Dead galaxies in Coma Cluster may be packed with dark matter

This artist's impression of the ‘quenching’ process shows how a normal blue (star-forming) galaxy lost its gas while falling into the Coma Cluster very early on in its formation. Credit: Cameron Yozin, ICRAR/UW. Hi-res image

The Coma Galaxy Cluster is a massive cluster of galaxies in the constellation Coma Berenices. Each point of light in this image may look like a star but in fact they are, mostly galaxies. With over 650 galaxies in the cluster, Abell 1656 is one of the densest collections of galaxies in the entire sky. Credit: Greg Parker, New Forest Observatory.

Galaxies in a cluster roughly 300 million light years from Earth could contain as much as 100 times more dark matter than visible matter, according to an Australian study.

The research, published today, used powerful computer simulations to study galaxies that have fallen into the Coma Cluster, one of the largest structures in the Universe in which thousands of galaxies are bound together by gravity.

"It found the galaxies could have fallen into the cluster as early as seven billion years ago, which, if our current theories of galaxies evolution are correct, suggests they must have lots of dark matter protecting the visible matter from being ripped apart by the cluster.”

Dark matter cannot be seen directly but the mysterious substance is thought to make up about 84 per cent of the matter in the Universe.

International Centre for Radio Astronomy Research PhD student Cameron Yozin, who led the study, says the paper demonstrates for the first time that some galaxies that have fallen into the cluster could plausibly have as much as 100 times more dark matter than visible matter.

Yozin, who is based at The University of Western Australia, says the galaxies he studied in the Coma Cluster are about the same size as our own Milky Way but contain only one per cent of the stars.

He says the galaxies appear to have stopped making new stars when they first fell into the cluster between seven and ten billion years ago and have been dead ever since, leading astrophysicists to label them “failed” galaxies.

This end to star formation is known as “quenching”.

“Galaxies originally form when large clouds of hydrogen gas collapse and are converted to stars—if you remove that gas, the galaxy cannot grow further,” Yozin says.

“
Falling into a cluster is one way in which this can happen. The immense gravitational force of the cluster pulls in the galaxy, but its gas is pushed out and essentially stolen by hot gas in the cluster itself.

“For the first time, my simulations have demonstrated that these galaxies could have been quenched by the cluster as early as seven billion years ago.

“They have however avoided being ripped apart completely in this environment because they fell in with enough dark matter to protect their visible matter.”

This research was motivated by the recent observational discovery of these galaxies by an American and Canadian team led Professor Pieter van Dokkum of Yale University. 

Using the data the North American team published last year, Yozin was able to create computer simulations to model how the galaxies evolved into what we can see today.

The study was released in the journal Monthly Notices of the Royal Astronomical Society, published by Oxford University Press.

Further Information  

ICRAR is a joint venture between Curtin University and The University of Western Australia with support and funding from the State Government of Western Australia.


Original publication details

'The quenching and survival of ultra-diffuse galaxies in the Coma cluster’ in Monthly Notices of the Royal Astronomical Society. Published online on 20/7/2015 at the MNRAS website.

Contact Details  

Cameron Yozin (ICRAR – UWA)
Ph: +61 8 6488 3819
E: cameron.yozin-smith@icrar.org
M: +61 423 941 128  

Pete Wheeler (Media Contact, ICRAR)
Ph: +61 8 6488 7758
E: pete.wheeler@icrar.org
M: +61 423 982 018

UWA Media Office
Ph: +61 8 6488 3229
E: uwamedia@uwa.edu.au

Source: ICRAR - International Centre for Radio Astronomy Research 

Measuring gas velocities in galaxy clusters with X-ray images

Fig 1: Schematic illustration of gas density distribution in a spherically symmetric cluster in perfect hydrostatic equilibrium (v=0, top) and in a slightly disturbed cluster (v ≠ 0, middle). Slow large-scale perturbations in a stratified cluster atmosphere can be interpreted as internal waves (as illustrated in the bottom panel), similar to waves in the ocean, where the velocity of water and the amplitude of waves are linked. In clusters, similar perturbations are caused by a variety of reasons, including minor mergers or the activity of the central supermassive black holes.

Fig 2: X-ray image of the Coma cluster as seen with Chandra observatory.

The substructure seen in the image implies that the X-ray emitting gas is not at rest. 

