Testing MOND over a Wide Acceleration Range in X-Ray Ellipticals

The gravitational fields of two isolated ellipticals, NGC 720 and NGC 1521, have been recently measured to very large galactic radii ($\sim 100$ and $\sim 200\mathrm{kpc}$), assuming hydrostatic balance of the hot gas enshrouding them. They afford, for the first time to my knowledge, testing modified Newtonian dynamics (MOND) in ellipticals with force and quality that, arguably, approach those of rotation-curve tests in disk galaxies. In the context of MOND, it is noteworthy that the measured accelerations span a wide range, from more than $10a_0$ to about $0.1a_0$, unprecedented in individual ellipticals. I find that MOND predicts correctly the measured dynamical mass runs (apart from a possible minor tension in the inner few kpc of NGC 720, which might be due to departure from hydrostatic equilibrium): The predicted mass discrepancy increases outward from none near the center, to $\sim 10$ at the outermost radii. The implications for the MOND-versus-dark-matter controversy go far beyond the simple fact of two more galaxies conforming to MOND.

Figure 1

NGC 1521: MOND predictions for the enclosed dynamical masses for several radii (filled squares), shown in comparison with the dynamical masses deduced by Ref. 14. Except for the MOND points, all the rest are reproduced from Ref. 14: Gray region, with its central solid (black) line, is the range of dynamical masses based on a model of the temperature and x-ray density profiles. The open circles are from their alternative derivation of the masses based on a “more traditional ‘smoothed inversion’ approach.” The dashed (red) line is the contribution of the stars $M^{*}(R)$, for their best fit $M^{*}/L_I=2.55(M/L{)}_{\odot }$, and the dot-dash (blue) line is that of the x-ray gas, $M_g(R)$. The MOND predictions are given by Eq. (3), with $M_b(R)=M^{*}(R)+M_g(R)$, and the interpolating function ${\nu }_1$.

Figure 2

NGC 720: The same as Fig. 1 (with results from Ref. 13) for two sets of MOND predictions: one for the same $M^{*}/L_K=0.54(M/L{)}_{\odot }$ preferred by the Newtonian analysis of Ref. 13 (small rings); the other for $M^{*}/L_K=0.8(M/L{)}_{\odot }$, which MOND prefers (filled squares). In the latter case, the curve for $M^{*}(r)$ should be raised by a factor 1.5.