Hubble Sees Galaxy With Dark Rings In New Light

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This NASA/ESA Hubble Space Telescope image features an uncommon galaxy with a striking appearance. NGC 7722 is a lenticular galaxy located about 187 million light-years away in the constellation Pegasus.

A lenticular, meaning "lens-shaped," galaxy is a type whose classification sits between more familiar spiral galaxies and elliptical galaxies. It is also less common than spirals and ellipticals - partly because these galaxies have a somewhat ambiguous appearance, making it hard to determine if it is a spiral, an elliptical, or something in between. Many of the known lenticular galaxies sport features of both spiral and elliptical. In this case, NGC 7722 lacks the defined arms of a spiral galaxy, while it has an extended, glowing halo and a bright bulge in its center like an elliptical galaxy. Unlike elliptical galaxies, it has a visible disk - concentric rings swirl around its bright nucleus. Its most prominent feature, however, is undoubtedly the long lanes of dark red dust coiling around the outer disk and halo.

This new Hubble image, the sharpest taken of NGC 7722, brings the galaxy's impressive dust lanes into sharp focus. Bands of dust like this are not uncommon in lenticular galaxies, and they stand out against the broad, smooth halo of light that typically surrounds lenticulars. Astronomers think NGC 7722's distinctive dust lanes are the result of a past merger with another galaxy, similar to other lenticular galaxies. Researchers do not fully understand how lenticular galaxies form, but they think mergers and other gravitational interactions play an important part in reshaping galaxies and exhausting their supplies of gas while bringing new dust.

While it doesn't host as many new, young stars as a spiral galaxy, there's still activity in NGC 7722: in 2020 it was host to the explosion of a star that astronomers detected from Earth. SN 2020SSF was a Type Ia supernova, an event that occurs when a white dwarf star in a binary system siphons enough mass away from its companion star that it grows unstable and explodes. These explosions output a remarkably consistent level of light: by measuring how bright they appear from Earth and comparing that to how bright they intrinsically are, astronomers can tell how far away they must be. Type Ia supernovae are one of the best ways to measure distances to galaxies, so understanding exactly how they work is of great importance for astronomy.

Taken with Hubble's Wide Field Camera 3, this Hubble image was obtained as part of an observing program (#16691, PI: R. J. Foley) that followed up on recent supernovae. SN 2020SSF, is not visible in this image. Researchers purposefully observed NGC 7722 two years after the supernova faded to witness the supernova's aftereffects and examine its surroundings, which can only be accomplished once the intense light of the explosion is gone. With Hubble's clear vision, astronomers can search for radioactive material created by the supernova, catalog its neighbors to help determine the original star's age, and look for the companion star it left behind - all from almost 200 million light-years away.

Text Credit: European Space Agency (ESA)