LOFAR VLBI studies at 55 MHz of 4C 43.15, a z=2.4 radio galaxy
Journal article, 2016

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 characterize 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 similar to 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 ultrasteep spectral index (alpha < -1; S-v similar to v(alpha)) between 365 MHz and 1.4 GHz. Using the International Low Frequency Array Low Band Antenna we achieve subarcsecond imaging resolution at 55MHz with very long baseline interferometry 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 +/- 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 ultrasteep spectral index is due to a combination of the steepening spectrum at high frequencies with a break at intermediate frequencies.



sky survey

flux densities



spectral index

ghz sources


radio continuum: galaxies

galaxies: active


galaxies: individual: 4C 43.15

galaxies: jets


L. K. Morabito

Leiden University

A. T. Deller

Netherlands Institute for Radio Astronomy (ASTRON)

H. Rottgering

Leiden University

G. Miley

Leiden University

Eskil Varenius

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

T. W. Shimwell

Leiden University

J. Moldon

University of Manchester

Neal Jackson

R. Morganti

University of Groningen

Netherlands Institute for Radio Astronomy (ASTRON)

R. J. van Weeren

Harvard-Smithsonian Center for Astrophysics

J. B. R. Oonk

Netherlands Institute for Radio Astronomy (ASTRON)

Leiden University

Monthly Notices of the Royal Astronomical Society

0035-8711 (ISSN) 1365-2966 (eISSN)

Vol. 461 3 2676-2687 stw1501

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences


Onsala Space Observatory



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