3. Results and conclusions
In addition to the program galaxies Leo 12 , Leo 16 , Leo 18 , Leo 44 and Leo 51 , spectroscopy was also obtained of the giant elliptical galaxies NGC 1600 and NGC 2300 as well as of M 32 itself for reference. Table 1 contains the spectroscopic data: the heliocentric radial velocity (), the velocity dispersion () and the absorption indices corrected for velocity dispersion broadening. The measurement errors of and are less than 10 km s-1. Our instrumental resolution is too low to get a correct value of for M 32 and that's why we use the measurement of Davies et al. (1987). These authors used an aperture size which is effectively the same as ours.
Table 1. Spectroscopic parameters
For one M 32 candidate - Leo 38 - we did not succeed in getting a spectrum but we obtained an image and it is included in Table 2. There, the effective radius (), the effective surface brightness at (SBe) and the total apparent magnitude () in the R band are listed. To convert this magnitude to the B band, we use a color index of 1.8 which is typical for an elliptical galaxy at (Bruzual and Charlot, 1993). We estimate the error in magnitude to be roughly 0.1 mag and in effective radius 0 :005. In Table 2, we also indicate the appearance of a substantial exponential component in the surface brightness profile.
Table 2. Photometric parameters
The Leo galaxies have all non-zero redshifts already indicating brightnesses too high for dwarf galaxies and placing them well beyond the Leo group of galaxies. All measured quantities lie within the parameter range typical for normal elliptical galaxies. In particular, the Leo galaxies obey the well-known Faber-Jackson (), Fundamental Plane and Mg relation of elliptical galaxies. As an example, we show the location of the Leo galaxies in the surface brightness-absolute magnitude diagram in Fig. 1 in comparison to the compilation of dynamically hot galaxies given by Bender et al. (1993, BBF). The mean effective surface brightness within was computed according to with B being the total apparent B magnitude. It is evident that the Leo galaxies are not similar to M 32, but are giant or intermediate ellipticals in the nomenclature of BBF. Leo 12 could also well be classified as a bright dwarf elliptical. Leo 38 has a much too low surface brightness to be an M 32 analogue and, so, even without a spectrum we can safely conclude that it is either a high luminosity giant elliptical at or a dwarf spheroidal at .
In Fig. 2, we compare the absorption indices of the Leo galaxies to the sample of elliptical galaxies given by González (1993). Most of the measured values agree well with the González sample for the same velocity dispersion. The velocity dispersion of Leo 12 is so low that our instrumental resolution is insufficient to give an accurate value. Its small absorption indices indicate also the presence of a substantial stellar disk component that fills H with emission and produces an [O III ] emission line. Therefore, Leo 12 might even be a spiral galaxy seen face-on. The Mg absorption of Leo 44 is extraordinarily low which indicates low age and may be due to a disk component. This is supported by the H absorption that is just redshifted enough to be visible at the blue end of the spectrum.
Both, the spectroscopic and the photometric parameters of the observed Leo galaxies demonstrate that they are not M 32-analogues. M 32 is still a quite unique object with only very few other galaxies having similar properties and constituting the family of compact ellipticals, however none of these is as faint as M 32. If this family is taken as the natural extension of all elliptical galaxies to lower luminosities, the absence of M 32-type objects in Leo adds further evidence that the faint end of the luminosity function of elliptical galaxies falls off very steeply at absolute blue magnitudes fainter than -17 (Binggeli et al., 1988). Our results also show that in general only spectroscopy can prove whether a galaxy is of the type of M 32.
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998