Andromeda vs. Milky Way galaxies

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Summary:
Why are their masses and diameters so divergent?
Has their been any explanations found for why the Andromeda galaxy is so much bigger in diameter than the MW, yet at the same time it's lighter? Is the Andromeda a type of Low-Surface Brightness (LSB) galaxy?

Also maybe somewhat related, why is the Andromeda's central black hole so much heavier than the MW's? Everything about these two galaxies seems screwy and contradictory.

Does this seem like because the MW is so much denser than the Andro, that Andro will distort and fall apart faster than the MW, during their upcoming collision?

DiameterGalactic MassBH Mass
Andromeda220,000 light years1,230 billion M☉110 million M☉
Milky Way105,700 light years1,500 billion M☉4.5 million M☉
 
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  • #3
Bandersnatch
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I don't know where you got the numbers in the OP from, but they're suspect by being so definitive. If you look at papers working out the masses of either galaxy by various methods (see Wikipedia's references for example), the error bars are rather large, and as such they cannot be omitted. 1.5 billion solar masses for the MW can be found as the upper bound in some, but it's an outlier. The premise in the question is likely to be incorrect.
 
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  • #4
sophiecentaur
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Summary:: Why are their masses and diameters so divergent?

yet at the same time it's lighter
Is that actually true? I think that the Andromeda Galaxy may be more visible because it contrasts with the relatively dark background. The Milky Way covers a much larger area and there is less of a defined outline to it.
If you look at Andromeda with the naked eye or even with a big scope, you only see the inner portion. A long photographic exposure will show a structure that's at least ten times the diameter of the bright fuzzy object that we conventionally see. So I think it's basically down to our eyes.
 
  • #5
stefan r
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Summary:: ...

Does this seem like because the MW is so much denser than the Andro, that Andro will distort and fall apart faster than the MW, during their upcoming collision?...

They are going to merge and become Milkomeda.

Some stars will be ejected out of the galaxy (or ejected from both galaxies if you prefer). That will depend on the individual star's position and interaction sequence when the merger occurs. Both cores will eject a stream. Like the tails on the mice galaxies:
_4676_%28captured_by_the_Hubble_Space_Telescope%29.jpg
or the antennae on the antennae galaxy:
NGC4038_Large_01.jpg


The interstellar gas and dust will do its own thing in the collision. That is the only part that actually collides.
 
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  • #6
sophiecentaur
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That is the only part that actually collides.
The enormous space between stars is often forgotten when you see images like the above. Actual star - star collisions will be very rare. However, there would be significant deflection of the stars' galactic orbits as they go 'close' to the visiting stars. It would depend on the relative momenta of the two galaxies compared with the internal momentum of each galaxy. This simulation from NASA shows a predicted result of the collision.
 
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  • #7
pinball1970
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The enormous space between stars is often forgotten when you see images like the above. Actual star - star collisions will be very rare. However, there would be significant deflection of the stars' galactic orbits as they go 'close' to the visiting stars. It would depend on the relative momenta of the two galaxies compared with the internal momentum of each galaxy. This simulation from NASA shows a predicted result of the collision.
That’s very cool.
Is there any reason whey the light moved redder @55 seconds?
Red shift? Loss of energy? Or just to denote its now a new galaxy?
 
  • #8
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I don't know where you got the numbers in the OP from, but they're suspect by being so definitive. If you look at papers working out the masses of either galaxy by various methods (see Wikipedia's references for example), the error bars are rather large, and as such they cannot be omitted. 1.5 billion solar masses for the MW can be found as the upper bound in some, but it's an outlier. The premise in the question is likely to be incorrect.
This is what I got with a simple Google search. Google itself presents those sizes and masses, without even referring to any specific external sites.
 
  • #9
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Is that actually true? I think that the Andromeda Galaxy may be more visible because it contrasts with the relatively dark background. The Milky Way covers a much larger area and there is less of a defined outline to it.
If you look at Andromeda with the naked eye or even with a big scope, you only see the inner portion. A long photographic exposure will show a structure that's at least ten times the diameter of the bright fuzzy object that we conventionally see. So I think it's basically down to our eyes.
No, I don't think that's right, as the table above shows the MW has a diameter of 105,000 LY while the Andro has a 220,000 LY diameter, so the Andro has a bit more than double the size. But Andro has got a nearly even amount of mass, even slightly less than MW. Just based on back of envelope calculations, that would make Andro way less dense.

Apparently the MW's and Andro's outer halos are already touching, so the collision is already underway. Would this mean that the MW's halo is bigger and more massive than Andro's? Or perhaps these masses also include the galaxies' Dark Matter? So would that mean MW has more DM than Andro, which would present even more questions about why?
 
  • #10
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They are going to merge and become Milkomeda.

Some stars will be ejected out of the galaxy (or ejected from both galaxies if you prefer). That will depend on the individual star's position and interaction sequence when the merger occurs. Both cores will eject a stream. Like the tails on the mice galaxies:
Yes, it's understood that there's going to some distortion of the galaxies, but which galaxy will distort more? If the MW is indeed more massive and compact than the Andro, then wouldn't that imply that most of the stars of the MW will stay together longer than the stars in the Andro, during the collision?
 
