The interacting-galaxy pair NGC 1531/2 in Eridanus, in a color-composite CCD image (1532 is the big spiral). This is one of the nearest strongly interacting pairs, and has been relatively little studies, first because of its southern declination, and second because its orientation makes projection effects difficult to untangle. (However, a nice photograph from Las Campanas did appear in the NASA Galaxy Atlas by Sandage and Bedke and in the Carnegie Atlas). From our simulation atlas, it seems that this is very much an M51-like system, with the companion in a direct but significantly inclined orbit. The countertail (as seen on the south side of M51 and even more extensively in neutral hydrogen in that system) is almost straight in projection in NGC 1532, extending well outside this field toward the southwest (lower right). The tidal arm extending toward the companion NGC 1531 is likewise probably much straighter than it appears from our vantage point.
Like many interacting systems, we see active star formation in NGC 1531/2, especially in the brilliant clusters in the front arm of NGC 1532. In addition, material may have been transferred to NGC 1531, which shows an S-shaped dust lane crossing its center (probably invisible in this display, which was tailored for fainter regions). This kind of symmetry is characteristic of a plane of material precessing in a distributed mass, as seen in such galaxies as Centaurus A.
This color composite was produced starting with B and I exposures using a Tektronix 2048 CCD at the f/13.5 Cassegrain focus of the 1.5m telescope at Cerro Tololo Inter-American Observatory, by Bill Keel, Ray White III, and Chris Conselice. This display has been averaged to blocks of 4x4 original pixels, with a logarithmic scale to show detail over the wide brightness range from faint to brightest stars. The field covers an area slightly more than 8 arcminutes square.