Stars as Laboratories for Fundamental Physics - MPP Theory Group

492 Chapter 12
contribution to the cosmic mass density would be Ω a h 2 = 0.082 m a /eV
(Turner 1987; Ressell 1991) and so one can expect that clusters of galaxies
contain substantial amounts of axions even if they are not the main
dark matter component.
Observations of the diffuse extragalactic background radiation limit
the axion mass to values below about 8 eV unless ξ is very small (Ressell
1991). Overduin and Wesson (1993) found ξ < 0.43, 0.07, and 0.02 for
m a /eV = 5.3, 8.6, and 13, respectively (Fig. 12.23). Moreover, some
axions would reside in the halo of our own galaxy so that their decays
would light up the night sky. Its brightness yields a conservative bound
of ξ < (6 eV/m a ) 5 (Ressell 1991).
Fig. 12.23. Constraints on axion decays in galaxies and galaxy clusters; ξ
parametrizes the coupling to photons with ξ = 1 corresponding to common
axion models. (a) Line emission from clusters A2256 and A2218 (Bershady,
Ressell, and Turner 1991; Ressell 1991). (b) Diffuse extragalactic background
radiation according to Ressell (1991) and (c) Overduin and Wesson (1993).
(d) Our galaxy (Ressell 1991).
The most interesting limits arise from a search for axion decay lines
from the intergalactic space in the clusters of galaxies A2256 and A2218.
Bershady, Ressell, and Turner (1991) and Ressell (1991) found ξ < 0.16
0.078, 0.039, 0.032, 0.016, and 0.011 for m a /eV = 3.5, 4.0, 4.5, 5.0,
6.0, and 7.5 respectively (Fig. 12.23). These limits are placed into the
context of other constraints in Fig. 5.9.
yields Ω a h 2 ≈ (10 −5 eV/m a ) 1.175 —see Eq. (14.5). While the overall coefficient of
this expression is very uncertain it is clear that Ω a = 1 saturates for m a somewhere
between 1 µeV and 1 meV. In this range axions never achieved thermal equilibrium.