Lifetime and Evolution of Giant Clumps
Roughly 60% of star forming galaxies at redshifts z~2-3 are clumpy (Guo et al 2015). In these clumpy galaxies, roughly 20% of the UV light is in several giant clumps, each containing a few percent of the total galaxy mass. This is robustly reproduced in high resolution cosmological simulations (e.g. Mandelker et al 2014), and is associated with a general state of violent disk instability (VDI).
There has been much controversy in the literature regarding the fate of these giant clumps. Some models predict that intense stellar feedback should destroy these clumps within a free fall time, making them short lived-transient features. Others predict clumps to be stable, long lived structures. In such models, clumps migrate towards the galaxy centre on orbital timescales, where they contribute to bulge growth and perhaps feed supermassive black holes at the centres of galaxies.
Using a “bathtub” toy model for the evolution of clumps in galactic disks, my collaborators and I show that clumps are predicted to survive for a broad range of parameters. Simulations of galaxy formation, both of isolated disks using RAMSES and cosmological using ART, confirm the simple toy model.
By comparing simulations with and without radiative feedback (the VELA simulations) I identify the effect of feedback on clumps. Feedback tends to make clumps less dense and more oblate, making the low mass ones more susceptible to disruption. However, high mass clumps survive and are rather unaffected by feedback as they migrate towards the galaxy centre. With the aid of the simulations, I make testable predictions regarding gradients of clump properties during migration, and suggest observations to distinguish between in situ and ex situ clumps.
Face on view of a snapshot from one of the VELA galaxies with (right) and without (left) radiative feedback. Feedback causes the gas disk to expand, reduces the amount of gas in the bulge and causes low mass clumps to disrupt. However, massive clumps survive even with stronger feedback.
Mass function of clumps in simulations with (red) and without (blue) radiative feedback. The Y axis is the average number of clumps per snapshot. The X axis is the clump mass (left) or the clump mass divided by the disk mass (right). Below roughly 10^8 solar masses, feedback causes clumps to disrupt, though above this mass the clumps are largely unaffected.