X-ray observations provide us with detailed information on the density and temperature of the hot gas inside galaxy clusters. The other major gas characteristic that still needs to be measured is the gas velocity. While current generation X-ray observatories lack the required energy resolution to measure velocities directly, future observatories such as ASTRO-H and ATHENA will address this limitation.

An international team including MPA scientists has shown that the power spectrum of the velocity field can inferred indirectly from existing X-ray images of relaxed clusters. Numerical simulations confirm this simple theoretical idea, opening a way of probing gas velocities using already existing X-ray data. 

Galaxy clusters are the largest gravitationally bound structures in the present Universe. Hot gas (with temperatures of 10 to 100 million Kelvin) fills their gravitational potential wells and shines in the X-ray band, making the clusters an easy target for orbital X-ray observatories. Both the density and the temperature of the gas in clusters is routinely measured using X-ray data, while it is notoriously difficult to directly measure the turbulent motion of the gas via the Doppler shift of X-ray lines. Since the information on the turbulent gas velocities would have profound implications for the mass determination of clusters and for determining the plasma microphysics, new approaches have been developed to indirectly measure the gas velocities using existing X-ray data. One of these approaches is based on the analysis of small-scale fluctuations in X-ray images as described below. 

In relaxed clusters the gas approaches the state of hydrostatic equilibrium, when all thermodynamic properties are aligned along surfaces with equal gravitational potential, making X-ray images smooth and round (see Fig.1). These stratified and stable atmospheres of cluster gas bear much similarity to the Earth's atmosphere or to water in the oceans where cold and dense material tends to be below hotter and lighter material due to the combined action of gravity and buoyancy. Slow subsonic perturbations of such atmospheres can be represented as a combination of internal (gravity) waves, very much like waves in the ocean (bottom panel of Fig.1). In oceans there is a simple relation: the larger the amplitude of the waves, the higher the velocity of water. Is the same true for gas in galaxy clusters? Both theoretical analysis and numerical simulations have shown that this is indeed the case.

The main idea is that gas is disturbed on large scales and that this results in a cascade of waves. In clusters these waves are creating perturbations in the gas density that are visible in X-ray images as small-scale fluctuations of the surface brightness relative to a smooth global model. Our analysis shows that there is a simple linear relation between the gas velocities and density perturbations. Moreover, this relation holds for a broad range of scales: on large scales, where buoyancy effects dominate (internal waves), as well as on small scales where the isotropic turbulent cascade usually develops. At these small scales, the entropy of the gas acts as a passive scalar advected by the velocity field and makes the gas displacement visible in X-rays. 

Based on these arguments one can expect that in relaxed clusters (i.e. clusters which are only slightly disturbed) the power spectrum of the velocity field can simply be recovered from the power spectrum of density fluctuations. The latter can be straightforwardly estimated from X-ray images. 

Numerical simulations (cosmological simulations of cluster formation and pure hydrodynamic simulations with turbulence) confirm this conclusion and open an interesting possibility to use gas density power spectra as a proxy for the velocity power spectra in relaxed clusters. 

Once the gas velocities can be measured directly with future X-ray observatories, it will be possible to push this analysis further and search for differences between the density and velocity power spectra. Strong departures of the two power spectra from the universal behavior described above can then be used to constrain physical effects such as conductivity or viscosity in the gas.

Eugene Churazov (MPA), Massimo Gaspari (MPA), Irina Zhuravleva (Stanford), Alex Schekochihin (Oxford), Rashid Sunyaev (MPA)

References:
 

Zhuravleva I., Churazov E., Schekochihin A. A., Lau E. T., Nagai D., Gaspari  M., Allen S. W., Nelson K., Parrish I. J., The Relation between Gas Density and Velocity Power Spectra in Galaxy Clusters: Qualitative Treatment and Cosmological Simulations, 2014, ApJL, 788, 13

 

Gaspari M., Churazov E., Nagai D., Lau E. T., Zhuravleva, I., The relation between gas density and velocity power spectra in galaxy clusters: high-resolution hydrodynamic simulations and the role of conduction,2014, A&A, 569A, 67

 

Gaspari M., Churazov E., Constraining turbulence and conduction in the hot ICM through density perturbations, 2013, A&A, 559A, 78

 

Churazov E.; Vikhlinin A.; Zhuravleva I.; Schekochihin A.; Parrish I.; Sunyaev R.; Forman W.; Böhringer H.; Randall S., X-ray surface brightness and gas density fluctuations in the Coma cluster, 2012, MNRAS, 421,1123

Source:  Max Planck Institute for Astrophysics