jan 302013

January 30, 2013

NGC 520, a pair of colliding galaxies in Pisces

Arp 157

Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and B. Whitmore (STScI)

NGC 520 (also known as Arp 157) is a pair of highly disturbed colliding spiral galaxies of more than 100,000 light-years across, located some 90.7 million light-years away in the constellation of Pisces (the Fish). NGC 520 is speeding away from us at approximately 2,281 kilometers per second.

The galaxy pair is included in Arp’s Atlas of Peculiar galaxies — a catalog of 338 peculiar galaxies produced by Halton Arp in 1966 — as Arp 157.

The two disk galaxies, seen edge-on, are crashing into each other, melding and forming a new giant elliptical galaxy. This happens slowly, over millions of years — the whole process started some 300 million years ago.

NGC 520 is now in the middle stage of the merging process, as the two nuclei haven’t merged yet, but the two disks have. This is the fate that awaits the Milky Way and the Andromeda Galaxy (M 31) in about 5 billion years.

The galaxy pair shows very obvious tidal tails of stars (not visible in this close-up image) and a prominent dust lane that runs diagonally across the center of the image and obscures the galaxy.

The merger is giving rise to massive star formation. The primary nucleus is known as a HII nucleus, and has the most active star formation activity. The emission and absorption features seen in its spectra are consistent with a very young stellar population. NGC 520 contains a number of radio sources and is also a source of infrared radiation, that also indicates massive star formation.

Although the speeds of stars are fast, the distances are so vast that the interacting pair will surely not change its shape noticeably during our lifetimes.

NGC 520 is one of the brightest interacting galaxies in the sky — after the Antennae galaxies (NGC 4038 and NGC 4039) — and can be observed with a small telescope, having the appearance of a comet.

This image is taken by the Hubble Space Telescope.

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