Su, Yuanyuan, Kraft, Ralph P., Nulsen, Paul E. J., Roediger, Elke, Forman, William R., Churazov, Eugene, Randall, Scott W., Jones, Christine, and Machacek, Marie E. 2017. "Capturing the 3D Motion of an Infalling Galaxy via Fluid Dynamics." The Astrophysical Journal 835:19. https://doi.org/10.3847/1538-4357/835/1/19
The Fornax Cluster is the nearest (<=slant 20 Mpc) galaxy cluster in the southern sky. NGC 1404 is a bright elliptical galaxy falling through the intracluster medium (ICM) of the Fornax Cluster. The sharp leading edge of NGC 1404 forms a classical "cold front" that separates 0.6 keV dense interstellar medium and 1.5 keV diffuse ICM. We measure the angular pressure variation along the cold front using a very deep (670 ks) Chandra X-ray observation. We are taking the classical approach-using stagnation pressure to determine a substructure's speed-to the next level by not only deriving a general speed but also directionality, which yields the complete velocity field as well as the distance of the substructure directly from the pressure distribution. We find a hydrodynamic model consistent with the pressure jump along NGC 1404's atmosphere measured in multiple directions. The best-fit model gives an inclination of 33° and a Mach number of 1.3 for the infall of NGC 1404, in agreement with complementary measurements of the motion of NGC 1404. Our study demonstrates the successful treatment of a highly ionized ICM as ideal fluid flow, in support of the hypothesis that magnetic pressure is not dynamically important over most of the virial region of galaxy clusters.