APOD: Galaxy Wars: M81 versus M82 (2016 Feb 03)

[APOD Robot]

Galaxy Wars: M81 versus M82

Explanation: In the lower left corner, surrounded by blue spiral arms, is spiral galaxy M81. In the upper right corner, 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. This big battle is seen from Earth through the faint glow of an Integrated Flux Nebula, a little studied complex of diffuse gas and dust clouds in our Milky Way Galaxy. In a few billion years only one galaxy will remain.

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[Ann]

This is a brilliant image! Thanks to everyone involved!

The IFN, the Integrated Flux Nebula, adds to the drama of the picture, and it is sometimes hard to know what is foreground IFN and what is nebulas involved in the M81-M82 dramatic interaction. Note that the red clouds of ionized gas from M82 appear to be outlined in grayish IFN colors. Are the ionized gases expelled by M82 surrounded by an older nebula of less ionized gas?

Click to view full size image

Holmberg IX. Photo: Chris Kochanek.

Note Holmberg IX, the small, brilliantly blue satellite galaxy of M81.

Click to view full size image

M81, M82 and NGC 3077. Source:
http://messier.seds.org/more/m081gr.html

Let's not forget that a third galaxy is involved in the M81-M82 battle, NGC 3077. NGC 3077 is an interesting galaxy, basically an elliptical galaxy with a nuclear starburst.

Ann

[Guest]

Looking at this image, and reading the description made me wonder. I am assuming there is a massive black hole at the center of these galaxies. Given that these black holes are massive, would the balance of the mass of the rest of the respective galaxies have any real effect on the cross-galaxy interactions as a whole? Are the star system in orbit around these black holes just along for the ride and add only decorative splendor to those eventually around to watch the merger, or do they have any real effect on the merger? Can the interaction between these galaxies be simulated accurately using just the mass, position and relative motion of the black holes seen as point gravitational sources and the rest of the stars involved ignored because their presence is inconsequential to the whole system; beyond the stars that are flung out from the system as the galaxies 'collide' taking mass, energy and momentum with them, of course, if that would amount to much in relative terms. Your thoughts would be interesting.

[avdhoeven]

As a first order approximation your thought can be right, but I think when you look at a more detailed simulation you do need the stars also. The black hole is about 70 million solar masses. This is about 0.07% (assuming only stars of solar mass..., probably it's a lot lower percentage) of the total mass of the galaxy. So in my humble opinion you can't just ignore the rest of the mass of the galaxy...

[Markus Schwarz]

As a first order approximation your thought can be right, but I think when you look at a more detailed simulation you do need the stars also. The black hole is about 70 million solar masses. This is about 0.07% (assuming only stars of solar mass..., probably it's a lot lower percentage) of the total mass of the galaxy. So in my humble opinion you can't just ignore the rest of the mass of the galaxy...

You should also keep in mind that the visible matter, e.g. stars, is only a fraction of the total mass of the galaxy. There is about 5 times more dark matter than there is ordinary matter. So, my guess is that the central black hole is irrelevant for collision simulations.

[avdhoeven]

You have a very good point there that I completely forgot

[rstevenson]

225 exposures, through four telescopes, by three astrophotographers, totalling 34 hours and 35 minutes of exposure time plus untold hours of processing = an incredible amount of work yielding an extraordinary result. Fascinating!

Rob

[starsurfer]

Let's not forget that a third galaxy is involved in the M81-M82 battle, NGC 3077. NGC 3077 is an interesting galaxy, basically an elliptical galaxy with a nuclear starburst.

Ann

You took the words out of my mouth!

[Chris Peterson]

225 exposures, through four telescopes, by three astrophotographers, totalling 34 hours and 35 minutes of exposure time plus untold hours of processing = an incredible amount of work yielding an extraordinary result. Fascinating!

Images like this can mislead if we don't recognize how the data were collected and processed, however. This image is scientifically interesting (and incidentally aesthetically interesting), as it shows so much luminous matter and gives us clues about its distribution. But we need to know that in order to see this, the intensity profile required radical modification. The IFN is many orders of magnitude less luminous than the central regions of the galaxies, but in order to display it with the equipment we have, their relative intensities have been compressed to just an order of magnitude or two. So even though this information is represented in an approximation of true color, our eyes would never remotely be able to see this scene. This is not a criticism! It's a demonstration of the power of imaging.

There are many ways that the data in this image could be represented. One approach that would be useful would be to use a false color palette to paint the luminance information. That would replace intensity variations with color, allowing subtle differences in intensity to be shown without the need to compress the intensity space.

[Ann]

There are many ways that the data in this image could be represented. One approach that would be useful would be to use a false color palette to paint the luminance information. That would replace intensity variations with color, allowing subtle differences in intensity to be shown without the need to compress the intensity space.

