The correlation between radio spectral index and redshift has been exploited to discover high redshift radio galaxies , but its underlying cause is unclear .
It is crucial to characterise the particle acceleration and loss mechanisms in high redshift radio galaxies to understand why their radio spectral indices are steeper than their local counterparts .
Low frequency information on scales of \sim 1 arcsec are necessary to determine the internal spectral index variation .
In this paper we present the first spatially resolved studies at frequencies below 100 MHz of the z = 2.4 radio galaxy 4C 43.15 which was selected based on its ultra-steep spectral index ( \alpha < -1 ; S _ { \nu } \sim \nu ^ { \alpha } ) between 365 MHz and 1.4 GHz .
Using the International Low Frequency Array ( LOFAR ) Low Band Antenna we achieve sub-arcsecond imaging resolution at 55 MHz with VLBI techniques .
Our study reveals low-frequency radio emission extended along the jet axis , which connects the two lobes .
The integrated spectral index for frequencies < 500 MHz is -0.83 .
The lobes have integrated spectral indices of -1.31 \pm 0.03 and -1.75 \pm 0.01 for frequencies \geq 1.4 GHz , implying a break frequency between 500 MHz and 1.4 GHz .
These spectral properties are similar to those of local radio galaxies .
We conclude that the initially measured ultra-steep spectral index is due to a combination of the steepening spectrum at high frequencies with a break at intermediate frequencies .