APOD: Three Galaxies and a Comet (2023 Dec 21)

[APOD Robot]

Three Galaxies and a Comet

Explanation: Distant galaxies abound in this one degree wide field of view toward the southern constellation Grus (The Crane). But the three spiral galaxies at the lower right are quite striking. In fact, all three galaxies are grouped about 70 million light years away and sometimes known as the Grus Triplet. They share the pretty telescopic frame, recorded on December 13, with the comet designated C/2020 V2 ZTF. Now outbound from the inner Solar System and swinging below the ecliptic plane in a hyperbolic orbit, the comet was about 29 light-minutes from our fair planet in this image. And though comet ZTF was brighter when it was closest to the Sun last May and closest to Earth in September of 2023, it still shines in telescopes pointed toward southern night skies, remaining almost as bright as the Grus Triplet galaxies.

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[jks]

Nice APOD today! I usually like to click on the image to see if I can zoom in (with a subsequent click on the image). Currently, however, the link to the image appears to be broken (although I can download the image from the page).

[Ann]

Nice APOD today! I usually like to click on the image to see if I can zoom in (with a subsequent click on the image). Currently, however, the link to the image appears to be broken (although I can download the image from the page).

I, too, would like to enlarge the image. Anyway...

Three Galaxies and a Comet. Image Credit & Copyright: Dan Bartlett

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The composition of the APOD is great. The long sweeping tail of the comet makes it look like some kind of outrageously distorted galaxy or, alternatively, a cosmic sea monster in the process of absorbing one of the prominent galaxies, NGC 7599.

Note that the distant galaxies in the APOD are very obviously redshift-reddened. I have marked two of the background galaxies, whose designations are PGC 71042 and PGC 71043. They are very obviously redder than the Grus Triplet galaxies. They move away from us much faster, too. The Grus Triplet galaxies move away from us at some 1,600 kilometers per second, but the small-looking PGC galaxies move away at some 16,000 kilometers per second. Ten times faster! That's the expansion of the Universe right there, and the redshift reddening that accompanies it.

The extremely colorless nature of the comet surprises me, however. Note that the galaxies are more colorful than the comet. And indeed, I found another picture of the comet with the Grus Triplet galaxies, where the comet looks decidedly more green:

Comet C/202 V2 ZTF and the Grus Triplet. Credit: D. Peach

The orientation of the APOD is the conventional one, where north is up and east to the left.

Ann

[goodfoodstove]

The image is available at

https://apod.nasa.gov/apod/image/2312/C ... tt1024.jpg

but it is not expandable.

[johnnydeep]

Anne wrote:

Note that the distant galaxies in the APOD are very obviously redshift-reddened. I have marked two of the background galaxies, whose designations are PGC 71042 and PGC 71043. They are very obviously redder than the Grus Triplet galaxies. They move away from us much faster, too. The Grus Triplet galaxies move away from us at some 1,600 kilometers per second, but the small-looking PGC galaxies move away at some 16,000 kilometers per second. Ten times faster! That's the expansion of the Universe right there, and the redshift reddening that accompanies it.

I suppose you used the calculated redshift values to determine the recession speeds?

[johnnydeep]

Three Galaxies and a Comet

Explanation: Distant galaxies abound in this one degree wide field of view toward the southern constellation Grus (The Crane). But the three spiral galaxies at the lower right are quite striking. In fact, all three galaxies are grouped about 70 million light years away and sometimes known as the Grus Triplet. They share the pretty telescopic frame, recorded on December 13, with the comet designated C/2020 V2 ZTF. Now outbound from the inner Solar System and swinging below the ecliptic plane in a hyperbolic orbit, the comet was about 29 light-minutes from our fair planet in this image. And though comet ZTF was brighter when it was closest to the Sun last May and closest to Earth in September of 2023, it still shines in telescopes pointed toward southern night skies, remaining almost as bright as the Grus Triplet galaxies.

The Grus Triple, or is it a quartet?

Northeast of Theta Gruis are four interacting galaxies known as the Grus Quartet.[49] These galaxies are NGC 7552, NGC 7590, NGC 7599, and NGC 7582.[50] The latter three galaxies occupy an area of sky only 10 arcminutes across and are sometimes referred to as the "Grus Triplet," although all four are part of a larger loose group of galaxies called the IC 1459 Grus Group.[51] NGC 7552 and 7582 are exhibiting high starburst activity; this is thought to have arisen because of the tidal forces from interacting.[50] Located on the border of Grus with Piscis Austrinus,[48] IC 1459 is a peculiar E3 giant elliptical galaxy. It has a fast counterrotating stellar core, and shells and ripples in its outer region.[52] The galaxy has an apparent magnitude of 11.9[53] and is around 80 million light-years distant.[48]

Is the fourth galaxy not visible in this FOV?

