NGC4438 and its environment at radio wavelengths

Mon. Not. R. Astron. Soc. 000, ??–?? ( ) Printed 1 February 2008 A (MN L TEX style le v1.4) NGC 4438 and its environment at radio wavelengths Ananda Hota1,2,3 , D.J. Saikia2 and Judith A. Irwin4 1 2 3 4 Joint Astronomy Programme, Indian Institute of Science, Bangalore 560 012, India National Centre for Radio Astrophysics, TIFR, Pune University Campus, Post Bag 3, Pune 411 007, India Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 106, Taiwan Department of Physics, Queen’s University, Kingston K7L 3N6, Canada arXiv:0706.3174v1 [astro-ph] 21 Jun 2007 Accepted. Received Dec. ABSTRACT We present multi-frequency radio-continuum and Hi observations of NGC 4438, the highly-disturbed, active galaxy in the Virgo cluster, with the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT). High-resolution observations of the central 1 kpc with the VLA at 4860 and 8460 MHz show the presence of an inverted-spectrum radio nucleus located between the highly asymmetric lobes of radio emission. This demonstrates that these lobes which are seen in radio continuum, Hα and x-ray wavelengths and are located at ～230 and 730 pc from the nucleus arise due to an active galactic nucleus (AGN) rather than a compact nuclear starburst. The lowfrequency radio continuum observations made with the GMRT detect the extended emission on the western side of the galaxy whose spectral index is atter at higher frequencies and suggests that it is a mixture of thermal and non-thermal emission. The Hi observations show an elongated structure which is displaced by ～4.1kpc on the western side of NGC 4438 and has a size of ～9.8 kpc and a mass of 1.8×108M⊙ . The velocity eld suggests systematic rotation, consistent with earlier observations. These observations also detect Hi emission from the disk of the galaxy with a mass of 1.2×108M⊙ . We detect a faint Hi-tail towards the north of NGC 4438 close to a stellar tail seen earlier in deep optical observations. This Hi-tail has a total extent of ～50 kpc and a mass of 1.4×108 M⊙ if it is at the distance of NGC 4438. The velocity of the Hi tail is ～ 10 km s 1 similar to that of Hi emission from IC 3355, but the possibility that the tail could be foreground Galactic emission cannot be ruled out. We discuss the di erent structures in the light of di erent models which have been suggested for this disturbed galaxy, namely ram pressure stripping, tidal and ISM-ISM interactions. Key words: galaxies: individual: NGC 4438 – galaxies: individual: IC 3355 – galaxies: nuclei – galaxies: interactions – galaxies: ISM – galaxies: kinematics and dynamics 1 INTRODUCTION The galaxy, NGC 4438 (VV188, Arp 120), which is located only about 1 (～300 kpc) from the centre of the Virgo cluster, is highly inclined (80 ) and has a very disturbed disk (Fig. 1) with various components of the interstellar medium (ISM) being visible on the western side of the disk. The basic properties of the galaxy are summarised in Table 1. Highresolution x-ray observations with the Chandra telescope show emission from a ～700 pc nuclear region, a ～2.3 kpc spherical bulge and a network of laments extending 4 10 kpc to the west and south-west of the galaxy (Machacek, Jones & Forman 2004). These regions are well correlated with similar features seen in Hα (Kenney & Yale 2002; hota@asiaa.sinica.edu.tw (AH), djs@ncra.tifr.res.in (DJS), irwin@astro.queensu.ca (JAI) c RAS Chemin et al. 2005). Di use extended radio emission at 1.4 GHz has been seen extending up to ～10 kpc on the western side of the galaxy. Some have suggested that these features on the western side might be due to interactions with the intracluster medium (ICM) of the Virgo cluster since the relative velocity of NGC 4438 is ～1000 km s 1 . A tidal encounter with the SB0 galaxy NGC 4435 at a projected distance of ～4.3 arcmin (～20 kpc) is also likely to have a ected NGC 4438. Kenney et al. (1995) have suggested that most of the features of the disturbed ISM are likely due to a highvelocity ISM-ISM collision between NGC 4438 and 4435. Numerical simulations suggest that the centres of these two galaxies have passed within 5 10 kpc of each other ～108 yr ago (Vollmer et al. 2005). Radio continuum observations of the nuclear region by Hummel & S

aikia (1991) have shown two well-de ned lobes of radio emission in this galaxy which is unusual, though not unique, for a spiral galaxy. These