Blazar GB 1508+5714 radio emission observed with the ILT using the longest baselines available in 2015. Left: Overlay on an optical image, Right: Zoom showing the core of the emission coming from the central supermassive black hole, and the two hot spots created by jets colliding with the intergalactic medium. Image credit: Universität Würzburg.

By 2022 the array of stations of the European-wide International LOFAR Telescope (ILT) extends on its longest baselines between Irbene in Latvia and Birr in Ireland more than 3000 km. The structure of the ionosphere varies significantly over the geographic distances seperating the more than 50 stations of the ILT. These therefore measure the same astronomical signal, but influenced by the ionosphere in different extents for different stations. These ionospheric effects must be corrected for the astronomical signal to be recovered. This is a major challenge at low radio frequencies, especially in the long baseline regime. But the results are rewarding, as the sharpness of the radio images (the spatial resolution), depends crucially on the involvement of stations participating in the longest baselines.

LOFAR scientists are well on their way to exploit the capabilities of the ILT using it's High-Band Antenna (HBA). Recently, a team of astronomers led by Alexander Kappes (Universität Würzburg) successfully imaged one of the most distant blazars currently known to us, GB 1508+5714 with a redshift of ~4.30, at a central frequency of around 144 MHz (λ ~2.1m). At this redshift, the age of the universe was only about one tenth of the current age. Blazars are a special type of galaxies, which accrete mass on the supermassive black hole at its center. The accretion process creates fast outflows expelling a small part of the mass surrounding the black hole back to the intergalactic medium, called extragalactic jets. By coincidence, the radio emission from one of these two jets is beamed in our direction, which makes us see them from very far away.

In the case of the blazar GB 1508+5714, the ILT enables the creation of an image (see figure) whose information traveled for about 12 billion years until it was captured by the telescope. Due to the expansion of the universe, the then young galaxies are now very distant and therefore faint and can hardly be detected with today's telescopes. Relativistic effects and the close alignment of the jet with the line of sight increase the luminosity of the radio emission by orders of magnitude, making it easier to detect such distant objects and eventually "see" them with the ILT. Blazars are therefore promising targets for studying the evolution of galaxies in the young universe.

In the center of the image is a core of radio emission containing about 86% of the total energy detected during the observation. The remaining energy contribution is distributed between two emission regions located to the east and west of the core. While the projected diameter of the entire structure is about 41 kpc, the actual object size probably extends to about 800 kpc, which is about 50 times the diameter of the Milky Way. The core is identified as the center of the galaxy, while the two emission regions are likely lobes formed by the jets, also containing the termination regions of the jets, known as hot spots. The  separation between the features in the east and west of the central region suggests that jets ejected in opposite directions are not perfectly aligned with the line of sight to the Earth.

The success of this observation has convinced A. Kappes and his collaborators that the study of the galaxy population in the early universe using blazars as probes is now within reach. A. Kappes: "Future observations will lead to data of much higher quality than was obtained for this observation in 2015.  Now, by expanding the sample of high-shift blazars to be observed by the ILT, we can perform a statistical study of the population that will allow us to understand the environment and evolution of galaxies in the early universe."