Astronomers use giant galaxy cluster to magnify X-ray emissions of an early-forming galaxy

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 0.5–7 keV (left) and Hubble optical (right) images of the giant arc at a scale 1.5 times larger.

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 objects in the universe. Because they are so large and composed of many galaxies all bound together by gravity, their gravitational pull is often so strong that it can bend space-time. When this happens, light may no longer move in straight lines as its path is warped by the gravitational pull of the cluster. As a result, the light from background objects is magnified. When this phenomena is used to astronomers’ advantage, it is called gravitational lensing.

Galaxy clusters are the most massive gravitationally bound objects in the Universe, composed of dark matter, hot gas, 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 emission 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.”

by Nicholle Cardinal

Source: LSA/Astronomy - University of Michigan/News

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More Information:

• Professor Keren Sharon
• Link to Published Study
• Link to MIT Press Release

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Massive Cluster Galaxies Move in Unexpected Ways

Figure 1. MS0440+02 galaxy cluster. The central galaxy is a multi-component BCG formed by six bright elliptical spheroids, all at the same redshift. This is a color composite GMOS South image (g, r, i) of the clusters. The size of the image is 2.6 x 2.6 arcmin² (N up, E left). Credit: R. Carrasco (Gemini Observatory/AURA) and Tomás Verdugo (UNAM).  Full resolution TIFF

Figure 2. The slope η of the velocity dispersion profile is plotted against the central velocity dispersion σ₀ for galaxies in multiple different samples. The blue points represent brightest galaxies in groups (BGGs) of high (square) and low (circles) density, while the green, red, and yellow points represent brightest cluster galaxies (BCGs) in various samples of galaxy clusters. The grey points indicate generic “early-type galaxies” (ETGs). The slope η is negative if the velocity dispersion decreases with radius and positive if it rises. Thus, massive BCGs tend to have rising profiles, with the stellar velocities responding to the cluster potential at larger radii. [Reproduced from Loubser et al. 2018, MNRAS, in press.]  Full resolution JPEG

Astronomers using data from both of the Gemini Multi-Object Spectrographs (GMOS - North and South) measured the motions of stars within a sample of 32 massive elliptical cluster galaxies and found the stellar motions inconsistent with these galaxies’ solitary cousins. 

The galaxies chosen are known as brightest cluster galaxies (BCGs) because they are the brightest members of large galaxy clusters. The international team of astronomers obtained Gemini spectra to find the relative velocities of stars within each galaxy and then determine the central stellar velocity dispersions and radial dispersion profiles for each galaxy. “This is similar to what we see in our own Solar System with the different velocities of the planets around the Sun,” said John Blakeslee, Gemini Observatory's Head of Science. “We use the planets’ velocities to determine our Solar System’s mass distribution and it is also how we know the Sun’s mass accurately.”

The researchers discovered a surprising variety in the shapes of the velocity dispersion profiles for the BCGs, with a large fraction showing rising dispersion profiles (Figure 2). A rising velocity dispersion profile means that the stars within these galaxies are moving faster as you look further from the galaxy’s core in response to an increasing gravitational force. In comparison, rising velocity dispersion profiles are much rarer in other massive ellipticals that are not BCGs, including many brightest galaxies in groups (BGGs).

“You would naively think that massive elliptical galaxies are a homogeneous, well-behaved class of objects, but the most massive beasts, those in the centers of groups and clusters, continue to surprise us,” said Ilani Loubser, an astronomer at North-West University in South Africa and the lead author of the study, which has been accepted for publication in Monthly Notices of the Royal Astronomical Society. She also noted, “The quality, and the wealth of information we can measure from the GMOS spectra (even in poor weather), is remarkable!”

BCGs tend to reside near the centers of their respective clusters, and are therefore generally embedded within extended distributions of both light and dark matter. The sample of BCGs in this study included some of the most massive known galaxies in the Universe out to a distance of about 3.2 billion light years (z ~ 0.3).

The study also found that the slopes of the velocity dispersion profiles correlate with the galaxy luminosity, in the sense that the increase in the speed of the stars is greater in brighter BCGs, as well as BGGs. Whether the full diversity in the observed velocity dispersion profiles is consistent with standard models for the growth of massive galaxies is not yet clear. More detailed comparisons with velocity dispersion profiles in cosmological simulations are needed.

Source:

Gemini Observatory