Galaxy Bulges as Tests of CDM versus 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 per unit mass of gas is ~2 × 10−10 m s−2 during the starburst 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 covariant versions of MOND, which can create the mirage of a Newtonian effective dark halo of density Π r−1 near the center, where the characteristic surface density Π = 130α−1 M☉ pc −2 and α is a fundamental constant of order unity fixed by the Lagrangian of the covariant 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 black holes, bulges, and ellipticals would respect tight correlations like the Mbul − MBH − σ relations. In general, MOND tends to produce tight correlations in galaxy properties, because its effective halo has less freedom and scatter than CDM halos.