Possible Dark Matter annihilation in the galaxy cluster

Possible Dark Matter annihilation in the galaxy cluster A 520 P. Marchegiani, S. Colafrancesco, N. Khanye Wits University (Johannesburg, SA) HEASA 2018

Radio halos in galaxy clusters • Diffuse radio emission • Produced by relativistic electrons interacting with magnetic fields (synchrotron emission) [Coma radio halo: image from Brown & Rudnick 2011] Several models for the origin of these electrons: Baryonic: • • • injections by galaxies [Jaffe 1977; Colafrancesco et al. in prep. ] acceleration by shocks or turbulences [Tribble 1993; Brunetti et al. 2001] production in hadronic collisions (secondary electrons) [Blasi & Colafrancesco 1999; Marchegiani et al. 2007] From Dark Matter annihilation processes [Colafrancesco & Mele 2001; Colafrancesco, Profumo & Ullio 2006]

In some clusters, hot gas (red) and Dark Matter (blue) have different spatial distributions Bullet cluster [Clowe et al. 2006] A 520 [Mahdavi et al. 2007] In these clusters a comparison between the spatial distribution of the radio emission and the hot gas and the DM distributions can help us to understand if DM annihilation has some role in producing the relativistic particles [e. g. Marchegiani & Colafrancesco 2015 for the Bullet]

A 520: X-rays (colours) vs. weak lensing (contours) images Main X-ray halo centred in the cluster centre Two main DM peaks far from the cluster centre [X-ray: Chandra 0. 8 -4 ke. V; Wang et al. 2016] A strip of cold gas is left from the motion of the Brightest Cluster Galaxy (BCG) outwards

Radio emission: diffuse emission + several radio galaxies Diffuse component after sources subtraction BCG [VLA 1. 4 GHz images from Wang et al. 2018] The diffuse emission can be divided in two components, on the NE and the SW of the cluster

The shape of the diffuse radio emission is more similar to the gravitational lensing image than to the X-ray one Is it possible that the radio emission is produced by DM annihilation?

Considering the whole cluster, let us compare two models: 1. Hadronic model: secondary electrons produced in interactions between cosmic ray protons and thermal gas nuclei 2. Dark Matter model: electrons produced in Dark Matter annihilation processes

NFW profile with parameters derived from the DM halo mass [Bullock et al. 2001]

The whole radio halo DM 43 Ge. V [VLA data from Vacca et al. 2014] DM 9 Ge. V

• We conclude that in the whole cluster the DM radio emission is subdominant compared to the baryonic one • Is it possible that the DM emission is dominant in some particular region of the cluster? • Radio maps show a subpeak in a region in the NE part of the cluster, far from the X-ray peak • The radio peak is close to a gravitational lensing peak, and does not correspond to any visible radio galaxy Contours: NVSS Colours: Chandra

Zoom of the radio peak region: • • Image: ESO-DSS White: NVSS Red: TGSS Black: gravitational lensing [Wang et al. 2016] Distance between the lensing peak and the TGSS peak is smaller than the TGSS resolution (25 arcsec) VLSSr Estimation of the flux coming from a region centred on the peak: TGSS NVSS

For a reference magnetic field of 5 m. G (constant in the region): DM 43 Ge. V Hadronic model DM 9 Ge. V