PKS 2126-158 is a radio source that was discovered by Parkes Observatory in Australia in the 1960s [1]. As best as I can tell, the letters PKS in this object's name stand for Parkes, although I'm not 100% certain about that, and the digits refer to the obejct's location in the 1950 coordinate system.
Like a lot of radio sources discovered by Parkes Observatory, PKS 2126-158 is a quasar, or a type of galaxy containing an active galactic nucleus (AGN). To review very quickly, an AGN consists of a supermassive black hole, a disk of gas falling into that black hole, and jets of gas emerging above the black holes that come from gas in the disk that got very hot and then was deflected away from the black hole's event horizon by its magnetic fields. One of the jets from PKS 2126-158 appear to be aimed roughly in the direction of the Earth, making the quasar appear very bright and very point-like.
One of the things that makes PKS 2126-158 distinctly different from most (but not all) of the other active galactic nuclei that I've mentioned in past episodes is that the light from the quasar is passing through huge amounts of intergalactic gas to reach the Earth [2, 3, 4, 5, 6]. Keep two key concepts in mind while I discuss this topic. First, because of the expansion of the universe, almost all galaxies appear to be moving away from our own, and things that are further away appear to be moving away faster than things nearby. This will stretch out the wavelengths of light from these objects. Second, individual elements within gas in space will absorb and emit light at very specific wavelengths. So, for example, hydrogen gas will absorb and emit light at 1216, 4861, and 6563 Å, among other wavelength.
PKS 2126-158 is located at a distance where and the light from the quasar has been stretched out to 4.29 times its original size [7]. The light has been travelling for 11.8 billion years to reach the Earth, but for complex reasons involving the curvature of space-time, this can't exactly be described as a distance of 11.8 billion light years. The hydrogen gas within PKS 2126-158 emits light at 1216 Å that is seen at 5212 Å on Earth. Sitting between PKS 2126-158 and the Earth are a bunch of intergalactic gas filaments as well as a few galaxies, and because those galaxies are not as far away, they aren't moving as quickly. The hydrogen gas in these filaments and galaxies will absorb light at 1216 Å, but as seen on Earth, the absorbed light is at various wavelengths between 1216 and 5212 Å. By looking at where light is absorbed in a spectrum of the quasar, astronomers can reconstruct the structure of interstellar gas between the quasar and the Earth.
This could be thought of as analogous to looking at someone else shining a flashlight through a room filled with cobwebs and the occasional spider. The flashlight in this case represents the quasar PKS 2126-158. The spiderwebs between the flashlight and you represent the intergalactic filaments absorbing light from the quasar. The occasional spider might represent occasional galaxies between the quasar and Earth, and they would absorb more light than the average filament. However, in space, galaxies are expected to form from intergalactic filaments of gas, whereas spiders would have made the spiderwebs in the hypothetical room I'm talking about, so this analogy breaks down a little. Also, that room with the spiderwebs and the person with the flashlight would have to be expanding to make this analogy work.
Anyway, one of the really cool things that astronomers found between us and PKS 2126-158 is an entire group of galaxies [6]. Going back to the analogy with the room filled with spider webs, this could be thought of as like finding a nest of spider eggs in between us and the flashlight, but fortunately, the spider eggs are moving away from us. Anyhow, this group of galaxies is located at a distance where the light has travelled 6.1 billion years to reach the Earth, and the group absorbs light at wavelengths of light that appear to be stretched out to 1.66 times their original size [7]. Interestingly, magnesium and calcium within the interstellar or intergalactic gas within this cluster is also absorbing light from PKS 2126-158 [7].
Based on what I could see in the scientific literature, it has become rather popular to look for galaxies in front of quasars that contain lots of spiders. No, wait, ignore the spiders part. It has become rather popular to look for galaxies in front of quasars that contain lots of magnesium absorbing light from those quasars, and the group of galaxies in front of PKS 2126-158 is now a major part of that research.