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)