[Paper Review] The Halo Occupation Distribution of HI Galaxies

J. Stuart, M. J. I. Brown+2 more|ArXiv.org|Aug 20, 2009

Galaxies: Formation, Evolution, Phenomena2 references6 citations

TL;DR

This study uses the halo occupation distribution (HOD) formalism to analyze the spatial clustering of HI-selected galaxies from the HIPASS survey, revealing that less than 20% are satellites and that HI-rich satellites preferentially reside in group-mass halos (M < 10^13.7–14.3 M⊙), not clusters. The results indicate HI galaxies are rare in dense environments and that future deeper surveys will enable detailed study of HI distribution with environment via clustering.

ABSTRACT

We perform an analysis of the spatial clustering properties of HI selected galaxies from the HI Parkes All Sky Survey (HIPASS) using the formalism of the halo occupation distribution (HOD). The resulting parameter constraints show that the fraction of satellite galaxies (i.e. galaxies which are not the central member of their host dark matter halo) among HIPASS galaxies is &lt;20%, and that satellite galaxies are therefore less common in HIPASS than in optically selected galaxy redshift surveys. Moreover the lack of fingers-of-god in the redshift space correlation function of HIPASS galaxies may indicate that the HI rich satellites which do exist are found in group mass rather than cluster mass dark matter halos. We find a minimum halo mass for HIPASS galaxies at the peak of the redshift distribution of M~10^11 solar masses, and show that less than 10% of baryons in HIPASS galaxies are in the form of HI. Quantitative constraints on HOD models from HIPASS galaxies are limited by uncertainties introduced through the small survey volume. However our results imply that future deeper surveys will allow the distribution of HI with environment to be studied in detail via clustering of HI galaxies.

Motivation & Objective

• To quantify how HI-selected galaxies populate dark matter halos using spatial clustering data from the HIPASS survey.
• To determine the fraction of satellite galaxies among HI-selected galaxies and their host halo mass distribution.
• To investigate whether environmental overdensities suppress HI content in galaxies, especially in clusters.
• To constrain the minimum halo mass required to host HI galaxies and assess the role of environment in HI depletion.
• To evaluate the potential of future deep HI surveys to map HI distribution with environment using clustering statistics.

Proposed method

• Applies the halo occupation distribution (HOD) formalism to decompose the real-space and redshift-space correlation functions into 1-halo and 2-halo terms.
• Uses the observed real-space correlation function of HIPASS galaxies to constrain the amplitude of the 1-halo term, indicating significant clustering within individual halos.
• Employs redshift-space clustering to detect fingers-of-god effects, which are sensitive to satellite velocity dispersion and thus halo mass.
• Imposes constraints on HOD parameters, including the minimum halo mass (M_min ≈ 10^11 M⊙) and the upper limit on halo mass hosting HI satellites (M < 10^13.7–14.3 M⊙).
• Performs model fitting to the correlation function while accounting for systematic uncertainties due to the small survey volume.
• Compares model predictions with observed clustering to infer the environmental dependence of HI content in galaxies.

Experimental results

Research questions

• RQ1 What fraction of HI-selected galaxies in HIPASS are satellites, and how does this compare to optically selected samples?
• RQ2 What is the characteristic halo mass of the host dark matter halos for HI galaxies, particularly for those hosting HI-rich satellites?
• RQ3 Do HI-rich galaxies show a preference for group-scale or cluster-scale halos, and what does this imply for HI stripping mechanisms?
• RQ4 How does the environment influence the HI content of galaxies, especially in cluster regions?
• RQ5 To what extent do current clustering measurements from HIPASS constrain the HOD of HI galaxies, and what are the limitations due to survey volume?

Key findings

• The fraction of satellite galaxies among HIPASS galaxies is less than 20%, significantly lower than in optically selected redshift surveys.
• The lack of prominent fingers-of-god in the redshift-space correlation function suggests that HI-rich satellites, if present, are hosted in group-mass halos (M < 10^13.7–14.3 M⊙), not cluster-mass halos.
• The minimum halo mass required to host an HI galaxy at the peak of the redshift distribution is approximately 10^11 M⊙.
• Less than 10% of the baryons in HIPASS galaxies are in the form of HI, indicating that HI is a minor component of the total baryonic content.
• Degeneracies in HOD parameter space limit quantitative constraints from HIPASS, but the redshift-space correlation function helps break these by constraining satellite halo masses via velocity dispersion.
• Future deeper surveys, such as those with ASKAP, will enable more precise measurements of HI clustering and detailed study of HI distribution with environment.