by alter-ego » Wed Feb 13, 2019 3:47 am
Anthony Barreiro wrote:MikeJ wrote:While there is no up or down in space, every time I see the Andromeda or other galaxy at a steep angle like this I wonder are we looking up at the bottom or down at the top. In other words is the edge to the right top closer to us or is it the bottom left and at that distance can we really know for sure?
It looks to me like the left hand edge is closer. You can see faint blue and green blotches of hotter gas to the lower left of that edge, against the blackness of intergalactic space. These look like small objects orbiting the galaxy -- globular clusters perhaps? On the opposite right hand edge, the faint blue and green blotches are superimposed against the much more saturated red spirals, as if they are seen against the background of the galactic disk. Any faint blue and green splotches behind the galactic disk would be hidden by the disk itself.
That's just how it looks to me. Maybe somebody else actually knows for sure.
...
Finally we now know.
Over the years since this APOD, I've been keeping an eye out for a definitive answer. At the time of this post, two competing ideas were: 1) The right (NW) edge because the inner dust lane(s) visible,
http://www.noao.edu/image_gallery/html/im0424.html wrote:We can see that the western (right) side of M31 is closer to us, by the fact that the dark dust lanes belonging to the inner spiral arms show up in silhouette against the nucleus on that side only.
and 2) The
GALEX UV image suggests the left edge is closer because the UV sources are brighter from less extinction. Even models/animations that simulate the present spiral structure and M32 didn't agree. In 2015, Dr. Marion Dierickx (
Signatures of the M31-M32 Galactic Collision (2014)) responded to my M31 rotation question. He compared several publications, but his summary succinct:
M. Dierickx wrote:The questions you ask are very relevant. In our work on M31 we found that there is quite a bit of confusion even amongst specialists about the rotation and spiral structure. Below I've pasted notes that my collaborator Laura Blecha wrote on on 3/30/14. As you can see it's quite a mess. The high inclination angle of the disk makes any attempt at deprojection difficult.
In the pair below left, the "normal" visual component does tend to show more silhouetting on the right edge, the GALEX image on the right:
![APOD 24 Jul 15_AndromedaGalex_2048.jpg (1.51 MiB) Viewed 2019 times GALEX](./download/file.php?id=34918&t=1&sid=dc64b6d336685a2aac8e0720d1961b85)
- GALEX
Both concepts are good and supported by observations, but both rely on assumptions: M31 is warped and it's not clear that more the nearest edge
has to be more transparent, and concluding the closer edge by the silhouette is based on visual interpretation. Since spiral arms can be leading or trailing, interpreting rotation direction from spiral structure is also not conclusive (exacerbated, as stated, by M31's high inclination angle).
Well the answer is, in fact, the right edge. GAIA has provided the data to better predict local group kinetics (and galactic rotation).
http://asterisk.apod.com/viewtopic.php?f=31&t=39150
I was pleasantly surprised. I wasn't thinking about GAIA measuring stars in other galaxies. Instead I thought precise Earth-based precise water maser tracking would provide the answer. (Tangential motion ~70 µarcsec/year)
Proper Motion of the Andromeda Galaxy: The Keystone of Local Group Dynamics Proper Motion (2011).
Using the M31 velocity field, and the convenient
S&T graphic, the definitive counterclockwise rotation identifies the right edge is the closest to the Milky Way.
![Andromeda Rotation_GAIA.JPG (91.69 KiB) Viewed 2019 times M31 Rotation - GAIA](./download/file.php?id=34916&t=1&sid=dc64b6d336685a2aac8e0720d1961b85)
- M31 Rotation - GAIA
![M31 Velocity Field_2.JPG (58.35 KiB) Viewed 2019 times M31 Velocity Field](./download/file.php?id=34917&t=1&sid=dc64b6d336685a2aac8e0720d1961b85)
- M31 Velocity Field
[quote="Anthony Barreiro"][quote="MikeJ"]While there is no up or down in space, every time I see the Andromeda or other galaxy at a steep angle like this I wonder are we looking up at the bottom or down at the top. In other words is the edge to the right top closer to us or is it the bottom left and at that distance can we really know for sure?[/quote]
It looks to me like the left hand edge is closer. You can see faint blue and green blotches of hotter gas to the lower left of that edge, against the blackness of intergalactic space. These look like small objects orbiting the galaxy -- globular clusters perhaps? On the opposite right hand edge, the faint blue and green blotches are superimposed against the much more saturated red spirals, as if they are seen against the background of the galactic disk. Any faint blue and green splotches behind the galactic disk would be hidden by the disk itself.
