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Clustering of z>7 galaxies
Where did the first galaxies form?
How are they different from present day galaxies?
Using the recent BlueTides simulation, we were able to (for the first time) probe the clustering properties of z>7 galaxies (a.k.a. cosmic dawn epoch); is a regime which we are only beginning to explore. Current observations of z > 7 galaxies largely come from the Hubble space telescope and Hyper Suprime Cam. Upcoming facilities such as JWST and WFIRST will revolutionize the discovery potential at these early epochs. Both simulations and observations indicate that the observable galaxies at z > 7 are very strongly clustered. The clustering signal is consistent between simulation vs. observation, marking a significant success of galaxy formation at these redshifts.
The dark matter distribution (background panel) at z=8 from the BlueTides simulation. Zoom ins show gas distribution within two of the most massive halos. Red circles show positions of currently observable galaxies using HST. Green circles show galaxies observable using JWST
Comparison of z > 7 clustering measurements between simulations (solid lines) and observations (black points). The left plot shows the angular correlation function. The right plot shows the linear galaxy bias.
z>7 observable galaxies have a bias of ~6-7, which is 2-3 times higher than the highest bias measurements of SDSS Luminous Red Galaxies at z~1. These galaxies therefore probe some of the rarest peaks of the underlying density field, not accessible using low redshift (z<1) galaxies.
Current observations do not probe the small scale (one-halo) clustering. These are contributed by satellite galaxies, which exceedingly rare. A few satellites that do exist, are largely concentrated around the halo centers. Density profiles of satellites around centrals is therefore substantially steeper compared to that of the underlying dark matter. This is in contrast to low redshift simulated galaxies where satellites tend to significantly better tracers of dark matter.
Examples of halos containing centrals and satellite galaxies at z=8. Grey histogram is the dark matter distribution within the halos. Red and blue histograms show stellar distributions around central and satellite galaxies respectively.
Radially averaged distributions of satellite galaxies around central galaxies within halos of different masses. The circles show simulation predictions. The dashed lines are NFW profiles with the same overall normalization.
For more details, please refer to Bhowmick et al. 2018a, Bhowmick at al 2018b
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