4 Effects of optical surface brightness and type

Above we briefly described the relations of basic global parameters of nearby galaxies with their blue luminosity or rotational velocity. [1, Bothun et al. (1997)] and some other authors argued that galaxies of high and very low surface brightness have essentially different conditions for star formation. This must lead to a difference in their HI content and global structure. In Fig. 5 the hydrogen mass-to-luminosity ratio for the LV galaxies is plotted versus the mean blue surface brightness. These data show that the relative content of hydrogen drops apparently with increasing surface brightness. It may be caused by evolutionary transformation of the gaseous component of the galaxy into its stellar population [20, (McGaugh & de Blok 1997).]

 

Figure 5: The HI mass-to-blue luminosity ratio versus the mean blue surface brightness (in mag/arcsec2). The solid line shows the least-squares regression. Some galaxies with extreme parameters, like NGC 205, are indicated with their name in the figure. The quantities "r" and "k" in a corner correspond to the regression parameters in the Cols. (4) and (7) of Table 1

 

Figure 6: The total mass-to-luminosity ratio as a function of optical surface brightness. The solid line shows the least-squares regression. Some galaxies with extreme parameters, like NGC 205, are indicated with their name in the figure. The quantities "r" and "k" in a corner correspond to the regression parameters in the Cols. (4) and (7) of Table 1

On average the hydrogen mass-to-luminosity ratio varies by a factor of $\sim30$with a maximum variation of the mean surface brightness by 5 magnitudes. A similar diagram for the total mass-to-luminosity ratio (Fig. 6) reveals only a slight correlation with the optical surface brightness of the galaxy.

According to [23, Roberts & Haynes (1994)] the mean ratios, $M_{\rm HI}/L$ and $M_{\rm HI}/M_{25},$increase smoothly from early morphological types towards late ones, but the ratio $\langle M_{25}/L\mid T\rangle$ appears to be approximately constant. In the considered volume-limited sample the early type galaxies are represented only in small numbers. Nevertheless, the regression coefficients in lines 20-22 of Table 1 agree well with the data from Roberts & Haynes derived for the Local Supercluster sample.

 

Figure 7: The total mass-to-luminosity ratio against the morphological type. The solid line shows the least-squares regression, the dashed line is the relation for a purely stellar population of galaxies caused by their evolution according to [8, Hoffman et al. (1996)]

In Fig. 7 a plot of the M₂₅/L versus morphological type is shown for our sample, where the straight line represents the linear regression. The dashed line refers to the variation of M₂₅/L along the morphological sequence for a purely stellar population of galaxies caused by their evolution [19, (Larson & Tinsley 1978;] [8, Hoffman et al. 1996).]