Despite still not being fully operational, the Vera C. Rubin Observatory is already making waves. The observatory, which features the world’s biggest and most sensitive digital camera, the 3.2-gigapixel LSSTCam, recently allowed astronomers to discover a stream of stars wrapped around the distant galaxy Messier 61 (M61).
This stream is 10,000 light-years wide and around 170,000 light-years long, exceeding the width of the visible Milky Way, which is estimated to be about 100,000 light-years wide. The team behind the discovery believes that the stream is what remains of a dwarf galaxy once cannibalized by M61 that is now getting a measure of revenge by causing turbulence at the heart of this spiral galaxy. The sheer size of this stellar stream and the fact that M61, also known as NGC 4303 and located around 55 million light-years away, is well studied, make it extraordinary that this stellar stream or “tidal tail” hasn’t been discovered before.
“The M61 stream is actually relatively bright, and it is surprising that it hadn’t been noticed before, around such a nearby, well-known galaxy. It has a brightness around 100 million times that of the sun. I think this highlights the difficulty of detecting even the brighter streams, as special equipment and techniques are needed,” discovery team member Aaron J. Romanowsky of San José State University told Space.com. “The discovery of this stream also highlights how much is still unknown about the life histories of galaxies, how the hidden universe shapes the realms that we can see.”
M61 is a barred spiral galaxy, meaning that it is similar to the Milky Way in morphology. Like the Milky Way, this galaxy also has a supermassive black hole at its heart. Whereas our home galaxy’s black hole is quiet and dormant, the supermassive black hole at the heart of M61 is voraciously feeding and blasting out power outflows of matter.
“M61 is a spiral galaxy that is like our own Milky Way, except it hosts a ‘galactic storm’ of intense gas star formation feeding an eruption of its central supermassive black hole,” Romanowsky said. “We now appear to have identified the culprit for this episode: a ‘stellar stream’ that marks a disrupting dwarf galaxy whose gravitational forces in turn are in turn shaking up M61.”
Romanowsky explained that this vast stellar stream is likely what remains of a dwarf galaxy that has drifted too close to the much larger M61. This would have exposed the dwarf galaxy to powerful tidal forces that unraveled it.
“As smaller galaxies fall into bigger ones, they get ripped apart by gravitational tides, and this appears to be what is happening with the M61 stream,” Romanowsky added.
The cannibalization of dwarf galaxies in this way is believed to be a major factor in how massive galaxies like the Milky Way and M61 grow. That means that the team is expecting Rubin to deliver many more previously hidden streams of stars flowing around massive galaxies when it begins its ten-year mission, the Legacy Survey of Space and Time (LSST).
“We are expecting that when Rubin obtains very deep imaging of galaxies, we will see them surrounded by a faint network of stellar streams,” Romanowsky concluded. “The discovery of the stream shows the excellent characteristics of Rubin for making such observations, and points to a rich future of similar discoveries as vast areas of the sky are mapped out.”
The team’s research is available on the paper repository site arXiv.