| Using the International Low Frequency Array (LOFAR) Low Band Antenna we achieved sub-arcsecond imaging resolution at 55 MHz with VLBI techniques.
The image of radio galaxy 4C 43.15 on the left was made using 15.6 MHz of bandwidth centred on 55 MHz. We used the multiscale function of the CLEAN task in CASA with Briggs weighting (robust -1.5) and no inner uv cut. The image noise achieved is 59 mJy/bm while the expected noise given the amount of flagged data and image weighting is 25 mJy/bm. The final restoring beam is 0.9 arcsec × 0.6 arcsec with PA -33 deg.
The image on the right is the same image, but smoothed with a Gaussian kernel 1.2 times the size of the restoring beam. The contours in both images are drawn at the same levels, which are 3σ, 5σ, 10σ, and 20σ of the unsmoothed image.
This radio galaxy was selected as part of a pilot study of 10 high redshift radio galaxies. 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 ~1 arcsec are necessary to determine the internal spectral index variation. This daily image shows 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 (α < −1; S_ν ∼ ν^α) between 365 MHz and 1.4 GHz. 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+/-0.03 and -1.75+/-0.01 for frequencies >=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.
Published in "LOFAR VLBI studies at 55 MHz of 4C 43.15, a z= 2.4 radio galaxy", 2016, MNRAS, 461, 2676
|L. Morabito, A. Deller, H. Rottgering, et al.