Cool, Relaxed, but Way Out of Its Lane: The Most Distant Cooling-Flow Galaxy Cluster Yet Observed

Title: SPT-CL J2215-3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster

Authors: Michael S. Calzadilla, Lindsey E. Bleem, Michael McDonald, Michael D. Gladders, Adam B. Mantz, Steven W. Allen, Matthew B. Bayliss, Anna-Christina Eilers, Benjamin Floyd, Julie Hlavacek-Larrondo, Gourav Khullar, Keunho J. Kim, Guillaume Mahler, Keren Sharon, Taweewat Somboonpanyakul, Brian Stalder, Antony A. Stark

First Author’s Institution: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA USA

Status: Accepted to ApJ [open access]

The largest gravitationally-bound structures in the universe are galaxy clusters – hundreds to thousands of individual galaxies, bound together by gravity and surrounded by dark matter and gas. Lurking at the center of these galaxy clusters are the brightest galaxies in the universe: the aptly-named Brightest Cluster Galaxies (BCGs).

Because BCGs are right at the center of their galaxy cluster’s gravitational field, the clusters themselves act like a well, funneling new material onto the BCGs. This means that BCGs grow very large, and that the evolution of the BCG is intimately linked to the evolution of the full galaxy cluster. Traditionally, it has been thought that the clusters feed the BCG with other galaxies full of pre-made stars (this mechanism is referred to as, no joke, galactic cannibalism – here’s a video on the topic). However, many BCGs have been observed to actively grow new stars. In these cases, it seems like the cluster is feeding free-floating gas known as Intracluster Medium (ICM) to the BCG. This ICM contains the ingredients for star formation (mostly hydrogen gas), and the BCG can process these ingredients into new stars itself.

Going with the Flow

In order for this second type of BCG growth to be happening, the galaxy cluster needs to obey some fairly specific criteria. There needs to be one specific BCG in the cluster, and it needs to be sitting at the center of the cluster’s gravitational field, in order to direct the ICM onto the BCG. The cluster itself also needs to be large enough to have a lot of concentrated ICM, and old enough that a lot of the initial heat (kinetic energy) in the ICM has had time to escape (if not, it will be moving around too fast to get caught by the BCG’s gravity). These types of clusters  are known as ‘cool-core’ clusters, and the flow of ICM onto the BCG is known as a ‘cooling flow’. All of these things typically happen naturally in clusters, but they all require time, so they’re far more common in much older clusters at times much closer to the present day. These clusters are called ‘relaxed.’ That’s what makes today’s paper so exciting – the authors of this paper have found a relaxed cluster funneling material onto its BCG at redshift 1.16 (only about 5.3 billion years after the big bang). This is the earliest example of such a cluster found to date – this cluster must have relaxed faster than previously thought possible.

A series of five progressively more zoomed-in images. The first, corresponding to the 0.9 GHz ASKAP radio survey, shows a blurry unresolved blob about 500 kpc in diameter. The second shows Chandra X-ray emission covering around 400 kpc, strongly peaked in the central 120 kpc. The third shows a multicolour HST image of the cluster, with the BCG in the central 25 kpc. The fourth shows the 25 kpc BCG as a blurry ellipse in the HST optical imaging, and the fifth is showing HST UV imaging on the scale. In the UV imaging, the BCG appears smaller, and several filaments extend outwards from the BCG.
Figure 1: Some of the wide variety of observations required to study this galaxy cluster. Clockwise from the top left, they are: the ASKAP radio observations showing the AGN, the Chandra X-ray observations showing the ICM, an HST composite imaging showing the cluster, and zoom-ins on the BCG in optical and UV wavelengths (respectively), also from HST. Figure 1 of the paper.

