Infall of the Elliptical Galaxy NGC 1404 into the Fornax Cluster

We use three Chandra observations, totalling 134.3 ks, to constrain thedynamical motion of NGC1404 falling towards the dominant elliptical NGC1399through the Fornax ICM. NGC1404's surface brightness profile shows a sharp edge\~8 kpc from its center in the direction of NGC 1399, characteristic ofjump-like temperature and density discontinuities from ram-pressure strippingof 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 brightnessprofile to fit the edge position, power law index of the density distributioninside the edge, and density discontinuity at the edge. The electron densityinside the edge (3.9-4.3 x 10^{-3} cm^{-3}) depends strongly on the gasabundance; while the density of the ICM (7-8 x10^{-4} cm^{-3}) depends stronglyon 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 Machnumber 0.83-1.03 and velocity 531-657 km/s for NGC1404 relative to the ICM. Theinclination angle of the motion inferred using the relative radial velocitybetween NGC1404 and NGC1399 as representative of that between NGC1404 and theICM, is uncomfortably large (>40 deg.) given the sharpness of the edge,suggesting either a non-zero impact parameter between the galaxies, or thatNGC1399 is moving radially with respect to the cluster ICM.Comment: 20 pages, 7 postscript figures, submitted to Ap