Some implications of the leptonic annihilation of dark matter: possible galactic radio emission signatures and the excess radio flux of extragalactic origin

Abstract

We give theoretical predictions for the radio emission of a dark matter candidate annihilating into 2-lepton and 4-lepton final states. We then compare our results with the known radio measurements of the sky temperature as a function of the frequency. In particular, we calculate the radio emission for some dark matter candidates annihilating into intermediate bosons that subsequently decay into a 4-lepton channel with a thermal annihilation cross-section. We show that within the range of frequencies from 20 MHz to 5 GHz, this channel can produce a stronger signature than direct annihilation into leptons.A combination of cosmological and astrophysical observations dating back decades has provided strong evidence for the existence of dark matter (DM). This evidence includes data from rotational curves of galaxies, the dynamics of galaxy-galaxy interactions, spatial temperature fluctuations in the cosmic microwave background (CMB) radiation, and gravitational lensing. The luminous matter observed in spiral galaxies is not large enough by itself to account for the rotation curves of the galaxies (see, e.g.[1]). The CMB fluctuation results infer that DM component of the universe is Ω dm h 2 = 0.120 ± 0.001 while the baryonic component is Ω b h 2 = 0.0224 ± 0.0001 [2]. Observations of gravitational lensing phenomena also imply the presence of the DM, showing that evidence of a significant amount of mass where nothing is observed optically (see, e.g., [3,4]).We note that strong limits on the direct interaction of DM particles with standard model particles have been obtained in the laboratory [5]-[7]). Other attempts to search for DM were performed by looking for the secondary γ-rays produced by DM annihilation and pion decay, as well as from processes such as internal bremsstrahlung [8,9] and synchrotron radiation [10]. The secondary radiation consists of three kinds: (1) radio waves due to synchrotron radiation of secondary electron-positron pairs (e ± ) interacting with the galactic magnetic field, (2) bremsstrahlung γ-rays, and (3) and γ-rays from Compton interactions with photons of interstellar radiation fields [11,12].There have been various studies of astrophysical γ-ray observations with the hope of finding γ-ray signals of dark matter annihilation. These studies included the galactic center region [13]. However, the interpretation of the observed excess of γ-rays from the galactic center [14] is complicated by other possible contributions from sources such as millisecond pulsars. Studies of γ-ray emission from dwarf galaxies [15] and the nearby galaxies M31 and M33 [16] have lead to mass-dependent constraints on DM annihilation.The observed galactic radio spectrum used to estimate the galactic temperature is a result of several measurements. We particularly highlight the measurements performed at frequencies above 60 GHz by COBE/FIRAS instrument, some surveys at 22, 45, 408 and 1420 MHz [19] and the measurements of the balloon-borne experiment ARCADE-2, which has measured radio signals of the sky temperature...

The Second Radio Synchrotron Background Workshop: Conference Summary and Report

“…Such constraints have led various authors to investigate potential origins such as supernovae of massive population III stars (Biermann et al 2014), emission from Alfvén reacceleration in merging galaxy clusters (Fang & Linden 2016), an enhancement in the Local Bubble (Sun et al 2008, although see Section 4.15 of this report), annihilating dark matter (DM) in halos or filaments (Fornengo et al 2011;Hooper et al 2012;Fang & Linden 2015;Fortes et al 2019) or ultracompact halos (Yang et al 2013), "dark" stars in the early universe (Spolyar et al 2009), dense nuggets of quarks (Lawson & Zhitnitsky 2013), injections from other potential particle processes as discussed in Cline & Vincent (2014) and Pospelov et al (2018) and here in Sections 4.10 and 4.20, and accretion onto primordial black holes (PBHs) as discussed here in Sections 4.13 and 4.14.…”