I see your point, Chris. I would prefer a very unsaturated RGB image. In a very pale RGB color image, few colors would be visible at all. Maybe none would be visible. I would be fine with that, because it would be a way of saying that the (true) colors are there if we could only see them, but we can so rarely see colors in galaxies. (Individual stars can and do indeed show colors.)

I wouldn't mind a very unsaturated RGB image of M81 and M82 where luminance was not "stretched". That would show us the "comparatively true" brightness of every part of these galaxies. It seems certain that the lovely blue dwarf galaxy, Holmberg IX, would be invisible if the bulge of M81 was not overexposed. An image like that would clearly show us the relative brightness of M81 and M82.

I would find such an image very fascinating, and I would love to compare it with an image like today's APOD. For aesthetic reasons, I would be unhappy with a false color image.

Ann

[Chris Peterson]

There are many ways that the data in this image could be represented. One approach that would be useful would be to use a false color palette to paint the luminance information. That would replace intensity variations with color, allowing subtle differences in intensity to be shown without the need to compress the intensity space.

I see your point, Chris. I would prefer a very unsaturated RGB image.

Personally, I would not use "prefer" for any one image. What I "prefer" is seeing a dataset represented in as many creative ways as possible, each intended to enhance information in different ways, to show things that other representations cannot.

[Ann]

I think it would be great if a whole series of images could be made from the same (RGB) dataset. You could start out with an image which had been "dimmed" so that the nucleus of M81 showed up as a white point against the rather wan (inner) bulge. And then you could progress until, gradually, not only the entire bulge but the inner spiral arms suffered a complete whiteout and the IFN started to appear.

That would give us a brilliant idea of how bright various parts of this skyscape is.

Ann

[avdhoeven]

Please let's not forget this image was not made for scientific purposes, but to show the IFN in this region together with the galaxies. As said this is the power of imaging. If you want to do science one should use the raw data and can do scientific measurements. The brightness difference is huge, that's what I can say for sure

[JohnD]

We think of the Universe, let alone our own Galaxy, as inevitably, somewhere, possessing life.
But of whole galaxies "glow in X-rays" won't that sterilise them?

John

[Chris Peterson]

We think of the Universe, let alone our own Galaxy, as inevitably, somewhere, possessing life.
But of whole galaxies "glow in X-rays" won't that sterilise them?

X-rays are attenuated by thick atmospheres, and blocked completely by oceans. So there would be lots of opportunities for life.

[neufer]

Click to play embedded YouTube video.

We think of the Universe, let alone our own Galaxy, as inevitably, somewhere, possessing life. But of whole galaxies "glow in X-rays" won't that sterilise them?

X-rays are attenuated by thick atmospheres, and blocked completely by oceans. So there would be lots of opportunities for life.

[Dad is watching]

As a first order approximation your thought can be right, but I think when you look at a more detailed simulation you do need the stars also. The black hole is about 70 million solar masses. This is about 0.07% (assuming only stars of solar mass..., probably it's a lot lower percentage) of the total mass of the galaxy. So in my humble opinion you can't just ignore the rest of the mass of the galaxy...

You should also keep in mind that the visible matter, e.g. stars, is only a fraction of the total mass of the galaxy. There is about 5 times more dark matter than there is ordinary matter. So, my guess is that the central black hole is irrelevant for collision simulations.

We read this and thought that the black hole kept the galaxies together. But now it seems to us that the mass of the galaxy itself, without the black hole, is sufficient for that purpose. Is the black hole in the middle just a 'garbage collector' for slower in-falling debris? And, perhaps, acting as a sort of storehouse for rotational/angular momentum? Could you have a big galaxy without a central black hole or are they inevitable formations that are the result of having a galaxy rather than the cause of galaxy formation?

[Chris Peterson]

We read this and thought that the black hole kept the galaxies together. But now it seems to us that the mass of the galaxy itself, without the black hole, is sufficient for that purpose. Is the black hole in the middle just a 'garbage collector' for slower in-falling debris? And, perhaps, acting as a sort of storehouse for rotational/angular momentum? Could you have a big galaxy without a central black hole or are they inevitable formations that are the result of having a galaxy rather than the cause of galaxy formation?

There are a few galaxies that don't have central black holes, but most appear to. Their origin and role are not well understood. However, it's likely that if they could be magically removed from their host galaxies, we wouldn't see a significant dynamical change (that is, a change in stellar orbits or gross structure).

[Ann]

Please let's not forget this image was not made for scientific purposes, but to show the IFN in this region together with the galaxies. As said this is the power of imaging. If you want to do science one should use the raw data and can do scientific measurements. The brightness difference is huge, that's what I can say for sure

I wholeheartedly agree, Andre. I love your image.

As for the IFN, have you any idea if some extra-thick IFN is superimposed just on the edges of the well-known fantastic Ha nebulas of M82, or are we really seeing the remnants of an older or just less ionized bona fide M82 nebula there?

