From the January 2024 issue

Wild’s Triplet

By | Published: January 1, 2024

Here we have what was once thought to be a trio of interacting spiral galaxies located in the constellation Virgo the Maiden. The three galaxies are PGC 36733, which glows at magnitude 13.6, PGC 36723 (magnitude 14.1), and PGC 36742 (magnitude 14.8). (The designations come from the Catalogue of Principal Galaxies, published in 1989.)

Commonly known as Wild’s Triplet, this group is named for Paul Wild, the British-born Australian astronomer who discovered them in the 1950s. It later was cataloged as the 248th object in American astronomer Halton C. Arp’s Atlas of Peculiar Galaxies, published in 1966.

Originally, Wild, Arp, and other astronomers assumed that all three galaxies were interacting gravitationally. But recent studies have shown that only the brightest two galaxies are interacting, connected by a 200,000-light-year-long bridge of material.

The bridge is composed of gas, dust, and stars, and is usually referred to as a tidal tail. Such features form when the gravity and tidal forces of two or more galaxies pull material from the outer regions of the objects. The tail is most impressive when two spiral galaxies merge because these contain lots of gas and dust.

The pair lies some 200 million light-years away. The smaller spiral below the central bridge is much farther than that. Astronomers estimate that in about a billion years, the two interacting galaxies will combine to form a single spiral.

If you’d like to try to observe Wild’s Triplet, point the largest scope you can use 53/4° due south of Zavijava (Beta [β] Virginis). Don’t use any filter; galaxies are full-spectrum objects, and a filter will simply cut the overall brightness.