NGC 3314a/b and NGC 3312: Ram pressure stripping in Hydra I cluster substructure

Abstract

Cluster substructure and ram pressure stripping in individual galaxies are among the primary pieces of evidence for the ongoing growth of galaxy clusters as they accrete galaxies and groups from their surroundings. We present a multiwavelength study of the center of the Hydra I galaxy cluster, including exquisite new MeerKAT H I and DECam Hα imaging which reveal conclusive evidence for ram pressure stripping in NGC 3312, NGC 3314a, and NGC 3314b through compressed H I contours, well-defined H I tails, and ongoing star formation in the stripped gas. In particular, we quantify the stripped material in NGC 3312, and NGC 3314a, which makes up between 8% and 35% of the gas still in the disk, is forming stars at ∼0.5 M⊙ yr−1, and extends ∼30−60 kpc from the main disk. The estimated stellar mass in the tails is an order of magnitude less than the H I mass. A fourth “ring” galaxy at the same velocity does not show signs of ram pressure in H I. In addition, we used the H I and stellar morphologies, combined with a Beta model of the hot intracluster medium, to constrain the real distances of the galaxies to the cluster center, and we used the chance alignment of NGC 3314b behind NGC 3314a to break the degeneracy between whether the galaxies are in front or in back of the cluster. The drag seen in the H I tails supports our preferred scenario that NGC 3312 and NGC 3314a are moving toward us as part of a foreground substructure which has already passed its pericenter and is on “out fall” from the cluster. The high surviving H I content of the galaxies may suggest that the substructure or intragroup medium can protect them from the harshest effects of ram pressure, or that the galaxies are in fact on more tangential orbits.