5.6 Size and Surface Brightness

Galaxy size is measured via the isophotal, Kron, total and effective apertures. The isophotal and total radii are shown in comparison to the integrated flux (Figure 14) demonstrating the linear trend (in log-log), and hence similar mean surface brightness for most galaxies. Note that NGC 104 is clearly deviant from this surface brightness correlation, even though the light distribution is "spheroidal", Figure 9--Milky Way globular clusters have a much higher surface brightness than galaxies, consistent with their compact and dynamically truncated nature.

The effective (or half-light) radius is compared to the isophotal radius in Figure 15, again showing a tight relation for galaxies grouped according to their morphology or Hubble type. The corresponding mean surface brightness, µJe, µHe, and µKe, is shown in Figure 16. The effective size tends to be small for early types compared to late types¨early types have a higher surface brightness relative to late-types. Interestingly, although NGC104 has a similar morphology and effective radius to that of most E/S0 types (consistent with other globulars, from the work of Frogel et al 1978), its central surface brightness is much larger than any galaxy in the sample. This result clearly demonstrates the difference in stellar radial distribution between Milky Way globulars and spheroidal/lenticular galaxies. Finally, the concentration index (Figure 17) shows no correlation with integrated flux, but does delineate early from late-types. Compared to early-types, late-type galaxies have smaller indexes and hence, smaller nuclei relative to the bulge light.

There is one additional discrepant point that is worth pointing out. The angularly large Andromeda satellite galaxy, M110 (NGC 205), is classified as an E5 spheroidal, with a supplemental classification of "peculiar". It has also been described as a "dwarf spheroidal" and low luminosity (it is physically small compared to M31, but is also remarkable given that it has a number of orbiting globular clusters; cf. van den Bergh 1981, 1998). In the NIR bands, this galaxy does not appear to be early type or spheroidal (or even all that peculiar); see Figure 1. To be sure, its effective radius and surface brightness are discrepant for early-type galaxies (see Tables 1 & 2 and Figures 16 & 17), while better matching those of late-type galaxies. Based on the NIR morphology, the Hubble type for M110 appears to be Sc or later. Its radial surface brightness distribution (Figure 18) clearly shows that it is exponential disk-dominated, most unlike E5 ellipticals, while much like Sb/Sc spirals. A late-type classification is also consistent with the observed dust lanes (hence the unusual or "peculiar" elements for an elliptical galaxy), gas content (Sage & Wrobel 1989) and nuclear star formation and dust properties (Hodge 1973; Fich & Hodge 1991; Hodge 1996; see also the review by Mateo 1998).

Contents     back     next