Euclid hunts down stars gone wild

Title: Euclid: Early Release Observations – The intracluster light and intracluster globular clusters of the Perseus Cluster

Authors: M. Kluge, N.A. Hatch, M. Montes, J.B. Golden-Marx, A.H. Gonzalez, J.-C. Cuillandre, M. Bolzonella, A. Lançon, et al

First Author’s Institution: Max Planck Institute for Extraterrestrial Physics, Garching, Germany

Status: Submitted to A&A [open access]

It’s always super exciting when a new telescope goes online. It opens up a whole new world of scientific possibility, that both professional scientists and the public are excited to get in on! Often, a new telescope shares a few things to prove its capabilities: some press-ready photos, for the general public (here are JWST’s!), and some early papers, for scientists. These papers are designed to show the scientific community what the telescope is uniquely good at, and get them thinking about how they might like to use the telescope in the future. In today’s bite, we’re looking into one of the suites of ‘Early Release Observations’ released for Euclid, the new space telescope launched by the European Space Agency (ESA). Euclid launched just over one year ago, and is already providing incredible scientific data.

One of the things that Euclid is good at (especially compared to other space telescopes like Hubble or JWST) is low surface-brightness (LSB) astronomy. This is the study of things that are both faint and big on the sky (have large angular scales) – things like ultra-diffuse galaxies. LSB astronomy requires a very wide field of view and tight control over the point spread function of the telescope, since traditional techniques for measuring brightness require calibrating against the background of an image. This isn’t possible when the background of the image is the thing you’re trying to measure!

Figure 1: A colour image of the Perseus cluster of galaxies, as imaged by Euclid.

In particular, today’s paper focuses on the low surface-brightness intracluster light (ICL) in the Perseus cluster of galaxies (shown in Figure 1). This is light from the galaxy cluster that isn’t associated with any galaxy. Instead, it comes from stars (or entire globular clusters) that used to belong to the galaxies making up the galaxy cluster, but have since been stripped of their outskirts or completely destroyed by the process of galaxy cluster formation. Because stars aren’t really affected by their outside conditions once they’re already formed (unlike things like gas, which heats and shocks), the ICL is a good representative of what these stars looked like when they were formed, and thus what kinds of galaxies had to fall into the galaxy cluster to create the ICL. Because of this, the ICL is a great fossil record for the galaxy cluster as a whole.

Studying the ICL in the Perseus cluster is a unique challenge for two reasons – the galaxy cluster’s size and location. The Perseus cluster is enormous on the plane of the sky. It covers about 82 arcminutes, which is almost 3 times the size of the moon! This means it’s very difficult to get the full ICL of the Perseus cluster into the field of view of a single telescope image. Perseus is also located very near to the galactic plane, which means studying its LSB parts is significantly complicated by galactic cirrus emission – emission from dust in clouds in our own Milky Way. Still, the Perseus cluster is a very well-known and well-studied galaxy cluster (here’s another astrobite looking at Perseus), so it’s very worth it to look into it more.

Thanks to Euclid’s wide field of view (nearly 25x that of JWST) the authors were able to get almost all of the Perseus cluster into the same image, and thus get a great picture. They also were able to clean out the galactic cirrus using imaging from WISE, an infrared space telescope. Most of the light in a 12 μm infrared image comes from galactic cirrus, so the authors were able to use known relationships between cirrus emission at these infrared wavelengths and the optical and near-infrared wavelengths covered by Euclid to remove the cirrus contamination nearly entirely. The whole process of reducing the image is shown in Figure 2. 

Figure 2: The data reduction process for the Euclid image of the Perseus cluster. Panel a) shows the raw image, panel b) shows the image after correction for some stray light getting into the telescope, and panel c) shows the image after the removal of the galactic cirrus contamination using panel f). Panel d) is the final reduced image, and panel g) shows that image with all galaxies except for the central Brightest Cluster Galaxy (BCG) masked.

Overall, the authors found that about 35% of the total light in the cluster belongs to the ICL, for a total ICL mass of 1.5 x 1012 solar masses. This is a lot of mass, and it begs the question of exactly what kind of galaxies the ICL came from. It’s actually too much mass to have come solely from dwarf galaxies being destroyed by the cluster, which is one common scenario for the formation of the ICL. Dwarf galaxies just aren’t common enough in the universe to fall into the cluster at that rate, so there must be some other source of stars to make up the ICL. One other place where stars could have come from is the outskirts of more massive galaxies (galaxies that are too big to be pulled apart entirely by the cluster).

Figure 3: The colour gradient of the ICL in the Perseus galaxy cluster, plotted as a function of distance from the center of the cluster. The galaxies in the cluster are plotted as points over top. In both plots, the top of the plot has a redder colour and the bottom of the plot has a bluer colour. This can track onto the metallicity of the stars making up the ICL, where bluer implies a lower metallicity. In the outskirts of the cluster, the ICL has a very low metallicity (lower than the sun).

This is backed up by the evidence shown in Figure 3, which shows the colour of the ICL as a function of distance from the center of the cluster. Assuming the ICL is old, and thus mostly made of mature stars, the colour of the ICL is determined by the metallicity of these stars, where bluer stars have lower metallicity. In Figure 3, the authors show that the farther you move from the center of the cluster, the bluer the stars get, and thus the lower their metallicity is. Far out in the cluster, the stars are lower-metallicity than even our sun. Very low-metallicity stars are mostly found in dwarf galaxies and the outskirts of massive galaxies, so this also points to these being the places from which the ICL came.

Thanks to this improved understanding of how the ICL formed, the authors know a little more about how the Perseus cluster as a whole came to be, and thus how cluster formation works in general. The source of the ICL is still debated, so any clue as to its origin is super useful . In any case, studying the diffuse, low-surface brightness emission in the Perseus cluster is no easy feat, and Euclid has proved it is  up to the task!

Astrobite edited by Cole Meldorf

Featured image credit: Euclid artist’s concept: NASA, Perseus Galaxy Cluster: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi

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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