Editing Andromeda Galaxy (section)

== Structure ==
{{Wide image|Andromeda Galaxy M31 - Heic1502a Full resolution.jpg|600px|''[[Zooming In on the Andromeda Galaxy]]'' – A panorama of foreground stars and the Andromeda Galaxy's nucleus. The image is the largest ever taken by the Hubble Space Telescope.}}
[[File:A Swift Tour of M31.ogv|thumb|A narrated tour of the Andromeda Galaxy, made by NASA's [[Swift satellite]] team]]
Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the [[Galaxy morphological classification#De Vaucouleurs system|de Vaucouleurs–Sandage extended classification system]] of spiral galaxies.<ref name="ned"/> However, infrared data from the [[2MASS]] survey and the [[Spitzer Space Telescope]] showed that Andromeda is actually a [[barred spiral galaxy]], like the Milky Way, with Andromeda's bar major axis oriented 55 degrees anti-clockwise from the disc major axis.<ref name="Beaton 2006"/>

There are various methods used in astronomy in defining the size of a galaxy, and each method can yield different results concerning one another. The most commonly employed is the D<sub>25</sub> standard, the [[isophote]] where the photometric brightness of a galaxy in the B-band (445&nbsp;nm wavelength of light, in the blue part of the [[visible spectrum]]) reaches 25 mag/arcsec<sup>2</sup>.<ref name="GalaxySize">{{Cite web|url=https://ned.ipac.caltech.edu/level5/PROPERTIES/dog.html|title=Dimensions of Galaxies|website=ned.ipac.caltech.edu}}</ref> The Third Reference Catalogue of Bright Galaxies (RC3) used this standard for Andromeda in 1991, yielding an isophotal diameter of {{convert|46.56|kpc|ly|sigfig=3|abbr=off}} at a distance of 2.5&nbsp;million light-years.<ref name="RC3"/> An earlier estimate from 1981 gave a diameter for Andromeda at {{convert|54|kpc|ly|sigfig=3|abbr=off}}.<ref name="AtlasofAndromeda">{{Cite web|url=https://ned.ipac.caltech.edu/level5/ANDROMEDA_Atlas/frames.html|title=Atlas of the Andromeda Galaxy|website=ned.ipac.caltech.edu}}</ref>

A study in 2005 by the [[Keck telescopes]] shows the existence of a tenuous sprinkle of stars, or [[galactic halo]], extending outward from the galaxy.<ref name="Chapman et al 2006"/> The stars in this halo behave differently from the ones in Andromeda's main galactic disc, where they show rather disorganized orbital motions as opposed to the stars in the main disc having more orderly orbits and uniform velocities of 200&nbsp;km/s.<ref name="Chapman et al 2006"/> This diffuse halo extends outwards away from Andromeda's main disc with the diameter of {{convert|67.45|kpc|ly|sigfig=3|abbr=off}}.<ref name="Chapman et al 2006"/>

The galaxy is inclined an estimated 77° relative to Earth (where an angle of 90° would be edge-on). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.<ref name="UC Santa Cruz 2001"/> A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy. The Galaxy [[Triangulum Galaxy|M33]] could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the [[Galaxy rotation curve|rotational velocity of the Andromeda Galaxy]] as a function of radial distance from the core. The rotational velocity has a maximum value of {{cvt|225|km/s}} at {{convert|1300|ly|e6AU|lk=on|abbr=unit}} from the core, and it has its minimum possibly as low as {{cvt|50|km/s}} at {{convert|7000|ly|e6AU|abbr=unit}} from the core. Further out, rotational velocity rises out to a radius of {{convert|33000|ly|e9AU|abbr=unit}}, where it reaches a peak of {{cvt|250|km/s}}. The velocities slowly decline beyond that distance, dropping to around {{cvt|200|km/s}} at {{convert|80000|ly|e9AU|abbr=unit}}. These velocity measurements imply a concentrated mass of about {{Solar mass|6{{e|9}}|link=y}} in the [[Galaxy nucleus|nucleus]]. The total mass of the galaxy increases [[linearity|linearly]] out to {{convert|45000|ly|e9AU|abbr=unit}}, then more slowly beyond that radius.<ref name="Rubin & Ford 1970"/>

