X-ray isophote shapes and the mass of NGC 3923

David A. Buote 0 2 Claude R. Canizares 1 0 Present address: Board of Studies in Astronomy, University of California at Santa Cruz , Santa Cruz, CA95064, USA 1 Department of Physics and Center for Space Research 37-241, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, MA 02139, USA 2 The distribution of dark matter in spiral galaxies is also far from being a solved problem - see, e.g., Broeils (1997) A B S T R A C T 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 ex 0:150:09-0:21 (90 per cent confidence) for semimajor axis a , 10 h701 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, emass 0:35-0:66 (90 per cent confidence), which exceed the intensity-weighted ellipticity of the R-band optical light, heRi 0:30 (eRmax 0:39). We conclude that mass density profiles with r , r2 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 emass 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 Sx ~ SR is significantly less than that indicated by the hard spectral component measured by ASCA. I N T R O D U C T I O N The structure of the dark matter haloes of galaxies provides important clues to their formation and dynamical evolution (e.g. Sackett 1996; de Zeeuw 1996, 1997). For example, in the cold dark matter (CDM) scenario (e.g. Ostriker 1993) there is evidence that the density profiles of haloes have a universal form essentially independent of the halo mass or Q0 (Navarro, Frenk & White 1997; though see Moore et al. 1997). The intrinsic shapes of CDM haloes are oblate-triaxial with ellipticities similar to the optical isophotes of elliptical galaxies (e.g. Dubinski 1994). The global shape of a halo also has implications for the mass of a central black hole (e.g. Merritt & Quinlan 1997). At present accurate constraints on the intrinsic shapes and density profiles of early-type galaxies are not widely available (e.g. Sackett 1996; Olling & Merrifield 1997).1 Stellar dynamical analyses that have incorporated the information contained in high-order moments of stellar velocity profiles have made important progress in limiting the uncertainty in the radial distribution of gravitating mass arising from velocity dispersion anisotropy (Rix et al. 1997; Gerhard et al. 1997). However, as indicated by the paucity of such stellar dynamical measurements, the required observations to obtain precise constraints at radii larger than ,Re are extensive, and the modelling techniques to recover the phase-space distribution function are complex. It is also unclear whether this method can provide interesting constraints on the intrinsic shapes since only weak limits on the range of possible shapes have been obtained from analysis of velocity profiles out to ,2Re (e.g. Statler 1994). Interesting measurements of the ellipticity of the gravitating mass have been obtained for two polar ring galaxies (Sackett et al. 1994; Sackett & Pogge 1995) and from statistical averaging of known gravitational lenses (e.g. Keeton, Kochanek & Falco 1997), but owing to the rarity of these objects it is possible that the structures of their haloes are not representative of most early-type galaxies. Moreover, gravitational lenses, which are biased towards the most massive galaxies, only give relatively crude constraints on the ellipticity and radial mass distribution for any individual system and only on scales similar to the Einstein radius (e.g. Kochanek 1991). The X-ray emission from hot gas in isolated early-type galaxies (Forman, Jones & Tucker 1985; Trinchieri, Fabbiano & Canizares 1986; for a review see Sarazin 1997) probably affords the best means for measuring the shapes and radial mass distributions in these systems (for a review see Buote & Canizares 1997b; also see Schechter 1987 and the original application to the analogous problem of the shapes of galaxy clusters by Binney & Strimple 1978). The isotropic pressure tensor of the hot gas in early-type galaxies greatly simplifies measurement of the mass distribution over stellar dynamical methods. Moreover, since the shape of the volume X-ray emission traces the shape of the gravitational potential independent of the (typically uncertain) gas temperature profile (Buote & Canizares 1994, 1996a), the shape of the mass distribution can be accurately measured in a way that is quite robust to the possible complicating effects of multiphase cooling flows and magnetic fields (see Buote & Canizares 1997b). Presently, X-ray measurements of the mass distributions in earlytype galaxies are inhibited by limitations in the available data. The ROSAT (Tru mper 1983) Position Sensitive Proportional Counter (PSPC) (Pfeffermann et al. 1987) has inadequate spatial resolution [point spread function (PSF) ,30 arcsec FWHM] to map the detailed mass distributions for all but the largest nearby galaxies, and the limited spectral resolution and bandwidth complicate interpretation of the measured temperature profiles (Buote & Canizares 1994; Trinchieri et al. 1994; Buote & Fabian 1998). Although equipped with superior spatial resolution (PSF ,4 arcsec FWHM), the ROSAT High Resolution Imager (HRI) (David et al. 1997) has too small an effective area and too large an internal background to provide images of sufficient quality for many galaxies for radii r * Re. Among the few galaxies with detailed measurements of their radial mass profiles are NGC 507 (Kim & Fabbiano 1995), 1399 (Rangarajan et al. 1995; Jones et al. 1997), 4472 (Irwin & Sarazin 1996), 4636 (Trinchieri et al. 1994), 4649 (Brighenti & Mathews 1997), and 5044 (David et al. 1994). The shape of the gravitating mass has been measured via X-ray analysis for the E4 galaxy NGC 720 and the E7/S0 galaxy NGC 1332 and found to be at least as elongated as the optical isophotes (Buote & Canizares 1994, 1996a, 1997a). For NGC 720, which has more precise constraints, the ellipticity of the gravitating matter is emass 0:44 0:68 (90 per cent confidence) compared with the intensity-weighted ellipticity of the o (...truncated)