That's just how it looks to me. Maybe somebody else actually knows for sure.
...[/quote]
[b][color=#4000FF]Finally we now know[/color][/b].
Over the years since this APOD, I've been keeping an eye out for a definitive answer. At the time of this post, two competing ideas were: 1) The right (NW) edge because the inner dust lane(s) visible,
[quote="[url]http://www.noao.edu/image_gallery/html/im0424.html[/url]"]We can see that the western (right) side of M31 is closer to us, by the fact that the dark dust lanes belonging to the inner spiral arms show up in silhouette against the nucleus on that side only.[/quote]
and 2) The [url=https://apod.nasa.gov/apod/ap150724.html]GALEX[/url] UV image suggests the left edge is closer because the UV sources are brighter from less extinction. Even models/animations that simulate the present spiral structure and M32 didn't agree. In 2015, Dr. Marion Dierickx ([url=https://arxiv.org/abs/1405.3990]Signatures of the M31-M32 Galactic Collision (2014)[/url]) responded to my M31 rotation question. He compared several publications, but his summary succinct:
[quote=M. Dierickx]The questions you ask are very relevant. In our work on M31 we found that there is quite a bit of confusion even amongst specialists about the rotation and spiral structure. Below I've pasted notes that my collaborator Laura Blecha wrote on on 3/30/14. As you can see it's quite a mess. The high inclination angle of the disk makes any attempt at deprojection difficult.
[/quote] In the pair below left, the "normal" visual component does tend to show more silhouetting on the right edge, the GALEX image on the right:
[float=left][imghover=https://lh5.googleusercontent.com/-DutX_GN7C3M/USGabMozH0I/AAAAAAAAB2U/BZ8NgXrQNaM/s576/HerschelAnd_APOD.jpg]https://lh5.googleusercontent.com/-1aBwDa3Sm3g/USGaFOSnF_I/AAAAAAAAB18/Aeo9R8pyKxg/s576/Visible%2520Overlay.JPG[/imghover]
[color=#0000FF]
[/float]
[float=right][attachment=0]APOD 24 Jul 15_AndromedaGalex_2048.jpg[/attachment][/float]
[hr][/hr]
Both concepts are good and supported by observations, but both rely on assumptions: M31 is warped and it's not clear that more the nearest edge [i]has[/i] to be more transparent, and concluding the closer edge by the silhouette is based on visual interpretation. Since spiral arms can be leading or trailing, interpreting rotation direction from spiral structure is also not conclusive (exacerbated, as stated, by M31's high inclination angle).
Well the answer is, in fact, the right edge. GAIA has provided the data to better predict local group kinetics (and galactic rotation). [url]http://asterisk.apod.com/viewtopic.php?f=31&t=39150[/url]
I was pleasantly surprised. I wasn't thinking about GAIA measuring stars in other galaxies. Instead I thought precise Earth-based precise water maser tracking would provide the answer. (Tangential motion ~70 µarcsec/year)
[url=https://science.nrao.edu/newscience/9-Wed/15-Darling/darling-v2.pdf]Proper Motion of the Andromeda Galaxy: The Keystone of Local Group Dynamics Proper Motion (2011)[/url].
Using the M31 velocity field, and the convenient [url=https://www.skyandtelescope.com/astronomy-news/milky-way-andromeda-merger-pushed-back/]S&T[/url] graphic, the definitive counterclockwise rotation identifies the right edge is the closest to the Milky Way.
[float=left][attachment=2]Andromeda Rotation_GAIA.JPG[/attachment][/float][float=right][attachment=1]M31 Velocity Field_2.JPG[/attachment][/float]