Infall of the Elliptical Galaxy NGC 1404 into the Fornax Cluster

Abstract

We use three Chandra observations, totalling 134.3 ks, to constrain the dynamical motion of NGC1404 falling towards the dominant elliptical NGC1399 through the Fornax ICM. NGC1404's surface brightness profile shows a sharp edge \~8 kpc from its center in the direction of NGC 1399, characteristic of jump-like temperature and density discontinuities from ram-pressure stripping of the galaxy gas. We find the temperature of the galaxy gas inside the edge is \~2.8 times cooler (kT=0.55^{+0.01}_{-0.02} keV; abundance A = 0.73^{+0.65}_{-0.16} solar) than the cluster gas (kT,A = 1.53^{+0.10}_{-0.13} keV, 0.42^{+0.2}_{-0.13} solar). We use the shape of the surface brightness profile to fit the edge position, power law index of the density distribution inside the edge, and density discontinuity at the edge. The electron density inside the edge (3.9-4.3 x 10^{-3} cm^{-3}) depends strongly on the gas abundance; while the density of the ICM (7-8 x10^{-4} cm^{-3}) depends strongly on the assumed relative distance between NGC1404 and NGC1399. The pressure jump (1.7-2.1) across the edge and the cluster free-stream region implies a Mach number 0.83-1.03 and velocity 531-657 km/s for NGC1404 relative to the ICM. The inclination angle of the motion inferred using the relative radial velocity between NGC1404 and NGC1399 as representative of that between NGC1404 and the ICM, is uncomfortably large (>40 deg.) given the sharpness of the edge, suggesting either a non-zero impact parameter between the galaxies, or that NGC1399 is moving radially with respect to the cluster ICM.