The Interacting Galaxy Pair
NGC 6872 / IC 4970

HI observations were carried with the ATCA in five different configurations. We used a bandwidth of 16 MHz divided in 512 channels, which corresponds to a total velocity coverage of 3431 km/s and a velocity resolution of 6.7 km/s. The data were reduced in MIRIAD and AIPS using standard procedures. The final maps obtained by combining the various data sets have a synthesised beam of 42.4" x 38.8" and a noise level per channel of 0.90 mJy, which is very close to the theoretical value of 0.85 mJy. Moment maps (zeroth order, first and second order) were obtained using signals above 2 mJy/beam.

C979: ATCA observations (EW352, 750B, 750D, 1.5B, 1.5D); beam = 42.4" x 38.8", robust (r=0.5) weighting. For details see Horellou & Koribalski (2007).

The atomic neutral hydrogen gas - as traced by the 21-cm HI line - is confined to the interacting galaxies NGC 6872 / IC 4970. There is no evidence for the presence of extended HI gas around the galaxies or in the region between NGC 6872 and the massive elliptical NGC 6876. We measure an HI flux of 17.5 Jy km/s toward NGC 6872 / IC 4970, which is less than the published single-dish measurements within the 15' beam of the Parkes antenna: 20.1 +- 0.5 Jy km/s (Horellou & Booth 1997); 21.7 Jy km/s (Meyer et al. 2004, HIPASS J2016-70 in the HICAT catalog). Since the single-dish flux is around 21 Jy km/s, as much as ~3.5 Jy km/s could be distributed in an extended area outside the main galaxies and below the detection limit of our interferometric observations.

The central region of NGC 6872 is devoid of atomic gas. This is not unusual in spiral galaxies, where most of the gas in the center is often in molecular form. From observations of the CO(1-0) line toward the center of NGC 6872, Horellou & Booth (1997) inferred a mass of molecular hydrogen, M_H2, of 9.6 x 10⁸ Msun within the central 45 arcsec. The companion galaxy, IC 4970, is clearly detected in HI (Fig. 3). It contains 1.3 x 10⁹ Msun of atomic gas.

• Stars and Gas in the Very Large Interacting Galaxy NGC 6872
  Horellou C., & Koribalski B., 2007, A&A 464, 155.
• Gas and Star Formation in the Interacting Galaxies NGC 6872/IC4970
  Horellou C., & Koribalski B., 2004, ASPC 320, 69.
• Stars and Gas in the Large Interacting Galaxy NGC 6872
  Horellou C., & Koribalski B., 2004, IAU 217, 422.
• The Gigantic Interacting Galaxy NGC 6872
  Horellou C., & Koribalski B., 2003, Ap&SS 284, 499.
• The Gigantic Interacting Galaxy NGC 6872
  Horellou C., Koribalski B., 2002, Proceedings of the Kiel conference "The Evolution of Galaxies. III- From Simple Approaches to Self-Consistent Models", eds G. Hensler, G. Stasinska, S. Harfst, P. Kroupa, C. Theis.
• The Gigantic Barred Galaxy NGC 6872
  Horellou C., Koribalski B., 2002, Proceedings of the meeting of the Societe Francaise d'Astronomie et d'Astrophysique, p. 389, eds. F. Combes and D. Barret.