Clustering, Proximity, and Balrog Eric Suchyta CCAPP Summer Series 01 July 2014

Outline Brief introduction to galaxy clustering New software called Balrog, particularly as it relates to clustering What happens to clustering when we include proximity effects? Simplified demonstration of the problem, connected with real data, which we hope to solve with Balrog

The Universe’s matter is clustered Credit: Millenium Simulation Project Credit: SDSS

Clustering is often measured with correlation functions Guo et al. 2013 e.g. the correlation function of galaxy number density has been measured

Anderson et al. 2013 Clustering measurements contribute to almost uncountable Astronomy analyses Parejko et al. 2013 Morrison et al. 2012

Galaxies are a biased tracer of the dark matter distribution Kaiser (1984) showed that objects which occupy only the highest density fluctuations are more strongly clustered compared to the overall matter distribution, by a ~ constant multiplicative bias factor Guo et al. 2013

Science results are not robust until systematics are understood Science results are not robust until systematics are understood. This is where Balrog enters.

The Balrog methodology is a very simple idea Start with real imaging data Draw GalSim (Jarvis et al. in prep) galaxy atop the image Simulated galaxy inherits the possibly hard to model image properties by construction Original Balrog

The Balrog methodology is a very simple idea Iterate many times to build up statistics For any quantity, do we we get back what we put in? (Balrog is quite general.) In today’s scope: are randomly placed Balrog galaxies harder to detect around the real galaxies? i.e. How do proximity effects influence clustering?

Proximity effects are not negligible in real data DES - Preliminary

The usual way of thinking about galaxy clustering

But other galaxies also influence detection y encodes the proximity effects, e.g.Galaxies missed because of excess sky subtraction

All the terms in the observed galaxy-galaxy correlation function

All the terms in the observed galaxy-galaxy correlation function

All the terms in the observed galaxy-galaxy correlation function

Let us insert Balrog galaxies (B) at random points (i. e Let us insert Balrog galaxies (B) at random points (i.e. no clustering to B)

Difficult, we can’t solve for all of these exactly Let us insert Balrog galaxies (B) at random points (i.e. no clustering to B) Difficult, we can’t solve for all of these exactly

Math aside, we know what the answer must look on scales >> proximity scale Proximity effects are pronounced in the densest regions, reminiscent of how galaxies form in the densest regions. Kaiser (1984) showed this leads to a bias on large scales between and Drawing analogy, we expect a ~ constant bias factor between and on scales significantly larger than the proximity effect scale Smaller scales…???

Test with simple simulations Generate log normal density field Proximity effects modeled by subtracting a portion of adjacent galaxies’ fluxes (improper background subtraction) Making a flux cut causes some galaxies to now not be detected because of the proximity effects

Test with simple simulations Recover a nearly constant large scale bias on large scales

Test with simple simulations Recover a nearly constant large scale bias on large scales But the bias between true and observed is much larger than you might naively guess from measuring Balrog-Balrog autocorrelation or Balrog-Observed cross correlation

Test with simple simulations Recover a nearly constant large scale bias on large scales But the bias between true and observed is much larger than you might naively guess from Balrog-Balrog autocorrelation or Balrog-Observed cross correlation

Connect with real data DES - Preliminary Simulation We’ve used Balrog on real data Balrog x Observed simulation has been tuned to same order of magnitude we measure in DES data DES - Preliminary Simulation

Wrapping up… Proximity effects can introduce difficult to measure, non- negligible bias on clustering The bias should be constant on large scales We need to nail this down in the math, with suitable approximations, and then solve it with Balrog ? ?

Backup Slides

Detailed Balrog algorithm

Building a realistic simulation source population We sample our galaxy properties from the COSMOS mock catalog (Jouvel et al., 2009) COSMOS is very deep, but intentionally devoid of large, bright objects For now one-component Sérsic models. No intrinsic shapes yet.

Test with simple simulations The bias appears to very nearly be the fraction of galaxy’s missing This is what one might naively guess for a constant bias factor: