Rose Galaxy (Arp 273)

This Hubble Space Telescope image shows a group of interacting galaxies called Arp 273. The larger of the spiral galaxies, known as UGC 1810, has a disk that is tidally distorted into a rose-like shape by the gravitational tidal pull of the companion galaxy below it, known as UGC 1813. A swath of blue jewels across the top is the combined light from clusters of intensely bright and hot young blue stars. These massive stars glow fiercely in ultraviolet light. The smaller, nearly edge-on companion shows distinct signs of intense star formation at its nucleus, perhaps triggered by the encounter with the companion galaxy. A series of uncommon spiral patterns in the large galaxy is a tell-tale sign of interaction. The large, outer arm appears partially as a ring, a feature seen when interacting galaxies actually pass through one another. This suggests that the smaller companion actually dived deep, but off-center, through UGC 1810. The inner set of spiral arms is highly warped out of the plane with one of the arms going behind the bulge and coming back out the other side. How these two spiral patterns connect is still not precisely known. A possible mini-spiral may be visible in the spiral arms of UGC 1810 to the upper right. It is noticeable how the outermost spiral arm changes character as it passes this third galaxy, from smooth with lots of old stars (reddish in color) on one side to clumpy and extremely blue on the other. The fairly regular spacing of the blue star-forming knots fits with what is seen in the spiral arms of other galaxies and is predictable based on instabilities in the gas contained within the arm. The larger galaxy in the UGC 1810 – UGC 1813 pair has a mass that is about five times that of the smaller galaxy. In unequal pairs such as this, the relatively rapid passage of a companion galaxy produces the lopsided or asymmetric structure in the main spiral. Also in such encounters, the starburst activity typically begins in the minor galaxies earlier than in the major galaxies. These effects could be due to the fact that the smaller galaxies have consumed less of the gas present in their nucleus, from which new stars are born. Arp 273 lies in the constellation Andromeda and is roughly 300 million light-years away from Earth. The image shows a tenuous tidal bridge of material between the two galaxies that are separated by tens of thousands of light-years from each other. The interaction was imaged on December 17, 2010, with Hubble’s Wide Field Camera 3 (WFC3). Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) (CC BY 2.0)

Rose Galaxy (Arp 273), image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona (CC BY-SA 3.0 US)

This interesting pair of interacting spiral galaxies was first described in the Catalogue of Peculiar Galaxies, compiled by Halton Arp in 1966. It is also pair number 64 in Igor Karachentsev’s catalog of binary galaxies. The larger member is strongly tidally distorted, looking almost as though one side of the galaxy has been placed under a magnifying glass. The edge-on companion, however, retains a relatively undisturbed spiral disk, but has a luminous, heavily obscured but infrared-bright, star-burst nucleus. The nucleus of the large spiral, by way of a contrast, contains a low-ionization nuclear emission-line region (LINER), which is indicative of much less activity than the bright nuclear HII region of its companion. We thus see an interesting example of the very different responses that different galaxies can have to interaction with their companions: the large galaxy has a shredded disk but essentially nothing in its nucleus, while the small galaxy has an undisturbed disk but a very active nucleus. Credit: WIYN/NOIRLab/NSF (CC BY 4.0)

Location of the Rose Galaxy (Arp 273), image: Stellarium