View Full Version here: : M31 - a noticeably peculiar galaxy
09-10-2011, 11:28 AM
Some of our correspondents more inclined towards physical/quantitative science may be interested in my recent posts in the thread
"M 31 repro"(by Marco) in the "Deep Space" forum, as regards the ring structures within M31 and the many peculiarities and non-planarities that exist in the morphology of the Andromeda Galaxy.
In this regard, I quote from Francois Schweizer in the symposium "Galaxies: Interactions and Induced Star Formation"(eds, Kennicutt, Schweizer, & Barnes) :
“Any large-field photograph of M31 shows not only the satellites M32 and NGC 205, but also the warped disk (e.g. Sandage 1961) and, when properly masked, the strikingly misaligned bulge. The misalignment against the major axis of the outer disk is of order 10 degrees (see Hodge 1992). “
This idea of a bulge misalignment is interesting, but the disk of M31 is so warped and bent in various places that there are several possible planes to measure the bulge misalignment from. Incidentally, I do have several examples in mind of specific edge-on disk galaxies in which the bulge component does appear to be at a noticeably different p.a. to the disk component.
On a related issue, I note that the strong tidal distortion of NGC 205 by M31 is well shown in Deep Space forum thread called "M110 - Satellite of the Andromeda Galaxy" by Leonardo70. There have been a few papers on this tidal distortion in the last 10 years, but modeling and observation are not very well advanced, as yet.
11-10-2011, 01:14 AM
The bends or twists in the disk component of M31 (that is, position angle changes) are well shown in this Burrell Schmidt image by P.Guhathakurta
(for the larger original, see his website:
There is more than one change in P.A., as we move outwards along the major axis of this galaxy.
[For some reason the above link does not work, but typing in the URL does.]
18-10-2011, 01:38 PM
Is there any computer genrated images of M31 that provide a more 'face on' illustration of the galaxy? Might give some insight to the physics involved if we can see the distribution of the arms around the core.
Might there be some images of M31 using other wavelengths that may pick up any jetisoned material? Again, this may also give some insight to the gravitational tidal distortion you've mentioned.
If my memory serves me right, even our Milky Way isn't vinyl record flat either. The Magellanic Clouds are seeing to that, and there is also residual material around the MM from other 'recent' mergings with smaller galaxies.
19-10-2011, 01:21 AM
In answer to your question, there have indeed been several attempts (in the literature) to deproject M31 to a face-on orientation, but it turns out to be a distinctly non-trivial task to do this kind of thing.
The morphology of M31 turns out to be unusual and very complex!!
Most "face on" views of the Andromeda Galaxy are not well fitted by any pattern of Very Large Scale coordinated spiral structure, but are better fit by a single very-large-scale ring structure, together with some minor spiral arms.
Because the disk of M31 is not very planar, even the definition of "face on" is somewhat indistinct. (As I have remarked in other posts, non-planarity is the rule in the disk components of galaxies, rather than being the exception.)
I am just trying to come to grips with the current literature on M31, and my current impression is that there is a gigantic amount of existing multi-wavelength observational material without there being any secure interpretation of the complex morphology and history of this enigmatic galaxy. Oddly, relatively "normal" and relatively nearby galaxies such as M31 have actually only recently become fashionable amongst extragalactic astronomers..... who were usually off chasing distant exotica such as quasars, primeval galaxies, etc.
There are at least two ring structures within M31, as is well shown in the near- and mid- infrared imaging data , for instance this image from the Spitzer Telescope:
The above is an 8 micron image from Spitzer, showing a large-scale ring of emission from dust, and also a smaller dust ring that is around the centre of this galaxy. The larger of the two rings is, intriguingly, split or broken near to the apparent position of the companion galaxy M32.
These structures have had some analysis in the literature, with various fanciful and interesting scenarios being posited, such as a pass through of M32 through the disk of M31.
