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Astron. Astrophys. 343, 23-32 (1999)

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2. General characteristics of NGC 3923

The main characteristics of NGC 3923 are summarized in Table 1. This is the dominant galaxy of a small group of five objects (LGG255, Garcia 1993); the other members are one elliptical (NGC 3904), one lenticular and two spirals. The velocity dispersion of the group is 184 km s-1 (Huchra & Geller 1982). No extended X-ray emission has been detected other than that associated with the galaxy, from a possible intragroup medium (Forman et al. 1985). NGC 3923 is one of the best known examples of an elliptical galaxy with shells (e.g., Carter et al. 1998). In NGC 3923 the shells are expected to be the result of two possible processes: a low-velocity merger with a less massive galaxy which is eventually disrupted, on a long time scale of several Gyrs (Quinn 1984); or a weak interaction, which took place a couple of Gyrs ago, with a passing galaxy of much smaller mass (Thomson 1991). NGC 3923 shows no rotation on its major axis (e.g., Pellegrini et al. 1997), and small minor axis rotation (Carter et al. 1998), which is consistent with either the presence of a kinematically decoupled core, established during a merger, or triaxiality (Franx et al. 1989). The modest content of cold gas and dust in NGC 3923 is consistent with a merger with a dwarf spheroidal; this would also explain the absence of radio activity, or of enhanced star formation (Forbes 1991). In case, any enhanced activity is expected to have occurred on a shorter time-scale than that of the long-lived shells. In all other respects except the presence of shells, NGC 3923 appears to be an ordinary elliptical galaxy, whose projected image on the sky is quite flat.


Table 1. General characteristics of NGC 3923
a from de Vaucouleurs et al. 1991. PA is the position angle of the optical major axis; [FORMULA] is the total B magnitude, corrected for galactic and internal extinction; [FORMULA] is the effective radius, and [FORMULA] is the minor to major axes ratio.
b distance from Fabbiano et al. (1992), who adopt a Hubble constant of 50 km s-1 Mpc-1.
c total B-band luminosity [FORMULA], derived using the indicated distance and [FORMULA].
d central stellar velocity dispersion from McElroy (1995).
e Galactic neutral hydrogen column density from Stark et al. (1992).

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999