We analyze a sample of 23 supermassive elliptical galaxies (central velocity dispersion larger than 330 km s-1), drawn from the SDSS. For each object, we estimate the dynamical mass from the light profile and central velocity dispersion, and compare
it with the stellar mass derived from stellar population models. We show that these galaxies are dominated by luminous matter within the radius for which the velocity dispersion is measured. We find that the sizes and stellar masses are tightly correlated, with Re ~ M*^{1.1}$, making the mean density within the de Vaucouleurs radius a steeply declining function of M*: rho_e ~ M*^{-2.2}. These scalings are easily derived from the virial theorem if one recalls that this sample has essentially fixed (but large) sigma_0. In contrast, the mean density within 1 kpc is almost independent of M*, at a value that is in good agreement with recent studies of z ~ 2 galaxies. The fact that the mass within 1 kpc has remained approximately unchanged suggests assembly histories that were dominated by minor mergers -- but we discuss why this is not the unique way to achieve this. Moreover, the total stellar mass of the objects in our sample is typically a factor of ~ 5 larger than that in the high redshift (z ~ 2) sample, an amount which seems difficult to achieve. If our galaxies are the evolved objects of the recent high redshift studies, then we suggest that major mergers were required at z > 1.5, and that minor mergers become the dominant growth mechanism for massive galaxies at z < 1.5.
We consider for the first time the implications on the modified gravity MOND model of galaxies, of the presence of dark baryons, under the form of cold molecular gas in galaxy discs. We show that MOND models of rotation curves are still valid and uni
versal, but the critical acceleration a0 separating the Newtonian and MONDian regimes has a lower value. We quantify this modification, as a function of the scale factor c between the total gas of the galaxy and the measured atomic gas. The main analysis concerns 43 resolved rotation curves and allows us to find the best pair (a0 = 0.96 10e-10 m.s-2, c = 3), which is also compatible to the one obtained from a second method by minimizing the scatter in the baryonic Tully-Fisher relation.
The stability of spiral galaxies is compared in modified Newtonian Dynamics (MOND) and Newtonian dynamics with dark matter (DM). We extend our previous simulations that involved pure stellar discs without gas, to deal with the effects of gas dissipat
ion and star formation. We also vary the interpolating function between the MOND and Newtonian regime. Bar formation is compared in both dynamics, from initial conditions identical in visible component. One first result is that the MOND galaxy evolution is not affected by the choice of the mu-function, it develops bars with the same frequency and strength. The choice of the mu-function significantly changes the equivalent DM models, in changing the dark matter to visible mass ratio and, therefore, changing the stability. The introduction of gas shortens the timescale for bar formation in the DM model, but is not significantly shortened in the MOND model. As a consequence, with gas, the MOND and DM bar frequency histograms are now more similar than without gas. The thickening of the plane occurs through vertical resonance with the bar and peanut formation, and even more quickly with gas. Since the mass gets more concentrated with gas, the radius of the peanut is smaller, and the appearance of the pseudo-bulge is more boxy. The bar strength difference is moderated by saturation, and feedback effects, like the bar weakening or destruction by gas inflow due to gravity torques. Averaged over a series of models representing the Hubble sequence, the MOND models have still more bars, and stronger bars, than the equivalent DM models, better fitting the observations. Gas inflows driven by bars produce accumulations at Lindblad resonances, and MOND models can reproduce observed morphologies quite well, as was found before in the Newtonian dynamics.
We compare N-body simulations performed in MOND with analogs in Newtonian gravity with dark matter (DM). We have developed a code which solves the Poisson equation in both gravity models. It is a grid solver using adaptive mesh refinement techniques,
allowing us to study isolated galaxies as well as interacting galaxies. Galaxies in MOND are found to form bars faster and stronger than in the DM model. In Newton dynamics, it is difficult to reproduce the observed high frequency of strong bars, while MOND appears to fit better the observations. Galaxy interactions and mergers, such as the Antennae, are also simulated with Newton and MOND dynamics. In the latter, dynamical friction is much weaker, and merging time-scales are longer. The formation of tidal dwarf galaxies in tidal tails are also compared in MOND and Newton+DM models.
We investigate how different models that have been proposed for solving the dark matter problem can fit the velocity dispersion observed around elliptical galaxies, on either a small scale (~ 20kpc) with stellar tracers, such as planetary nebulae, or
large scale (~ 200kpc) with satellite galaxies as tracers. Predictions of Newtonian gravity, either containing pure baryonic matter, or embedded in massive cold dark matter (CDM) haloes, are compared with predictions of the modified gravity of MOND. The standard CDM model has problems on a small scale, and the Newtonian pure baryonic model has difficulties on a large scale, while a fit with MOND is possible on both scales.
The LCDM model is the most commonly admitted to describe our Universe. In spite of a great success with regard to the large scale structure formation, some problems are still unresolved at galactic scales. Alternative scenarios have to be explored su
ch as modified gravity. We have developed an N-body code able to solve in a self consistent way the galactic dynamics in MOND. The first version of the code consists in solving the modified Poisson equation on a uniform Cartesian grid to derive the gravitational force on each particle. With it, we study the evolution of isolated galaxies, like the bar instability, the angular momentum transfer, etc. Galaxies in MOND are found to form stronger bars, faster than in Newtonian dynamics with dark matter. In a second step, we implement an adaptive mesh refinement technique in the code, allowing to run more contrasted simulations on larger scales, like interacting galaxies. During an interaction, the dynamical friction forces are less important in MOND, and merging times are longer than in DM models. The different morphologies of interacting galaxies in the two models are discussed. All simulations are performed in both frameworks of modified gravity and Newtonian gravity with dark matter with equivalent initial conditions.
