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Dark rings and new light For this Picture of the Month from the NASA/ESA Hubble Space Telescope, we have a sight of an uncommon galaxy with a striking appearance. This is NGC 7722, a lenticular galaxy located about 187 million light-years away in the constellation Pegasus.
A 'lenticular', meaning 'lens-shaped', galaxy is a type that sits in between the more familiar spiral galaxies and elliptical galaxies. It is also less common than these – partly because when a galaxy has an ambiguous appearance, it can be hard to determine if it is actually a spiral, an elliptical galaxy, or something in between. Many of the known lenticular galaxies sport features of both spiral and elliptical galaxies. 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 the centre similar to an elliptical galaxy. Unlike elliptical galaxies, it has a visible disc – 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 disc and halo.
This new Hubble image, the sharpest yet taken of NGC 7722, brings the impressive dust lanes into sharp focus. Bands of dust like this are notuncommon in lenticular galaxies, and they stand out against the broad, smooth halo of light that typically surrounds lenticular galaxies. The distinctive dust lanes of NGC 7722 are thought to result from a merger with another galaxy in the past, similar to other lenticular galaxies. It is not yet fully understood how lenticular galaxies form, but mergers and other gravitational interactions are thought to play an important part, 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 could be detected from Earth. SN 2020SSF was a Type Ia supernova, an event which 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 against how bright they really are, it’s possible to 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 to astronomers.
Taken with Hubble’s Wide Field Camera 3, this Hubble image was obtained as part of an observing programme (#16691, PI: R. J. Foley) that followed up on recent supernovae. SN 2020SSF is not visible in this image, as it was actually taken two years later, when the supernova had long faded. This was on purpose: the aim of the observations was to witness the aftereffects of the supernova and examine its surroundings, which can only be done once the intense light of the explosion is gone. With Hubble’s clear vision, astronomers can search for radioactive material created by the supernova, catalogue its neighbours to see how old the star likely was, and look for the companion star it left behind – all from almost 200 million light-years away.
[Image Description: A disc-shaped galaxy. It glows brightly at the centre and shines a faint white light all around it. The disc is made up of tightly-packed rings of dust, some darker and some lighter. Wide, long lanes of dark reddish dust cross the galaxy in front of its edge, blocking out some of its light; the long strands twist and break apart at each side. A couple of nearby stars and distant galaxies are also visible on the black background.]