Ann

[Ann]

We read this and thought that the black hole kept the galaxies together. But now it seems to us that the mass of the galaxy itself, without the black hole, is sufficient for that purpose. Is the black hole in the middle just a 'garbage collector' for slower in-falling debris? And, perhaps, acting as a sort of storehouse for rotational/angular momentum? Could you have a big galaxy without a central black hole or are they inevitable formations that are the result of having a galaxy rather than the cause of galaxy formation?

There are a few galaxies that don't have central black holes, but most appear to. Their origin and role are not well understood. However, it's likely that if they could be magically removed from their host galaxies, we wouldn't see a significant dynamical change (that is, a change in stellar orbits or gross structure).

Wikipedia wrote about M33:
The nucleus of this galaxy is an H II region,[27] and it contains an ultraluminous X-ray source with an emission of 1.2 × 1039 erg s−1, which is the most luminous source of X-rays in the Local Group of galaxies. This source is modulated by 20% over a 106-day cycle.[37] However, the nucleus does not appear to contain a supermassive black hole, as an upper limit of 3,000 solar masses is placed on the mass of a central black hole based upon the velocity of stars in the core region.

My impression is that small galaxies don't necessarily contain supermassive central black holes. We have no indication that the Large Magellanic Cloud contains a supermassive central black hole, or even a definite nucleus in the first place.

But I also think we have every reason to believe that basically all large galaxies, the size and mass of the Milky Way or larger, contain supermassive central black holes. But these black holes are probably more a consequence of, than a reason for, the large mass of the galaxy. Or to put it differently: I believe that large massive galaxies were born out of huge mass concentrations, including a lot of dark matter. These huge mass concentrations were probably the cause of both the collapse of matter near the nucleus into central supermassive black holes and of the ongoing accretion of more mass to the galaxies themselves from the outside.

But the black hole itself plays virtually no part at all in holding the galaxy together.

Ann

[Chris Peterson]

My impression is that small galaxies don't necessarily contain supermassive central black holes. We have no indication that the Large Magellanic Cloud contains a supermassive central black hole, or even a definite nucleus in the first place.

True, I was talking about typical spirals and ellipticals that evolve from them. The general thinking has been that dwarf galaxies don't host supermassive black holes. That thinking has changed in recent years, however, as quite a few dwarf galaxies with massive black holes (on the order of a million solar masses) have now been identified, and since only a small number would be active at any time (and therefore readily detectable), even more must have them. It is also false to suggest that there is no evidence of a supermassive black hole in the LMC. While none has been detected directly, we know of a star that was ejected from the LMC and- to date- only a supermassive black hole interaction can explain that.

[Ann]

Chris wrote:
It is also false to suggest that there is no evidence of a supermassive black hole in the LMC. While none has been detected directly, we know of a star that was ejected from the LMC and- to date- only a supermassive black hole interaction can explain that.

I will not argue about the math underlying that claim.

My point is that black holes arise out of mass concentrations. My gut feeling, for what it is worth, is that huge mass concentrations more easily occur when there is a lot of free gas available, than when most of the gas has been turned into stars. That would make the LMC a pretty fertile ground for black hole making. The Tarantula Nebula and R136 anyone?

On the other hand, I wouldn't expect the LMC to ever form a black hole the mass of the Milky Way's central supermassive black hole. The LMC doesn't pack the punch, or more importantly, the mass to do that.

When really supermassive black holes have been found in quite small galaxies, I believe we are mostly talking about galaxies that for some reasons have lost most of their "outlying mass", so that only the inner bulge is left. The tiny dwarf galaxy M60-UCD1, whose mass is only 1/1000th of the mass of the Milky Way, nevertheless contains a central black hole of 21 million solar masses, more than five times the mass of the central black hole of our own galaxy.

But M60-UCD1 is clearly a galaxy that has been robbed of most of its mass by its big bully neighbor, M60. So having a monstrous central black hole didn't help M60-UCD1 hold on to its mass.

Ann

[avdhoeven]

I wholeheartedly agree, Andre. I love your image.

As for the IFN, have you any idea if some extra-thick IFN is superimposed just on the edges of the well-known fantastic Ha nebulas of M82, or are we really seeing the remnants of an older or just less ionized bona fide M82 nebula there?

Ann

I don't dare to say. My gut feeling is that there is no relation.

[NGC3314]

Some of the IFN seen immediately around M82 may in fact belong to it. There was a polarization study with the 8m Subaru telescope showing that some of the emission-line radiation in the bright filaments is scattered from dust particles in the outlawing wind, which makes it also possible that regions of the wind that remain less ionized, or are in regions that don't see much ionizing far-UV from the central starburst region, could show reflection from the optical light generated by the starburst in the core. Also, some of the bluish filaments around M82 are excellent matches for the UV structure (pretty much all reflection) seen in the ultraviolet GALEX image (for example in this older APOD).