APOD: Gravity's Grin (2022 May 11)

[APOD Robot]

Gravity's Grin

Explanation: Albert Einstein's general theory of relativity, published over 100 years ago, predicted the phenomenon of gravitational lensing. And that's what gives these distant galaxies such a whimsical appearance, seen through the looking glass of X-ray and optical image data from the Chandra and Hubble space telescopes. Nicknamed the Cheshire Cat galaxy group, the group's two large elliptical galaxies are suggestively framed by arcs. The arcs are optical images of distant background galaxies lensed by the foreground group's total distribution of gravitational mass. Of course, that gravitational mass is dominated by dark matter. The two large elliptical "eye" galaxies represent the brightest members of their own galaxy groups which are merging. Their relative collisional speed of nearly 1,350 kilometers/second heats gas to millions of degrees producing the X-ray glow shown in purple hues. Curiouser about galaxy group mergers? The Cheshire Cat group grins in the constellation Ursa Major, some 4.6 billion light-years away.

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[Roger]

How did astronomers ascertain the speed by which the two galaxy groups are approaching one another?

[orin stepanek]

Cheshire Cat galaxy group; or how about Emoji in space!
Very interesting how gravity creates lensing!

[Chris Peterson]

How did astronomers ascertain the speed by which the two galaxy groups are approaching one another?

That's a line-of-sight speed based on redshift differences and the assumption that the galaxies are at the same distance from us (that is, that the redshift difference is Doppler, not cosmological).

[AVAO]

i like halloween too

Chandra reveals that the right eye is the central galaxy. Hubble reveals that this has a double core, or two elliptical galaxies are in close merger. If these are part of the gravitational center, they must be very "heavy", which in my logic would mean that they would also have to be spinning very quickly. We could possibly also measure this with the WEBB. I think that would be exciting to better understand the mechanisms of galaxy lenses, since double core systems in the center are still relatively common there.


https://live.staticflickr.com/65535/520 ... 1bfd_b.jpg
Jac Berne (flickr)

[johnnydeep]

How did astronomers ascertain the speed by which the two galaxy groups are approaching one another?

That's a line-of-sight speed based on redshift differences and the assumption that the galaxies are at the same distance from us (that is, that the redshift difference is Doppler, not cosmological).

Wait, we only know their speed away from us, in our line of sight. How does having only that allow us to calculate their speed toward or away from each other? ...Oh, never mind: it's because we know the two vectors starting at us and ending at each large elliptical "eye" galaxy and simple vector math can calculate the vector that is the third side of the triangle. Correct?

[Chris Peterson]

How did astronomers ascertain the speed by which the two galaxy groups are approaching one another?

That's a line-of-sight speed based on redshift differences and the assumption that the galaxies are at the same distance from us (that is, that the redshift difference is Doppler, not cosmological).

Wait, we only know their speed away from us, in our line of sight. How does having only that allow us to calculate their speed toward or away from each other? ...Oh, never mind: it's because we know the two vectors starting at us and ending at each large elliptical "eye" galaxy and simple vector math can calculate the vector that is the third side of the triangle. Correct?

No, I don't think so. They're in orbit around each other, so if we know their masses and their line-of-sight speeds, we can estimate their orbital parameters. Keep in mind that their centers aren't likely to be colliding, but it's more like a glancing collision, an overlap of two extended objects.

[johnnydeep]

That's a line-of-sight speed based on redshift differences and the assumption that the galaxies are at the same distance from us (that is, that the redshift difference is Doppler, not cosmological).

Wait, we only know their speed away from us, in our line of sight. How does having only that allow us to calculate their speed toward or away from each other? ...Oh, never mind: it's because we know the two vectors starting at us and ending at each large elliptical "eye" galaxy and simple vector math can calculate the vector that is the third side of the triangle. Correct?

No, I don't think so. They're in orbit around each other, so if we know their masses and their line-of-sight speeds, we can estimate their orbital parameters. Keep in mind that their centers aren't likely to be colliding, but it's more like a glancing collision, an overlap of two extended objects.

And how do we know they're orbiting each other?

[Chris Peterson]

Wait, we only know their speed away from us, in our line of sight. How does having only that allow us to calculate their speed toward or away from each other? ...Oh, never mind: it's because we know the two vectors starting at us and ending at each large elliptical "eye" galaxy and simple vector math can calculate the vector that is the third side of the triangle. Correct?

