1. The clustering of galaxies detected by neutral hydrogen emission Sean Passmoor Prof. Catherine Cress Image courtesy of NRAO/AUI and Fabian Walter, Max Planck Institute for Astronomy The clustering of galaxies detected by neutral hydrogen emission

2. Introduction Why Measure Clustering: –Compare galaxy populations & relationship with dark matter –Evolution of clustering dependent on (Ω Λ ; Ω m ) –Constrains nature of Dark Matter (e.g. Hot Dark Matter Evolves Differently) –Need to know bias to make predictions for SKA experiments

3. 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

4. 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 :

5. Angular Correlation Function Probability of Finding 2 sources separated by a given angle For Random Samples ω(θ)=0

6. Illustration: Angular Correlation Function for an Artificial Distribution The fake data clustered on ±10° scale The Random Data is evenly distributed over the field

7. 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

8. The Real Spatial Correlation Function from the Angular Correlation Function The Limber Equation Assume: Isotropic The Real Space Clustering Obtained HIPASS R 0 = 2.86 ± 0.12Mpc/h & γ = 1.65 ± 0.04 ALFALFA R 0 = 2.29 ± 0.33 Mpc/h & γ = 1.63 ± 0.05

9. 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

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

11. 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 R 0 from the Angular correlation function R 0 from the Projected Spatial Correlation function

12. 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.

13. 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)