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Hubble captures a dusty galaxy that holds a luminous secret

The image from the Hubble Space Telescope this week shows the dusty galaxy NGC 7172, located 110 million light-years away in the constellation of Piscis Austrinus (the southern fish). This might look like a typical galaxy from this angle, but in fact, it holds a secret.

“The lane of dust threading its way across NGC 7172 is obscuring the luminous heart of the galaxy, making NGC 7172 appear to be nothing more than a normal spiral galaxy viewed from the side,” Hubble scientists write. However, on closer inspection astronomers found something unexpected: “When astronomers inspected NGC 7172 across the electromagnetic spectrum they quickly discovered that there was more to it than meets the eye: NGC 7172 is a Seyfert galaxy — a type of galaxy with an intensely luminous active galactic nucleus powered by matter accreting onto a supermassive black hole.”

Tendrils of dark dust threading across the heart of the spiral galaxy NGC 7172.
This image from the NASA/ESA Hubble Space Telescope reveals tendrils of dark dust threading across the heart of the spiral galaxy NGC 7172. The galaxy lies approximately 110 million light-years from Earth in the constellation Piscis Austrinus. ESA/Hubble & NASA, D. J. Rosario, A. Barth; Acknowledgment: L. Shatz

Hubble views objects like this galaxy in the visible light wavelength, which is the same as what the human eye can see. This image was taken using two of its instruments, the Advanced Camera for Surveys and Wide Field Camera 3.

To understand more about the structure of this galaxy, though, it was necessary to look through a different wavelength. In the 1980s astronomers observed the galaxy in the infrared wavelength, which can look through clouds of dust to observe structures beneath. Observations at these wavelengths revealed the brightly glowing heart of the Seyfert galaxy.

The Hubble image uses data collected for a study of active galactic nuclei, a group that includes Seyfert galaxies. Active galactic nuclei are brightly-glowing regions at the heart of galaxies that seem to be brighter than can be accounted for due to the density of stars there. These regions can be so bright that they are brighter than the entire rest of the galaxy.

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