lane, which is a hint for recent or ongoing star formation. This is unusual in elliptical galaxies, which are usually fairly low in dust and gas.

In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.[43]

Andromeda's satellites discovered before 1900
Name Type Distance
from Sun
(Mly)		 Magnitude Discovered by Year
discovered
M32 cE2 2.65 ± 0.10 +9.0 Guillaume Le Gentil 1749
M110 E5 pec 2.69 ± 0.09 +8.9 Charles Messier 1773
NGC 185 dSph/dE3 2.08 ± 0.15 +10.1 William Herschel 1787
NGC 147 dSph/dE5 2.67 ± 0.18 +10.5 John Herschel 1829

[edit] See also

[edit] Notes

  1. ^ average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± ((182 + 402 + 442 + 252)0.5 / 4) = 778 ± 17
  2. ^ Apparent Magnitude of 4.36 - distance modulus of 24.4 = −20.0
  3. ^ J00443799+4129236 is at celestial coordinates R.A. 00h 44m 37.99s, Dec. +41° 29′ 23.6″.
  4. ^ distance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter

[edit] References

  1. ^ a b c d e f g NASA/IPAC Extragalactic Database. Results for Messier 31. Retrieved on 2006-11-01.
  2. ^ a b c d Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. 
  3. ^ a b I. D. Karachentsev, V. E. Karachentseva, W. K. Hutchmeier, D. I. Makarov (2004). "A Catalog of Neighboring Galaxies". Astronomical Journal 127: 2031–2068. 
  4. ^ a b c I. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward (2005). "First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy". Astrophysical Journal 635: L37-L40. 
  5. ^ a b McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. (2005). "Distances and metallicities for 17 Local Group galaxies". Monthly Notices of the Royal Astronomical Society 356 (4): 979-997. 
  6. ^ Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; Liu, Michael C.; Rieke, Marcia J.; Ajhar, Edward A.; Blakeslee, John P. (February 2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations". Astrophysical Journal 583 (2): 712-726. 
  7. ^ "Dark matter comes out of the cold", BBC News, February 5, 2006. Retrieved on 2006-05-24. 
  8. ^ Young, Kelly (2006-06-06). Andromeda galaxy hosts a trillion stars (English). NewScientistSpace. Retrieved on 2006-06-08.
  9. ^ Frommert, H.; Kronberg, C. (August 22, 2007). Messier Object Data, sorted by Apparent Visual Magnitude. SEDS. Retrieved on 2007-08-27.
  10. ^ a b c d Kepple, George Robert; Glen W. Sanner (1998). The Night Sky Observer's Guide, Volume 1. Willmann-Bell, Inc., 18. ISBN 0-943396-58-1. 
  11. ^ W. Herschel (1785). "On the Construction of the Heavens". Philosophical Transactions of the Royal Society of London 75: 213–266. 
  12. ^ William Huggins (1864). "On the Spectra of Some of the Nebulae". Philosophical Transactions of the Royal Society of London 154: 437–444. 
  13. ^ Roberts, Isaac (1899). A Selection of Photographs of Stars, Star-clusters and Nebulae, Vol. II. London: The Universal Press. 
  14. ^ V.M. Slipher (1913). "The Radial Velocity of the Andromeda Nebula". Lowell Observatory Bulletin 1: 2.56–2.57. 
  15. ^ Heber D. Curtis (January 1988). "Novae in Spiral Nebulae and the Island Universe Theory". Publications of the Astronomical Society of the Pacific 100: 6. 
  16. ^ E. P. Hubble (1929). "A spiral nebula as a stellar system, Messier 31". Astrophysical JournalEngl 69: 103–158. 
  17. ^ W. Baade (1944). "The Resolution of Messier 32, NGC 205, and the Central Region of the Andromeda Nebula". Astrophysical Journal 100: 137. 
  18. ^ Malik, Tariq (2002-05-07). Crash Course: Simulating the Fate of Our Milky Way (English). SPACE.com. Retrieved on 2006-09-18.
  19. ^ Dr. John Dubinski, University of Toronto. A Collision Between The Milky Way And The Andromeda Galaxy. Retrieved on 2006-05-24.
  20. ^ "The Grand Collision". The Sky At Night. 2007-11.
  21. ^ N. W. Evans & M. I. Wilkinson (2000). "The mass of the Andromeda galaxy". Monthly Notices of the Royal Astronomical Society 316 (4): 929–942. 
