A Telescope Search for Decaying Relic Axions

TL;DR

This work leverages strong-lensing mass maps of galaxy clusters and VIMOS IFU spectroscopy to search for optical line emission from decaying relic axions in the 4.5–7.7 eV window. By density-weighting the IFU data with lensing-derived mass maps and performing robust sky subtraction, the authors set new 95% upper limits on the two-photon coupling $\xi$ (0.003–0.017 depending on mass), excluding canonical KSVZ/DFSZ in this range. They also revise past telescope constraints, validate their technique with simulations, and discuss implications for sterile-neutrino scenarios. The study demonstrates a promising IFU+lensing approach for probing decaying relics and motivates extending the search to higher-redshift clusters to access heavier axion masses ($8$–$14$ eV).

Abstract

A search for optical line emission from the two-photon decay of relic axions was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from the VIMOS (Visible Multi-Object Spectrograph) integral field unit at the Very Large Telescope. New upper limits to the two-photon coupling of the axion are derived, and are at least a factor of 3 more stringent than previous upper limits in this mass window. The improvement follows from larger collecting area, integration time, and spatial resolution, as well as from improvements in signal to noise and sky subtraction made possible by strong-lensing mass models of these clusters. The new limits either require that the two-photon coupling of the axion be extremely weak or that the axion mass window between 4.5 eV and 7.7 eV be closed. Implications for sterile-neutrino dark matter are discussed briefly also.

Figures (11)

• Figure 1: Image of the Abell 2667 cluster core imaged with HST in the F450W, F606W, and F814W filters. The white (thin yellow) square shows the IFU field of view, which is $54"\times 54"$. North is to the top and east is to the left. Note the strongly magnified gravitational arc north-east of the central galaxy. The white curves correspond to iso-mass contours from the lens model; the dark gray (red) line is the critical line at the redshift of the giant arc. The field of view is centered on $\alpha_{J2000}$=23:52:28.4, $\delta_{J2000}$=$-$26:05:08. At a redshift of $z=0.233$, the angular scale is $3.661$ kpc/arcsec.
• Figure 2: Image of the Abell 2390 cluster core imaged with HST in the F450W, F606W, and F814W filters. The white (thin yellow) squares correspond to the IFU field of view in different pointings. The white curves correspond to iso-mass contours from the lens model. The dark gray (red) line is the critical line at the redshift of the giant arc, labelled 1. Each square is $54"\times 54"$. North is to the top and east is to the left. The field of view is centered on $\alpha_{J2000}$=21:53:36.970, $\delta_{J2000}$=+17:41:44.66. At a redshift of $z=0.228$, the angular scale is $3.601$ kpc/arcsec.
• Figure 3: Mass map of A2667. The intensity of the image scales with density (in units of $10^{12} M_{\odot}~\rm{pix}^{-2}$), where $1~\rm{pix}=0.50"$. A density scale is provided on the bottom of the image. The horizontal extent of this map is $222.6"$. The vertical extent is $200.0"$. The thick black line indicates the spatial extent of the IFU head on the mass map.
• Figure 4: Mass map of A2390. The intensity of the image scales with density (in units of $10^{12} M_{\odot}~\rm{pix}^{-2}$), where $1~\rm{pix}=0.50"$. A density scale is provided on the bottom of the image. The horizontal spatial extent of the map is $157.5"$. The vertical extent is $150.0"$. The thick black lines indicate IFU pointings used to construct our data cubes.
• Figure 5: Average one-dimensional sky subtracted spectra of clusters A2667 and A2390. Intensity is in units of $10^{-18}~{\rm ergs}~{\rm cm}^{-2}~{\rm s}^{-1}~{{\rm \AA}}^{-1}~\hbox{arcsec}^{-2}$. Poorly subtracted sky emission lines at $5577\rm{\AA}$, $5894\rm{\AA}$, and $6300\rm{\AA}$ have not been removed.