The Curious Case of NGC 6908

Abstract

The object NGC 6908 was once thought to simply be a surface brightness enhancement in the eastern spiral arm of the nearby spiral galaxy NGC 6907. Based on an examination of near-infrared imaging, the object is shown to in fact be a lenticular S0(6/7) galaxy hidden in the optical glare of the disk and spiral structure of the larger galaxy. New radial velocities of NGC 6908 [3060 ± 16 km s^(-1) (emission); 3113 ± 73 km s^(-1) (absorption)] have been obtained at the Baade 6.5 m and the du Pont 2.5 m telescopes at Las Campanas, Chile, placing NGC 6908 at the same expansion velocity distance as NGC 6907 (3190 ± 5 km s^(-1)), eliminating the possibility of a purely chance line-of-sight coincidence. The once-enigmatic asymmetries in the disk and outer spiral structure of NGC 6907 are now explained as being due to an advanced merger event. Newly discovered tails and debris in the outer reaches of this galaxy further support the merger scenario for this system. This pair of galaxies is a rather striking example of two objects discovered over 100 years ago, whose true nature was lost until modern detectors operating at infrared wavelengths gave us a new (high-contrast) look. Other examples of embedded merger remnants may also reveal themselves in the growing samples of near-infrared imaging of nearby galaxies, and a pilot study does reveal several other promising candidates for follow-up observations.

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