No, I don't think so. They're in orbit around each other, so if we know their masses and their line-of-sight speeds, we can estimate their orbital parameters. Keep in mind that their centers aren't likely to be colliding, but it's more like a glancing collision, an overlap of two extended objects.

And how do we know they're orbiting each other?

Because those relative speeds aren't high enough for them to be in an open (hyperbolic) orbit. Basically, the speed difference is less than the escape velocity.

[johnnydeep]

No, I don't think so. They're in orbit around each other, so if we know their masses and their line-of-sight speeds, we can estimate their orbital parameters. Keep in mind that their centers aren't likely to be colliding, but it's more like a glancing collision, an overlap of two extended objects.

And how do we know they're orbiting each other?

Because those relative speeds aren't high enough for them to be in an open (hyperbolic) orbit. Basically, the speed difference is less than the escape velocity.

But that would imply we already know their (approximate) masses. Hmm, do we know that from the characteristics of the gravity induced lensing arcs?

[ Sorry, but I'm realizing I've been taking a lot of this "how astronomers know stuff" about far away stuff for granted lately! ]

[Chris Peterson]

And how do we know they're orbiting each other?

Because those relative speeds aren't high enough for them to be in an open (hyperbolic) orbit. Basically, the speed difference is less than the escape velocity.

But that would imply we already know their (approximate) masses. Hmm, do we know that from the characteristics of the gravity induced lensing arcs?

[ Sorry, but I'm realizing I've been taking a lot of this "how astronomers know stuff" about far away stuff for granted lately! ]

Most galaxies have reasonably accurate masses derived from their luminosity. Where there is gravitational lensing, the masses can be even more accurately assessed.

[WWW]

.

Sure, it cute... But this is at least the second, if not the third time this has shown up on an APOD.
How about a new survey using the webb telescope? Now that would be interesting.

.

[Sa Ji Tario]

With the JWST it will be possible to adjust or correct the known parameters

[Chris Peterson]

.

Sure, it cute... But this is at least the second, if not the third time this has shown up on an APOD.
How about a new survey using the webb telescope? Now that would be interesting.

Survey of what?

It's unclear what sort of similar data we might get from JWST. All of these lensing structures involve a nearby lensing body (which the JWST will not provide a lot more information about) and one or more very distant objects, typically galaxies. JWST can potentially see farther by virtue of detecting objects with a greater redshift... maybe even individual early stars. We'll see. No doubt there will be surprises.

[johnnydeep]

Because those relative speeds aren't high enough for them to be in an open (hyperbolic) orbit. Basically, the speed difference is less than the escape velocity.

But that would imply we already know their (approximate) masses. Hmm, do we know that from the characteristics of the gravity induced lensing arcs?

[ Sorry, but I'm realizing I've been taking a lot of this "how astronomers know stuff" about far away stuff for granted lately! ]

Most galaxies have reasonably accurate masses derived from their luminosity. Where there is gravitational lensing, the masses can be even more accurately assessed.

Alright. This kind of thing still seems like a case of having too many unknowns, but I'm sure that's just my lack of understanding speaking. Time for some more googling...

[Chris Peterson]

But that would imply we already know their (approximate) masses. Hmm, do we know that from the characteristics of the gravity induced lensing arcs?

[ Sorry, but I'm realizing I've been taking a lot of this "how astronomers know stuff" about far away stuff for granted lately! ]

Most galaxies have reasonably accurate masses derived from their luminosity. Where there is gravitational lensing, the masses can be even more accurately assessed.

Alright. This kind of thing still seems like a case of having too many unknowns, but I'm sure that's just my lack of understanding speaking. Time for some more googling...

There are few "unknowns". There are quantitative values, with varying degrees of uncertainty.

[Zuben L. Genubi]

The explanation states: "Of course, that gravitational mass is dominated by dark matter." Why "of course"? Aren't dark matter and dark energy still hypothetical constructs devised to explain an observed (apparent) acceleration of the Universe? Also, is the dark matter hypothesis necessary to counteract the expansion phenomenon in light of the obvious evidence of local aggregation of "white matter" due to gravity? What if there were a different explanation for the apparent accelerating expansion of the Universe? (I hope that came out as English !)

