<p>It is a strongly lensed, <em>and triply mirrored</em>, galaxy.</p>
<p>Close to (but not <em>at</em>) the so-called <em>critical curves</em> of the lens, which is where you in principle will have infinite magnification, on either side you may see multiple images of the same background source. Two images are common, but often you even see three or more images.</p>
<p>Yesterday (yes, things are happening fast right now), <a href="https://ui.adsabs.harvard.edu/abs/2022arXiv220707102P" rel="noreferrer">Pascale et al. (2022)</a> presented a model (built upon the popular Light-Traces-Mass model) of the gravitational lensing of the cluster SMACS J0723.3-7327. They identify 15 new sets of multiply imaged galaxies. The one you ask about is #7 in this set, and the images are then called 7.1, 7.2, and 7.3:</p>
<p><a href="https://i.sstatic.net/ClGMI.jpg" rel="noreferrer"><img src="https://i.sstatic.net/ClGMI.jpg" alt="gal7" /></a>
<sup>From <a href="https://ui.adsabs.harvard.edu/abs/2022arXiv220707102P" rel="noreferrer">Pascale et al. (2022)</a>.</sup></p>
<p>The bright dots in the arc are star-forming regions within the galaxy.</p>
<p>Based on SED fitting (with <a href="https://www.cfht.hawaii.edu/%7Earnouts/LEPHARE/lephare.html" rel="noreferrer">LePhare</a>) to images with JWST's NIRCam, the galaxy has a photometric redshift of <span class="math-container">$z_\mathrm{phot} = 0.38_{-0.13}^{+0.12}$</span>, which would put it very close to the lensing cluster. However, the lensing model returns a redshift <span class="math-container">$z_\mathrm{model} = 2.35_{-0.30}^{+0.33}$</span>, which seems more realistic (although the uncertainties on the photometric redshift is large enough that it could be well behind the cluster).</p>
<p>This redshift corresponds to a distance of
<span class="math-container">$d = 18.9_{-1.3}^{+1.3}$</span> billion lightyears, whereas the foreground cluster is &quot;only&quot; 5.1 billion lightyears away.</p>

CC BY-SA 4.0
2022-07-15T18:25:23.700Z
wow! Amazing!  Two questions: Why does “7.3” appear a little outside of the curve? If I follow the path by eye, I would expect it to appear a little lower. Just an optical illusion?
chaimp
CC BY-SA 4.0
2022-07-15T18:26:48.657Z
and second question: 18.9 billion light years? I though our view of the universe is limited to around 13 billion.
chaimp
CC BY-SA 4.0
2022-07-15T19:49:48.187Z
@chaimp The exact position on the sky depends on the mass distribution of the lens, which can be quite inhomogeneous. Sometimes a third and a fourth image appear on the other side of the cluster, as in the case of [SN-Requiem](https://cosmicdawn.dk/news/astronomers-see-the-same-supernova-three-times-and-predict-a-fourth-to-appear-in-16-years).
pela
CC BY-SA 4.0
2022-07-15T19:54:03.820Z
@chaimp Our "lookback time" is limited to the age of the Universe (13.8 bn yr), but because space expands, galaxies at cosmological distances are carried much farther away than the same number in lightyears. For a galaxy at redshift 2.35, it was 5.6 bn lightyears away when it emitted the light that we see today, but since that galaxy and the Milky Way recede from each other, it took more that 5.6 bn years for the light to reach us. In fact it took 11 bn years. In the meantime, the distance between the two galaxies increased to a current separation of 18.9 bn lightyears.
pela
CC BY-SA 4.0
2022-07-18T02:13:51.147Z
amazing. so much to ponder. Thank you for the clear answers!
chaimp
CC BY-SA 4.0
2022-07-18T07:44:05.510Z
@chaimp My pleasure :)
pela
CC BY-SA 4.0
2022-07-15T13:03:12.767Z
2022-07-15T13:03:12.767Z
pela
<p>Any guesses what this might be?</p>
<p><a href="https://i.sstatic.net/ckJgR.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ckJgR.jpg" alt="Comparison of Webb (left) and Hubble (right)" /></a></p>
<p>Source: <a href="https://twitter.com/c_victor_astro/status/1546641504323633152" rel="nofollow noreferrer">tweet</a></p>

CC BY-SA 4.0
2022-07-12T16:26:02.667Z
I guess it is a very lensed galaxy, similar to others in the picture that are very stretched. I don't know what else it could be
Prallax
CC BY-SA 4.0
2022-07-12T19:38:35.567Z
Agree with @Prallax, it's a lensed galaxy. By eye, it looks like three distinct, star-forming clumps, which are mirrored across a caustic that runs close to the nearby foreground galaxy.
pela
CC BY-SA 4.0
2022-07-12T21:25:54.637Z
Cool question.Please credit the source of the image in your question by adding a link. *Thanks!*
uhoh
CC BY-SA 4.0
2022-07-13T16:14:36.870Z
Thanks for the suggestions.

