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4.2. NGC 4826

NGC 4826 (Fig. 4) is another prototypical example (Kormendy 1993). Sandage (1961) calls it the earliest-type Sb galaxy in the Hubble Atlas; normally such objects contain a bulge that resembles an elliptical galaxy. Although partially obscured by dust, the central brightness profile is approximately an r1/4 law. Also, the central brightness is normal for a bulge and much higher than in typical disks. But NGC 4826 does not have the velocity dispersion of a bulge, as would have been implied by the published sigma = 160 ± 16 km s-1 (see Whitmore, McElroy, and Tonry 1985). In fact, the central velocity dispersion is very low, sigma = 90 ± 5 km s-1 (Fig. 5).

Figure 4

Figure 4. A V-band image of NGC 4826 taken with the CFHT. The scale is 0.21" pixel-1; this panel is 210" wide. The Gaussian seeing dispersion radius is sigma* = 0.40". Brightness and contrast have been adjusted to illustrate the dust disk; the central part of the bulge is saturated in this print. Compare the excellent photograph in the Hubble Atlas.

Figure 5

Figure 5. Absorption-line rotation velocities V and velocity dispersions sigma along the major axis of NGC 4826 (Kormendy 1993).

Figure 6 shows the Faber-Jackson (1976) correlation between sigma and luminosity. NGC 4826 is well below the scatter for normal bulges. Whitmore, Kirshner, and Schechter (1979) and Whitmore and Kirshner (1981) long ago showed that some bulges have smaller dispersions than ellipticals of the same MB. Kormendy and Illingworth (1983) found that most of these are in barred galaxies. There were two prominent examples among unbarred galaxies, NGC 1172 (E/S0) and NGC 7457 (S0, see section 4.3). NGC 4826 is very like these.

Figure 6

Figure 6. Correlation between central velocity dispersion sigma and absolute magnitude MB for elliptical galaxies and for bulges of unbarred (SA) and barred (SB) disk galaxies. The solid line is a fit to the galaxies in the middle panel; the dashed line is a fit to the ellipticals. Except for the NGC 4826 point, this figure is from Kormendy and Illingworth (1983).

A small velocity dispersion is characteristic of disks. Kormendy and Illingworth (1983) and Kormendy (1982b) interpreted abnormally cold bulges as disk-like. A more definitive conclusion is provided by the V / sigma - epsilon diagram (Fig. 3). Like NGC 4736, NGC 4826 is above the oblate line. Therefore much of the steep central brightness profile is coming from a cold component. A bulge may also contribute, but it does not dominate the light. Kormendy (1993) therefore concludes that the central disk light in NGC 4826 has the r1/4-law brightness profile of a bulge.

Kormendy (1982b) found that many "bulges" of barred galaxies also are well above the oblate line in the V / sigma - epsilon diagram. In particular, NGC 3945 and NGC 4371 are as dominated by rotation as NGC 4736. In all of these objects, the small sigma (Fig. 6, bottom) and large V / sigma (Fig. 3) show that the central components that we thought were bulges are really largely disk light.

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