The Picture(s) of Relaxation

This cluster, known as SPT-CL J2215-3537, or SPT2215, was originally found using the Sunyaev-Zeldovich effect in a South Pole Telescope survey. Optical and UV imaging (Figure 1) from the Hubble Space Telescope and the Magellan Telescopes were used to find the galaxies associated with the cluster, and optical spectroscopy from Magellan was used to make sure that the galaxies were all associated with the cluster in all three dimensions. This optical spectroscopy also measured the distance to the cluster, using redshifting of spectral lines, and therefore confirmed that we’re observing this cluster earlier in the universe’s history than any other cluster of its kind. A faint radio-wavelength source (probably an Active Galactic Nucleus, or AGN) was also found to be associated with the cluster using the ASKAP array (Figure 1).

A Cool Customer

A plot of radius (in kpc) versus the Boltzmann constant k times temperature T (in keV). Grey points show actual measurements of the cluster, which start at 6 keV at 10 kpc, and increase steadily to 16 keV at 500 kpc. The green fit overlaps very well with the data points.
Figure 2: The temperature profile of the ICM of the galaxy cluster, measured from the X-ray observations shown in Figure 1. The temperature is shown in energy units, because in this case it’s essentially a measure of the kinetic energy of the gas. The grey line shows the actual data points, and the green region is a fit to a known model of the temperature profile in cool-core clusters. Adapted from Figure 3 of the paper.

The ICM is very diffuse, and it isn’t typically visible in optical or UV measurements. In order to measure this cluster’s ICM properties, the authors had to take observations using the Chandra X-ray observatory. From this, they noticed that the ICM is distributed extremely regularly in the cluster, and that the ICM’s luminosity peaks very strongly in the center. As mentioned above, both of these characteristics are good indicators that the cluster is relaxed. The authors also measured the spectrum of the X-rays in order to determine the temperature of the ICM. By measuring different X-ray spectra at different distances from the center of the cluster, the authors developed a temperature profile (Figure 2). This showed that the ICM in the middle of the cluster in particular had a very low temperature, making it a cool-core cluster. Filaments of gas are also visible surrounding the BCG in the UV imaging from HST, suggesting that ICM is indeed falling onto the BCG.

Relaxed, but Working Hard

Finally, the authors measured a Spectral Energy Distribution (SED) of the BCG itself. This is a technique where the amount of light emitted from a galaxy is measured at as many different wavelengths as possible, and then the luminosity at these different wavelengths is compared. Different components of a galaxy (such as new stars, old stars, or gas) emit light at different wavelengths, so scientists can estimate how fast a galaxy forms stars by fitting measurements to models of these different components. In this case, the authors used the HST and Magellan measurements mentioned above (at optical and UV wavelengths), additional near-infrared Magellan measurements, and far-infrared (very long-wavelength) Spitzer Space Telescope observations to construct their SED. From the SED, they determined that the BCG in this cluster was forming 320 solar masses worth of new stars every year (about 300x the Milky Way’s rate)!

A plot of flux density (ergs per second per centimeter squared per angstrom) as a function of wavelength (angstrom). There are nine measurements, stretching from 4000 angstrom to 50000 angstrom, and an optical spectrum. The fitted model points agree very well with the observed points.
Figure 3: The Spectral Energy Distribution of the BCG inside SPT2215. The blue points show the observed values for this galaxy, and the red points show the model that was fit to these values. Using this technique, the authors determined that the BCG is forming stars at a much higher rate than expected. Figure 4 of the paper.

All of this evidence seems to point to a BCG forming stars out of fuel from the cluster itself. If this is the case, this will be the earliest-ever example of such a cluster, and it has some pretty exciting implications. The authors suggest that clusters relaxing this quickly may have a totally separate mechanism for BCG formation, independent from the cannibalism-driven growth we expect. It also implies that AGN (such as the one seen in the ASKAP imaging of this cluster in Figure 1) could start powering on earlier than expected, feeding energy back into the BCG and the cluster and disrupting star formation. The authors are working with more X-ray observations to characterize the physics of this cluster more precisely, and hopefully figure out some of the specifics of these implications.

Astrobite edited by William Lamb

Featured image credit: Galaxy Optical: National Optical Astronomy Observatory/Kitt Peak, Galaxy X-ray: NASA/Chandra X-ray Center/IoA 

About Delaney Dunne

I'm a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency Line Intensity Mapping experiment based in California's Owens Valley.

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