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Angular momentum, black holes and the characteristic mass of galaxies

One of my PhD students, Yetli Rosas-Guevara, has just submitted a very exciting paper. In it, we discuss the important of angular momentum in black hole accretion.  In effect, our new model that takes account of the whirlpool that forms as matter tries to fall into the black hole!

This can have a major impact on the accretion rates of black hole and strongly affects how black holes affect the galaxies that host them. The graph below shows an example of how one black hole grows during the simulation. The colouring of the points indicates the importance of angular momentum (redder colours mean the angular momentum is important). Focus on the upper panel which shows the rate at which matter falls in. Notice how the accretion onto the black hole is concentrated into a few events, these are triggered by galaxy mergers. As the system grows in mass, however, the gas halo around the galaxy takes longer and longer to recover from the outburst. The effect of concentrating the accretion into a few events is to create a characteristic size for galaxies. We live in a galaxy of this “characteristic size” — the Milky Way. In the near future, we’ll publish a detailed comparison between the simulations and observations as part of the EAGLE project.

Here’s the abstract of the paper:

“Feedback from energy liberated by gas accretion onto black holes (BHs) is an attractive mechanism to explain the exponential cut-off at the massive end of the galaxy stellar mass function (SMF). Semi-analytic models of galaxy formation in which this form of feedback is assumed to suppress cooling in haloes where the gas cooling time is large compared to the dynamical time do indeed achieve a good match to the observed SMF. Furthermore, hydrodynamic simulations of individual halos in which gas is assumed to accrete onto the central BH at the Bondi rate have shown that a self-regulating regime is established in which the BH grows just enough to liberate an amount of energy comparable to the thermal energy of the halo. However, this process is efficient at suppressing the growth not only of massive galaxies but also of galaxies like the Milky Way, leading to disagreement with the observed SMF. The Bondi accretion rate, however, is inappropriate when the accreting material has angular momentum. We present an improved accretion model that takes into account the circularisation and subsequent viscous transport of infalling material and include it as a “subgrid” model in hydrodynamic simulations of the evolution of halos with a wide range of masses. The resulting accretion rates are generally low in low mass ($\lsim 10^{11.5} \msun$) halos, but show outbursts of Eddington-limited accretion during galaxy mergers. During outbursts these objects strongly resemble quasars. In higher mass haloes, gas accretion occurs continuously, typically at  10 % of the Eddington rate, which is conducive to the formation of radio jets. The resulting dependence of the accretion behaviour on halo mass induces a break in the relation between galaxy stellar mass and halo mass in these simulations that matches observations.”

You can down load the paper from astro-ph …… where you will find the full caption to the figure at the top!