The 1000 Brightest HIPASS Galaxies: The HiMass Function and_Hi

Abstract

We present a new accurate measurement of the H I mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest H I peak flux densities in the southern (δ < 0 • ) hemisphere (Koribalski et al. 2003). This sample spans nearly four orders of magnitude in H I mass (from log(M HI ⁄ M ⊙ ) + 2 log h 75 = 6.8 to 10.6) and is the largest sample of H I selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies, and show that this is a robust method, insensitive to the effects of large scale structure. The resulting H I mass function can be fitted satisfactorily with a Schechter function with faint-end slope α = −1.30. This slope is found to be dependent on morphological type, with later type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the H I mass function, including peculiar motions of galaxies, large scale structure, selection bias, and inclination effects, and quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: Ω HI = (3.8 ± 0.6) × 10 −4 h −1 75 . Low surface brightness galaxies contribute only ∼ 15% to this value, consistent with previous findings.

The simulated H I sky at low redshift

“…We subsequently calculate the atomic density as function of the thermal pressure The average H i mass density at z = 0 is plotted against the threshold thermal pressure, where atomic hydrogen is assumed to recombine from ionised, while also satisfying the condition that τ rec < τ s (and accounting for a molecular density of ρ H 2 = 1.8 × 10 7 h M Mpc −3 ). At a pressure of P/k = 155 cm −3 K (dashed line), ρ HI = 6.1 × 10 7 h M Mpc −3 (dotted line), which is consistent with the H i density from Zwaan et al (2003).…”

“…4. It is empirically found, that a threshold value of P/k = 155 cm −3 K results in an H i density of ρ = 6.1 × 10 7 h M Mpc −3 , that is similar to the derived value in Zwaan et al (2003). We will adopt this threshold value for our further analysis.…”

“…It is beyond the scope of this paper to revisit these determinations, therefore we will adopt a value of Ω H 2 /Ω HI = 0.3 that falls within the error bars of current estimates. Given the observed local value of the atomic mass density, of ρ HI = 6.1 × 10 7 h M Mpc −3 (Zwaan et al 2003), this implies a molecular mass density of ρ H 2 = 1.8×10 7 h M Mpc −3 .…”

“…The H i density is very well determined from the 1000 brightest HIPASS galaxies in Zwaan et al (2003) They deduce an H i density due to galaxies in the local universe ofρ HI = (6.9 ± 1.1) × 10 7 h M Mpc −3 orρ HI = (6.1 ± 1.0) × 10 7 h M Mpc −3 when taking into account biases like selection bias, Eddington effect, H i self absorption and cosmic variance. From Rosenberg & Schneider (2002) a value ofρ HI = 7.1 × 10 7 h M Mpc −3 can be derived for the average H i density in the universe.…”

Tracing Hi Beyond the Local Universe

“…This can either be simply summed directly from the measured H I mass function data points (providing a sufficient mass range is spanned and making sure to correct values to those appropriate for the log M H I binwidths actually used), or through the following analytic solution expressed in terms of the fitted Schechter parameters and the complete Gamma function, (Zwaan et al 2003):…”