|Abstract : || |
Radio synchrotron halos generated from the cosmic rays, on about ten thousand light-year extents, were known around star-forming galaxies. Recently, a few radio halos on about a million light-year extent were detected, either being hosted in some quiescent galaxies or appearing host-less, in a radio survey with an unprecedented sensitivity achieved by precursors of Square Kilometer Array-SKA radio telescopes. The large sizes of halos and quiescent nature of the host galaxies do not fit into our present understanding of formation of such halos. Formation of these halos popularly termed as 'Odd Radio Circles' remained a mystery. Our recent works using basic ideas and arguments can explain (i) host-less halos as either thermonuclear supernovae remnants in the nearby intergalactic space and (ii) the hosted halos as due to supernovae-like shock energy deposition by a large number of tidal disruption events of Sun-like stars in close proximity of intermediate mass black-holes in galaxies. This mechanism can work even after the cessation of star formation in galaxies.