Accretion mode may determine galaxy morphology.
This figure is from an analysis of gas accretion at z~0 onto 17 galaxies simulated by the FIRE collaboration. We found that thin disks in those galaxies all formed via hot-mode accretion, as visualized in the figure. Small panels along the figure’s top and right sides show evolution in the temperature and spatial distribution of accreting gas elements. The large panel at the bottom left shows representative trajectories for three of those accreting gas elements. Red, white, and blue colors indicate gas temperatures of 106 K, 105 K, and 104 K, respectively. An interactive 3-D visualization of the trajectories is available on our website.
For each accreting gas element, the time is defined to be the moment when it last cools below 105 K. In the sequence of small panels, all gas-element trajectories are synchronized to that moment. Accretion remains hot (~106 K) while the gas contracts quasi-spherically, up until 150 Myr prior to the moment of cooling below 105 K. Over the next 300 Myr, the accreting gas trajectories then change from from quasi-spherical to quasi-circular, settling into a cool disk by 150 Myr after the moment of cooling below 105 K.