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 .