The Interacting Galaxy Pair
NGC 6872 / IC 4970

Key paper: Stars and Gas in the Very Large Interacting Galaxy NGC 6872 by Cathy Horellou & Bärbel Koribalski (2007, A&A 464, 155)

Abstract. The dynamical evolution of the large (>100 kpc), barred spiral galaxy NGC 6872 and its small companion IC 4970 in the southern group Pavo is investigated. We present N-body simulations with stars and gas and 21 cm HI observations carried out with the Australia Telescope Compact Array (ATCA) of the large-scale distribution and kinematics of atomic gas. HI is detected toward the companion, corresponding to a gas mass of ~1.3 x 10⁹ Msun. NGC 6872 contains ~1.4 x 10¹⁰ Msun of HI gas, distributed in an extended rotating disk. Massive concentrations of gas (~10⁹ Msun) are found at the tip of both tidal tails and towards the break seen in the optical northern arm near the companion. We detect no HI counterpart to the X-ray trail between NGC 6872 and NGC 6876, the dominant elliptical galaxy in the Pavo group located ~8 arcmin to the southeast. At the sensitivity and the resolution of the observations, there is no sign in the overall HI distribution that NGC 6876 has affected the evolution of NGC 6872. There is no evidence of ram pressure stripping either. The X-ray trail could be due to gravitational focusing of the hot gas in the Pavo group behind NGC 6872 as the galaxy moves supersonically through the hot medium. The simulations of a gravitational interaction with a small nearby companion on a low-inclination prograde passage are able to reproduce most of the observed features of NGC 6872, including the general morphology of the galaxy, the inner bar, the extent of the tidal tails and the thinness of the southern tail.

NOTES: We adopted a distance of 61 Mpc, equivalent to 200 million ly; at that distance 1 arcmin = 17.75 kpc = 58 thousand ly; the known optical diameter at the 25th B-magnitude is 107 kpc which corresponds to 350 million ly. With the ATCA we detected 21-cm neutral hydrogen (HI) emission over the same extent, but none associated with the tidal dwarf galaxy candidate to the north-east. - Msun is the mass of the Sun = 2 x 10^30 kg.

Figure caption: This composite image of the giant barred spiral galaxy NGC 6872 and its small companion IC 4970 combines visible light images from ESO's VLT with GALEX far-ultraviolet and Spitzer 3.6-micron infrared data. A previously unsuspected tidal dwarf galaxy candidate (circled) is seen in DSS optical images, but appears much more prominent in the GALEX FUV images. - Image Credit: NASA's Goddard Space Flight Center/ESO/JPL-Caltech/DSS.

Figure caption: Computer simulations of the collision between NGC 6872 and IC 4970 reproduce the basic features of the galaxies as we see them today. They indicate that IC 4970's closest encounter occurred 130 million years ago and that the smaller galaxy followed a path (dashed curve) close to the plane of the spiral's disk and in the same direction it rotates. - Image Credit: NASA's Goddard Space Flight Center, after C. Horellou (Onsala Space Observatory) and B. Koribalski (CSIRO's Australia Telescope National Facility) - see original below.

Figure 8 from Horellou & Koribalski (2007) - caption: Distribution the stars (top figure) and the gas (bottom figure) in the simulation 130 Myr after perigalacticon, when the simulated system resembles the observed one most. The position of the companion is indicated by a star. - We also simulate the velocity field.

Figure caption: A VLT three-colour composite, reproduced from one blue (B), one green-yellow (V) and one red (R) exposure, obtained with FORS1 at ANTU in the morning of Mar 29, 1999. The field size is 6.8 arcmin x 6.8 arcmin. North is to the upper right and East is to the upper left. - This is also the 25 May 1999 Astronomy Picture of the Day.

The spectacular galaxy pair NGC 6872 / IC 4970 - an introduction by Horellou & Koribalski (2007):

The southern galaxy NGC 6872 is one of the largest spiral galaxies known. Star formation is traced all along the arms, which extend over more than 100 kpc at our adopted distance of 61 Mpc. The galaxy belongs to a small group, Pavo (see our Fig. 1a). NGC 6872 is likely to be affected by tidal perturbations from the nearby companion IC 4970, a small lenticular galaxy located 1.1 arcmin to the north. Machacek et al. (2005) have discovered a more than 100 kpc long X-ray trail extending between NGC 6872 and the neighbor galaxy NGC 6876, the dominant elliptical in the group which lies about 8 arcmin (~142 kpc) to the southeast. The radial velocity of NGC 6876 is ~800 km/s lower than that of NGC 6872 (see our Table 1). The X-ray trail is hotter (~1 keV) than the undisturbed Pavo intergalactic medium (~0.5 keV) and has a low metal abundance. The authors interpret the trail as partly due to gravitational focusing of the intracluster gas into a Bondi-Hoyle wake, as the spiral galaxy moves supersonically through the intracluster medium; they point out that the trail could also consist of a mixture of intracluster gas and gas removed from NGC 6872 by turbulent viscous stripping.

Mihos et al. (1993) have modeled the dynamic evolution of NGC 6872, as it is gravitationally perturbed during the passage of the small companion IC 4970 on a low-inclination, prograde orbit. They were able to reproduce most of the observed features, although the simulated tidal tails were not as thin as the observed ones. The model predicted an accumulation of gas toward the center, which was subsequently detected in CO emission (Horellou & Booth 1997).

Notes. NGC 6872: Type = SB(s)b pec, optical diameter = 10' x 2'.
IC 4970: Type = SA0 pec:, optical diameter = 0.6' x 0.2'.

Link to NED for the pair NGC 6872 / IC 4970, NGC 6872, and IC 4970
Dave Malin's deep optical image of the galaxy pair NGC 6872 / IC 4970 and the neighbouring elliptical galaxy NGC 6876
Link to NED Images for NGC 6872, and IC 4970
2MASS JHK-color composite for NGC 6872 (7.3' x 7.3')
Astronomy Picture of the Day (25 May 1999)
Astronomy Picture of the Day (3 April 2011)