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

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 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:

• Relativistic jets: they are composed of relativistic particles directly connected to the accretion disk of the central supermassive black hole (see pink emission in fig. 1 for an example of radio emission from the jets).
• Halo: diffuse emission located at the center of the cluster and of typical scales of 1-2 Mpc. Main hypothesis for the acceleration of electrons is: turbulence induced by fusion of clusters.
• Mini-halo: diffuse emission located at the center of the cluster and of typical scales of 100-500 kpc. Main hypotheses for the acceleration of electrons are: 1) produced as secondary products from the interaction between relativistic cosmic-ray protons and thermal protons in the ICM; 2) turbulence induced by the movement of ICM itself induced by fusion of galaxies with the cluster.
• Radio relic: diffuse emission located at the outskirts of the cluster and of typical scales of 0.5-2 Mpc. Main hypothesis for the electron acceleration: turbulence induced at shocks occuring in fusion of clusters.

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.

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).

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.

Here are some key features of MS 0735.6+7421:

• Redshift: z = 0.216 (3.5 kpc per arcsec).
• Mass: M₅₀₀ = (8.4±4.4)⋅10¹⁴ M⊙ (Gitti et al. 2007), which places it among the massive galaxy clusters.
• Central entropy: K₀ = (12.6±0.6) keV cm² (Vantyghem et al. 2014), which places it among the cool-core clusters.
• Mean power of the jets: P_jets = (1.7±0.6)⋅10⁴⁶ erg/s (Vantyghem et al. 2014).

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.

• No diffuse extended radio emission was discovered as part of this McNamara et al. 2009 paper. This is not unexpected given the configuration that was used (VLA A configuration observations benefit from higher spatial resolution, but lower sensitivity to radio flux density).
• Imaging of the central part of the cluster with higher sensitivity to flux density should, in theory, allow to detect a diffuse emission (see the box titled ''Why observing MS 0735.6+7421?'').

• In order to image objects in radio wavelengths, we use radiotelescopes which are operating as interferometers.
• For my project, we used data from the Karl G. Jansky Very Large Array (JVLA) which is a radiotelescope exploiting 27 antennas distributed over a ''Y'' region of approximately 20 km in radius and located in the Plains of San Agustin in New-Mexico, USA (see fig. 4 for an example of the distribution of antennas of the JVLA).
• Calibration and deconvolution were conducted using the software Common Astronomy Software Applications (CASA). A script was specifically developped to take into account the high radio frequency intereference (RFI) in the frequencies between 230 MHz and 2 GHz.
• Our data set consists of 5 hours of L-band (1-2 GHz) C configuration and 5 hours of P-band (230-470 MHz) C configuration JVLA observations of MS 0735.6+7421.

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).

• The L-band (1-2 GHz) image shows extended diffuse emission larger than the previously detected jets. However, it is not yet clear if this new diffuse emission is attributable to the jets or to a more extended diffuse emission. We note that the extended emission is on the order of 500 kpc which is typical for a mini-halo structure.
• By comparing the total flux density of the old P-band (230-470 MHz) A configuration image (S_{Aconfig,350MHzs} = 0.38±0.05 Jy) with the new P-band (230-470 MHz) C configuration image (S_{Cconfig,350MHz} = 0.68±0.07 Jy), we find a clear increase of S_{diffuse,350MHz} = 0.30±0.09 Jy that can be explained by new extended diffuse emission detection.
• By scaling the flux density of the new diffuse emission to 1.4 GHz (using a typical spectral index for mini-halos) in order to compare it to litterature values, we find S_{diffuse,1.4GHz} = 0.08±0.03 Jy which corresponds to a radio power of P_{diffuse,1.4GHz} = (1.3±0.5)⋅10²⁵ W/Hz which is comparable to the most luminous mini-halos detected in the review of Giacintucci et al. 2019. The hypothesis of the mini-halo can't be excluded.

• We discovered a diffuse radio emission with a radio power that is consistent with the diffuse emission being a very luminous mini-halo (see Giacintucci et al. 2019).
• At this time, we are generating a spectral index map between P-band (230-470 MHz) and L-band (1-2 GHz) in order to constrain the nature of this extended diffuse radio emission. A spectral index map will give insight on physical processes responsible for the emission.
• An article is currently being written to summarize our results: Discovery of extended radio emission in the galaxy cluster MS0735.6+7421 with the Karl G. Jansky Very Large Array (Bégin et al. in preparation). Stay tuned!!!!

[1] An energetic AGN outburst powered by a rapidly spinning supermassive black hole or an accreting ultramassive black hole, ApJ, McNamara et al. 2009

[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