NGC 7552 is a face-on barred spiral galaxy in the constellation Grus at a distance of 69 million light years (21 Mpc) [1]. The bar, which is about 30 thousand light years long and about half the diameter of its optical disk, is a relatively prominent feature in this galaxy [2], and two spiral arms emerge from the ends of the bar. Although the bar has already set this galaxy apart from ordinary spiral galaxies, what makes this galaxy more interesting is its circumnuclear starburst ring.
Astronomers initially had difficulty understanding exactly what was happening in the center of NGC 7552. It was originally identified as an active galactic nucleus (AGN) [3]. An active galactic nucleus consists of a supermassive black hole with millions or billions of times the mass of the Sun, an accretion disk of gas and dust that is falling into the black hole, and jets of ionized gas emerging from above the poles of the black hole. Eventually, through a combination of observations in near-infrared [4] and X-ray [5], nobody could find the presence of this AGN, suggesting instead that the center of NGC 7552 is dominated by the circumnuclear starburst ring.
The circumnuclear ring formed a lot of bright blue stars, which produce a lot of light. Therefore, the center of this galaxy looks very bright. The diameter of the circumnuclear ring is about 3 thousand light years long, which is only 5% of the diameter of the galaxy's optical disk. However, the star formation within the ring is extremely high compared to the other components of NGC 7552. 50% to 80% of all the stars currently forming within the galaxy are located within the ring [2].
The concentration of the interstellar gas and dust within the ring provides lots of materials for creating new stars, which can explain why there are lots of bright stars within the circumnuclear ring. Nevertheless, the specific conditions under which gas and dust accumulate in the ring are still subject to debate [2]. What is clear is that there is a large amount of gas and dust present in the bar in addition to the circumnuclear ring and that the dust lanes in the bar intersect the ring.
The interstellar dust absorbs lots of ultraviolet and optical light from the hot bright stars within the star-forming regions and emits infrared light. Since there are many stars-forming regions in the circumnuclear ring, it is abnormally bright at infrared wavelengths. Therefore, astronomers classify this galaxy as a luminous infrared galaxy (LIRG) [6].
NGC 7552 is also a member of a group of galaxies named the Grus Quartet, with the other three members being NGC 7582, NGC 7590, and NGC 7599. NGC 7552 and NGC 7582 are the brightest of the four galaxies. Radio observations of these galaxies have shown the presence of hydrogen gas in between the galaxies that was probably stripped stript away from the galaxies by gravitational interactions between them [7].