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The term "HI content" of a galaxy has often lent itself to some confusion. The simplest and most obvious meaning is the mass of HI within the entire galaxy. As with absolute magnitude, however, this quantity is usually uncertain by a factor that goes with the square of the assumed distance scale. The ratio of the hydrogen mass to optical luminosity, MH / L, being equal to the ratio of the related fluxes, is conveniently distance independent, and the label "HI content" has frequently been applied also to this quantity, contributing a measure of ambiguity. Here we shall maintain the term for the total HI mass, MH, or, more properly, for the scaled quantity h2 MH, where h is related to the distance scale parameter in the form H0 = 100 h km s-1 Mpc-1. Remember that the total mass, derived from the maximum rotational velocity and a disk radius, scales linearly with h, and hence we will utilize the quantity h MT. In this section, we will address the following question: how do global properties of a galaxy which are inferred from 21-cm observations, i.e., h2 MH, hMT, and the scale length of the HI distribution, correlate with global properties, such as luminosity, linear size, and morphological type, which are measured at other wavelengths? A caveat is necessary at this stage: it has been found that the relationships between those parameters which are intrinsic to a galaxy's structure are not unambiguously defined, unless account is taken of the peculiarities of the environment surrounding that galaxy. Galaxies that live in high-population-density regions exhibit notable deviations in their behavior and possibly in their evolution. In Section 12.6, we discuss the evidence that such perturbations occur. Here we will discuss the properties of unperturbed galaxies - those that have most likely spent their entire existence in relative isolation - and will assume, somewhat arbitrarily, that they define the standards of normalcy.

12.5.1. Relations Between Global Properties for Spirals (Sa and Later)

Derived from the sample of isolated galaxies of Haynes and Giovanelli (1984), Table 12.1 lists the average values of log(h2 MH) separately for various morphological-type groupings. The units of mass and luminosity are solar; the units of diameter are kpc and the errors are the standard deviations appropriate for individual objects in each sample (not for the mean). The number of objects used in each determination is also given. The HI mass, like other global properties of galaxies within a morphological-type group, exhibits wide variations that correlate reasonably well with blue luminosity, and less so with near-infrared ones. The scatter about the mean values of the ratio between HI mass and blue luminosity is reduced, with respect to that about HI mass means alone. As shown in Table 12.1, these two parameters exhibit variation not only within a single morphological class but also between different classes. For example, the MH / LB, ratio is about twice as large for Sc's as for Sa's. However, the difference between these mean values for even the two extremes of spiral morphology (Sa, later than Sc) is only of the same order as the rms scatter within the various type groups. This large scatter is intrinsic to each statistical grouping, not the result of measurement errors. It arises in part because integrated luminosities contain the contributions of both a disk stellar component and a bulge, which are quite different from both a dynamical and an evolutionary, viewpoint; the HI mass, on the other hand, is strictly a disk, extreme Population I component. Galaxies with widely different disk-to-bulge luminosity ratios coexist even within groups of objects sharing the same morphological label. The size of the major axis of the optical image of a galaxy, on the other hand, most likely represents a disk property, and one may expect a tight correlation with the HI mass. The logarithm of the ratio of the HI mass to the square of the linear blue major diameter, also a distance-independent ratio, exhibits a much reduced scatter about the mean for each type grouping, as listed in Table 12.1. In addition, the strong morphological-type dependence seen in MH / LB averages is greatly reduced. Involving as it does an HI mass and an optically defined surface area, the MID' ratio is a hybrid form of mean HI surface density. As discussed in Section 12.1, there is little evidence for a morphological-type dependence of the ratio between HI and blue radius; thus; the relative type independence of MH / D2 suggests that the mean surface density of HI is nearly invariant (within a factor of 2) among spiral galaxies.

Table 12.1. Average global properties.

Type(s) N log(h2 MH) log(MH / LB) log(MH / D2)

Sa, Sab 38 9.37 + 0.47 -0.55 ± 0.41 6.69 + 0.32
Sb 74 9.67 + 0.42 -0.44 + 0.37 6.83 + 0.26
Sbc 38 9.54 + 0.43 -0.32 + 0.32 6.85 + 0.19
Sc 72 9.40 + 0.40 -0.28 ± 0.31 6.79 ± 0.19
> Sc 40 8.93 + 0.73 -0.04 + 0.33 6.87 + 0.17

HI content can also be measured as a fraction of the total dynamical mass, the latter as obtained from the inclination-corrected HI velocity width and an assumed width-mass relation (the rotation curve). The mean values of this ratio for the various morphological classes are not well determined. Inclinations are sometimes difficult to estimate, samples are reduced by the exclusion of nearly face-on galaxies, and the adoption of standard rotation curves to estimate total masses may be inappropriate for very late galaxies (such as dwarfs and irregulars), the gravity of which may be balanced by disordered, random motions rather than by rotation. Despite these caveats, the HI-to-total mass ratio varies between about 1.3% for Sa galaxies and 2.2% for Sc's, more or less monotonically through the Hubble sequence. The dispersions around these mean values are rather large, on the order of a factor of 2, comparable with the difference between mean values at the extremes of the spiral sequence. Because the current data do not conclusively indicate that the HI-to-total mass ratio is either type invariant or a good discriminator among the spiral classes, this ratio is not presently as useful an observational parameter reflecting HI content as MH / D2.

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