NGC 3166

Lore & Background

NGC 3166 has a bright, elongated nucleus surrounded by an elliptical bulge oriented nearly east-west. Within the central 6–7 arcseconds lies a circumnuclear star disk, with nuclear stars no older than two billion years. A low-surface-brightness bar lies nearly perpendicular to the bulge, while the fainter parts of the bulge are nearly circular, about one arcminute in diameter, and show spiral whorls. Beyond the bulge is a low-surface-brightness disk without spiral arms or knots, crossed by dust lanes. Stars in the disk have sub-solar metallicity, indicating ages greater than 8 billion years.

In ultraviolet light, the galaxy appears asymmetric, with stronger emission west of the nucleus. That western region, at the tidal arm, has a spectrum similar to an HII region, suggesting young stars. Hydrogen emission is present east of the nucleus. This asymmetry may result from recent gas accretion, and a complete star formation ring has not yet formed. An H-alpha arm is visible 100 arcseconds west of the nucleus, and the outer regions appear lopsided.

One supernova, SN 2012cw (Type Ic, magnitude 16.5), was discovered by Kōichi Itagaki on 14 June 2012. Its progenitor could not be detected in Hubble Space Telescope archive images, indicating an absolute magnitude fainter than -7. NGC 3166 forms an interacting pair with NGC 3169 at a separation of 7.5 arcminutes, and NGC 3165 lies 4.5 arcminutes to the southwest. A tidal tail connecting NGC 3166 and NGC 3169, visible in radio hydrogen line emission, extends south of the galaxy. At the tail's end, a tidal dwarf galaxy (AGC 208457) has formed, along with other gas clumps, three of which are classical dwarf irregular galaxies. These galaxies, together with NGC 3156, form the NGC 3169 Group, part of the Leo II Groups in the Virgo Supercluster.

Reader's Guide

NGC 3166 is significant as a lenticular galaxy exhibiting complex internal structure and ongoing interaction with its neighbors. Its bright, elongated nucleus and circumnuclear star disk reveal a relatively young stellar population in the core, while the older disk stars indicate a prolonged star formation history. The ultraviolet asymmetry and HII-region-like spectrum in the western tidal arm suggest recent gas accretion and localized star formation, though a complete star formation ring has not yet developed. The galaxy's interaction with NGC 3169 is evidenced by a tidal tail detected in radio hydrogen emission, which has spawned a tidal dwarf galaxy (AGC 208457) and several dwarf irregular galaxies. This system provides a laboratory for studying galaxy interactions, tidal dwarf formation, and the evolution of lenticular galaxies. The detection of SN 2012cw, a Type Ic supernova whose progenitor was undetected in Hubble images, adds to its legacy as a site of massive stellar death. NGC 3166 is part of the NGC 3169 Group within the Leo II Groups, placing it in the broader context of the Virgo Supercluster.

Did You Know?

• The galaxy has a circumnuclear star disk in its central 6–7 arcseconds, with stars no older than two billion years.
• A tidal tail connecting NGC 3166 and NGC 3169 has formed a tidal dwarf galaxy called AGC 208457.
• Supernova SN 2012cw (Type Ic) was discovered in NGC 3166 on 14 June 2012 by Kōichi Itagaki.