I thought I did include the source, but it is here: https://twitter.com/c_victor_astro/status/1546641504323633152?s=20&t=DKh-s4Bws3ISvnckB5px2A
chaimp
CC BY-SA 4.0
2022-07-13T21:17:57.113Z
and so I've gone and actually included your image credit in your post. Comments don't count because they are temporary and can be deleted at any time. In Stack Exchange we should include credits for images and other material directly in the post itself. *Thanks!*
uhoh
<hubble-telescope><james-webb-space-telescope>
What is this curve, captured both in the Webb and Hubble images?
chaimp
# What is this curve, captured both in the Webb and Hubble images?

(Asked by: username_0 on 2022-07-12)

Any guesses what this might be?

[![Comparison of Webb (left) and Hubble (right)](https://i.sstatic.net/ckJgR.jpg)](https://i.sstatic.net/ckJgR.jpg)

Source: [tweet](https://twitter.com/c_victor_astro/status/1546641504323633152)

**Tags:** hubble-telescope, james-webb-space-telescope
> **Comments:**
> **username_1**: I guess it is a very lensed galaxy, similar to others in the picture that are very stretched. I don't know what else it could be  
> **username_2**: Agree with @Prallax, it's a lensed galaxy. By eye, it looks like three distinct, star-forming clumps, which are mirrored across a caustic that runs close to the nearby foreground galaxy.  
> **username_3**: Cool question.Please credit the source of the image in your question by adding a link. *Thanks!*  
> **username_0**: Thanks for the suggestions.  I thought I did include the source, but it is here: https://twitter.com/c_victor_astro/status/1546641504323633152?s=20&t=DKh-s4Bws3ISvnckB5px2A  
> **username_3**: and so I've gone and actually included your image credit in your post. Comments don't count because they are temporary and can be deleted at any time. In Stack Exchange we should include credits for images and other material directly in the post itself. *Thanks!*  

---

## 1 answers

### Answer by username_2 on 2022-07-15. Score: 7 | Accepted answer

It is a strongly lensed, *and triply mirrored*, galaxy.

Close to (but not *at*) the so-called *critical curves* of the lens, which is where you in principle will have infinite magnification, on either side you may see multiple images of the same background source. Two images are common, but often you even see three or more images.

Yesterday (yes, things are happening fast right now), [Pascale et al. (2022)](https://ui.adsabs.harvard.edu/abs/2022arXiv220707102P) presented a model (built upon the popular Light-Traces-Mass model) of the gravitational lensing of the cluster SMACS J0723.3-7327. They identify 15 new sets of multiply imaged galaxies. The one you ask about is #7 in this set, and the images are then called 7.1, 7.2, and 7.3:

[![gal7](https://i.sstatic.net/ClGMI.jpg)](https://i.sstatic.net/ClGMI.jpg)
From [Pascale et al. (2022)](https://ui.adsabs.harvard.edu/abs/2022arXiv220707102P).

The bright dots in the arc are star-forming regions within the galaxy.

Based on SED fitting (with [LePhare](https://www.cfht.hawaii.edu/%7Earnouts/LEPHARE/lephare.html)) to images with JWST's NIRCam, the galaxy has a photometric redshift of $z\_\mathrm{phot} = 0.38\_{-0.13}^{+0.12}$, which would put it very close to the lensing cluster. However, the lensing model returns a redshift $z\_\mathrm{model} = 2.35\_{-0.30}^{+0.33}$, which seems more realistic (although the uncertainties on the photometric redshift is large enough that it could be well behind the cluster).

This redshift corresponds to a distance of
$d = 18.9\_{-1.3}^{+1.3}$ billion lightyears, whereas the foreground cluster is "only" 5.1 billion lightyears away.

> **Comments:**
> **username_0**: wow! Amazing!  Two questions: Why does “7.3” appear a little outside of the curve? If I follow the path by eye, I would expect it to appear a little lower. Just an optical illusion?  
> **username_0**: and second question: 18.9 billion light years? I though our view of the universe is limited to around 13 billion.  
> **username_2**: @chaimp The exact position on the sky depends on the mass distribution of the lens, which can be quite inhomogeneous. Sometimes a third and a fourth image appear on the other side of the cluster, as in the case of [SN-Requiem](https://cosmicdawn.dk/news/astronomers-see-the-same-supernova-three-times-and-predict-a-fourth-to-appear-in-16-years).  
> **username_2**: @chaimp Our "lookback time" is limited to the age of the Universe (13.8 bn yr), but because space expands, galaxies at cosmological distances are carried much farther away than the same number in lightyears. For a galaxy at redshift 2.35, it was 5.6 bn lightyears away when it emitted the light that we see today, but since that galaxy and the Milky Way recede from each other, it took more that 5.6 bn years for the light to reach us. In fact it took 11 bn years. In the meantime, the distance between the two galaxies increased to a current separation of 18.9 bn lightyears.  
> **username_0**: amazing. so much to ponder. Thank you for the clear answers!  
> **username_2**: @chaimp My pleasure :)  

---

