Kippenhahn Award for two junior MPA scientists

October 24, 2017
Two junior MPA scientists receive the Kippenhahn Award for the best MPA student publication in 2016. Titouan Lazeyras receives the prize for his paper "Large-scale assembly bias of dark matter halos" which presents high-precision measurements of halo assembly bias. Dijana Vrbanec was awarded for her paper "Predictions for the 21 cm-galaxy cross-power spectrum observable with LOFAR and Subaru"; finding that a clear anti-correlation should be seen on scales larger than the typical separation distance between galaxy clusters.

Dark matter halos play a key role in our understanding of the large-scale structure. They host the galaxies that we observe, and their clustering is thus a key stepping stone to understand the clustering of galaxies themselves. It is by now well established that the clustering of halos depends not just on halo mass, but on the halo formation history and other properties as well, such as halo shape, density profile and angular momentum. This is known as assembly bias.

Models that relate halo clustering to galaxy clustering thus need to consider more than just the halo mass. This is an issue which is expected to be a significant source of systematic error in cosmological constraints inferred from ongoing and future surveys. Conversely, it could offer rich insights into the physics of galaxy formation and how that process is connected to the galaxies' host halos.

Titouan's paper presents clean, high-precision measurements of halo assembly bias. For this, he used the novel separate universe simulation technique developed by the MPA cosmology group. His results improve upon previous results in this field in several ways: first, this technique cleanly extracts the truly large-scale clustering. Second, he showed that assembly bias exists in higher-order bias parameters as well. Third, he added several new trends, and, for the first time, was able to show assembly bias with respect to two halo properties at the same time. A key conclusion to take away from Titouan's results is that halo assembly bias is a complex phenomenon, which is not simply explained by one additional variable, such as the formation time. There is thus a lot more to explore in the field of assembly bias.


While the detection of the 21cm signal from neutral hydrogen during the Epoch of Reionization remains the best observational probe of the reionization history and the physical state of the IGM at high redshift (z>7), the weakness of the signal makes its detection extremely challenging. It has been proposed that using 21cm observations in combination with observations in other frequency bands could not only help in confirming the origin of the signal itself, but also maximizing the success of its extraction with respect to e.g. systematic effects.

In the context of the LOFAR-SUBARU collaboration, Dijana has investigated the potential of cross-correlating the 21cm signal with Lyman Alpha Emitters, using simulations of reionization to evaluate the theoretical cross-correlation, and including the characteristics of both instruments to produce mock observations and assess the feasibility of the experiment.

Dijana played the role of PI in the definition and execution of the project, and this has gained her a core membership in the EoR LOFAR Key Science Project. Her main finding is that clear anti-correlation should be seen on scales larger than ~60 Mpc/h, allowing an estimate of the typical dimension of ionized regions at various redshifts.

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