A team of astronomers, including Professor Keren Sharon from the University of Michigan, have utilized a massive cluster of galaxies in order to look back in time to the first generation of galaxies. The work of Sharon and her coauthors, led by Matthew Bayliss, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research, was published recently in Nature Astronomy.
An experiment of gravitational lensing
In previous studies, strong gravitational lensing by galaxy clusters was leveraged to observe faint galaxies at optical and infrared wavelengths. However, this is the first time gravitational lensing has been used to peer into distant star formation in the X-ray.
While conducting their research, the astronomers detected an infant galaxy, about 1/10,000 the size of the Milky Way, in its first high-energy stage of star formation. According to Sharon and her colleagues, the detection of this distant galaxy supports the idea that scientists can use galaxy clusters to observe phenomena dating back to the universe’s early history – in this case, nearly 9.4 billion years ago.
Galaxy clusters are the most massive gravitationally bound objects in the Universe, composed of dark matter, hot gas, and hundreds of galaxies. Because they are so massive, their gravity bends space-time, an effect predicted by Einstein’s general relativity. When this happens, light may no longer move in straight lines as its path follows the warped space-time. As a result, the image of background objects appears distorted and magnified. When this phenomena is used to astronomers’ advantage, it is called gravitational lensing.
“We have observed that the Phoenix cluster is causing lensing of several background sources, magnifying them and creating multiple images,” said Sharon. “I can take those observations and use them to solve for how much mass there is in this intervening object and determine the degree of magnification.”
Gravitational lensing essentially allows astronomers to use galaxy clusters as enormous magnifying glasses. If they are able to approximate the mass of a cluster, they can estimate the gravitational effects it may have on background light sources. For instance, the light from an object positioned behind the cluster would travel directly towards the cluster before bending around it and continuing towards the observer, appearing as distorted, magnified images of the object. This helps astronomers study galaxies that would otherwise be too faint for present-day telescopes.
Sourcing the X-rays
While studying gravitationally-lensed faint galaxies is now routine, faint X-ray sources behind galaxy clusters pose an additional challenge. The cores of galaxy clusters are filled with hot gas, which emits brightly in the X-ray. The X-ray light of the background source, although magnified, can be lost in the X-ray light of the foreground cluster.
While examining X-ray data of the Phoenix cluster, Dr. Bayliss realized that there was X-ray emission originating from one of the gravitationally-lensed background galaxies that appeared as a point source.
“This radiation is likely coming not from something that is very X-ray luminous, but from a stellar object or X-ray binary,” said Sharon. “This is very hard to see at such large distances and is only visible in this case because it is being magnified by the Phoenix cluster at a factor of 60. This is the farthest away it’s ever been seen.”
Isolating the background object
In order to achieve a better understanding of the identified X-ray emissions, the team tested whether they could isolate the fainter X-rays coming from the background object. Using data taken by NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, and the Magellan telescope in Chile, the team developed a model to precisely measure the X-ray emissions from the Phoenix cluster and subtract it from the data. What was left were the lensed emissions that they were able to trace back to a tiny dwarf galaxy, originating approximately 9.4 billion years ago.
The researchers further examined the the X-ray, optical, and infra-red emissions coming from this galaxy and were able to measure its physical conditions, such as age, metallicity (the fraction of heavy elements), star formation rate, and mass. The gravitational lensing analysis, led by Sharon, enabled the team to convert the observed measurements to the intrinsic ones one would have measured without lensing magnification. Because this galaxy has low mass and low metallicity, and because X-rays are typically produced during extreme events, the researchers conclude that this galaxy may by similar to the first generation of galaxies formed in the very early universe, and responsible for ionizing it.
“The universe’s first stars and galaxies had enough energetic photons to create larger and larger spheres for ionized materials. After that epoch, light was free to travel through the universe,” said Sharon. “It’s interesting to find galaxies that are analogs of those first galaxies in an epoch where we can see them.”