Annu. Rev. Astron. Astrophys. 1997. 35: 637-675
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3.2. Bulges and Disk

Astronomical gospel declares that bulges are red and disks are blue. This is generally presumed to be derived from studies of nearby bulges. Unfortunately, there are very few data on which these rather strong statements are based. The observations were difficult to make before the advent of CCD cameras and have been lacking since then until very recently - perhaps because the problem was considered to be solved. Full two-dimensional imaging is needed for accurate bulge/disk decomposition and for exclusion of dusty areas, and large surveys with multicolor information are still rare. Notable exceptions are the recent studies of the colors of "normal" spiral galaxies by de Jong (1995, 1996) and by Balcells & Peletier (1994, Peletier & Balcells 1996).

A relationship between bulges and disks is seen clearly in their colors. We show in Figure 5 the correlation between bulge color and the color of the disk of the same galaxy, for the data of Peletier & Balcells (1996), taken from their table 1. The disk color is measured at two major axis scale lengths, and the bulge color at half an effective radius, or at 5 arcsec, whichever is the larger. Note that bulges are more like their disk than they are like each other, and the very wide range of colors evident. This sample consists of luminous (MR ltapprox -21) nearby disk galaxies that span the range S0-Sbc.

Figure 5

Figure 5. The correlation between bulge color and the color of the disk of the same galaxy, for the data of Peletier & Balcells (1996), taken from their table 1. The disk color is measured at two major axis scale lengths, and the bulge color at half an effective radius, or at 5 arcsec, whichever is the larger. Note that bulges are more like their disk than they are like each other, and the very wide range of colors evident.

The color range for the bulges is noticeably large - almost as large as is the range of colors for the disks. Furthermore, although some bulges are quite red, blue bulges clearly exist, as do red disks. The sample of de Jong (1996) includes the later morphological types of disk galaxies (Sc and Sd) and shows a similar relationship between the colors of bulge and inner disks. These data show that there is little support for sweeping statements such as "bulges are red, and disks are blue." Color data for the "hidden" disks in elliptical galaxies would be very interesting.

Furthermore, the similarity in color between inner disk and bulge has been interpreted as implying similar ages and metallicities for these two components and an implicit evolutionary connection (de Jong 1996, Peletier & Balcells 1996). Given the difficulties of disentangling the effects of age and metallicity even with resolved bulges, any quantification of "similar" must be treated with caution (see Peletier & Balcells 1996, who derive an age difference of less than 30%, assuming old populations with identical metallicities). We notice in passing that the ages of ellipticals have not been determined yet to high accuracy. Measurements of various absorption line strengths have been interpreted to indicate a wide range of ages of the central regions of ellipticals, with no correlation between age and luminosity (Faber et al 1995), but this is far from rigorously established because of the coupling of age and metallicity in their effects on line strengths.

A close association between bulges and disks has been suggested by Courteau et al (1996), on the strength of a correlation between the scale lengths of the bulge and disk; they find that bulges have about one-tenth the scale length of disks. This correlation shows considerable scatter, especially for earlier galaxies of type Sa, and relies upon an ability to measure reliably bulge scale lengths that are a small fraction of the seeing. More and better data are anticipated.

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