Simulating Galaxies for Fun and Profit

Title: Galaxy Formation with Self-consistently Modeled Stars and Massive Black Holes. I: Feedback-regulated Star Formation and Black Hole Growth
Authors: Ji-Hoon Kim, John H. Wise, Marcelo A. Alvarez, Tom Abel
First Author’s Institution: Kavli Institute for Particle Astrophysics and Cosmology; Stanford University

In previous astrobites posts, we’ve talked about how black holes eat gas and the relationship between black hole growth and galaxy evolution. We know that galaxies and black holes grow during their evolution and that something must couple the growth of galaxies and black holes to produce the observed M-σ relation between the mass of supermassive black holes and the typical velocities in galactic bulges. Today, we’ll discuss a new attempt to understand the coupled growth of galaxies and supermassive black holes by directly simulating the growth of a high redshift disk galaxy and its central black hole.

Using the cosmological hydrodynamics code enzo, the authors have come up with novel prescriptions for simulating the birth of stars and the feedback of black holes. In this simulation, molecular clouds form when gas cools and collapses. Molecular clouds in turn slowly convert a small fraction of their mass into stars, which can then explode in supernovae, supplying kinetic energy for turbulent gas motions. This is in contrast with previous simulations where gas is converted directly into stars and is more consistent with observations of star formation in the Milky Way and nearby galaxies.

The black hole can ionize, heat, and exert forces on the gas in its surroundings via both radiation pressure and by ejecting a collimated jet. This is also a significant improvement compared to previous work in which only thermal feedback was included by dumping an enormous amount of thermal energy in the center of the simulated galaxy – equivalent to setting off a bomb in the vicinity of the SMBH. This approach only works for black holes that are accreting well below the theoretical maximum accretion rate given by the Eddington limit, so-called radio-mode AGN feedback.  Future work from this group will include simulations of galaxies hosting black holes accreting near their Eddington limit, so-called Quasar-mode feedback.  For this reason, the galaxy in this simulation does not launch a wind into the IGM surrounding the galaxy.  This is a little bit troubling, since galaxy winds are ubiquitously observed around high redshift star forming galaxies.

The gas temperature in the simulation with (right) and without (left) feedback from the black hole. The black hole heats the gas in the center of the galaxy, inhibiting star formation and accretion onto the black hole.

This novel AGN feedback and star formation prescription has the net effect of producing a galaxy that can regulate its own growth. Including slow star formation in molecular clouds inhibits the runaway collapse of gas into stars. Including AGN feedback heats the gas in the core of the galaxy, making it more stable to runaway fragmentation and collapse. This significantly reduces the amount of star formation in the core of the galaxy and inhibits the accretion rate onto the black hole.  The work by Kim and collaborators is a significant improvement in both resolution and modeling compared to older simulations of the coupled evolution of galaxies and their central black holes.

About Nathan Goldbaum

Nathan is a third year graduate student in Astrophysics at UCSC. His interests include the local ISM, molecular clouds, and the role of star formation in galactic evolution.

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  1. what software was used for making the plots?

    • I believe Ji-Hoon made all of his plots in YT, a visualization package for the Enzo code.



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