The Warm-Hot, Extended, Massive Circumgalactic Medium of NGC 3221: An XMM-Newton Discovery

Abstract

Using Suzaku data, we had found a 3.4σ evidence for the X-ray emitting warm-hot circumgalactic medium (CGM) in the L^⋆ galaxy NGC 3221. Here we present XMM-Newton data and outline an efficient, rigorous, and well-defined method to extract the faint CGM signal. We confirm the CGM detection at 4σ significance within 30-200 kpc of the galaxy. We claim with 99.62% confidence that the CGM is extended beyond 150 kpc. The average temperature of the CGM is ${2.0}_{-0.3}^{+0.2}\times {10}^{6}$ K, but it is not isothermal. We find suggestive evidence for a declining temperature gradient out to 125 kpc and for supervirial temperature within 100 kpc. While a supervirial temperature component has been detected in the Milky Way CGM, this is the first time a temperature gradient has been observed in the warm-hot CGM of any spiral galaxy. The emission measure profile is well-fit with either a β-model or a constant-density profile. Deeper data are required to constrain the temperature and density profiles. We also confirm the Suzaku result that the warm-hot CGM is one of the most massive baryon components of NGC 3221 and can account for the missing galactic baryons.