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 ε x  = 0.15 (0.09–0.21) (90 per cent confidence) for semimajor axis a  ∼ 10  h −1 70 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  ∝  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, ε mass  = 0.35–0.66 (90 per cent confidence), which exceed the intensity‐weighted ellipticity of the R ‐band optical light, 〈ε R 〉 = 0.30 (ε max R  = 0.39).  We conclude that mass density profiles with ρ ∼  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 ≲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 ε 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 Σ x  ∝ Σ R is significantly less than that indicated by the hard spectral component measured by ASCA .