M31, M32 and M33: Radial Gradients

For M31 and M33 (Figures 3 & 4, respectively) Thilker et al. [13] have determined the UV radial light and color profiles and compared them with the distribution of ionized gas as traced by H emission. It is found that the extent of the UV emission, in both targets, is greater than the extent of the observed HII regions and diffuse ionized gas. In addition, the ultraviolet diffuse fraction in M33 using the FUV observations has been compared to the H diffuse fraction obtained from wide-field narrow-band imaging. The FUV diffuse fraction appears to be remarkably constant near 0.65 over a large range in galactocentric radius, with departures to higher values in circumnuclear regions and, most notably, at the limit of the H disk. The authors suggest that the increase in FUV diffuse fraction at large galactocentric radii could indicate that a substantial portion of the diffuse emission beyond this point is not generated in situ but rather it is scattered from dust, after originating in the vicinity of the disk's outermost HII regions. A radial variation of the H diffuse fraction was also measured: in general the H diffuse fraction is near 0.4 but it rises up to 0.6 toward the galaxy center.

Figure 3. GALEX Montage of M33. A single pointing of GALEX is capable of imaging almost the entire main body of M33. Because M33 has only a faint background population of old red stars its morphology in the UV is not too dissimilar to its appearance at the optical wavelengths. The primary difference being the contrast of the arms with respect to the disk.

Figure 4. In Gil de Paz et al. [12] the luminosity profiles and color gradients have been derived for the compact elliptical galaxy M32, a nearby companion to Andromeda. The left panel shows that there is only a very slight color gradient in this galaxy which begins only in its outer regions, if at all. Surface photometry of this galaxy is especially difficult given that it is projected on the complex background of spiral structure in M31. The right-hand panel shows the divergence of photometry based on earlier photographic photometry (circles) and the GALEX observations (triangles).

For M32, the high-surface-brightness elliptical companion to the Andromeda galaxy, Gil de Paz et al. [12] have undertaken a detailed analysis of its UV light and color profile. Early studies of the UV properties of this galaxy, first by O'Connell et al. [9] and then by Ohl et al. [10] cast some doubt on M32 as being a truly typical elliptical galaxy. Based on UIT (Ultraviolet Imaging Telescope) data it was claimed that M32 had a strong FUV-optical color gradient, but that it was inverted (negative) with respect to the gradients observed in the vast majority of other elliptical galaxies. If true, M32 would be extremely anomalous. The imaging data for M32 had to be carefully decomposed so as to properly account for the complicated background contamination by the disk of M31. The results are shown in Figure 5 where it can be seen that M32 has a very modest (positive) color gradient making M32 fully consistent in its UV properties as compared to those observed in luminous ellipticals.

Figure 5. M83: (a) FUV image (from the sky background to 22.3 AB mag arcsec-2), (b) NUV image (from the sky background to 22.7 AB mag arcsec-2), (c) UV spectral slope , from -2.5 (white) to 0.3 (black). (d)H / H ratio, from H / H = 2 to 6. White areas in this plot correspond to regions where either H or H was below 3 sky. 8" × 8" median filters were applied in the case of panels (c) & (d). (Taken from Boissier et al. [3]