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Any guesses what this might be?

Comparison of Webb (left) and Hubble (right)

Source: tweet

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    $\begingroup$ 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 $\endgroup$
    – Prallax
    Jul 12, 2022 at 16:26
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    $\begingroup$ 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. $\endgroup$
    – pela
    Jul 12, 2022 at 19:38
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    $\begingroup$ Cool question.Please credit the source of the image in your question by adding a link. Thanks! $\endgroup$
    – uhoh
    Jul 12, 2022 at 21:25
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    $\begingroup$ Thanks for the suggestions. I thought I did include the source, but it is here: twitter.com/c_victor_astro/status/… $\endgroup$
    – chaimp
    Jul 13, 2022 at 16:14
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    $\begingroup$ 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! $\endgroup$
    – uhoh
    Jul 13, 2022 at 21:17

1 Answer 1

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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) 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 From Pascale et al. (2022).

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

Based on SED fitting (with LePhare) 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.

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  • $\begingroup$ 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? $\endgroup$
    – chaimp
    Jul 15, 2022 at 18:25
  • $\begingroup$ and second question: 18.9 billion light years? I though our view of the universe is limited to around 13 billion. $\endgroup$
    – chaimp
    Jul 15, 2022 at 18:26
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    $\begingroup$ @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. $\endgroup$
    – pela
    Jul 15, 2022 at 19:49
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    $\begingroup$ @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. $\endgroup$
    – pela
    Jul 15, 2022 at 19:54
  • $\begingroup$ amazing. so much to ponder. Thank you for the clear answers! $\endgroup$
    – chaimp
    Jul 18, 2022 at 2:13

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