How to turn two galaxies into ten

How to turn two galaxies into ten

Title: A trace of galaxies without dark matter from a cold-blooded collision

Author: Pieter van Dokkum, Zili Shen, Michael A. Keim, Sebastian Trujillo-Gomez, Shany Danieli, Dhruba Dutta Chowdhury, Roberto Abraham, Charlie Conroy, JM Diederik Kruijssen, Daisuke Nagai, Aaron Romanowsky

First author’s institution: Yale University, New Haven, CT, USA

Status: Published in Nature [open access]available on arXiv

The nature of dark matter is one of physics’ greatest mysteries. All we really know about this strange type of matter is that it does not experience anyone forces other than gravity. This makes it totally invisible and allows it to pass through other matter, such as stars, planets and us, almost completely unnoticed. Despite this, evidence from recent decades means that we know that dark matter exists, and that there is a lot of it – roughly six times more than all visible matter in our universe! In addition, we know that most major astronomical structures, such as galaxies and galaxhoparare embedded in large, spherical glorior of dark matter.

One of the most striking evidences of dark matter is Bullet Cluster, shown in Figure 1. This image is actually the result of two galaxy clusters that have crashed into each other and now lie side by side. I rosa, den hot, visible gas in these clusters are shown, which have been drawn towards the middle of the two clusters of features forces. Using weak gravitational lensing to study the light from galaxies in the background of this image, we can conclude where matter is in these clusters, as shown by the blue areas. The fact that the distributions of hot gas and mass do not overlap tells us that even though the usual substance in these clusters is stirred and drawn into the middle, there is some extra cluster material that flies past unhindered – our old friend, dark matter!

Photograph of two galaxy clusters, one on the left and one on the right.  Superimposed are two pink regions, close to each other in the center of the image, and two blue regions, separated by a larger distance and so farther from the center.

Figure 1: The Bullet Cluster, displayed as three separate superimposed images. The galaxies of the two colliding clusters (as well as background galaxies) are displayed in optical (visible) wavelengths. The two pink areas near the center of the image show the X-rays emitted by the hot cluster gas, indicating that these two clouds have collided and been dragged to the central region. The two blue circular areas show where the majority of the cluster’s mass is located, indicating that much of their mass is made of invisible matter. Credit: Chandra / Magellan / NASA / STScI / ESO.

The same process can in principle take place on a smaller scale. Today’s newspaper provides evidence of two dwarf galaxies has previously experienced a collision similar to the ball cluster, and goes on to discuss how this may explain some of the strangest types of galaxies we observe in our universe.

A glare

This magazine is looking at NGC1052 galax groupand in particular two dwarf galaxies in this group, called DF2 and DF4. These galaxies are remarkable because they are ultradiffuse galaxies, which contains very few stars but has a diameter similar to other galaxies, which means that they are very weak. Many ultra-diffuse galaxies, including DF2 and DF4, also contain very little dark matter, and the reason why they differ from other galaxies in this way is still unknown.

But, we may be approaching an answer! The authors suggest that a collision between two galaxies, the predecessors of DF2 and DF4, occurred about 8 billion years ago. Figure 2 shows what such a collision would have looked like and what the result of it would be today.

Schematic diagram showing a galaxy (

Figure 2: Schematic view showing collision between DF2 and DF4, near the galaxy NGC1052, located in the center of a galaxy group. In this scenario, a galaxy (ancestor 1) collided with a member of the NGC1052 group (parent 2) about 8 billion years ago. This removed the stars from the halos of dark matter from ancestors 1 and 2, which later became the galaxies DF2 and DF4, respectively. Parts of these galaxies were pulled away by the collision and became other, small galaxies between these two. Adapted from figure 1 in today’s newspaper.

Such a collision can result in positions and velocities of DF2 and DF4 that are consistent with observations: that they are separated by a distance of 2.1 Mpc (about 7 million light-years) and move away from each other at 358 km / s. In addition, this collision can cause the dark matter halos and stellar components of these galaxies to separate, in the same way that occurred in the Bullet Cluster, leaving two galaxies with very little dark matter left: DF2 and DF4. The authors also predict that parts of these galaxies would be stripped away, leading to an arc with about 10 smaller galaxies, as well as two “dark galaxies“, Almost entirely made of dark matter!

A cosmic mess

To back up their claims, the authors present observational data for DF2 and DF4, as well as the area between them. The image of this region is shown in Figure 3, which has an eerie resemblance to Figure 2! Incredibly, the observations match exactly what is predicted by a ball-cluster-like collision between two galaxies: a chain of small, weak galaxies located on a line between DF2 and DF4 can be seen, as well as a companion galaxy near each of them. two, both of which have been shown to consist almost entirely of dark matter.

Image showing a long row of 13 objects, each identified as a galaxy, similar to the diagram in Figure 2. A bright galaxy in the center, the central group galaxy NGC1052, is marked in a dotted red circle.  Two smaller galaxies on either side of this are marked as DF2 and DF4.  Near each of these is a very faint galaxy marked with a blue square, indicating that these are the dark matter-rich companions to DF2 and DF4 that were predicted to exist.  Zoomed images of each of the galaxies are embedded, indicating that they are usually faint, spheroidal galaxies.

Picture 3: Image of the NGC1052 group and nearby galaxies. The central galaxy NGC1052 is marked with a dotted red circle, the ultra-diffuse galaxies DF2 and DF4 with solid green circles, and two galaxies consisting almost entirely of dark matter (RCP32 and DF7) are shown by blue squares. Several other faint galaxies are marked in small white squares, along a line between DF2 and DF4, which reflects the prediction in Figure 2. Adapted from Figure 3 in today’s newspaper.

This work provides an origin story for the galaxies in this region of the universe, and explains their unusual properties – specifically why some have so little dark matter, and some have much more than average. Similar collisions may explain other galaxies with an atypical content of dark matter. Crucially, however, the formation of these systems will depend on the properties of the galaxies’ dark matter halos. Studying galaxies that have been torn apart like this can help us limit ourselves properties of dark matterand get one step closer to finally understanding this elusive part of the universe.

Astrobite edited by Sarah Bodansky

Selected image credit: Figure 2 from today’s newspaper, van Dokkum et al., 2022

About Roan Haggar

I am a doctoral student at the University of Nottingham and work with hydrodynamic simulations of galaxy clusters to study the evolution of incident galaxies. I also run a portable planetarium that we take around to schools in the area. My more down-to-earth hobbies include rock climbing and going to music venues that I have not been to before.

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