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The Hot, Warm and Cold Gas in Arp 227 - an Evolving Poor Group R The hot, warm and cold gas in Arp 227 - an evolving poor group R. Rampazzo, P. Alexander, C. Carignan, M.S. Clemens, H. Cullen, O. Garrido, M. Marcelin, K. Sheth, G. Trinchieri To cite this version: R. Rampazzo, P. Alexander, C. Carignan, M.S. Clemens, H. Cullen, et al.. The hot, warm and cold gas in Arp 227 - an evolving poor group. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2006, 368, pp.851. 10.1111/j.1365- 2966.2006.10179.x. hal-00083833 HAL Id: hal-00083833 https://hal.archives-ouvertes.fr/hal-00083833 Submitted on 13 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Mon. Not. R. Astron. Soc. 368, 851–863 (2006) doi:10.1111/j.1365-2966.2006.10179.x The hot, warm and cold gas in Arp 227 – an evolving poor group R. Rampazzo,1 P. Alexander,2 C. Carignan,3 M. S. Clemens,1 H. Cullen,2 O. Garrido,4 M. Marcelin,5 K. Sheth6 and G. Trinchieri7 1Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy 2Astrophysics Group, Cavendish Laboratories, Cambridge CB3 OH3 3Departement´ de physique, Universite´ de Montreal,´ C. P. 6128, Succ. centre-ville, Montreal,´ QC, Canada H3C 3J7 4GEPI, Observatoire de Paris, 5 Place Janssen, 92195 Meudon Cedex, France 5Observatoire Astronomique de Marseille Provence, 2 Place Le Verrier, 13248 Marseille Cedex 04, France 6 Division of Physics, Mathematics, and Astronomy, California Institute of Technology, 770 South Wilson Avenue, Pasadena, CA 91125, USA Downloaded from https://academic.oup.com/mnras/article/368/2/851/985382 by guest on 13 December 2020 7Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy Accepted 2006 February 6. Received 2005 November 22; in original form 2005 May 6 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,HI 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 obser- vations 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 sec- ondary 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. Key words: galaxies: evolution – galaxies: interactions – galaxies: individual: NGC 470 – galaxies: individual: NGC 474 – galaxies: kinematics and dynamics. affects the evolution of most galaxies. Whether it is the cluster or 1 INTRODUCTION the group environment that most strongly affects galaxy evolution The latest results from large spectroscopic surveys (Lewis et al. is still an open and debatable question because our knowledge of 2002) conclude that environmental influences on galaxy properties galaxy evolution within low-density environments is still very poor. are effective well outside cluster cores, at local galaxy densities more Poor galaxy aggregates deserve special attention because they typical of the group environment (Bower & Balog 2004). Given that are the simplest systems where one can investigate the effects of such environments today contain a substantial fraction of the mass interaction on their evolution. Pairs of galaxies represent about in the Universe, this would imply that the local environment strongly 10 per cent of the local non-cluster population and are particularly interesting because of the potentially accelerated effects on the sec- ular evolution compared with unpaired galaxies in low-density envi- E-mail: [email protected] ronments. Observational signatures of secular evolution are indeed C 2006 The Authors. Journal compilation C 2006 RAS 852 R. Rampazzo et al. present in binary galaxies. Optical and far-infrared (FIR) studies Table 1. Overview of Arp 227. of pairs (Sulentic 1989; Xu & Sulentic 1991; Hernandez–Toledo & Puerari 1999; Hernandez–Toledo, Dultzin-Hacyan & Sulentic 2001; NGC 470 NGC 474 References Nikolic, Cullen & Alexander 2004) show unambiguous evidence for Morphol. type SA(rs)b RSA(r)0 1 interaction-induced star formation in the spiral components of the Hel. Sys. Vel. (km s−1) 2374±3 2373±71 pairs at odds with, e.g. compact groups which show a low fraction Apparent magnitude of galaxies with signatures of a past interaction and no evidence and colours of enhanced FIR emission (Zepf & Whitmore 1991; Zepf 1993; BT 12.53±0.13 12.37±0.13 1 Verdes-Montenegro et al. 1998). (B − V)T 0.75±0.11 0.88±0.02 4 − ± ± Among physical pairs, mixed binaries, i.e. pairs composed of an (U B)T 0.10 0.05 0.38 0.03 1 − early- and a late-type galaxy, are interesting for two main reasons. (J H)2MASS 0.67 0.69 (H − K)2MASS 0.31 0.25 (i) They are clearly a site of galaxy evolution: recent Infrared Galaxy structure μ Downloaded from https://academic.oup.com/mnras/article/368/2/851/985382 by guest on 13 December 2020 Space Observatory (ISO) and Hα observations of 17 mixed pairs Effective surf. bright. e(B) 22.04 22.96 4 indicate that some of the early-type components are cross-fuelled Effective aperture Ae(B) (arcsec) 57.2 64.6 4 ε ± ± by their spiral companions (Domingue et al. 2003). Support for this Average ellipticity 0.31 0.09 0.16 0.12 4 PA (deg) ≈ 63 ≈33 4 comes from the evidence of a significant population of early-type Fine structure parameter () 5.26 5 components with active galactic nucleus/low-ionization nuclear Kinematical parameters emission-line region (AGN/LINER)/H II galaxy spectroscopic prop- −1 Vel. disp. σ 0 [km s ] stars 56±31 163±5 2 and 3 −1 erties in mixed pairs compared to the results from a similar survey Vel. disp. σ 0 [km s ] gas 84±82 −1 of spiral–spiral pairs (Xu et al. 2000). Longhetti et al. (1999, 2000) Max. rotation V max [km s ]30±63 analysed line-strength indices of 30 elliptical galaxies (E) mem- Arp 227 as a pair bers of interacting pairs suggesting that at least a fraction of them Projected separation (arcmin) (kpc) 5.4 (49.6) − should have experienced a secondary episode of star formation V (km s 1) ≈0 which could explain their apparent youth in the Hβ versus MgFe Adopted distance (Mpc) 31.6 plane. References: (1) de Vaucouleurs et al. (1991); (2) Vega Beltran et al. (2001); (ii) In a hierarchical evolution scenario, pairs could represent the (3) Simien & Prugniel (2000) and (4) Pierfederici & Rampazzo (2004). The debris of older associations evolving towards more compact but large position angle variation in NGC 470 is due to the bar presence. The poorer associations (Diaferio, Geller & Ramella 1994; Governato position angle and ellipticity variation with radius have been fixed in the et al. 1996). The elliptical in mixed pairs could be the result of NGC 474 outskirts due to the presence of shells. The adopted distance is = −1 −1 a merger (see e.g. Rampazzo & Sulentic 1992) and the pair itself obtained using H 0 75 km s Mpc . The fine structure parameter, , a way station towards isolated elliptical galaxies as suggested by is obtained from Schweitzer et al. (1990). simulations (see e.g. Barnes 1996; Struck 1999) and recently by Fig. 1 shows the shell system around NGC 474 only partially X-ray observations (see e.g. Mulchaey 2000). visible in Digitized Sky Survey (DSS) images. The complex system With the present work, we present a multiphase study of the gas of shells in NGC 474 extends for several arcmin. Some of the fea- in Arp 227, a prototypical example of a galaxy–galaxy encounter tures are more reminiscent of tail/loop structures than shells.
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