Non-Detection of X-Ray Emission From Sterile Neutrinos in Stacked Galaxy Spectra

TL;DR

This study searches for X-ray signals from sterile neutrino dark matter in galaxy spectra but finds no evidence, setting strong constraints on their properties and challenging previous claims of detection.

Contribution

The paper introduces a new formalism for maximizing dark matter signal detection and provides the first comprehensive non-detection analysis across a large galaxy sample.

Findings

No significant emission lines detected.

Constraints on sterile neutrino mixing angles are established.

Results challenge previous claims of a 3.5 keV line detection.

Abstract

We conduct a comprehensive search for X-ray emission lines from sterile neutrino dark matter, motivated by recent claims of unidentified emission lines in the stacked X-ray spectra of galaxy clusters and the centers of the Milky Way and M31. Since the claimed emission lines lie around 3.5 keV, we focus on galaxies and galaxy groups (masking the central regions), since these objects emit very little radiation above $\sim 2$ keV and offer a clean background against which to detect emission lines. We develop a formalism for maximizing the signal-to-noise of decaying dark matter emission lines by weighing each X-ray event according to the expected dark matter profile. In total, we examine 81 and 89 galaxies with Chandra and XMM-Newton respectively, totaling 15.0 and 14.6 Ms of integration time. We find no significant evidence of any emission lines, placing strong constraints on the mixing angle of sterile neutrinos with masses between 4.8-12.4 keV. In particular, if the 3.57 keV feature from Bulbul et al. (2014) were due to 7.1 keV sterile neutrino emission, we would have detected it at $4.4\sigma$ and $11.8\sigma$ in our two samples. The most conservative estimates of the systematic uncertainties reduce these constraints to $4.4\sigma$ and 7.8$\sigma$, or letting the line energy vary between 3.50 and 3.60 keV reduces these constraints to $2.7\sigma$ and $11.0\sigma$ respectively. Unlike previous constraints, our measurements do not depend on the model of the X-ray background or on the assumed logarithmic slope of the center of the dark matter profile.