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[1008.4583] A Halo Model with Environment Dependence: Theoretical Considerations
Authors:Héctor Gil-Marín, Raul Jimenez, Licia Verde
Abstract:We present a modification of the standard halo model with the goal of providing an improved description of galaxy clustering. Recent surveys, like the Sloan Digital Sky Survey (SDSS) and the Anglo-Australian Two-degree survey (2dF), have shown that there seems to be a correlation between the clustering of galaxies and their properties such as metallicity and star formation rate, which are believed to be environment-dependent. This environmental dependence is not included in the standard halo model where the host halo mass is the only variable specifying galaxy properties. In our approach, the halo properties i.e., the concentration, and the Halo Occupation Distribution --HOD-- prescription, will not only depend on the halo mass (like in the standard halo model) but also on the halo environment. We examine how different environmental dependence of halo concentration and HOD prescription affect the correlation function. We see that at the level of dark matter clustering, the concentration of haloes does not affect considerably to the dark matter correlation function. However the galaxy correlation function is extremely sensitive to the HOD details, even when only the HOD of a small fraction of haloes is modified. In particular, the galaxy correlation function is most sensitive to the minimum mass for a halo to host a galaxy and the number of satellite galaxies for a given halo mass and environment.
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Molly Swanson

Joined: 22 Jun 2009
Posts: 9
Affiliation: University College London

PostPosted: August 30 2010  Reply with quote

This paper presents an interesting new extension of the halo model. In the traditional halo model, the assumption is that the properties of halos and the galaxies that populate them only depend on the halo mass. This works well for low-redshift modeling of the 2 point correlation function, but may need to be extended to model higher order statistics, type-dependent clustering, higher redshift ranges, etc. Here the authors relax this assumption and explore a model that also allows dependence on the environment.

Their simple example application of the model defines "the environment" by classifying halos as either "node halos" or "filament halos" depending on their place in the large scale structure distribution and allows different halo properties and halo occupation distributions (HODs) for the galaxies in each type of halo.

The model is flexible in terms of how the environment is defined, but it seems to require the environment to be defined discretely (i.e., splitting into filaments and nodes, or perhaps a larger number of categories), not continuously (i.e., the density of galaxies in a surrounding sphere) though the latter could be done by binning. However, defining too many different halo types to parameterize the environment would lead to a proliferation of huge numbers of HOD parameters, which is probably not the most elegant way to do the modeling.

I think an important use of models such as this for cosmology will be the power to measure and model clustering properties beyond the two-point correlation function, which will in turn provide tools for modeling galaxy bias when using the galaxy power spectrum for cosmology. I'm particularly interested to see its application to marked correlation functions, as the authors promise to do in a forthcoming work.
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