The [[Spiral galaxy#Spiral arms|spiral arms]] of the Andromeda Galaxy are outlined by a series of [[H II region|HII regions]], first studied in great detail by [[Walter Baade]] and described by him as resembling "beads on a string". His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy.<ref name="Arp 1964"/> His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows<ref name="Bergh1991"/><sup>§pp1062</sup><!--
--><ref name="Hodge1966"/><sup>§pp92</sup>:
{| class="wikitable"
|+ Baade's spiral arms of M31
! Arms (N=cross M31's major axis at north, S=cross M31's major axis at south)
! Distance from center ([[Minute and second of arc|arcminutes]]) (N*/S*)
! Distance from the center (kpc) (N*/S*)
! Notes
|-
| align="left" |N1/S1|| 3.4/1.7|| 0.7/0.4|| Dust arms with no [[Stellar kinematics#OB associations|OB associations]] of [[H II region|HII region]]s
|-
| align="left" |N2/S2||8.0/10.0||1.7/2.1|| Dust arms with some OB associations
|-
| align="left" |N3/S3||25/30||5.3/6.3|| As per N2/S2, but with some HII regions too
|-
| align="left" |N4/S4||50/47||11/9.9|| Large numbers of OB associations, HII regions, and little dust
|-
| align="left" |N5/S5||70/66||15/14|| As per N4/S4 but much fainter
|-
| align="left" |N6/S6||91/95||19/20|| Loose OB associations. No dust is visible.
|-
| align="left" |N7/S7||110/116||23/24|| As per N6/S6 but fainter and inconspicuous
|}[[File:Andromeda galaxy Ssc2005-20a1.jpg|thumb|300x300px|Image of the Andromeda Galaxy taken by Spitzer in infrared, 24 [[micrometre]]s (Credit: [[NASA]]/[[JPL]]–[[Caltech]]/Karl D. Gordon, [[University of Arizona]])]]
Since the Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about {{convert|13000|ly|e6AU|lk=on|abbr=unit}} and that can be followed outward from a distance of roughly {{convert|1600|ly|e6AU|abbr=unit}} from the core. Alternative spiral structures have been proposed such as a single spiral arm<ref name="Simien1978"/> or a [[flocculent spiral galaxy|flocculent]]<ref name="Haas2000"/> pattern of long, filamentary, and thick spiral arms.<ref name="ned"/><ref name="Walterbos1988"/>

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites [[Messier 32|M32]] and [[Messier 110|M110]].<ref name="Gordon2006"/> This can be seen by the displacement of the [[H I region|neutral hydrogen clouds]] from the stars.<ref name="Braun 1991"/>

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 the galaxy are generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of {{cvt|32000|ly|kpc}} from the core,<ref name="ESA 1998-10-14"/> nicknamed by some astronomers the ''ring of fire''.<ref name="M31youngclusters"/> This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the Andromeda Galaxy is concentrated there.<ref name="Andromedan10kpcring"/>

Later studies with the help of the [[Spitzer Space Telescope]] showed how the Andromeda Galaxy's spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond the large ring mentioned above. Those arms, however, are not continuous and have a segmented structure.<ref name="Gordon2006"/>

Close examination of the inner region of the Andromeda Galaxy with the same telescope also 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 the Andromeda Galaxy 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.<ref name="Harvard-Smithsonian 2006-10-18"/>
It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the [[barycenter]], that suggested a nearly head-on collision with the satellite M32, a milder version of the [[Cartwheel Galaxy|Cartwheel encounter]].<ref name="Block2006"/>

Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "[[Metallicity|metal-poor]]", and increasingly so with greater distance.<ref name="Kalirai et al 2006"/> This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 100–200 low-mass galaxies during the past 12&nbsp;billion years.<ref name="Bullock & Johnston 2005"/> The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one-third the distance separating the two galaxies.

