Environment from cross-correlations:
linking cause to effect in satellite quenching

There is significant evidence suggesting that galaxies evolve differently depending on the environment they live in: dense regions of the universe host primarily red ellipticals, while blue spirals occupy the more under-dense regions. These properties are found to be governed by the star formation activity which is heavily influenced by the environment that a galaxy lives in. Despite decades of research, little progress has been made in determining which processes are driving this evolution. We hypothesise that the reason for this is that, until recently, it has not been possible to directly measure the local physical conditions around galaxies. Instead, existing studies have focussed on optical proxies for local environment - from galaxy observations alone - and compared these with observed galaxy properties. However, there has been a revolution in recent years; with large area, precise, and accurate galaxy surveys in addition to CMB and X-ray instruments, it is now possible to directly constrain the local hot gas and dark matter properties. The process can be carried out by employing map-based techniques, previously used exclusively in cosmology on CMB and lensing data. Cross-correlating these direct measures of ICM and halo properties with galaxy properties can effectively constrain the processes of environmental quenching.

My work primarily involves making corresponding maps of galaxy properties like the one shown above. I then compute two-point statistics to determine whether there is a correlation between any two given maps. By using one map of environmental measures and another of galaxy properties which respond to environmental effects, it is possible detect statistically significant signals from currently available data. These signals are the first direct detections of environmental cause and effect correlation.

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