APOD: Galaxy Wars: M81 versus M82 (2010 Mar 24)

[APOD Robot]

Galaxy Wars: M81 versus M82

Explanation: On the right, surrounded by blue spiral arms, is spiral galaxy M81. On the left, marked by red gas and dust clouds, is irregular galaxy M82. This stunning vista shows these two mammoth galaxies locked in gravitational combat, as they have been for the past billion years. The gravity from each galaxy dramatically affects the other during each hundred million-year pass. Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. But M81 left M82 with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays. In a few billion years only one galaxy will remain.

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[verkle2]

Sound like unlikely claims regarding the go-around of M81 and M82. Any real mathematics behind it?

[hstarbuck]

Sound like unlikely claims regarding the go-around of M81 and M82. Any real mathematics behind it?

I would guess that simplified n-body computer simulations--simply using Newton's Universal Law of Gravitation in 3-D vector form--on the largest supercomputer available would give some results. Run it backwards and forwards for billions of years at some small time interval. This mixed in with observations of other galaxy interactions of various stages/go-rounds. Again, just a guess.

[RJN]

ApJ: Intergalactic Stellar Distributions in the Interacting M81/M82 Galaxy Group
http://adsabs.harvard.edu/abs/2005ApJ...630L.133S

Recent measurements of distances to and radial velocities of M81 (3.63 Mpc and 48 km s-1, respectively) and M82 (3.9 Mpc and 296 km s-1) lend further support to the notion of a close passage between these two galaxies several hundred million years ago.

[jbmetz]

FROM TODAY'S APOD:
"The gravity from each galaxy dramatically affects the other during each hundred million-year pass. Last go-round, M82 's gravity likely raised density waves rippling around M81 , resulting in the richness of M81 's spiral arms. But M81 left M82 with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays . In a few billion years only one galaxy will remain ."

An Amateur's Question:
If EVERYTHING in the universe is rapidly expanding away from everything else, with the exception of very 'local' phenomena such as the relationships of bodies within our solar system, etc., then how can entire galaxies behave in cyclic patterns which cause them to pass close to each other repeatedly hundreds of millions of years apart?

[Amir]

An Amateur's Question:
If EVERYTHING in the universe is rapidly expanding away from everything else, with the exception of very 'local' phenomena such as the relationships of bodies within our solar system, etc., then how can entire galaxies behave in cyclic patterns which cause them to pass close to each other repeatedly hundreds of millions of years apart?

well, i think they are within those 'local' boundaries, simply they are closer than the distance that universe expansion could affect them. they are 150000Ly away from each other.

[star37]

M81 and M82, in fact, interact via NGC3077 as reported by Yun et al. in 1994 in Nature.

[Chris Peterson]

If EVERYTHING in the universe is rapidly expanding away from everything else, with the exception of very 'local' phenomena such as the relationships of bodies within our solar system, etc., then how can entire galaxies behave in cyclic patterns which cause them to pass close to each other repeatedly hundreds of millions of years apart?

Because on the scale of cosmic expansion, galaxy clusters are "local" in the same way the bodies in a solar system are. Even over the scale of a galaxy cluster, the force of gravity dominates the "force" of universal expansion.

[JohnD]

All,
The Apod features parts of the universe which are highly active, like M82, that is "so energetic the galaxy glows in X-rays", that one wonders if whole galaxies cannot hold life, becasue it would be irradiated out of existance.
Should M82 as a whole be excluded for being a possible place where there could be life?

The Drake Eqaution doesn't have a factor for 'galactic inhabitability', just for star system inhabitability, the size of the Goldilocks Zone. Are enough galaxies so active, so impossible as paces for life to exist, that there should be one?

John

[biddie67]

jbmetz: thanks for the question -- I was wondering the same thing.

[Chris Peterson]

The Apod features parts of the universe which are highly active, like M82, that is "so energetic the galaxy glows in X-rays", that one wonders if whole galaxies cannot hold life, becasue it would be irradiated out of existance.
Should M82 as a whole be excluded for being a possible place where there could be life?

