Galaxies fall into two clearly distinct types: active, blue-sequence galaxies that are rapidly forming young stars, and passive red-sequence galaxies in which star formation has almost completely ceased. These sequences are closely related to the visual galaxy classification system, based on the prominence of spiral arms first suggested by Edwin Hubble.
Numerical simulations by the EAGLE collaboration shows that these sequences, and thus the Hubble sequence of galaxies, are created by a competition between star formation-driven outflows and gas accretion onto the supermassive black hole at a galaxy’s centre.
In galaxies containing fewer than 30 billion solar-like stars, young stars, exploding as supernovae, are able to drive buoyant outflows that prevents high density gas building up around the black hole. As the galaxies increase in stellar mass, they convert a larger fraction of the inflowing gas into stars and grow along the blue sequence.
More massive galaxies, however, are surrounded by a hot corona and this has important ramifications. The supernova-driven outflow is no longer buoyant and star formation is unable to prevent the build up of gas in the galaxy, and around the black hole in particular. This triggers a strongly non-linear response from the black hole. Its accretion rate rises rapidly, heating the galaxy’s corona and disrupting the incoming supply of cool gas. The galaxy is starved of the fuel for star formation and makes a transition to the red sequence, and further growth predominantly occurs through galaxy mergers. Interestingly, our own Milky Way galaxy lies at the boundary between these two galaxy sequences suggesting that it will undergo a transition to a red and `dead’ early-type galaxy in the near future.
This model has deep implications for understanding the Universe. Far from being exotic predictions of General Relativity, the growth of supermassive black holes sets a fundamental limit to the mass of star forming galaxies.
The figure on the left shows formation timescales of galaxies as a function of stellar mass in the Eagle simulation (points) and observational data (Ilbert 2015). The separation of galaxies into blue (rapidly star forming) and red (passive) sequences is clearly seen. Most low mass galaxies follow the star forming blue galaxy sequence, doubling their stellar mass every 3 billion years, but more massive galaxies have much longer star formation growth timescales (a horizontal dotted line shows the present day age of the Universe; galaxies with longer star formation timescales are shown above the line). The transition between the sequences occurs at a stellar mass of round 30 billion solar masses. While it is not possible to reliably measure the masses of black holes in observed galaxies, it is possible to do this for galaxies in the EAGLE cosmological simulation. The simulated galaxies follow the observed data closely and points are coloured by the mass of the black hole relative to that of the host halo. In red-sequence galaxies, black holes account for around 0.01% of the halo mass, but the fraction is a factor of 100 smaller in blue-sequence galaxies. Around a galaxy mass of 30 billion solar masses, there is considerable scatter in the relative mass of the black hole and in the star formation growth timescale. However, at a fixed galaxy mass, systems with a higher black hole mass have substantially longer growth timescales, implying the existence of an intimate connection between black hole mass and galaxy type.