=== Nucleus ===
[[File:M31 nucleus (labels).jpg|thumb|[[Hubble Space Telescope]] image of the Andromeda Galaxy core showing P1, P2 and P3, with P3 containing M31*. [[NASA]]/[[European Space Agency|ESA]] photo]]
The Andromeda Galaxy is known to harbor a dense and compact star cluster at its very center, similar to the [[Milky Way]] galaxy. A large telescope creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, the [[Hubble Space Telescope]] was used to image the Andromeda Galaxy's inner nucleus. The nucleus consists of two concentrations separated by {{cvt|1.5|pc|lk=on}}. 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 an embedded star cluster, called P3,<ref name="Bender et al 2005"/> containing many [[Ultraviolet|UV]]-bright [[A-type main-sequence star|A-stars]] and the [[supermassive black hole]], called [[M31*]].<ref name=":0">{{Cite journal |last1=Garcia |first1=Michael R. |last2=Hextall |first2=Richard |last3=Baganoff |first3=Frederick K. |last4=Galache |first4=Jose |last5=Melia |first5=Fulvio |last6=Murray |first6=Stephen S. |last7=Primini |first7=F. A. |last8=Sjouwerman |first8=Loránt O. |last9=Williams |first9=Ben |date=1 February 2010 |title=X-ray and Radio Variability of M31*, The Andromeda Galaxy Nuclear Supermassive Black Hole |url=https://ui.adsabs.harvard.edu/abs/2010ApJ...710..755G |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=755–763 |arxiv=0907.4977 |bibcode=2010ApJ...710..755G |doi=10.1088/0004-637X/710/1/755 |hdl=1721.1/96091 |issn=0004-637X}}</ref><ref name=":1">{{Cite journal |last1=Yang |first1=Yang |last2=Li |first2=Zhiyuan |last3=Sjouwerman |first3=Loránt O. |last4=Yuan |first4=Feng |last5=Shen |first5=Zhi-Qiang |date=1 August 2017 |title=Very Large Array Multiband Monitoring Observations of M31* |journal=The Astrophysical Journal |volume=845 |issue=2 |pages=140 |arxiv=1707.08317 |bibcode=2017ApJ...845..140Y |doi=10.3847/1538-4357/aa8265 |doi-access=free |issn=0004-637X}}</ref> The black hole is classified as a low-luminosity [[Active galactic nucleus|AGN]] (LLAGN) and it was detected only in [[Radio astronomy|radio wavelengths]] and in [[X-ray astronomy|x-rays]].<ref name=":1"/> It was quiescent in 2004–2005, but it was highly variable in 2006–2007.<ref name=":0"/> An additional x-ray flare occurred in 2013.<ref>{{Cite journal |last1=DiKerby |first1=Stephen |last2=Zhang |first2=Shuo |last3=Irwin |first3=Jimmy |date=2025-03-01 |title=Fifteen Years of M31* X-Ray Variability and Flares |journal=The Astrophysical Journal |volume=981 |issue=1 |pages=50 |arxiv=2502.01365 |bibcode=2025ApJ...981...50D |doi=10.3847/1538-4357/adb1d5 |doi-access=free |issn=0004-637X}}</ref> The mass of M31* was measured at 3–5 × 10<sup>7</sup> {{Solar mass}} in 1993,<ref name="Lauer"/> and at 1.1–2.3 × 10<sup>8</sup> {{Solar mass}} in 2005.<ref name="Bender et al 2005"/> The [[velocity dispersion]] of material around it is measured to be ≈ {{cvt|160|km/s|round=10|lk=on}}.<ref name="Gebhardt et al 2000"/>

It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an [[orbital eccentricity|eccentric orbit]] around the central black hole.<ref name="Tremaine 1995"/> The eccentricity is such that stars linger at the orbital [[apsis|apocenter]], creating a concentration of stars. It has been postulated that such an eccentric disk could have been formed from the result of a previous black hole merger, where the release of gravitational waves could have "kicked" the stars into their current eccentric distribution.<ref>{{Cite journal |last1=Akiba |first1=Tatsuya |last2=Madigan |first2=Ann-Marie |date=1 November 2021 |title=On the Formation of an Eccentric Nuclear Disk following the Gravitational Recoil Kick of a Supermassive Black Hole |journal=The Astrophysical Journal Letters |volume=921 |issue=1 |pages=L12 |doi=10.3847/2041-8213/ac30d9 |arxiv=2110.10163 |bibcode=2021ApJ...921L..12A |s2cid=239049969 |issn=2041-8205 |doi-access=free }}</ref> P2 also contains a compact disk of hot, [[stellar classification|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.<ref name="hubblesite 1993-07-20"/>

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy "cannibalized" by the Andromeda Galaxy,<ref name="Schewe & Stein 1993"/> this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to [[tidal force|tidal disruption]] by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.<ref name="Tremaine 1995"/>