Non-detection of X-ray emission from sterile neutrinos in stacked galaxy spectra

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 centres 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 similar to 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, totalling 15.0 and 14.6Ms 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 and 12.4 keV. In particular, if the 3.57 keV feature from Bulbul et al. 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 centre of the dark matter profile.