Mon. Not. R. Astron. Soc. 348, 1255–1274 (2004) doi:10.1111/j.1365-2966.2004.07444.x Neutral hydrogen gas in interacting galaxies: the NGC 6221/6215 galaxy group , B¨arbel Koribalski1 and John M. Dickey2 1 1Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia 2University of Minnesota, Department of Astronomy, 116 Church Street, S.E., Minneapolis, MN 55455, USA Accepted 2003 November 18. Received 2003 November 11; in original form 2003 September 15 ABSTRACT Neutral hydrogen observations of the spiral galaxies NGC 6221 and 6215 with the Australia Telescope Compact Array (ATCA) reveal a wide, two-stranded bridge of at least 3 × 108 M which can be traced between the two galaxies over a projected distance of 100 kpc. The velocity gradient of the H I bridge provides a rough estimate for the time since the encounter of 500 Myr. For NGC 6221, the brightest and most massive galaxy of the group, we derive a dynamical 10 mass of Mtot = 8 × 10 M, while its companion NGC 6215 has a mass of only Mtot ∼ 2 × 109 M. Further, we find three low-surface-brightness dwarf galaxies (Dwarfs 1, 2 and 3) in the neighbourhood of NGC 6221/15 with H I masses of 3.3, 0.6 and 0.3 × 108 M, respectively. The smallest, previously uncatalogued galaxy, Dwarf 3, lies between NGC 6221 and 6215, and may have formed out of bridge material. The brightest part of the H I bridge lies roughly halfway between the interacting galaxies, indicating that bridge gas close to NGC 6221 and 6215 may have fallen back to the galaxies. The asymmetric extensions to the H I envelope of NGC 6221 are likely to be reaccreted gas, still settling in. Also, the peculiar velocity field of NGC 6215 may be explained by accreted bridge material settling into a plane offset from the old disc. Key words: galaxies: individual: NGC 6215 – galaxies: individual: NGC 6221 – galaxies: interactions. (about five times the optical diameter of NGC 6221). Adopting a 1 INTRODUCTION −1 −1 distance of D = 18 Mpc (H 0 = 75 km s Mpc ) for the NGC NGC 6221 and 6215 are a nearby pair of southern spiral galaxies 6221/15 galaxy group (1 arcsec = 87 pc) results in a projected (see Fig. 1). Their relative proximity, and the peculiar appearance of distance between the two main galaxies of ∼100 kpc. NGC 6221 in the optical, Hα and H I (see Pence & Blackman 1984) NGC 6221 (Fig. 2, top) is a barred spiral galaxy with an optical suggest they are interacting. Individually, both galaxies have been diameter of ∼3.5 × 2.5 arcmin2 (18 × 13 kpc2) and a position angle studied over a large range of wavelengths: e.g. CO (Harnett, Loiseau (PA) of 5◦. Large amounts of dust are visible in both spiral arms & Reuter 1990; Aalto et al. 1995), Hα (Durret & Bergeron 1987; as well as along the bar. NGC 6215 (Fig. 2, bottom) is generally Ryder & Dopita 1993; Vega Beltr´an et al. 1998), X-rays (Levenson classified as a non-barred spiral galaxy and has a smaller optical et al. 2001), deep B- and H-band images (Eskridge et al. 2002), as diameter of ∼2.1 × 1.8 arcmin2 (11 × 9 kpc2), PA = 78◦. Both well as Two-Micron All Sky Survey (2MASS) J-, H- and K-band galaxies are moderately inclined with inclination angles of i = 43◦ images (Jarrett et al. 2003). For stellar population studies of NGC and 38◦, respectively. Eskridge et al. (2002) give morphological 6221, see, for example, Cid Fernandes et al. (2003) and Bonatto classifications derived from deep B- and H-band images of NGC et al. (1998). Here we present a detailed study of the neutral hydro- 6221 (SABcd, SBb) and NGC 6215 (Scd, SABbc), respectively. gen (H I) gas distribution and kinematics in NGC 6221, 6215 and For a summary of the galaxy properties, see Table 1. their surroundings. Preliminary results were reported by Koribalski The bar in NGC 6221, which is clearly visible in optical and (1996a,b). infrared images (Laustsen, Madsen & West 1987; Sandage & Bedke ◦ The systemic velocities of NGC 6221 and 6215, vHI = 1481 ± 1994), lies at a PA of 118 (Pence & Blackman 1984) and has a length 11 and 1555 ± 7kms−1 (Reif et al. 1982; see also Table 1), differ of ∼1 arcmin (5.2 kpc). Two spiral arms extend symmetrically from by less than 100 km s−1. The galaxies are separated by 18.6 arcmin the ends of the bar for ∼1 arcmin. Beyond this, the northern arm sharply turns (>90◦) and continues for another ∼2 arcmin toward the southeast. This peculiar extension of the northern spiral arm E-mail: [email protected] is relatively faint in the