The actual x-ray intensity isn't that great. Our own galaxy, while currently less active, glows in x-rays as well. There are good arguments against complex life developing in the central part of galaxies in general- not just because of higher radiation and frequent exposure to supernovas, but because planetary systems might be fairly unstable due to gravitational perturbations. But planets with atmospheres and magnetic fields, further out in a galaxy like M82, ought to be okay.

[JohnD]

Thank you Chris!

But a magnetic field as X-ray protection?
Only charged particles, surely?

John

[DaveBone]

Regarding Chris's answer to the question (about which I, also, had wondered). I thought that the atmosphere was pretty much transparent to X-rays and that the X-rays being electro-magnetic waves were not affected by magnetic fields. It would seem that the atmosphere and earth's mag field wouldn't offer much protection from a strong X-ray source. Chris did say that the intensity was weak so maybe this is just nit-picking, but it was a question that jumped into mind as I read his answer.

Dave

[Chris Peterson]

But a magnetic field as X-ray protection?
Only charged particles, surely?

Yes, sorry if that was confusing. I was talking about protection in general from the products of an active galaxy. A thick atmosphere like ours is opaque to x-rays, so that is the source of protection from that source. The same processes that produce x-rays are likely to produce charged particles, and a magnetic field provides protection from those. So I think you'd want both if you were in an active galaxy.

[jjohnson]

Galaxies are not at "war" with one another. That is just hyperbole based on the movie title, "Star Wars".

There is no evidence that gravity waves from one are pounding the other. So far there is not a shred of evidence that there even are gravity waves, as reported regularly from the LIGO press announcements. Of course, they put a slight amount of spin (1/2, say) on it by praising themselves for setting "an upper limit on the intensity with which gravity waves could be reaching Earth." In People-speak that means, "we haven't detected any gravity waves yet, but if we get more funding we will keep trying because we really believe that we will, and it pays well."

There are no observations of either stars or galaxies colliding, other than in computer simulations and artists' "interpretations". Do any of you wonder why there are always a lot of artists' interpretations? It's because there are no actual (real; observed; gathered with telescopes of some type and imaged) images of such collisions during their occurrence. If you have such an image, please post. I'm skeptical, but I'll be open-minded about it, of course. The stuff you read about pairs of neutron stars and white dwarfs orbiting about one another in the famous "death spiral to doom", resulting in a supernova or a "black hole" or both, is just all made up. No one has yet filmed such a collision, in our galaxy and certainly not in another one. Effects imputed to this hypothetical scenario? Sure! Anyone can say that this supernova was caused by two white dwarfs (now gone missing!) or too close an encounter with a blalck hole's (unobservable) event horizon, or even because it ate too much lasagna. Observational evidence beats theories to the contrary almost every time, except in astronomy.

Due to the increasing suspicion regarding the accuracy of and assumptions behind the "standard candles" traditionally used to state positively how far away things are, outside our galaxy, and the knowledge that there are other things that cause redshifts besides relative velocity, astronomers can't actually and truthfully state how far away this or that galaxy is unless they are close enough for parallax measurements. - At least not yet. That means that two galaxies with overlapping star fields give a lot of doubt as to whether this star is in this galaxy or in that one, or behind both, etc. Even within our own galaxy only the Hipparcos geometric distance measurements of a large number (a paltry tens of thousands out of some 200 billion) of relatively nearby stars are considered to be accurate.

Calling out the colors associated with the galaxies is a red herring. Those colors are likely to be false colors representing wavelengths that we cannot see in colors which we can. There is nothing wrong with false imaging. It is an invaluable tool for letting us visualize the morphology of an object as caused by different mechanisms at different wavelengths. But it should not be dragged into the "war" as evidence of interactions.

