Astronomers have used a huge group of galaxies called the Phoenix cluster as a cosmic magnifying glass to detect X-ray emission from a very distant dwarf galaxy in its very first, high-energy stages of star formation.
False-color and X-ray images of the giant arc in SPT-CLJ2344-4243: the X-ray-emitting giant arc is shown relative to the center of the foreground lensing galaxy cluster in a false-color image at optical wavelengths; the inset shows Chandra X-ray (left) and Hubble optical (right) images of the giant arc at a scale 1.5 times larger; the optical colors here are given by Hubble imaging data in the F850LP (red), F775W (green) and F475W (blue) filters; two magenta circles indicate the locations of the X-ray emission from the giant arc in both inset panels; the lensing geometry of the giant arc is a pair of merging images, where the lower and upper halves of the arc are each a single image with mirror symmetry. Image credit: Bayliss et al, doi: 10.1038/s41550-019-0888-7.
Astronomers have used galaxy clusters to magnify objects at optical wavelengths, but never in the X-ray band of the electromagnetic spectrum, mainly because galaxy clusters themselves emit an enormous amount of X-rays. They have thought that any X-rays coming from a background source would be impossible to discern from the cluster’s own glare.
“If you’re trying to see an X-ray source behind a cluster, it’s like trying to see a candle next to a really bright light. So we knew this was a challenging measurement to make,” said Dr. Matthew Bayliss, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research.
Dr. Bayliss and his colleagues wondered: could they subtract that bright light and see the candle behind it? In other words, could they remove the X-ray emissions coming from the galaxy cluster, to view the much fainter X-rays coming from an object, behind and magnified by the cluster?
They tested this idea with observations taken by NASA’s Chandra X-ray Observatory. They looked in particular at Chandra’s measurements of the Phoenix cluster, also known as SPT-CLJ2344-4243, a distant galaxy cluster located 5.7 billion light-years from Earth.
“The idea is to take whatever your best X-ray telescope is — in this case, Chandra — and use a natural lens to magnify and effectively make Chandra bigger, so you can see more distant things,” Dr. Bayliss said.
The astronomers analyzed observations of the Phoenix cluster, taken continuously by Chandra for over a month.
They also looked at images of the cluster taken by two optical and infrared telescopes — the NASA/ESA Hubble Space Telescope and the Magellan telescope in Chile.
With all these various views, the team developed a model to characterize the cluster’s optical effects, which allowed the researchers to precisely measure the X-ray emissions from the cluster itself, and subtract it from the data.
They were left with two similar patterns of X-ray emissions around the cluster, which they determined were lensed by the cluster.
When they traced the emissions backward in time, they found that they all originated from a single, distant source: a tiny dwarf galaxy — about 1/10,000 the size of our Milky Way — from 9.4 billion years ago, when the Universe itself was roughly 4.4 billion years old.
The combination of Chandra and the Phoenix cluster’s lensing power enabled the team to see the tiny galaxy hiding behind the cluster, magnified about 60 times. At this resolution, they were able to zoom in to discern two distinct clumps within the galaxy, one producing many more X-rays than the other.
As X-rays are typically produced during extreme, short-lived phenomena, the astronomers believe that the first X-ray-rich clump signals a part of the dwarf galaxy that has very recently formed supermassive stars, while the quieter region is an older region that contains more mature stars.
“We’re catching this galaxy at a very useful stage, where it’s got these really young stars,” Dr. Bayliss said.
The team’s paper was published in the journal Nature Astronomy.
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M.B. Bayliss et al. An X-ray detection of star formation in a highly magnified giant arc. Nature Astronomy, published online October 14, 2019; doi: 10.1038/s41550-019-0888-7
