This week’s Hubble Space Telescope image shows the dusty galaxy NGC 7172, 110 million light-years away in the constellation Piscis Austrinus (the southern fish). This may look like a typical galaxy from this angle, but it actually holds a secret.
“The dust lane making its way through NGC 7172 obscures the galaxy’s luminous heart, making NGC 7172 appear to be nothing more than a normal spiral galaxy seen from the side,” Hubble scientists write. However, upon closer inspection, astronomers discovered 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 core powered by matter accreting on a supermassive black hole.”
This image from the NASA/ESA Hubble Space Telescope shows tendrils of dark dust running through the heart of spiral galaxy NGC 7172. The galaxy is located about 110 million light-years from Earth in the constellation Piscis Austrinus. ESA/Hubble & NASA, DJ Rosario, A. Barth; Credit: L. Shatz
Hubble views objects like this galaxy in the wavelength of visible light, which is the same as what the human eye can see. This photo was taken with two of its instruments, the Advanced Camera for Surveys and Wide Field Camera 3.
To understand more about the structure of this galaxy, however, it was necessary to look through a different wavelength. In the 1980s, astronomers observed the galaxy in the infrared wavelength, which can peer through dust clouds to observe underlying structures. 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 appear brighter than can be explained by the density of the stars there. These regions can be so bright that they are brighter than the rest of the galaxy.