NGC 520

Lore & Background

NGC 520 was discovered by astronomer William Herschel on 13 December 1784. Halton Arp called it the second-brightest very disturbed galaxy in the sky, and it is as bright in the infrared and radio bands as the Antennae Galaxies. Simulations indicate the object consists of two galactic disks that began interacting about 300 million years ago. The system is still in an early stage of its merger, showing two separate velocity systems in the spectra and two small tails. Two galactic nuclei have been detected, one of which is an H II nucleus.

The main galactic component is being viewed edge-on, making it fainter in the optical band. The secondary component is brighter but less massive than the main, located to the northwest, and they are separated by a dark lane of dust. The region outside the nuclei experienced a period of increased star formation roughly around the time they began to interact. Two dwarf objects are located in the vicinity; one, designated UGC 957, is in the northern tidal tail and may be the result of the interaction.

When viewed in the X-ray band, the interacting galaxies appear around half as luminous as expected given their merger state. Analysis of gas and molecular features suggests the secondary merger component is gas poor, so most star formation took place in the gas-rich main component to the southeast. Fifteen X-ray sources are detected within the merger, many displaying long-term variability. A large galactic wind is evident, driven by starburst activity. One supernova has been observed: SN 2025zjx (Type II, mag. 21.68), discovered by Pan-STARRS on 29 September 2025.

Reader's Guide

NGC 520 holds significance as a nearby example of an early-stage galactic merger, providing insight into the processes that shape galaxy evolution. Its status as the second-brightest very disturbed galaxy in the sky, as noted by Halton Arp, and its comparable brightness in infrared and radio to the Antennae Galaxies underscore its importance for multi-wavelength studies. The system's two distinct velocity systems and small tails indicate the interaction began roughly 300 million years ago, offering a snapshot of a merger still in progress. The detection of two galactic nuclei, one an H II nucleus, and the presence of a dark dust lane separating the components highlight the complex dynamics at play. The increased star formation outside the nuclei around the time of interaction, along with the dwarf object UGC 957 in the northern tidal tail, suggests that such mergers can trigger star formation and produce tidal dwarf galaxies. The lower-than-expected X-ray luminosity and the gas-poor nature of the secondary component challenge simple models of merger-driven activity, while the large galactic wind driven by starburst activity demonstrates feedback processes. The observation of supernova SN 2025zjx adds a transient event to the system's history. Overall, NGC 520 serves as a key laboratory for studying galaxy interactions, star formation, and feedback in a relatively nearby universe.

Did You Know?

• NGC 520 is also known as the Flying Ghost.
• It was discovered by William Herschel on 13 December 1784.
• One supernova, SN 2025zjx (Type II), was observed in NGC 520 on 29 September 2025.