Search for Dark Matter Annihilation and Decay with H$α$ Line Emission

Abstract

I present a new indirect search for dark matter (DM) using Hydrogen-$α$ (H$α$) recombination emission. DM annihilation or decay products can ionize neutral gas; subsequent recombination cascades generate H$α$ photons through the $3\rightarrow2$ transition. In quiet gas-rich dwarf galaxies, the $n{=}2$ population is negligible, so H$α$ is effectively unabsorbed and traces the DM-energy injection site. Using the non-detection of extended H$α$ emission in the Leo T dwarf galaxy with Multi Unit Spectroscopic Explorer (MUSE) observations, I derive the first H$α$-based limits on DM annihilation and decay, reaching leading sensitivity for parts of the eV-GeV mass range. Existing and upcoming telescopes can further extend this reach, establishing H$α$ imaging as a powerful DM search strategy.

Figures (6)

• Figure 1: Schematic of H$\alpha$ emission for DM discovery in quiet gas-rich galaxies. DM annihilation or decay products ionize neutral hydrogen, which then recombines to emit $\mathrm{H}\alpha$ radiation that is detectable with optical telescopes.
• Figure 2: New 95% C.L. limits on the DM mass and annihilation rate $\langle\sigma v\rangle$ or decay lifetime into photons or electrons, using $\mathrm{H}\alpha$ measurements of the Leo T galaxy with MUSE/VLT. Dark red shaded region is for a Burkert DM profile, light red shaded is an NFW DM profile. I also show simplified projections with $\mathrm{H}\alpha$ measurements with ELT's HARMONI instrument, assuming a stack of 10 dwarfs. The white dotted line approximates where Leo T is ionized at the level of $10\%$ and therefore would no longer have a neutral medium; see text for more details. Overlaid in blue are complementary constraints from Leo T Heating (updated in this work, see also Ref. Wadekar:2021qae), the cosmic microwave background (CMB) Slatyer:2015jlaBolliet:2020ofj, Voyager Boudaud:2016mosBoudaud:2018oya, Integral Cirelli:2020bpc, a compilation of X-rays and soft $\gamma$-rays "$X/\gamma$-ray" Essig:2013goa, and the intergalactic medium (IGM) Liu:2020wqz. Note differing axes.
• Figure S1: New 95% C.L. limits on the DM mass and $p$-wave annihilation rate $\langle\sigma v\rangle$ (evaluated at $v_{\rm ref}$) into photons or electrons, using $\mathrm{H}\alpha$ measurements of the Leo T galaxy with MUSE/VLT. Dark red shaded region is for a Burkert DM profile, light red shaded is an NFW DM profile. I also show simplified projections with $\mathrm{H}\alpha$ measurements with ELT's HARMONI instrument, assuming a stack of 10 dwarfs. The white dotted line indicates where Leo T would become ionized at the level of $10\%$ and therefore would no longer have a neutral medium; see text for more details. Overlaid in blue are complementary constraints from Leo T Heating (updated in this work, see also Ref. Wadekar:2021qae), Voyager Boudaud:2018oya, Integral Cirelli:2020bpc, a compilation of X-rays and soft $\gamma$-rays "$X/\gamma$-ray" Essig:2013goa, and the IGM Liu:2020wqz. Note differing axes.
• Figure S2: Number densities for the Leo T Galaxy. I show two DM density profiles (NFW and Burkert) for a benchmark 1 GeV DM mass, as well as the neutral gas H i, and ions $e^-$.
• Figure S3: Energy deposition probability for photons and electrons in Leo T. Comparison is shown for the MUSE aperture relevant for $\mathrm{H}\alpha$ limits (red solid), as well as if the whole WNM core is used (blue solid). I also show for comparison the result from Ref. Wadekar:2021qae, which assumes the full WNM relevant for DM heating, but uses the total attenuation / stopping rates as if they correspond to local energy deposition; see text for details.