The hot, warm and cold gas in Arp 227 – an evolving poor group

Abstract

Arp 227 represents a prototypical example of an interacting mixed pair of galaxies located in a low-density environment. We investigate the gas properties of the pair in the X-ray, Hα, H i and CO bands. We also detect two additional members of the group in H i which indicates that the pair constitutes the dominant members of a loose group. The H i distribution shows a tail of gas that connects the spiral member, NGC 470, to the lenticular, NGC 474, showing that the two main members are currently undergoing interaction. The Hα emission reveals the presence of secondary components at the centre of NGC 470, superposed on the main component tracing the rotation of the galaxy. This latter maps a nearly unperturbed velocity field. The dominant, nearly unperturbed trend of the kinematics is confirmed by CO observations, although restricted to the centre of the galaxy. The X-ray luminosity of NGC 470 is comparable with that of a 'normal' spiral galaxy. NGC 474 on the other hand is very gas-poor and has not been detected in Hα. Its X-ray luminosity is consistent with the low end of the expected emission from discrete sources. Arp 227 as a loose group shows several signatures of galaxy–galaxy interaction. Our observations suggest the presence of signatures of interaction in the overall kinematics of the spiral companion. The ongoing interaction is clearly visible only in the outer H i halo of NGC 470. While the large shell system of NGC 474 could be associated with an accretion event, the secondary components in the Hα profile in the centre of NGC 470 could be due to the interaction with the companion. The low X-ray luminosity of NGC 470 seems to be a characteristic of dynamically young systems. All the above evidence suggest that Arp 227 is an evolving group in the early phase of its evolution and that its drivers are the accretion of faint galaxies and the ongoing large-scale interaction between NGC 470 and 474.

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