Discovery of extended radio emission in the galaxy cluster MS 0735.6+7421

T. Bégin a, J. Hlavacek-Larrondo a, M.-L. Gendron-Marsollaisb, B. McNamarac, R. van Weerend
aDépartement de physique, Université de Montréal, Montréal, Québec, Canada
bEuropean Southern Observatory, Santiage, Chili
cDepartment of physics and astronomy, University of Waterloo, Waterloo, Ontario, Canada
dHarvard-Smithsonian Center for Astrophysics, Harvard College Observatory, Cambridge, Massachusetts, USA

The basics of galaxy clusters

What is a galaxy cluster?

A galaxy cluster is a structure consisting of hundreds to thousands of galaxies bound together by gravity.

What is the intracluster medium (ICM)?

The ICM is the gas that permeates the environment between the individual members (galaxies) of the cluster. It is heated to very high temperatures due to the intense gravitational potential well in the vicinity of the cluster.

Radio emission in galaxy clusters

Radio emission in galaxy clusters is mainly generated by synchrotron emission which is resulting from the radial acceleration of a charged particle in a magnetic field. The radio emission thus tracks the regions where there is a combination between the presence of a magnetic field and of a population of high-velocity electrons.

The main radio emission categories in galaxy clusters are:

Main takeaways: In terms of radio emission, a cluster that is not going through a fusion with another cluster could potentially host relativistic jets and a mini-halo.

X-ray emission in galaxy clusters

As was previously stated, the ICM is heated to extreme temperatures (up to 108 K) by the gravitational potential of the cluster. X-ray emission in galaxy cluster is the result of bremsstrahlung emission by these heated ICM particles (see blue emission in fig. 1 for an example of X-ray emission by the ICM).

Fig. 1 - Composite image of MS 0735.6+7421: Chandra X-ray Image (blue) which traces the ICM, VLA P-band (230-470 MHz) radio image (pink) which shows the relativistic jets, and optical Hubble Space Telescope F850LP image (white) which shows the galaxies members of the cluster. Credit: X-ray: NASA/CXC/Univ. Waterloo/B.McNamara; Optical: NASA/ESA/STScI/Univ. Waterloo/B.McNamara; Radio: NRAO/Ohio Univ./L.Bîrzan et al. 2008.

Why observing MS 0735.6+7421?

Why is MS 0735.6+7421 an important galaxy cluster to observe?

MS 0735.6+7421 is a massive and relaxed (not experiencing any form of merger with another cluster) cool-core cluster (very peaked central X-ray emission) so, according to current understanding of cluster's dynamics, it should contain a mini-halo (see fig. 2). Moreover, it exhibits the largest known X-ray cavities and jets (typical jets are on the scale of 20 kpc, while jets in MS 0735.6+7421 are approximately 200 kpc in size), and one of the most active supermassive black hole known, thus representing a key limiting case to understand galaxy clusters.

However, as presented in the box ''Previous radio imaging of MS 0735.6+7421'', no mini-halo was discovered by previous radio investigation of MS 0735.6+7421.

Fig. 2 - Figure from Giacintucci et al. (2017) illustrating cluster mass (M500) versus central entropy (K0). The authors used a statistically complete sample of 58 clusters drawn from the Planck SZ cluster catalog using a mass cut of M500 > 6⋅1014 M⊙ (circles). They supplement their sample with clusters that have good quality Chandra and radio observations (squares). Those with mini-halos are shown in magenta (confirmed) and cyan (candidates). We show MS 0735.6+7421 with the red star (uncertainty is represented as the red bar): it should be massive enough that it should contain a mini-halo associated with its cool core.

MS 0735.6+7421 in a few numbers

Here are some key features of MS 0735.6+7421:

Previous radio imaging of MS 0735.6+7421

P-band (230-470 MHz) A configuration from McNamara et al. 2009

The detailed morphology of the relativistic jets were imaged with the VLA in P-band (230-470 MHz) A configuration and published in McNamara et al. 2009. See fig. 3 for the detailed radio emission of the jets.

Fig. 3 - MS 0735.6+7421 jets detailed morphology as imaged by the VLA P-band (230-470 MHz) A configuration. The largest green contour encompasses emission above 3 times the ambient rms noise (σrms = 1.3⋅10-3 Jy/beam). Credits to McNamara, B.R. (McNamara et al. 2009).

Previous radio imaging of the cluster did not allow to detect a central diffuse emission


Telescopes and software

Fig. 4 - Radiotelescope Karl G. Jansky Very Large Array (JVLA).

Our new radio imaging

Our new P-band (230-470 MHz) C configuration and L-band (1-2 GHz) C configuration images

We produced two new JVLA radio images of MS 0735.6+7421 in L-band (1-2 GHz) C configuration (see fig. 5) and in P-band (230-470 MHz) C configuration (see fig. 6). The key advantage of these images is that they are more sensitive to the faint emission (thus allowing to detect emission that is fainter and more diffuse).

Fig. 5 - 1-2 GHz (L-band) JVLA C configuration image. The noise level in the vicinity of the cluster is σrms = 0.017 mJy/beam, the peak value is 7.92 mJy/beam and the beam size is 13.5'' x 19.2''. Contour levels are drawn at [1, 2, 6, 28, 155] x 3 σrms (5 levels in total).
Fig. 6 - 230-470 MHz (P-band) JVLA C configuration image. The noise level in the vicinity of the cluster is σrms = 0.40 mJy/beam, the peak value is 314 mJy/beam and the beam size is 59'' x 75''. Contour levels are drawn at [1, 2, 5, 17, 66, 262] x 3 σrms (6 levels in total).

Discussion, conclusion and future work


Conclusion and future work


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[2] Radiative efficiency and content of extragalactic radio sources: toward a universal scaling relation between jet power and radio power, ApJ, Bîrzan et al. 2008

[3] Diffuse Radio Emission from Galaxy Clusters, SSReviews, van Weeren et al. 2019

[4] Occurrence of radio minihalos in a mass-limited sample of galaxy clusters, ApJ, Giacintucci et al. 2017

[5] Cycling of the powerful AGN in MS 0735.6+7421 and the duty cycle of radio AGN in clusters, MNRAS, Vantyghem et al. 2014

[6] Cosmological Effects of Powerful AGN Outbursts in Galaxy Clusters: Insights from an XMM-Newton Observation of MS0735.6+7421, ApJ, Gitti et al. 2007

[7] Expanding the Sample of Radio Minihalos in Galaxy Clusters, ApJ, Giacintucci et al. 2019