The Distribution of Neutral Hydrogen in the Color-Magnitude Plane of Galaxies

TL;DR

This paper analyzes the distribution of neutral hydrogen in galaxies based on optical properties, revealing how HI content varies across different galaxy types and its relation to star formation, using a large survey dataset.

Contribution

It introduces the conditional HI mass function conditioned on optical properties and derives the distribution of HI density in the color-magnitude plane, revealing unbiased relations with star formation.

Findings

HI distribution peaks in the blue cloud at specific magnitudes and colors.

Luminous red, luminous blue, and faint blue galaxies dominate different mass regimes.

The distribution function of HI density is skewed with a long tail towards faint blue and luminous red galaxies.

Abstract

We present the conditional HI (neutral hydrogen) Mass Function (HIMF) conditioned on observed optical properties, $M_{\text{r}}$ ($r$-band absolute magnitude) and $C_{\text{ur}}$ ($u-r$ color), for a sample of 7709 galaxies from ALFALFA (40% data release - $\alpha.40$) which overlaps with a common volume in SDSS DR7. Based on the conditional HIMF we find that the luminous red, luminous blue and faint blue populations dominate the total HIMF at the high-mass end, knee and the low-mass end respectively. We use the conditional HIMF to derive the underlying distribution function of $\Omega_{\text{HI}}$ (HI density parameter), $p(\Omega_{\text{HI}})$, in the color-magnitude plane of galaxies. The distribution, $p(\Omega_{\text{HI}})$, peaks in the blue cloud at $M_{\text{r}}^{\text{max}}=$ $-19.25, C_{\text{ur}}^{\text{max}}=1.44$ but is skewed. It has a long tail towards faint blue galaxies and luminous red galaxies. We argue that $p(\Omega_{\text{HI}})$ can be used to reveal the underlying relation between cold gas, stellar mass and the star formation rate (SFR) in an unbiased way; that is the derived relation does not suffer from survey or sample selection.