HCG 90 — Four Galaxies Entwined by Gravity, 110 Million Light-Years Away
Hickson Compact Group 90 lies in the constellation Piscis Austrinus, about 110 million light-years from Earth.
NGC 7173 is an elliptical galaxy spanning about 65,000 × 45,000 light-years.
NGC 7176, another elliptical, measures roughly 64,000 × 50,000 light-years.
NGC 7174 is a barred spiral (SB(s)bc pec) about 74,000 × 35,000 light-years, strongly warped by tidal forces.
NGC 7172 is an unbarred spiral (SA(s)ab) seen edge-on, nearly 125,000 × 55,000 light-years in size. Its prominent dust lane hides much of the disk, but faint blue patches at the disk ends reveal star formation in its outer spiral arms, while the core hosts a Seyfert-2 active nucleus.
Together, these galaxies occupy an area of about 7′ × 5′ on the sky, corresponding to a physical span of 220,000 × 160,000 light-years. A faint halo of diffuse starlight envelopes the group — evidence of stars stripped from their galaxies by gravity. Remarkably, this intragroup light may account for as much as 40% of the system’s total starlight, showing how advanced the interactions have become.
The group is also poor in neutral gas, a sign that star formation is being quenched as material is redistributed. Faint dwarf companions have been detected as well, some likely born from tidal debris.
HCG 90 provides a striking view of galaxies in transition — where close encounters sculpt structure, strip stars into a common halo, and set the course for eventual merger into a single, larger system.
Imaged in LRGB on my Planewave CDK 1000 at Observatorio El Sauce, Chile.
Image acquisition and processing: Mike Selby