Telescope searches for decaying thermal relic axions in the galaxy cluster RDCS 1252


I successfully proposed ~20 hours of observation time to look for decaying relic axions in the high-z galaxy cluster RDCS 1252. The resulting upper limits are the best in existence in the mass window from 8-14 eV. In the course of project, I mentored Princeton undergraduate student Kai Zhong Khor (shown below).kai

Collaborators: Kai Zong Khor, Prof. Marc Kamionkowski, Giovanni Covone, Prof. Jean-Paul Kneib, and Eric Jullo.

This work is described in my successful VLT VIMOS proposal for ~20 hours of observation time to look for decaying relic axions in the high-z galaxy cluster RDCS 1252. The manuscript is nearly complete. The above plot shows limits to decaying relic axions in the 8-14 eV mass window, using optical IFU observations of a z~1 galaxy cluster. Although any faint signal would fall off with redshift, the higher-redshift observations would probe an axion mass m_{a}=24800\AA(1+z)~{\rm eV}/\lambda .  Since the intensity of the axion decay signal at fixed rest-frame wavelength scales as m_{a}^{3} here, the optical telescope axion searches in the 8-14 eV window would be roughly two orders of magnitude more sensitive (in terms of the model-dependent two-photon coupling parameter \xi) than existing limits, obtained using measurements of the diffuse extra-galactic background radiation (DEBRA) and limits derived using the lifetimes of helium-burning stars.

To this end, I put together a proposal to obtain 15-20 hours of VIMOS IFU spectroscopy of the galaxy cluster RDCS 1252, once the highest-redshift known galaxy cluster, in order to search for decaying relic axions. The proposal was approved and observations obtained by spring 2008. Using Rosati’s weak lensing maps, I have worked with Kai Khor (a Princeton undergradauate) and Eric Jullo to perform a search for decaying relic axions in this galaxy cluster. This window is particularly interesting, because axions in this mass window could be a considerable fraction of the cosmological dark matter, with thermal freeze-out abundance:

\Omega_{a}h^{2}\simeq \frac{m_{a}}{130~{\rm eV}}

Axions in this mass window are already highly constrained by stellar lifetimes and DEBRA measurements. Reports of their demise, however, are greatly exaggerated. The reason is that the two-photon coupling of the axion is highly dependent on the axion model ; the coupling also depends on the poorly-known up-down quark mass ratio (lattice QCD constraints still disagree by as much as 50%!). With a moderate amount of fine tuning on the theoretical side, existing experimental constraints still allow an axion in this mass window, and indeed significantly relax many astrophysical axion constraints . Thus, an axion in the 8-14 eV mass window, if weakly coupled to photons, could be almost all of the dark matter, and would (as discussed below) be sufficiently cold to escape limits from the galaxy correlation power spectrum and large-scale structure. We have used the RDCS 1252 data to impose the tightest limits (by 1-2 orders of magnitude to axions or axion-like particles in the 8-14 eV mass window).