In Figure 11, we study the relationship between the fraction difference between the two dispersion measurements and the dust mass, calculated from IRAS flux densities. If it is dust that causes the difference between infrared and optical dispersion, then it is certainly the relative amount of dust in a galaxy, not the absolute amount, that is important. To study this, we record the fraction difference in relation to the ratio between the IRAS dust mass and the total B-band brightness, as shown in RC3. This gives a rough estimate of the relative importance of dust for each galaxy. Using a K-band magnitude would be preferable here because dust attenuation greatly affects the B-band, but some of the galaxies in our sample do not have near-IR photometric measurements in the same filter system or not at all. Various spectral clues have been used to study the chemical history of galaxies. The Mg2 index is perhaps the most widely used studied for this purpose; it is a measure of the flow deficit in the lines relative to the neighboring continuum. Comparing this index with our infrared data could give us useful information about these aspects of galaxies. Twenty of the galaxies in our sample have Mg2 indices in the Golev & Prugniel compilation (1998) and updates to this compilation available on HYPERCAT.2 This catalogue of published absorption line Mg2 indices has been corrected from zero point and converted to a homogeneous system. This makes it easier to compare measurements by different authors. We used measures by Trager et al.
(1998) where available; Table 3 shows the Mg2 indices used and their sources in the literature. The actual values are extracted from HYPERCAT and transformed into a standard system. Figure 14 illustrates the relationships between the Mg2 index and the EQUIVALENT WIDTH of the CO band head, our velocity dispersion measurement and the fraction difference between optical and infrared dispersions. The Mg2 index appears to be closely correlated with velocity dispersion (P = 0.105, rs = 0.383); this is the well-known and well-studied Mg ratio. Further study of the scattering, slope, etc. of this relationship with infrared velocity dispersion measurements (and not optical, as is currently done) can reveal important information about this scale relationship. The equivalent width of the CO band head is most likely not correlated with the Mg2 index (P = 0.290, rs = -0.249). This is perhaps surprising because the CO equivalent width and mg2 index appear to be significantly correlated with velocity dispersion, which could lead them to be correlated with one another. We don`t see such a correlation, which could be due to the small size of our sample. We calculate the Spearman ranking correlation coefficient (rs) and probability (P) to study a statistical correlation here; P = 1 indicates that the data are completely uncorrelated, while P = 0 indicates a complete correlation; The rs sign indicates correlation or anticorrelation. .