X-ray Group and cluster mass profiles in MOND: Unexplained mass on the group scale

We investigate the mass profiles of clusters in MOND for a sample of galaxy groups and clusters with temperatures ranging from 0.7 to 8.9 keV. We confirm that a huge hidden mass component, about 1.5 to 4 times more massive than the total visible mass (at the last measured radii, dropping thereafter), is needed to account for the hydrostatic equilibrium of these clusters. For the massive systems (T > ∼ 5 keV) we show that neutrinos with masses just below the experimentally detectable limit (mν = 2eV ) can account for the bulk (> ∼ 90%) of this hidden mass, in agreement with Sanders (2007), and confirming the results of Pointecouteau & Silk (2005) that these neutrinos leave a further minor residual component in the centers (r< ∼ 120kpc) However, the fractional contribution of the residual mass to the total MOND mass increases subtantially with decreasing mass, reaching as high as ≈ 80% for MOND masses below 1013M⊙. In these lowest mass systems this residual mass cannot be explained by simply rescaling the stellar mass profile of the central galaxy. The stellar mass profiles not only have the wrong shape, but they also would require unlikely large K-band mass-to-light ratios between 2 and 11 for the BCG. Therefore, a MOND Universe filled with massive ordinary neutrinos cannot obviate the need for additional (maybe baryonic) dark matter in X-ray bright groups.