  22. ^ Moore, Sir Patrick (2002). Oxford Astronomy Encyclopedia. New York: Oxford University Press. ISBN 0-19-521833-7. 
  23. ^ W. Liller, B. Mayer (July 1987). "The Rate of Nova Production in the Galaxy". Publications Astronomical Society of the Pacific 99: 606–609. 
  24. ^ R.L. Beaton, E. Athanassoula, S.R. Majewski, P. Guhathakurta, M.F. Skrutskie, R.J. Patterson, M. Bureau (2006). "Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy". Astrophysical Journal Letters. 
  25. ^ S. C. Chapman, R. Ibata, G. F. Lewis, A. M. N. Ferguson, M. Irwin, A. McConnachie, N. Tanvir (2006). "A kinematically selected, metal-poor spheroid in the outskirts of M31". Astrophysical Journal.  Also see the press release, CalTech Media Relations (February 27, 2006). "Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to That of Milky Way". Press release. Retrieved on 2006-05-24.
  26. ^ UC Santa Cruz (January 9, 2001). "Astronomers Find Evidence of an Extreme Warp in the Stellar Disk of the Andromeda Galaxy". Press release. Retrieved on 2006-05-24.
  27. ^ V. C. Rubin, W. K. J. Ford (1970). "Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission". Astrophysical Journal 159: 379. 
  28. ^ H. Arp (1964). "Andromeda Nebula from a Spectroscopic Survey of Emission". Astrophysical Journal 139: 1045. 
  29. ^ R. Braun (1991). "The distribution and kinematics of neutral gas, HI region in M31". Astrophysical Journal 372, part 1: 54–66. 
  30. ^ Esa Science News (October 14, 1998). "ISO unveils the hidden rings of Andromeda". Press release. Retrieved on 2006-05-24.
  31. ^ "Busted! Astronomers Nab Culprit in Galactic Hit-and-Run", Harvard-Smithsonian Center for Astrophysics, October 18, 2006. Retrieved on 2006-10-18. 
  32. ^ J. S. Kalirai, K. M. Gilbert, P. Guhathakurta, S. R. Majewski, J. C. Ostheimer, R. M. Rich, M. C. Cooper, D. B. Reitzel, R. J. Patterson (2006). "The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)". Astrophysical Journal. 
  33. ^ J.S. Bullock and K.V. Johnston (2005). "Tracing Galaxy Formation with Stellar Halos I: Methods". Astrophysical Journal 635 (2): 931–949. 
  34. ^ Hubble news desk STScI-1993-18 (July 20, 1993). "Hubble Space Telescope Finds a Double Nucleus in the Andromeda Galaxy". Press release. Retrieved on 2006-05-26.
  35. ^ http://www.aip.org/pnu/1993/split/pnu138-2.htm
  36. ^ R., Barnard; U. Kolb; J.P. Osborne (August 2005). "Timing the bright X-ray population of the core of M31 with XMM-Newton". A&A. 
  37. ^ P. Barmby, J.P. Huchra (2001). "M31 Globular Clusters in the Hubble Space Telescope Archive. I. Cluster Detection and Completeness". Astrophysical Journal 122: 2458–2468. 
  38. ^ Hubble news desk STSci-1996-11 (April 24, 1996). "Hubble Spies Globular Cluster in Neighboring Galaxy". Press release. Retrieved on 2006-05-26.
  39. ^ G. Meylan, A. Sarajedini, P. Jablonka, S.G. Djorgovski, T. Bridges, R.M. Rich (2001). "G1 in M31 - Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?". Astrophysical Journal 122: 830–841. 
  40. ^ A.P. Huxor, N.R. Tanvir, M.J. Irwin, R. Ibata (2005). "A new population of extended, luminous, star clusters in the halo of M31" (PDF). Monthly Notices of the Royal Astronomical Society 360: 993–1006. 
  41. ^ K. Bekki, W.J. Couch, M.J. Drinkwater, M.D. Gregg (2001). "A New Formation Model for M32: A Threshed Early-type Spiral?". Astrophysical Journal 557 (1): L39–L42. 
  42. ^ R. Ibata, M. Irwin, G. Lewis, A.M. Ferguson, N. Tanvir (July 5, 2001). "A giant stream of metal-rich stars in the halo of the galaxy M31". Nature 412 (6842): 49–52. 
  43. ^ A. Koch and E.K. Grebel. "The Anisotropic Distribution of M 31 Satellite Galaxies: A Polar Great Plane of Early-Type Companions". Astronomical Journal 131 (3): 1405–1415. 

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