Sophie Koudmani
Virtually all large galaxies, like the Milky Way, host black holes at their centres. Some of these black holes are ‘active’, i.e. they are growing by devouring gas, releasing massive amounts of energy as the gas spirals inwards. Recent observations have revealed that at least some small galaxies (so-called dwarf galaxies) host black holes, too. This discovery represents a paradigm shift as black holes had previously not been included in dwarf galaxy models. I have used both isolated simulations of individual galaxies as well as cosmological simulations (encompassing tens of thousands of galaxies) to investigate the impact of active black holes on dwarf galaxies.
In the isolated dwarf simulations, I find that black hole activity has a clear effect on the gas outflows emanating from the galaxy disc, increasing both outflow temperatures and velocities. However, there is not a significant effect on star formation in the isolated set-up, indicating that active black holes most likely play an indirect role in regulating star formation, e.g. by hindering cosmic gas inflows and depleting the gas reservoir.
I further investigate this possibility using the cosmological simulation suite FABLE. The effect of black hole activity on outflows in dwarf galaxies persists in a realistic cosmological environment. Overmassive black holes drive hotter and faster outflows and lead to significantly reduced gas mass fractions. In the early Universe, overmassive black holes are also associated with the suppression of star formation. However, at late times, supernovae (explosions of massive stars) are the main process regulating star formation. Though a comparison between the simulated dwarfs and X-ray observations suggests that the supernovae in FABLE may be too powerful, stunting black hole activity, and that black holes might be able to affect star formation at late cosmic times, too.
Active black holes release massive amounts of energy as the gas spirals inwards. This heats up the surrounding gas and disrupts the metamorphosis of gas into stars. Until recently, it had been assumed that black hole activity was only important for star formation suppression in large galaxies. However, there is now mounting observational evidence that at least some dwarf galaxies host active black holes, too. I have used numerical simulations to determine the impact of black hole activity on dwarf galaxies. The simulations were carried out with the massively parallel AREPO (Springel, 2010) code, where fluid dynamics is discretized on a moving mesh (a slice of the mesh is shown in the centre of the poster).
In the isolated dwarf simulations (Koudmani+2019), I find that black hole activity has a clear impact on galaxy outflows, increasing both outflow temperatures and velocities. This heating effect can be seen in the gas temperature projections. However, the active black hole does not have a significant effect on star formation in the isolated set-up. This suggests that black hole activity could most likely have an indirect effect on star formation, for example by hindering cosmic gas inflows and depleting the gas reservoir, but this cannot be tested in an isolated (non-cosmological) setting.
I have used the cosmological simulation suite FABLE (Henden+2018) to investigate this possibility within a realistic cosmological environment (Koudmani+2020). Though note that, like the majority of cosmological simulations, FABLE employs strong supernovae to regulate star formation in dwarfs as supernovae had hitherto been assumed to be the main feedback process. Powerful supernovae can stunt black hole growth, so any effect from black hole activity in FABLE is likely a lower limit.
I find that galaxies with active black holes have large-scale outflows across the whole stellar mass range, as can be seen in the gas density projection of the whole simulation box which also shows the locations of low-mass galaxies with active black holes and their outflow rates as well as zoomed insets of the outflow velocity structures of three examples galaxies. To separate supernova from black hole feedback effects, I compare properties of low-mass galaxies with undermassive and overmassive black holes in the next figure. In agreement with the isolated set-up, I find that overmassive black holes in dwarf galaxies drive hotter and faster outflows. They are also associated with significantly higher mass outflow rates and lower galaxy gas mass fractions. In the early Universe, overmassive black holes can also suppress star formation in dwarfs, whilst supernovae become the dominant process regulating dwarf galaxies at late times (not shown). Though a comparison between the simulated dwarfs and X-ray observations suggests that the supernovae in FABLE may indeed be too powerful, as dwarf galaxies with highly active black holes are absent from FABLE at late times, which can be seen from the figure comparing the observed and simulated fractions of dwarf galaxies with highly active black holes throughout cosmic time. Follow-up simulations are required to test whether more realistic supernova energetics would allow the black holes to grow and radiate energy more efficiently. Otherwise the simulations are in good agreement with the observed X-ray data and we predict that future X-ray observations may uncover many more active black holes in dwarfs from earlier epochs.
References: Springel, 2010, MNRAS, 401, 791; Henden+2018, MNRAS, 479, 5385; Koudmani+2019, MNRAS, 484, 2047; Koudmani+2020, preprint (arXiv:2007.10342)