We report on 1.6 and 5.0 GHz observations of the ultraluminous infrared galaxy ( ULIRG ) Mrk 273 , using the European VLBI Network ( EVN ) and the Multi-Element Radio-Linked Interferometer Network ( MERLIN ) .
We also make use of published 1.4 GHz VLBA observations of Mrk 273 by Carilli & Taylor ( 2000 ) .
Our 5 GHz images have a maximum resolution of 5–10 mas , which corresponds to linear resolutions of 3.5–7 pc at the distance of Mrk 273 , and are the most sensitive high-resolution radio observations yet made of this ULIRG .
Component N1 , often pinpointed as a possible AGN , displays a steep spectral index ( \alpha = 1.2 \pm 0.1 ;S _ { \nu } \propto \nu ^ { - \alpha } ) ; hence it is very difficult to reconcile with N1 being an AGN , and rather suggests that the compact nonthermal radio emission is produced by an extremely high luminous individual radio supernova ( RSN ) , or a combination of unresolved emission from nested supernova remnants ( SNR ) , luminous RSNe , or both .
Component N2 is partly resolved out into several compact radio sources –none of which clearly dominates– and a region of extended emission about 30 pc in size .
The integrated spectral index of this region is flat ( \alpha = 0.15 \pm 0.1 ) , which can be interpreted as due to a superposition of several unresolved components , e.g. , RSNe or SNRs , whose radio emission peaks at different frequencies and is partially free-free absorbed .
Is it also possible that one of the compact components detected in this region is the radio counterpart of the AGN .
The overall extended radio emission from component N is typical of nonthermal , optically thin radio emission ( \alpha = 0.8 \pm 0.1 ) , and its 1.4 GHz luminosity ( L _ { 1.4 GHz } = ( 2.2 \pm 0.1 ) \times 10 ^ { 23 } WHz ^ { -1 } ) is consistent with being produced by relativistic electrons diffused away from supernova remnants in an outburst .
The southern component , SE , shows also a very steep spectrum ( \alpha = 1.4 \pm 0.2 ) , and extended radio emission whose origin and physical interpretation is not straightforward .