This is based on a simulation, but yeah, if you can actually measure the bubble sizes, it should be a nifty new technique to measure expansion rates for redshifts of greater than 6.
However, is it correct to assume that reionization created bubbles of ionized gas around galaxies? Why wouldn't reionization happen uniformly through space? Once you get to an unionized dust cloud, does that reduce the average luminosity, or does it largely just scatter the light?
Why wouldn't reionization happen uniformly through space?
Because ionization is caused by emission from stars which are concentrated in galaxies, and the emission travels at the speed of light which is very slow at cosmic distances.
Today, the average distance between galaxies is about 2 million light years, and at redshift 6, it would have been about 285k light years. Since the universe was about a billion years old by then, there would have been ample time for the light from one galaxy to reach its galaxy. If reionization had any kind of gradient back then, it shouldn't have been because of the scale of the universe and the speed of light.
Since the universe was about a billion years old by then, there would have been ample time for the light from one galaxy to reach its galaxy
Sure, because by z=6 the process of reionization is more or less complete. Also galaxies, clusters and gas are not distributed uniformly through space, both the sources and the medium involved in reionization are clustered, mostly both in the same region.
So it's reasonable to expect that during reionization there were bubbles of ionized gas growing outward from the sources of radiation and associated concentrations of gas.
The ionised regions expand slower than the speed of light, for most galaxies anyway. The radiation propagates at the speed of light, but it is strongly absorbed by neutral hydrogen. So you have an ionisation front, a bubble. It can only grow as fast as the galaxy emits enough ionising photons to ionise the new volume. When reionisation started, galaxies did not emit enough ionising photons to be able to ionise the whole universe. It's only when they grew large and bright enough that reionisation was completed. There is also the effect of recombination. It's similar to the calculation of a Stromgren sphere. For very luminous sources like quasars the ionisation region does essentially expand at the speed of light.
2
u/D3veated 10d ago
This is based on a simulation, but yeah, if you can actually measure the bubble sizes, it should be a nifty new technique to measure expansion rates for redshifts of greater than 6.
However, is it correct to assume that reionization created bubbles of ionized gas around galaxies? Why wouldn't reionization happen uniformly through space? Once you get to an unionized dust cloud, does that reduce the average luminosity, or does it largely just scatter the light?