X-RAY CONSTRAINTS ON THE INTRINSIC SHAPE OF THE LENTICULAR GALAXY NGC 1332

摘要

We have analyzed ROSAT PSPC X-ray data of the optically elongated S0 galaxy NGC 1332 with the purposes of constraining the intrinsic shape of its underlying mass and presenting a detailed investigation of the uncertainties resulting from the assumptions underlying this type of analysis. The X-ray isophotes are elongated with ellipticity 0.10-0.27 (90% confidence) for semimajor axes 75″-90″ and have orientations consistent with the optical isophotes (ellipticity ～0.43). The spectrum is poorly constrained by the PSPC data and is consistent with a single-temperature Raymond-Smith plasma (T ～ 0.6 keV). However, the spectrum cannot rule out sizeable radial temperature gradients ∣ (d ln T_(gas))/(d ln R) ∣ < 0.35 (99% confidence) or an emission component due to discrete sources equal in magnitude to the hot gas. Using (and clarifying) the geometric test for dark matter introduced by Buote & Canizares (1994), we determined that the hypothesis that mass-traces-light is not consistent with the X-ray data at 68% confidence and marginally consistent at 90% confidence independent of the temperature profile of the gas; similar results are obtained considering the effects of possible gas rotation and emission from discrete sources. Detailed analysis of the mass distribution gives constraints on the ellipticity of the underlying mass of ε_(mass) = 0.47-0.72 (0.31-0.83) at 68% (90%) confidence for isothermal and polytropic models. The total mass of the isothermal models within a = 43.6 kpc (D = 20 h_(80)~(-1) Mpc) is M_(tot) = (0.38-1.7) x 10~(12) h_(80)~(-1) solar mass (90% confidence) corresponding to total blue mass-to-light ratio γ_B = (31.9-143) h_(80) γ_☉; polytropic models yield mass ranges larger by a factor of ～2 due to the uncertainty in the temperature profile. Similar results are obtained when the dark matter is fitted directly using the known distributions of the stars and gas. When possible rotation of the gas and emission from discrete sources are included flattened mass distributions are still required, although the constraints on ε_(mass), but not the total mass, are weakened considerably.