Subscribers help me peer into the guts of the Universe

Galaxy clusters make my ape brain hurt.

A galaxy is an immense thing, a vast collection of billions — hundreds of billions — of stars, as well as gas, dust, and dark matter. They are thousands of light-years across, with our own Milky Way a soul-crushing 120,000 light-year diameter.

Yet, there are bigger structures. Galaxy clusters are themselves collections of galaxies, some with hundreds, some with over a thousand. They can stretch for millions of light-years and dominate all of space around them. 

Abell S1063 is one such cluster. It’s a long way off, about 4.5 billion light-years, so the light we see from it left the cluster when the cosmos itself was about 2/3^rd its present age. It is colossal, with at least 1,234 member galaxies.

Hubble observed it in 2016, and the image was amazing. But now JWST has taken a peek at it, and the image is jaw-dropping. Prepare your brain, but you cannot. You cannot.

See? SEE? I told you. [Grab a higher-res 7,000 x 6.600 pixel version here, because yegads.]

This image is composed of over 500 separate exposures using 9 different filters, with a total exposure time of about 120 hours — 5 days. It’s the longest, deepest exposure of a single target ever done by the infrared space observatory. Incidentally, this is only half of the science data: JWST NIRCAM (Near-Infrared Camera) has two modules, like having two cameras in one, that see two fields of view next to each the in the sky. This image is from is Module B, and you can get a glimpse of Module A in this paper about the observations.

In the center of the cluster is what’s called a cD galaxy, a central dominant galaxy. These are common in cluster cores, because galaxies in the cluster collide and fall to the center, so a galaxy situated there just grows and grows. I’m not sure what the glow around it is; it might be the extended halo of stars surrounding the cD or it might be material from all the other galaxies floating around the center; that’s where the gravity well peaks so stuff tends to fall into the central region.

Mind you, almost everything you see in this image is a galaxy. The sharp points with the six spikes are mostly stars, though some have fuzzy centers and those are almost certainly active galaxies: every galaxy has a supermassive black hole in its center, but if matter is falling into the black hole it piles up in a flat disk just outside the Point Of No Return. That material heats up due to friction and gets so hot and so bright it can easily outshine all the stars in the rest of the galaxy combined. There are quite a few in the image.

The arcs are amazing. Those are gravitational lenses, which I’ve described many times before in the newsletter, notably here and here. The combined mass of the galaxies in the cluster bends space, and light from a more distant background galaxy travels through that warped space on its way here. This can have many effects, such as creating multiple images of the background galaxy, and distorting it into arcs or even rings.

I worked on an early Hubble image of a cluster with lensed arcs in it, and thought it was so cool that we found 5 arcs in it. Five.

Yeah.

Some of the arcs in this JWST image are likely multiple images from the same background source, but there are so many arcs there must be quite a few more distant galaxies seen. In fact this is one reason this image was taken! Galaxies can be so far away that even JWST has little hope of seeing them, but if they happen to be on the other side of a huge cluster, their light can be magnified enough by gravitational lensing for us to see them. In fact, in the Module A image (the one not displayed here) are two incredibly faint galaxies that have been magnified modestly by the cluster, and are candidates for the most distant galaxies ever seen, with redshifts around z ~16 (see here for an explanation of what that means). We see them as they were just 400 million years or so after the cosmos itself was born! Mind you, this has not yet been confirmed… and confirming it will be tough since they’re so faint.

There’s a lot more to analyze in the image, too. The galaxies in it range from relatively close to us to incredibly distant, which means we can use them to probe what the Universe was like at different ages. Things were very different early on; the cosmos was opaque for a long time, then became transparent in a pair of events called recombination and reionization. Very early on the cosmos was so hot that electrons were easily stripped off of atoms by high-energy light. Free electrons are really good at absorbing light, so the Universe was opaque then. But, as the Universe expanded and cooled, the atoms and electrons could combine and stay that way (this is called recombination for historical reason). The cosmos became transparent.

Then, later on, powerful sources of ultraviolet light were created that zapped the gas in the Universe, ripping off electrons from their atoms once again. However, by that time the expansion had dropped the overall density of the Universe so much that there wasn’t as much material to absorb light, making it possible for the kind of light we see to travel long distances. That’s why we can see so far back in the first place! We don’t know if these UV sources were supermassive stars or black holes gobbling down matter, and observations like this one can possibly help us figure that out.

JWST has only been in space for 3.5 years, and is mostly used to look at specific objects for relatively short amounts of time, since a lot of astronomers want time on it. It’s unusual to get one with such a long exposure, but as time goes on and JWST keeps operating — assuming Trump doesn’t cancel it — it will likely do more deep images like this. What will we see next?

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