NGC 7552 belongs in NGC 7582 group, also known as the Grus group. Others members of the group include the spiral galaxies of NGC 7599, NGC 7590, NGC 7582,[12] which along with NGC 7552 form the Grus Quartet. A large tidal extension of HI reaches from NGC 7582 to NGC 7552, which is indicative of interactions between the group members,[12] yet NGC 7552 does not have highly disturbed morphology.[13]

[Chris Peterson]

Note that the distant galaxies in the APOD are very obviously redshift-reddened. I have marked two of the background galaxies, whose designations are PGC 71042 and PGC 71043. They are very obviously redder than the Grus Triplet galaxies. They move away from us much faster, too. The Grus Triplet galaxies move away from us at some 1,600 kilometers per second, but the small-looking PGC galaxies move away at some 16,000 kilometers per second. Ten times faster! That's the expansion of the Universe right there, and the redshift reddening that accompanies it.

Well, technically we are not seeing Doppler shift, but cosmological redshift. Which means the galaxies are not moving away from us, but rather, the space between us is expanding.

[johnnydeep]

Note that the distant galaxies in the APOD are very obviously redshift-reddened. I have marked two of the background galaxies, whose designations are PGC 71042 and PGC 71043. They are very obviously redder than the Grus Triplet galaxies. They move away from us much faster, too. The Grus Triplet galaxies move away from us at some 1,600 kilometers per second, but the small-looking PGC galaxies move away at some 16,000 kilometers per second. Ten times faster! That's the expansion of the Universe right there, and the redshift reddening that accompanies it.

Well, technically we are not seeing Doppler shift, but cosmological redshift. Which means the galaxies are not moving away from us, but rather, the space between us is expanding.

Hmm. So, what if an initially close galaxy (say, 500 Kly away) gets magically accelerated away from us at, say, 0.1c. Then over the next 5 billion years, the space between them expands due to cosmological redshift. Is there any way to distinguish the two components of the resulting redshift that we would measure? And how would you then determine its true distance?

[johnnydeep]

Answering my question above about the fourth member of the Grus Quartet: yes, it's outside of the FOV of this APOD:

Click to play embedded YouTube video.

It's NGC 7552, a barred spiral galaxy below right of the "P" in "Group" in the Youtube thumbnail.

[Chris Peterson]

Note that the distant galaxies in the APOD are very obviously redshift-reddened. I have marked two of the background galaxies, whose designations are PGC 71042 and PGC 71043. They are very obviously redder than the Grus Triplet galaxies. They move away from us much faster, too. The Grus Triplet galaxies move away from us at some 1,600 kilometers per second, but the small-looking PGC galaxies move away at some 16,000 kilometers per second. Ten times faster! That's the expansion of the Universe right there, and the redshift reddening that accompanies it.

Well, technically we are not seeing Doppler shift, but cosmological redshift. Which means the galaxies are not moving away from us, but rather, the space between us is expanding.

Hmm. So, what if an initially close galaxy (say, 500 Kly away) gets magically accelerated away from us at, say, 0.1c. Then over the next 5 billion years, the space between them expands due to cosmological redshift. Is there any way to distinguish the two components of the resulting redshift that we would measure? And how would you then determine its true distance?

No, cosmological redshift is indistinguishable from Doppler shift. The shift we see for nearby objects is Doppler; as objects get farther away, that shifts to cosmological. The cosmological redshift (z) we use for various distance calculations is actually distorted by whatever actual Doppler shift is present, which could be either towards blue or towards red. But we assume that at cosmological distances, the value is only slightly modified by Doppler shift, since we assume that nothing has a true speed relative to us greater than a small fraction of the large apparent speed resulting from the expansion of space.

[johnnydeep]

Well, technically we are not seeing Doppler shift, but cosmological redshift. Which means the galaxies are not moving away from us, but rather, the space between us is expanding.

Hmm. So, what if an initially close galaxy (say, 500 Kly away) gets magically accelerated away from us at, say, 0.1c. Then over the next 5 billion years, the space between them expands due to cosmological redshift. Is there any way to distinguish the two components of the resulting redshift that we would measure? And how would you then determine its true distance?

No, cosmological redshift is indistinguishable from Doppler shift. The shift we see for nearby objects is Doppler; as objects get farther away, that shifts to cosmological. The cosmological redshift (z) we use for various distance calculations is actually distorted by whatever actual Doppler shift is present, which could be either towards blue or towards red. But we assume that at cosmological distances, the value is only slightly modified by Doppler shift, since we assume that nothing has a true speed relative to us greater than a small fraction of the large apparent speed resulting from the expansion of space.

Thanks, good to know!

[Ann]

Well, technically we are not seeing Doppler shift, but cosmological redshift. Which means the galaxies are not moving away from us, but rather, the space between us is expanding.

