Is the actual comparison HST image being used here from the hubble deep field?
EDIT: Doesn't look like it is -- it is from a more recent 2019-published study of SMACS J0723.3-732 as part of the Reionization Lensing Cluster Survey (RELICS).
Although that paper does mention that this image is the deepest image of the Universe to date, and that the Fine Guidance Sensor image may be the second deepest, both exceeding the Hubble Deep Field image.
EDIT: Doesn't look like it is -- it is from a more recent 2019-published study of SMACS J0723.3-732 as part of the Reionization Lensing Cluster Survey (RELICS).
Some background info in this paper:
https://arxiv.org/pdf/2207.05007.pdf
Although that paper does mention that this image is the deepest image of the Universe to date, and that the Fine Guidance Sensor image may be the second deepest, both exceeding the Hubble Deep Field image.