THE HUBBLE SPACE TELESCOPE turns 21 today, and astronomers have celebrated by pointing it at an especially photogenic pair of interacting galaxies that together are called Arp 273.
The larger of the spiral galaxies, known as UGC 1810, has a main body that is distorted into a rose-like shape by the gravitational pull of the companion galaxy below it, known as UGC 1813.
The swathe of blue jewels across the top of UGC 1810 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 galaxy UGC 1813 shows distinct signs of intense star formation at its core, perhaps triggered by the encounter with UGC 1810.
A series of uncommon spiral patterns in UGC 1810 are a telltale sign of a close encounter between galaxies. The large, outer arm forms a partial ring around the galaxy, a feature that is seen when two galaxies actually pass through one another. This suggests that the smaller galaxy actually ‘dived’ deeply, but off-centre, through UGC 1810.
The inner set of spiral arms is highly warped out of the flat plane of the galaxy, 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.
The image also shows a tenuous tidal bridge of gas between the two galaxies, which are separated by tens of thousands of light-years from each other.
UGC 1810 has a mass about five times that of UGC 1813. In unequal pairs such as this, the relatively rapid passage of the smaller galaxy produces the lopsided structure in the big galaxy.
Also in such encounters, starburst activity typically begins earlier in the minor galaxy than in the major galaxy. This could be because to the fact that the smaller galaxy has consumed less of the gas present in its core, from which new stars are born.
The Arp 273 pair is roughly 300 million light-years from Earth.
Adapted from information issued by HEIC. Image courtesy NASA, ESA and the Hubble Heritage Team (STScI/AURA).
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