Using the Hubble Space Telescope, astronomers have discovered three new ultra-faint dwarf galaxies associated with the nearby spiral galaxy NGC 253. These are some of the faintest systems found outside the Local Group.
Ultra-faint dwarf galaxies (UFDs) are the least luminous star clusters known to be dominated by dark matter. They are also poorly developed chemically. Astronomers often refer to them as “fossils of the universe” because they appeared early in its origin.
The Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) project is one of the astronomical surveys aimed at finding such faint galaxies. As part of the PISCeS program, a group of astronomers led by Burchin Mutlu-Pakdil of the University of Chicago observed the field around the galaxy NGC 253 using the Hubble telescope. They looked for dwarf star systems next to it. NGC 253 lies 11.4 million light years from Earth. It is one of the brightest galaxies outside the Local Group.
Scientists visually examined all images in search of spatially compact clusters of stars around NGC 253 and discovered three new UFDs – Scl-MM-dw3, Scl-MM-dw4 and Scl-MMdw5 – in addition to two known ones discovered in 2014 and 2016. All three galaxies are equally old, with an estimated age of about 12 billion years. According to modern concepts, the observable Universe originated 13.799 ± 0.021 billion years ago.
Among the three galaxies, Scl-MM-dw3 is the smallest. Its elliptical half-light radius along the semi-major axis is estimated at about 362 light-years.
Astronomers have pointed out that the newly discovered dwarfs have luminosities between -7.5 and -7.24 magnitudes, making them one of the faintest galaxies identified outside our Local Group.