Object 61: SPT-CL J0546-5345
Podcast release date: 29 November 2021
Right ascension: 05:46:35.0
Corresponding Earth location: About 850 km east of Heard Island in the Indian Ocean
If you live in the Northern Hemisphere and you aren't familiar with the constellation Pictor, that is because the constellation is located very far to the south and isn't visible from most of the Northern Hemisphere. If you live in the Southern Hemisphere and you aren't familiar with the constellation Pictor, that is because it is a collection of stars fainter than magnitude 3 that don't look like anything. This constellation was invented by Nicolas Louis de la Caille in the eighteenth century, apparently to fill a relatively empty space in the sky. It's supposed to look like a painter's easel. It's another relatively stupid constellation, and I think it could be described as worse than the constellation Cetus, which you would have heard me complain about in a few previous episodes.
Anyway, the coordinates for this episode point specifically to the cluster of galaxies SPT-CL J0546-5345. The SPT stands for the South Pole Telescope. The CL stands for cluster, or at least I think that's what it's supposed to stand for. The letter J and the numbers at the end of the name for this object give the object's coordinates.
So let me take a few minutes to describe the South Pole Telescope, which discovered SPT-CL J0546-5345, before I discuss the object itself. If you live in the Northern Hemisphere and you aren't familiar with the South Pole Telescope, that is because the telescope is located very far to the south and isn't visible from most of the Northern Hemisphere. If you live in the Southern Hemisphere and... No, I already read that. Well, as you may or may not expect, the South Pole Telescope is located at the actual South Pole. It's one of the facilities at the Amundsen-Scott South Pole Station. The telescope has a 10 meter wide curved reflector that forms an L shape with a rectangular box-like structure containing the detectors and a curved shield that block radiation from the ground . The telescope is designed to observe submillimeter and millimeter emission, which are forms of electromagnetic radiation with wavelengths of either less than a millimeter (in the case of submillimeter radiation) or more than a millimeter (in the case of millimeter radiation). Since water vapor in the atmosphere can both absorb and emit this radiation, astronomers need to place submillimeter and millimeter telescopes in locations where the air is dry. Although Antarctica is covered in ice, the air above Antarctica is free of water vapor, so it's a good place for this type of telescope.
The South Pole Telescope has not been used to perform general observations of astronomical objects in the sky but instead has been used to make a few specific types of special observations, and one of the things that this telescope is used for is to find clusters of galaxies . Astronomers actually have three ways of finding these clusters. The most seemingly straightforward way to find them is to look at the sky in the visible part of the electromagnetic spectrum and to identify locations which have more galaxies than usual. However, clusters of galaxies also contain very thin but very hot ionized gas that produces lots of X-rays, so a second way to identify clusters of galaxies is to just search for this X-ray emitting gas. The South Pole Telescope uses a third technique that is slightly more complicated. The Big Bang produced an afterglow of light that we see in the form of millimeter radiation across the sky that is called the cosmic microwave background, but when that radiation passes through the ionized gas in clusters of galaxies, the wavelength of the radiation changes. This effect is called the Sunyaev-Zeldovich effect after the Russian astrophysicists who first described this phenomenon . If astronomers can find places where the cosmic microwave background has changed in wavelength, they can identify where clusters of galaxies are located.
So, in 2008, astronomers performed a survey of the southern sky with the South Pole Telescope to identify clusters of galaxies by looking specifically for places where, because of the Sunaev-Zeldovich effect, the cosmic microwave background had been altered by passing through the intracluster gas in the clusters . They found a total of 21 clusters of galaxies this way, and SPT-CL J0546-5345, the subject of this podcast episode, was one of them. However, this cluster (which I am going to call J0546 for short in the rest of the podcast) did not necessarily stand out right away, but the astronomers did a series of follow-up observations of J0546 in other parts of the electromagnetic spectrum to look at the individual galaxies in the cluster and to look at the X-ray emission from the gas in between the galaxies, and they discovered that J0546 was actually a very special cluster.
First of all, J0546 is located relatively far away for a cluster of galaxies. The distance could kind of be described as 8.1 billion light years, although because of relativity effects, it's more accurate to say that the light emitted from J0546 took 8.1 billion years to travel to our galaxy . Professional astronomers would actually express this distance in terms of how much the light from the cluster gets stretched out because of the expansion of the universe. Things located further from our galaxy appear to be moving faster from us than things closer by, so the amount that the wavelengths of light from an object is stretched is directly related to how far away the object is. This stretching is expressed in terms of an equation where the ratio of the stretched wavelength of light observed on Earth to the original wavelength size is equal to 1+z. z is in effect directly related to distance. In the case of J0546, z is equal to 1.067, which means that the cluster is located so far away in a place where the universe appears to be expanding so fast that the wavelengths of light reaching our galaxy are slightly more than twice as long as when they were emitted by the cluster.
Regardless of how the distance is described, this is actually quite far for a cluster of galaxies. In fact, J0546 was the first cluster identified using the Sunyaev-Zeldovich effect at a z greater than 1. Most other clusters are much closer to our galaxy. If you think of distance as inversely related to how old the universe was, then we are seeing J0546 when the universe was 5.6 billion years old, which means that it took less than this amount of time for it to form. I know that this sounds like a really big number, but in terms of forming something which is basically made of hundreds of galaxies, that's really fast. In contrast, we are seeing most other clusters of galaxies when the universe was older, which means that it took those clusters of galaxies a few extra billion years to form.
J0546 is also relatively large for a cluster of galaxies. It has a mass of about one quadrillion times the mass of our Sun, which is equivalent in size to some of the larger clusters of galaxies seen near our own galaxy . Keep in mind that the nearby clusters had more than 13 billion years to get to their current size, so for J0546 to get to the same size in 5.6 billion years, it had to grow very fast. Clusters of galaxies continue to grow in size over time, so if we could actually see J0546 in the present (or, in other words, 13.7 billion years after the Big Bang), then it would be one of the largest clusters in existence.
So, J0546 is a large cluster of galaxies that formed abnormally quickly and that we see from Earth at a time when the universe was much younger. It was one of the first examples of such a large cluster at these distances, although, since the discovery of J0546, astronomers have found many similarly large but distant clusters of galaxies. Like I said a minute or two ago, it normally takes a lot of time following the Big Bang to form a cluster of galaxies. J0546 and these other distant but large clusters do not quite break any theories about the Big Bang because everybody expects that the universe was a little lumpy after it formed, and large clusters like J0546 could have formed in place where the Big Bang left relatively large lumps of primordial matter. Having said that, these really large but distant clusters of galaxies provide constraints on models of how the universe formed from the Big Bang and how galaxies and clusters of galaxies have evolved over time. This makes J0546 very important in understanding the evolution of our universe.
 Carlstrom, J. E. et al., The 10 Meter South Pole Telescope, 2011, Publications of the Astronomical Society of the Pacific, 123, 568
 Sunyaev, R. A. and Zeldovich, Ya. B., The Observations of Relic Radiation as a Test of the Nature of X-Ray Radiation from the Clusters of Galaxies, 1972, Comments on Astrophysics and Space Physics, 4, 173
 Vanderlinde, K. et al., Galaxy Clusters Selected with the Sunyaev-Zel'dovich Effect from 2008 South Pole Telescope Observations, 2010, Astrophysical Journal, 722, 1180
 Brodwin, M. et al., SPT-CL J0546-5345: A Massive z>1 Galaxy Cluster Selected Via the Sunyaev-Zel'dovich Effect with the South Pole Telescope, 2010, Astrophysical Journal, 721, 90