Infall of the Elliptical Galaxy NGC 1404 into the Fornax Cluster

Abstract

We use three Chandra observations, totaling 134.3 ks, to constrain the dynamical motion of NGC 1404 falling toward the dominant elliptical NGC 1399 through the Fornax Cluster gas. The surface brightness profile of NGC 1404 shows a sharp edge at ~8 kpc from its center in the direction of NGC 1399, characteristic of jumplike temperature and density discontinuities from ram pressure stripping of the galaxy gas, caused by its motion through the surrounding intracluster medium (ICM). We find that the temperature of the galaxy gas inside the edge is ~2.8 times cooler (kT = 0.55 keV with abundance A = 0.73 Z☉) than the cluster gas (kT = 1.53 keV, A = 0.42 Z☉). We use the shape of the surface brightness profile across the edge to fit the position of the edge, the power-law behavior of NGC 1404's density distribution in the leading direction, and the density discontinuity at the edge. The electron density inside the edge (3.9-4.3) × 10-3 cm-3 depends strongly on the gas abundance, while the density of the ICM (7-8) × 10-4 cm-3 depends strongly on the assumed geometry (relative distance) between NGC 1404 and NGC 1399. The corresponding pressure jump of 1.7-2.1 across the leading edge of the galaxy and the cluster free-stream region implies near sonic motion (Mach number 0.83-1.03) for NGC 1404 with a velocity 531-657 km s-1 relative to the surrounding cluster gas. The inclination angle of the motion, inferred using the relative radial velocity between NGC 1404 and 1399 as representative of that between NGC 1404 and the cluster ICM, is uncomfortably large (40°) given the sharpness of the surface brightness edge, suggesting either a nonzero impact parameter between NGC 1404 and 1399 or that NGC 1399 is also moving radially with respect to the cluster ICM.