infrared images (see Fig. 2). The turning C 2004 RAS 1256 B. Koribalski and J. M. Dickey Figure 1. Optical UK Schmidt Telescope (UKST) B-band mosaic of the field containing the galaxies NGC 6221 (left) and 6215 (right), obtained from the Super COSMOS Sky Survey. The high density of stars in the foreground is due to the low Galactic latitude (b ∼−10◦) of the galaxies. East is to the left and North to the top. point (north of the nucleus) is marked by a blue H II region complex 1984; Vega Beltran´ et al. 1998) revealed very large non-circular mo- (Wray 1988) suggesting recent star formation, possibly induced by tions of the ionized gas, possibly as a result of streaming motions the interaction (see Section 3). In addition to this most peculiar, along the bar and tidal interactions with NGC 6215. Recent Hα south-turning arm, there are another two southern extensions which Fabry–Perot´ observations of NGC 6221 and 6215 by Oddone et al. are best seen in deep optical exposures (Pence & Blackman 1984): (1999) are promising, and we are looking forward to detailed ve- the southern spiral arm itself which seems to continue straight and, locity fields of the ionized gas. to the west of it, a faint stellar extension which has no obvious NGC 6221 and 6215 have both been detected in CO(1–0). connection to the spiral arms. All three features together give NGC Using the 15-m Swedish–ESO Submillimetre Telescope (SEST), −1 6221 a triangular appearance in deep optical images, with the base Harnett et al. (1990) obtained flux densities of I CO = 20.0 K km s toward the south. Ryder & Dopita (1993) point out that the southern (NGC 6221) and 7.1 K km s−1 (NGC 6215) at their respective arm may have split in two, with one part continuing on southward central positions. Aalto et al. (1995) measured slightly higher −1 and the other part almost joining the northern arm as if trying to CO(1–0) flux densities of I CO = 21.8 ± 0.3Kkms (NGC 6221) establish a ring. There appears to be a large amount of diffuse stellar and 8 ± 1 K km s−1 (NGC 6215). This corresponds to central = × 9 emission surrounding the peculiar southern features. molecular gas masses of MH2 1.2 10 M (NGC 6221) and Durret & Bergeron (1987) detect ionized gas from the nuclear 4.4 × 108 M (NGC 6215) assuming a standard conversion fac- / = × 20 −2 −1 −1 region (approximately the inner 10 arcsec) as well as numerous tor of N H2 I CO 2.3 10 H2 cm (K km s ) (Strong et al. H II regions along the spiral arms. The two brightest H II region 1988). complexes in the spiral arms, NE and N of the nuclear region, appear The galaxy NGC 6300, which was suggested as a group member as blue star-forming regions in the colour image by Wray (1988). by Harnett et al. (1990), has a systemic velocity of vHI = 1110 ± − The nuclear spectrum of NGC 6221 is the superposition of an H II 14 km s 1 (Reif et al. 1982; Ryder et al. 1996) and lies SE of region-like spectrum and a very weak Seyfert 2 nucleus (Veron,´ NGC 6221 at a projected distance of 278.5 arcmin (∼1.5 Mpc). Its Veron´ & Zuiderwijk 1981; Pence & Blackman 1984; Boisson & velocity and projected separation suggest that it is not part of the −1 Durret 1986; Levenson et al. 2001). Cid Fernandes et al. (2003) give NGC 6221/15 group. The galaxies NGC 6215A (vsys = 2740 km s ) 7.4±1.1 2 −1 a mean starburst age of 10 yr for the central 10 × 20 arcsec and ESO138–G001 (vsys = 2894 km s ) lie at projected distances of of NGC 6221. 11.0 and 16.2 arcmin, respectively, from NGC 6221. Their systemic Kinematical studies of NGC 6221 using several long-slit spectra velocities indicate that they are much more distant than NGC 6221 of the Hα line emission out to radii of 80 arcsec (Pence & Blackman and not dynamically related. C 2004 RAS, MNRAS 348, 1255–1274 Neutral hydrogen gas in interacting galaxies 1257 Table1. Some basic parameters of the observed spiral galaxies. References: 2 OBSERVATION AND DATA REDUCTION (1) de Vaucouleurs et al. (1991), RC3; (2) Buta (1995); (3) Boisson & Durret (1986); (4) Reif et al. (1982); (5) Harnett et al. (1990); (6) Pence & Blackman The neighbouring galaxies NGC 6221 and 6215 were observed with (1984); (7) Aalto et al. (1995); (8) Sanders et al. (1995); (9) Jarrett et al. the Australia Telescope Compact Array (ATCA) in 1995 March and (2003). August. H I line (IF 1) and wide-band 20-cm radio continuum (IF 2) data were obtained simultaneously using two ∼12-h observations NGC 6221 NGC 6215 Ref.