[Chris Peterson]

The explanation states: "Of course, that gravitational mass is dominated by dark matter." Why "of course"? Aren't dark matter and dark energy still hypothetical constructs devised to explain an observed (apparent) acceleration of the Universe? Also, is the dark matter hypothesis necessary to counteract the expansion phenomenon in light of the obvious evidence of local aggregation of "white matter" due to gravity? What if there were a different explanation for the apparent accelerating expansion of the Universe? (I hope that came out as English !)

Few would consider dark matter to be anything other than some form of matter. No other explanations hold up, and the idea that dark matter is a form of matter is well supported by multiple independent lines of evidence. (Dark energy is irrelevant to this discussion.)

[Zuben L. Genubi]

If I may ask a pretty basic question, if the Universe weren't apparently expanding, dark matter would still a necessary hypothetical construct? To explain what?

[johnnydeep]

If I may ask a pretty basic question, if the Universe weren't apparently expanding, dark matter would still a necessary hypothetical construct? To explain what?

Dark Matter explains the high rotational speed measured in galaxies that can't be accounted for by the gravity of the observed normal matter (predominantly stars). This "problem" is what led to dark matter being hypothesized in the first place.

[Chris Peterson]

If I may ask a pretty basic question, if the Universe weren't apparently expanding, dark matter would still a necessary hypothetical construct? To explain what?

The expansion of the Universe is not dependent upon dark matter, and dark matter is not a component of our understanding of the expansion of the Universe.

We observe dark matter through its gravitational influence on ordinary matter. On the structure of gravitational lenses. On the structure and rotation of galaxies. On the large scale structure of matter in the Universe. On the structure in the CMB. On the motion of galaxies in galaxy clusters. Dark matter is also a key feature of the dominant theory of the origin of the Universe, Lambda-CDM.

The observed change in the expansion rate of the Universe with time is explained by dark energy, which is entirely unrelated to dark matter.

[Zuben L. Genubi]

I hope you don't mind if I try to nail down a few edges here. I do understand that, in this APOD image, dark energy plays no part; however, as I understand it, the explanation given suggests that dark matter's influence is additive to the gravity of normal matter, but I'm not sure why the image itself implies an "of course." I guess that's just a given. And to clarify my overall understanding, dark matter contributes to the coherence of galaxies - it helps to pull together and hold together normal matter. Dark energy, on the other had, is theoretically necessary to explain the accelerating expansion of the Universe - so it seems to me that these two dark forces are in some way working in opposition to one another. Dark energy is supposedly, eventually going to rip all matter apart at the most elemental level. The Universe ends in a cold, dark whimper, not a bang. In this sense, is it not overcoming the gravitational contributions of dark matter in holding galaxies together? I appreciate your patience with a very amateur cosmologist wannabe.

[Chris Peterson]

I hope you don't mind if I try to nail down a few edges here. I do understand that, in this APOD image, dark energy plays no part; however, as I understand it, the explanation given suggests that dark matter's influence is additive to the gravity of normal matter, but I'm not sure why the image itself implies an "of course." I guess that's just a given. And to clarify my overall understanding, dark matter contributes to the coherence of galaxies - it helps to pull together and hold together normal matter. Dark energy, on the other had, is theoretically necessary to explain the accelerating expansion of the Universe - so it seems to me that these two dark forces are in some way working in opposition to one another. Dark energy is supposedly, eventually going to rip all matter apart at the most elemental level. The Universe ends in a cold, dark whimper, not a bang. In this sense, is it not overcoming the gravitational contributions of dark matter in holding galaxies together? I appreciate your patience with a very amateur cosmologist wannabe.

The mass of ordinary matter is not strong enough to create a gravitational lens like this. It is by mapping the optical paths that the much higher mass of invisible matter is required (or some other mechanism that creates a gravitational field... something for which no theory exists and which is largely inconsistent with all of our known physics).

Gravity (and therefore the effect of dark matter) works over fairly short distances- up to about the scale of galaxy clusters. Dark energy doesn't become significant in its impact until cosmological distances are reached: the distances between galaxy clusters and more. The two work in opposite directions, but in different domains, so they can't really be said to be working in opposition to each other.

[Zuben L. Genubi]

Thanks! I appreciate your time -- and your understanding!