X-ray isophote shapes and the mass of NGC 3923

We present an analysis of the shape and radial mass distribution of the E4 galaxy NGC 3923 using archival X-ray data from the ROSAT PSPC and HRI. The X-ray isophotes are significantly elongated with ellipticity epsilon(x) = 0.15 (0.09-0.21) (90 per cent confidence) for semimajor axis a similar to 10 h(70)(-1) kpc and have position angles aligned with the optical isophotes within the estimated uncertainties. Applying the Geometric Test for dark matter, which is independent of the gas temperature profile, we find that the ellipticities of the PSPC isophotes exceed those predicted if M proportional to L at a marginal significance level of 85 per cent (80 per cent) for oblate (prolate) symmetry. Detailed hydrostatic models of an isothermal gas yield ellipticities for the gravitating matter, epsilon(mass) = 0.35-0.66 (90 per cent confidence), which exceed the intensity-weighted ellipticity of the R-band optical light, [E-R] = 0.30 (E-R(max) = 0.39). We conclude that mass density profiles with rho similar to r(-2) are favoured over steeper profiles if the gas is essentially isothermal (which is suggested by the PSPC spectrum) and the surface brightness in the central regions (r less than or similar to 15 arcsec) is not modified substantially by a multiphase cooling flow, magnetic fields, or discrete sources. We argue that these effects are unlikely to be important for NGC 3923, (The derived epsilon(mass) range is very insensitive to these issues.) Our spatial analysis also indicates that the allowed contribution to the ROSAT emission from a population of discrete sources with Sigma(x) proportional to Sigma(R) is significantly less than that indicated by the hard spectral component measured by ASCA.