Do the gravity math. The pull between two extended objects on each other can be taken as coming from the center of gravity of each, and it falls off as the inverse square of the distance once you're roughly outside the arms, just as Earth's gravity drops linearly to zero at the center, but falls off as the inverse of the distance outside the surface. Someone figure out what the gravity acceleration is in m/s/s at the one galaxy by the other one, if you can estimate the distance between their centers and their overall masses, over that distance. If the gravity waves are rocking the near side of one galaxy, then how much weaker will their effect be on the far side of the galaxy, 50 to 100 thousand light years away? Do the tidal effects math and wonder why both galaxies don't look like they've been run through a Cuisinart, if the gravity pull is that strong. My guess is that in terms of m/s/s, the answer will have a negative exponent with two significant digits.

My opinion of this companion explanation of what the image is telling us is that it is hogwash (or a trick question to see if we are awake), "...full of sound and fury, signifying nothing. -The Bard of Avon" and am not sure why it was credited and, especially, copyrighted.

[Chris Peterson]

Galaxies are not at "war" with one another. That is just hyperbole based on the movie title, "Star Wars".

I think everybody gets that. There's nothing wrong with a bit of poetic expression in an APOD!

There is no evidence that gravity waves from one are pounding the other.

Nor is there any suggestion of gravity waves in the APOD caption. While there are plenty of good reasons to believe that gravity waves exist, there is nothing suggesting that they are responsible for any sort of structure seen in galaxies- colliding or otherwise.

There are no observations of either stars or galaxies colliding, other than in computer simulations and artists' "interpretations". Do any of you wonder why there are always a lot of artists' interpretations? It's because there are no actual (real; observed; gathered with telescopes of some type and imaged) images of such collisions during their occurrence. If you have such an image, please post.

This APOD is such an image. There are hundreds of images of colliding galaxies. Of course, we haven't observed for the hundreds of thousands of years necessary to see actual motion, but that doesn't mean we can't see the structural effects in galaxies caused by tides from nearby or colliding galaxies, or that we can't see the resulting density waves and their zones of new star formation. These things constitute powerful observational evidence that our interpretations of interacting galaxies are correct.

The stuff you read about pairs of neutron stars and white dwarfs orbiting about one another in the famous "death spiral to doom", resulting in a supernova or a "black hole" or both, is just all made up. No one has yet filmed such a collision, in our galaxy and certainly not in another one.

If you limit yourself to what can be observed in the visible spectrum, you'll never get far in understanding the Universe.

Due to the increasing suspicion regarding the accuracy of and assumptions behind the "standard candles" traditionally used to state positively how far away things are, outside our galaxy, and the knowledge that there are other things that cause redshifts besides relative velocity, astronomers can't actually and truthfully state how far away this or that galaxy is unless they are close enough for parallax measurements.

There is no "increasing suspicion regarding the accuracy" of standard candles. Quite the opposite; as the mechanisms behind various standard candles become better understood, the limits on accuracy are improving. Furthermore, virtually nobody believes that redshift can't be used to determine if two galaxies are the same distance- even if the error on the Hubble Constant means that the actual distance may be off by 5-10%. No galaxies can have their distances determined by parallax measurements. No mechanism exists to explain redshifts in objects like this except velocity (Doppler), which is largely insignificant at these distances, and the effect of cosmological expansion, which dominates.

Calling out the colors associated with the galaxies is a red herring. Those colors are likely to be false colors representing wavelengths that we cannot see in colors which we can.

This isn't a false color image. It was made through filters that approximate the response of the eye. The areas of red are associated with hydrogen alpha emission, which is associated with areas where tidal forces have concentrated hydrogen, and where we see a lot of new star formation. Again, this is powerful evidence of interaction between these galaxies.

Do the gravity math. The pull between two extended objects on each other can be taken as coming from the center of gravity of each, and it falls off as the inverse square of the distance once you're roughly outside the arms...

You do the math. But do the right math. You are less concerned here with the absolute forces than you are with tidal forces. And it doesn't matter if the actual forces are small, it only matters what they look like compared with the escape velocity of stars inside a galaxy- which are quite low.

My opinion of this companion explanation of what the image is telling us is that it is hogwash (or a trick question to see if we are awake), "...full of sound and fury, signifying nothing.

The vast majority of astronomers would disagree with your opinion.