The Milky Way seems to be missing some of its matter. A study of a powerful flash of cosmic radio waves called a fast radio burst (FRB) has shown that the area in and around our home galaxy has less normal, or baryonic, matter than we would expect, as compared with the amount of dark matter.
The Deep Synoptic Array (DSA), a set of 110 radio dishes in California, of which 63 have been operating so far, was designed specifically to spot FRBs and trace them back to their home galaxies. In doing so, it can also be used to calculate the amount of matter between those galaxies and the detector by measuring how that matter has absorbed some of the radiation from the FRBs 鈥� a bit like shining a light through fog to determine how thick the fog is.
at the California Institute of Technology presented one such measurement on 9 January at a meeting of the American Astronomical Society in Seattle. He and his colleagues used the light from a burst called FRB 20220319D, which originated in a galaxy about 163 million light years away, to examine the matter in the Milky Way鈥檚 halo 鈥� a roughly spherical cloud of stars, gas and dark matter in which the galaxy鈥檚 disc is embedded.
Advertisement
鈥淎lthough the universe on average has something like 16 per cent baryonic matter with the rest being dark matter, the baryon fraction of the Milky Way is much lower,鈥� Ravi said. The observations showed that the Milky Way鈥檚 halo is made up of about 9.6 per cent baryonic matter at most, 40 per cent less than the universe at large.
鈥淚t鈥檚 surprising in the sense that, for a fair while 鈥� at least within the FRB community 鈥� we assumed that [galactic] halos kind of keep the cosmic fraction of baryons that we would expect,鈥� said Ravi. 鈥淗owever, certainly it鈥檚 been the case that numerical simulations of galaxy formation show that various feedback mechanisms tend to remove mass.鈥�
These mechanisms include powerful winds caused by supernovae and massive stars, which blow matter out of galaxies as they form, as well as similar effects from the supermassive black holes at the centres of galaxies. If this measurement is borne out by future observations, it may be proof that our galaxy鈥檚 youth was tumultuous and characterised by matter being flung away into intergalactic space.
Reference:
