Well, I've been doubly beaten to the punch...

StupendousMan: your link is to a radial search page. Here is the link to the object's explore page (the top link from your link). The width of those emission lines says one thing: quasar, as ngc3314 says. My first thought was that that was a particularly large spiral galaxy, considering its angular size at a redshift of 0.24, but it's only ~35 kpc, which isn't that big.

Definitely a lot of galaxies shining through ngc4921, there. Wow. What an image. Thanks for the post.

For comparison, here's a 1-1 pixel image from SDSS, covering nearly the same field (it looks like the HST image might be rotated slightly). Same size of telescope (~2.4 meters), just inside the Earth's atmosphere, and only a 1 minute exposure instead of 17 hours.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report