Large scale structure in the HI Parkes All-Sky Survey: Filling the Voids with HI galaxies?

Abstract

We estimate the two-point correlation function in redshift space of the recently compiled HIPASS neutral hydrogen (HI) sources catalogue, which if modeled as a power law, $\xi (r)=(r_0/r{)}^{\gamma }$, the best-fitting parameters for the HI selected galaxies are found to be $r_0=3.3\pm 0.3h^{-1}$ Mpc with $\gamma =1.38\pm 0.24$. Fixing the slope to its universal value $\gamma =1.8$, we obtain $r_0=3.2\pm 0.2h^{-1}$ Mpc. Comparing the measured two point correlation function with the predictions of the concordance cosmological model, we find that at the present epoch the HI selected galaxies are anti-biased with respect to the underlying matter fluctuation field with their bias value being $b_0\simeq 0.68$. Furthermore, dividing the HI galaxies into two richness subsamples we find that the low mass HI galaxies have a very low present bias factor ($b_0\simeq 0.48$), while the high mass HI galaxies trace the underlying matter distribution as the optical galaxies ($b_0\simeq 1$). Using our derived present-day HI galaxy bias we estimate their redshift space distortion parameter, and correct accordingly the correlation function for peculiar motions. The resulting real-space correlation length is $r_0^{\mathrm{re}}=1.8\pm 0.2h^{-1}$Mpc and $r_0^{\mathrm{re}}=3.9\pm 0.6h^{-1}$Mpc for the low and high mass HI galaxies, respectively. The low-mass HI galaxies appear to have the lowest correlation length among all extragalactic populations studied to-date. Also, we have correlated the IRAS-PSCz reconstructed density field, smoothed over scales of 5$h^{-1}$ Mpc, with the positions of the HI galaxies, to find that indeed the HI galaxies are typically found in negative overdensity regions ($δρ/ρ_{\rm PSCz} \mincir 0$).

9 pages, 8 figures, MNRAS in press

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