Sean PassmoorProf. Catherine Cress

Image courtesy of NRAO/AUI and Fabian Walter, Max Planck Institute for Astronomy

The clustering of galaxies detected by neutral hydrogen emission

Introduction

• Why Measure Clustering:– Compare galaxy populations & relationship with dark

matter

– Evolution of clustering dependent on (ΩΛ; Ω

m)

– Constrains nature of Dark Matter (e.g. Hot Dark Matter Evolves Differently)

– Need to know bias to make predictions for SKA experiments

Current Neutral Hydrogen Surveys

HIPASS Survey (Blue)• Area = 20 000 deg²• Depth z ≈ 0.02• Has 4315 HI sources

ALFALFA Survey (Red)• Area ≈ 400 deg²• Depth z ≈ 0.06• Has 1796 HI sources

The Two Point Correlation Function

Two Point Correlation Function provides a simple statistical measure of clustering

Probability of finding 2 galaxies separated by a given angle/ spatial distance

Astronomical sources behave as a power law

While For Random samples :

Angular Correlation Function

Probability of Finding 2 sources separated by a given angle

For Random Samples ω(θ)=0

Illustration: Angular Correlation Function for an Artificial Distribution

•The fake data clustered on ±10° scale

• The Random Data is evenly distributed over the field

Angular Correlation Function for HIPASS & ALFALFA

• Not as different as they appear to be.• The Deeper ALFALFA has lower signal as clustering in-

front of one another washes reduces the value

The Real Spatial Correlation Function from the Angular Correlation Function

The Limber Equation

Assume: IsotropicThe Real Space Clustering Obtained HIPASS

R0 = 2.86 ± 0.12Mpc/h & γ = 1.65 ± 0.04

ALFALFA

R0 = 2.29 ± 0.33 Mpc/h & γ = 1.63 ± 0.05

The Projected Spatial Correlation function

π is the Radial Separation of the objects σ is the angular separation distance at the Avg. distance

This provides a second method to calculate the real space correlation function

The Projected Spatial Correlation function

This provides a second method to calculate the real space correlation function

π is the Radial Separation of the objects σ is the angular separation distance at the Avg. distance

Comparing the two correlation functions

• Angular Correlation Function – Uses redshift distribution– As the survey gets deeper the signal gets smaller

• Projected Spatial Correlation Function– Uses redshift information about each object

R0 from the Angular correlation function

R0 from the Projected Spatial Correlation function

Summary

We find agreement between the clustering strengths of the two surveys.

HI-selected galaxies less clustered than optically selected galaxies.- Stripping of could gas in dense environments

The Clustering and bias of HI is important for SKA / MeerKAT.

The clustering compared to Optical galaxies

2° declination strip

Mass limit for a 7σ detection of a galaxy width of 100 km.s-1

Note that due to RFI, ALFALFA is blind to cosmic emission between about 15000 and 16000 km.s−1

Amélie Saintonge et al. (2008)