  • #11
DaveC426913
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That’s very cool.
Is there any reason whey the light moved redder @55 seconds?
Red shift? Loss of energy? Or just to denote its now a new galaxy?
Stellar evolution / aging?
 
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  • #12
pinball1970
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Stellar evolution / aging?
I played it back and I the change is not as abrupt as I thought it was.

The light moves towards yellow then red over a few billion years.
 
  • #13
Keith_McClary
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Is there any reason whey the light moved redder @55 seconds?
Most astrophotos we see are not true colour. Here are some purported to be:
http://astronomy.swin.edu.au/~dforbes/images.html
I don't know whether the video purports/attempts to be.

Here are amateur attempts at true colour Andromeda:
https://www.dpreview.com/forums/post/63269043
https://www.cloudynights.com/topic/742795-true-colors-of-andromeda-what-are-they/

Even if they are true colour, they are often photoshopped to decrease the contrast between the bright core and the faint outer parts.

The blue colors are hot bright stars, which have short lifespans. I speculate that the collision disrupts the gas and dust and the spiral arms where new stars form, so after millions of years there is less blue.
EDIT:
But:
Mergers are also locations of extreme amounts of star formation.[4] The star formation rate (SFR) during a major merger can reach thousands of solar masses worth of new stars each year, depending on the gas content of each galaxy and its redshift.[5][6] Typical merger SFRs are less than 100 new solar masses per year.[7][8] This is large compared to our Galaxy, which makes only a few new stars each year (~2 new stars).[9] Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds.[citation needed] These collisions then induce condensations of these clouds into new stars.
Wikipedia
 
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  • #14
sophiecentaur
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No, I don't think that's right, as the table above shows the MW has a diameter of 105,000 LY while the Andro has a 220,000 LY diameter, so the Andro has a bit more than double the size
I would have said that it's the density of light per unit angle subtended that would count and not the diameter in LY. As we are inside the MW, there is very little contrast to make what we would 'call' the Milky Way that we see. Otoh, Andromeda is a fairly bright localised object against a dark background. Whatever the quantitative details of the light flux from the two galaxies, we are getting a very different subjective impression of the two.
If you look up in the right direction for Andromeda (and if the conditions are ok, in that rough direction) then you can feel quite smug about having spotted it, (At least, I do and, once seen, it seems to stay visible and easy to find in that session) but the MW takes up a large swathe of the sky and you can very often only see parts of it in the presence of cloud and light pollution near the ground. It's a totally different viewing experience - from UK at least. On your holidays, with good dark skies, things could be very different.
Astrophotographs are an entirely different thing because photographers always cheat!
 
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  • #15
Keith_McClary
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Astrophotographs are an entirely different thing because photographers always cheat!
😲
Andromeda unprocessed:
 
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  • #16
sophiecentaur
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Stellar evolution / aging?
The effect will have been 'introduced' into the graphics, rather than being the result of what the simulation could have generated, I'm sure. (Nothing of the sort would have been 'revealed' by the simulation)
 
  • #17
DaveC426913
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The effect will have been 'introduced' into the graphics, rather than being the result of what the simulation could have generated, I'm sure. (Nothing of the sort would have been 'revealed' by the simulation)
At first I agreed with you, but adding age as a property to each element isn't unfeasible. Colour could be derived from age and mass.
 
  • #18
sophiecentaur
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At first I agreed with you, but adding age as a property to each element isn't unfeasible. Colour could be derived from age and mass.
Good for adding information to the display but the colour change does little more than remind people of the effect of age on stars - it's a bolt on and not 'revealed information', as you can sometimes get from simulations.
There's such a lot of stuff available about star formation etc. which is making use of the fantastically increased capacity of modern computers. How does their use on Bitcoin Mining compare for value??
 
  • #19
Vanadium 50
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Colour could be derived from age and mass.
Only if they are simulating individual stars. They are surely not - that's over a trillion stars, the majority of which's age, position and momentum need to be invented out of whole cloth. Why wouild anyone want do that?

If you are looking at voxels, the mass is perhaps well-defined, although the relation to color not so much, but age is surely not. The stars in a voxel will have different ages.

Just accept it - the color is an after-the-fact add on.
 
  • #20
Keith_McClary
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Note, the video is 9 years old (which is a long time in cosmology :smile: ).
Since the second data release (DR2) of the European Space Agency's Gaia mission there has been a revolution in astrometry, including measuring the motion of the Andromeda Galaxy.

On February this year [2019] van der Marel et al (also in ArXiv) published interesting results on that matter by using Gaia's DR2 measurements. The results reveal that the collision is going to happen 600 million years later than the previous estimate (in 4.5 Gyr instead of 3.9 Gyr). Also Andromeda appears to have more tangential motion than previously thought and thus "the galaxy is likely to deliver more of a glancing blow to the Milky Way than a head-on collision".
https://astronomy.stackexchange.com/a/33093/17743
 
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  • #21
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I would have said that it's the density of light per unit angle subtended that would count and not the diameter in LY. As we are inside the MW, there is very little contrast to make what we would 'call' the Milky Way that we see. Otoh, Andromeda is a fairly bright localised object against a dark background. Whatever the quantitative details of the light flux from the two galaxies, we are getting a very different subjective impression of the two.
If you look up in the right direction for Andromeda (and if the conditions are ok, in that rough direction) then you can feel quite smug about having spotted it, (At least, I do and, once seen, it seems to stay visible and easy to find in that session) but the MW takes up a large swathe of the sky and you can very often only see parts of it in the presence of cloud and light pollution near the ground. It's a totally different viewing experience - from UK at least. On your holidays, with good dark skies, things could be very different.
Astrophotographs are an entirely different thing because photographers always cheat!
If the Andro is a LSB galaxy, it would still look pretty bright when its nearby, like it is, I suppose. But I don't know if it is an LSB or not. It may not be, because another known feature of Andro is that it is the host of about 1 trillion stars, whereas the MW is host to only 400 billion stars. Is the Andro host to more red dwarfs per capita than MW? I mean what would explain its apparent lack of mass compared to the the MW, despite it being so much more than the MW in many other metrics?
 
  • #22
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What is this "Andro"? I have never heard an astronomer call it that. It's either "Andromeda" or "M31". More commonly the latter.
 
  • #23
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Oh, and M31 is not a LSB galaxy. And the surface brightness doesn't depend on distance.
 
  • #24
sophiecentaur
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Oh, and M31 is not a LSB galaxy. And the surface brightness doesn't depend on distance.
I looked again in the OP and it contains the word "lighter". I assumed that referred to visibility from Earth so was surface brightness the term that was meant? It's the sort of confusion that's common in threads where standard terms are not used.
 
  • #25
Bandersnatch
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It's pretty clear to me that 'lighter' in the OP meant 'less massive'.

@bbbl67 This is important: there is currently little reason to think MW more massive than M31. The numbers in the OP cannot be taken at face value. They're at best bad reporting. You are attempting to draw conclusions about the structure of the galaxies from faulty assumptions.

I think it likely the MW mass is taken from this recent study (albeit with error bars conspicuously omitted), given how its press releases dominate the search results:
https://iopscience.iop.org/article/10.3847/1538-4357/ab089f/pdf
Various headlines tout it, as they often do, along the lines of: 'scientists accurately measure MW mass'. But it's just one of many such studies, and it's not that clear-cut.
First thing to notice here, is that the mass estimate of ~1.5 trillion M☉ (1.54 +0.75 - 0.44, so what it actually says is: 'between 1.1 and 2.3' trillion) is for the virial mass. This is basically all the mass that can be considered bound to the galaxy, that is not another galaxy, while being sufficiently denser than the intergalactic medium to count for anything. Various studies will have this extend to different radii, but this one assumes everything within 300 kpc. That's ten times larger than the luminous disc radius. This will inevitably be mostly non-luminous halo of gas and dark matter.
Whereas the mass enclosed within the radius comparable to what is given in the OP (~40 kpc) is measured at ~0.42 (+0.07 -0.06) trillion. So you're already using incompatible values for even one galaxy. I.e. it's not true that the study claims there to be that much mass inside that much radius.
Furthermore, if you read through the discussion section, you'll see the results compared with other studies, and some limitations mentioned, which should hint at why this shouldn't be taken as be-all-end-all result.
But what you really want is a review study, like this one:
https://arxiv.org/pdf/1912.02599.pdf
Which highlights all the issues with various methods used, and how the results differ. It's a hefty read, but at least look at the summary and discussion section.
Now, it's true that with the advent of GAIA the results started to converge on a much narrower range of values than just a few years earlier, but it's still a range, as shown in fig.5:
1622206836673.png

The study referred to above is the Watkins19 one (showing lower value here due to the virial mass being rescaled so that all the studies use the same definition and can be meaningfully compared).

All this uncertainty is just for the MW, for which we have much better data. I wasn't able to track down the source for the M31 mass estimate (unless Facts For Kids counts). But, given what was discussed above, questions should immediately arise: What are the error bars? Is that the luminous mass or the virial mass? And if it's the latter, what is the definition in use? How does it look like in the context of other studies? Are we comparing apples and oranges? Etc.
If I were to guess, given that the same 'astronomy facts' websites that cite the number from the OP also give stellar population of M31 at 1 trillion (which also looks suspiciously like a half-arsed estimate from an intro astronomy class): 1 trillion times 1 solar mass + central black hole mass is almost the same number. So, at best, it's just what's in the disc and the bulge. No dark matter, no baryonic gas out to 300 kpc. Apples and oranges.

There's another point here, that the review study highlights - while it's difficult to accurately measure and compare the mass of either galaxy, the dynamics of the cluster as a whole provide a means to estimate the ratio of their masses (section 7). These suggest M31 being more massive than the MW by a factor of anywhere between ~1.2 and 4. I'd rely on this rather than on googled numbers of dubious provenance.

The lesson here being: be suspicious of precise numbers, don't jump to conclusions, check the sources, look at the bigger picture. The usual mantra of the information age.
 
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