NGC 5291: a massive, gas-rich and highly peculiar lenticular in the IC 4329 cluster

Abstract

NGC 5291, previously classified as a peculiar elliptical, is the brighter of an interacting pair of galaxies in the western outskirts of the IC 4329 cluster. New deep direct plates show that the pair are flanked by complexes of H II regions which extend 4 arcmin to both north and south; NGC 5291 is classified S0+ on the new plates. Spectroscopic observations confirm that the H II complexes are definitely associated with NGC 5291 and the whole system extends 200 kpc from north to south (if H0 = 50 km s –1 Mpc–1). Radio H I observations show that a very large mass (~ 1011 M⊙) of neutral hydrogen is present in the system at a range of radial velocities equal to those observed optically. Its spatial distribution is similar to that of the optical features but slightly displaced to the west. A simple analysis of the kinematics suggests that NGC 5291 has a remarkably high mass |$(\gtrsim 2 \times {10}^{12} {M}_{\odot})$| and mass/light ratio |$(\gtrsim 33 {M}_{\odot}/L_{B \odot}).$| Several possible interpretations of these observations are discussed: the preferred explanation of the extraordinary properties of the system involves the compression of an extended gaseous disc around NGC 5291 by interaction with an intracluster medium. We suggest that the outlying H II regions, some of which exceed the whole LMC in size and luminosity, may eventually become detached and form a population of blue dwarf irregular galaxies within the cluster.