Galaxy Bulges As Tests of CDM vs MOND in Strong Gravity

Abstract

The tight correlation between galaxy bulges and their central black hole masses likely emerges in a phase of rapid collapse and starburst at high redshift, due to the balance of gravity on gas with the feedback force from starbursts and the wind from the black hole; the average gravity on per unit mass of gas is ~ 2 x 10^-10 m/sec^2 during the star burst phase. This level of gravity could come from the real r^{-1} cusps of Cold Dark Matter (CDM) halos, but the predicted gravity would have a large scatter due to dependence on cosmological parameters and formation histories. Better agreement is found with the gravity from the scalar field in some co-variant versions of MOND, which can create the mirage of a Newtonian effective dark halo of density Pi r^{-1} near the center, where the characteristic surface density Pi=130alpha^{-1} Msun pc^{-2} and alpha is a fundamental constant of order unity fixed by the Lagrangian of the co-variant theory if neglecting environmental effects. We show with a toy analytical model and a hydrodynamical simulation that a constant background gravity due to MOND/TeVeS scalar field implies a critical pressure synchronizing starbursts and the formation of galaxy bulges and ellipticals. A universal threshold for the formation of the brightest regions of galaxies in a MONDian universe suggests that the central BHs, bulges and ellipticals would respect tight correlations like the M_{bulge}-M_{BH}-sigma relations. In general MOND tends to produce tight correlations in galaxy properties because its effective halo has less freedom and scatter than CDM halos.