The Whirlpool galaxy is often seen as the poster child for spiral galaxies, its winding spiral arms slowly stealing material from its spherical companion. But a deeper analysis reveals a more complex, fascinating picture and possibly new clues on how galaxies evolve.
A study published in The Astrophysical Journal presents new X-ray observations of the Whirlpool galaxy and its companion, technically known as M51a and M51b, respectively. The system is a galaxy merger – those that typically have powerful supermassive black holes at their cores, gobbling up material and shining brightly in X-rays. Curiously, this has not been the case for M51.
Researchers have suspected that both of the supermassive black holes in the Whirlpool system could be shrouded in thick layers of gas. The observations presented in the study penetrated through the layers of gas and discovered that, in fact, the black holes are unusually dim.
"The hard X-ray data used in the study was essential to allow us to pierce through any obscuring material that could be making them appear weaker than they are," Dr Murray Brightman of Caltech told IFLScience. "This result is surprising since the galaxies are merging, which is expected to drive material onto the black holes, leading to higher accretion rates. The low accretion rates are probably due to the black hole ‘flickering’, where instantaneous snapshots, like the one we got, don’t necessarily reveal the long-term activity of the black holes."
The work was possible thanks to NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array), a space-based X-ray telescope that captured X-ray photons coming from the galaxy. These high-energy “shiny crumbs” didn’t just provide information about the black holes, they also informed the researchers of an incredible neutron star.
Neutron stars are one form massive stars might become at the end of their lives. They are extremely dense objects, up to a few times the mass of the Sun but not much wider than a city. They can be really bright, so much so they outshine active supermassive black holes. There is at least one such neutron star in the Whirlpool system and it is brighter than the two dim supermassive black holes. There are also other ultraluminous X-ray sources in M51, some of which could be neutron stars or massive black holes, but they are not as big as the ones at the core of the two galaxies. Previous ultraluminous X-rays sources were discovered to be neutron stars thanks to coherent pulsations.
"This particular neutron star in M51 has not been found to pulsate (yet), but we identified a cyclotron resonance scattering feature in its X-ray spectrum, which is produced by the interaction of charged particles in the presence of a magnetic field," Dr Brightman continued. "The implied strength of the magnetic field was so high that it could not possibly be produced by a black hole, and must therefore be a neutron star and may actually be the reason it shines so brightly."The team has already collected new data to look for pulsations of this and other ultraluminous X-ray sources in the system. They are also planning new observations of the supermassive black holes.