Now when we consider the older stellar population emphasized in this 3.4 microns image from WISE, we see that the large scale ring is actually broken rather than split:
Thus far, in the literature, there has been little in the way of interpretation of this break in the ring.
Here is the ring (and also a central spiral!) in a continuum-subtracted Hydrogen Alpha bandpass:
I think that the morphology & history & kinematics of M31 is going to be a tough problem to crack.....the "pros" haven't got it sorted out yet! Proper motions of M32 and NGC 205, which are observationally feasible, would help to sort out the influence on M31 of these satellite galaxies.
(NGC 205 is tidally distorted, in its outer parts)
Note added in a later edit:
On the issue of warped disks in spiral galaxies ("bent" disks), Leo Blitz has some good data about the very complex non-planar shape of the Milky Way's gas layer; though not much has been published of his data, as far as I know.
Furthermore, Thomas Dame and Patrick Thaddeus have recently identified a probable outer spiral arm that is not exactly in the same plane as the rest of the Milky Way.
(This is not surprising, as outer arms are seen in other galaxies that are lifted somewhat out of the principal plane)
19-10-2011, 07:44 AM
Further to previous post, here is an attempt to provide a face-on view of M31, based on the 24 micron image from the Spitzer Space Telescope.
(This deprojection assumes that M31 is 15 degrees from edge-on.)
This image is taken from the reference: K.D. Gordon et al, (2006), ApJ, Volume 638, L87
19-10-2011, 09:42 AM
Mad'man (:lol:, I couldn't help the abreviation! Kindred spirit I see, ;)), thanks for that info. The pictures you've provided really lend to consider that M31 has been very busy in merging with other galaxies, and would seem to be in a 'settling down' phase. It would seem to be developing a distinct ring structure with an internal barred spiral system. I've seen a few pictures of galaxies of a similar nature, but at a more developed stage.
Any other X-ray images of M31 that may suggest the existence of other blackholes to hint at the cores of consumed galaxies? I'm taking this from the hypothisis that Omega Centauri, 47 Tuc and M2 are consisdered the remnant cores of captured galaxies within the MW. If there is, this/these remnant cores could be influencing the formation of this ring structure, much like the shepherd moons in Saturn's rings.
19-10-2011, 12:15 PM
Agreed, Alexander, that the ring structures in M31 are plausibly due to mergers or pass-throughs of other smaller galaxies .
Gordon et al. (as mentioned below) hypothesized that M32 passed through the plane of M31, leading to a large-scale disturbance in the interstellar medium of M31, which could have set off a moving ring-shaped wave of star formation. However...... the orbit of M32 is not, as far as I know, well determined at the present time.
Can't understand why people still call M31 a spiral galaxy; there was never good evidence for anything apart from some minor spiral arms!
The "bar" seen in the extrapolated face-on version of M31 at 24 microns (in the mid-infrared regime) may or may not be real. It is plausible that it is actually real, but the near edge-on orientation of M31 makes this structure very foreshortened......
The bulge component of M31 is actually somewhat square (that is, the elliptical isophotes of the bulge are somewhat "boxy"), which is often taken to be the sign of a bar that is seen in an edge-on orientation.
For instance do a google search for a paper entitled : "Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy"
[I just downloaded this and some other work about the bar of M31....I am getting data indigestion!]
Here is the boxy bulge of M31, as seen in the near infrared regime, from the 2MASS survey:
Your point about various candidate nuclei of disrupted galaxies being some of today's globular clusters, is a good one.
20-10-2011, 12:07 PM
Further to the previous posts regarding the nature and origin of the ring structures seen in M31, I note that D.L. Block et al,
in (2006), Nature, Volume 443, p.832 provide some modeling that shows the physical plausibility of the two Mid-IR rings seen by Spitzer Telescope having an origin in an impact of M32 with the disk of M31. They interpret both rings as being density waves caused by an almost head-on collision of Andromeda Galaxy with M32, with the smaller galaxy plunging through the centre of M31.
I attach, here, the preprint of their paper in Nature:
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