Hmm. So, what if an initially close galaxy (say, 500 Kly away) gets magically accelerated away from us at, say, 0.1c. Then over the next 5 billion years, the space between them expands due to cosmological redshift. Is there any way to distinguish the two components of the resulting redshift that we would measure? And how would you then determine its true distance?

No, cosmological redshift is indistinguishable from Doppler shift. The shift we see for nearby objects is Doppler; as objects get farther away, that shifts to cosmological. The cosmological redshift (z) we use for various distance calculations is actually distorted by whatever actual Doppler shift is present, which could be either towards blue or towards red. But we assume that at cosmological distances, the value is only slightly modified by Doppler shift, since we assume that nothing has a true speed relative to us greater than a small fraction of the large apparent speed resulting from the expansion of space.

Exactly, Chris. So for all intents and purposes they are moving away from us, if only in the sense that raisins in a dough that is rising are also moving away from one another.

The raisins are moving away from each other as the dough in which they are embedded grows larger in volume.

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Ann

[Ann]

Answering my question above about the fourth member of the Grus Quartet: yes, it's outside of the FOV of this APOD:

Click to play embedded YouTube video.

It's NGC 7552, a barred spiral galaxy below right of the "P" in "Group" in the Youtube thumbnail.

Indeed. And there are some great pictures of the "outlier", NGC 7552!

NGC 7552 by Judy Schmidt and NASA.

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NGC 7552 by Mark Hanson and Mike Selby.

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Ann

[johnnydeep]

Hmm. So, what if an initially close galaxy (say, 500 Kly away) gets magically accelerated away from us at, say, 0.1c. Then over the next 5 billion years, the space between them expands due to cosmological redshift. Is there any way to distinguish the two components of the resulting redshift that we would measure? And how would you then determine its true distance?

No, cosmological redshift is indistinguishable from Doppler shift. The shift we see for nearby objects is Doppler; as objects get farther away, that shifts to cosmological. The cosmological redshift (z) we use for various distance calculations is actually distorted by whatever actual Doppler shift is present, which could be either towards blue or towards red. But we assume that at cosmological distances, the value is only slightly modified by Doppler shift, since we assume that nothing has a true speed relative to us greater than a small fraction of the large apparent speed resulting from the expansion of space.

Exactly, Chris. So for all intents and purposes they are moving away from us, if only in the sense that raisins in a dough that is rising are also moving away from one another.

The raisins are moving away from each other as the dough in which they are embedded grows larger in volume.

---

Ann

Yes, but…. if space weren’t expanding, and yet instead, all the galaxies were moving away from each other through space at the same speeds and directions, we would be unable to discern the difference. Though I still think there might be some way other than via spectrum shifts.

[Ann]

No, cosmological redshift is indistinguishable from Doppler shift. The shift we see for nearby objects is Doppler; as objects get farther away, that shifts to cosmological. The cosmological redshift (z) we use for various distance calculations is actually distorted by whatever actual Doppler shift is present, which could be either towards blue or towards red. But we assume that at cosmological distances, the value is only slightly modified by Doppler shift, since we assume that nothing has a true speed relative to us greater than a small fraction of the large apparent speed resulting from the expansion of space.

Exactly, Chris. So for all intents and purposes they are moving away from us, if only in the sense that raisins in a dough that is rising are also moving away from one another.

The raisins are moving away from each other as the dough in which they are embedded grows larger in volume.

---

Ann

Yes, but…. if space weren’t expanding, and yet instead, all the galaxies were moving away from each other through space at the same speeds and directions, we would be unable to discern the difference. Though I still think there might be some way other than via spectrum shifts.

Physicists would be busy trying to explain the antigravity of galaxies!

Ann

[Chris Peterson]

Hmm. So, what if an initially close galaxy (say, 500 Kly away) gets magically accelerated away from us at, say, 0.1c. Then over the next 5 billion years, the space between them expands due to cosmological redshift. Is there any way to distinguish the two components of the resulting redshift that we would measure? And how would you then determine its true distance?

No, cosmological redshift is indistinguishable from Doppler shift. The shift we see for nearby objects is Doppler; as objects get farther away, that shifts to cosmological. The cosmological redshift (z) we use for various distance calculations is actually distorted by whatever actual Doppler shift is present, which could be either towards blue or towards red. But we assume that at cosmological distances, the value is only slightly modified by Doppler shift, since we assume that nothing has a true speed relative to us greater than a small fraction of the large apparent speed resulting from the expansion of space.

Exactly, Chris. So for all intents and purposes they are moving away from us, if only in the sense that raisins in a dough that is rising are also moving away from one another.

Nevertheless, they are very different. If they were actually moving away, we would have to explain the force that accelerated them. Indeed, for objects beyond the edge of the observable universe, that would require infinite energy (since those bodies would have to be treated as moving at faster than c). So in terms of the math that actually describes how the Universe works, everything we see is stationary with respect to us, except for small local relative motion, and the redshift is the product of expanding space... not of actual relative motion.