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The birth of radio galaxies

obrazek: New radio emission that appeared a few years ago is presented on a spectra (each spectrum peaks at a few GHz), and color maps obtained with a long baseline interferometry. Optical images of the sources are in the bottom right corner of radio maps. [fot. Aleksandra Wołowska | Magdalena Kunert-Bajraszewska | SDSS catalogue]
New radio emission that appeared a few years ago is presented on a spectra (each spectrum peaks at a few GHz), and color maps obtained with a long baseline interferometry. Optical images of the sources are in the bottom right corner of radio maps. photo: Aleksandra Wołowska | Magdalena Kunert-Bajraszewska | SDSS catalogue

Astronomers managed to observe the birth of radio galaxies and trace their development during the first several years of their lives.

Until now, the word "young" was used to describe radio galaxies of age below 1000 years. Thanks to the new method called 'time domain observations',  that is, regular and repeated observations of the same parts of the sky, it was possible to directly record the birth of radio sources. The results of those observations are presented in the latest publication by astronomers from NCU: PhD student Aleksandra Wołowska and prof. Magdalena Kunert-Bajraszewska, in cooperation with an international group of seven other authors (https://arxiv.org/abs/2103.08422).

This article is a follow-up of the Caltech-NRAO Stripe 82 Survey (CNSS) series. The aim of  CNSS was to systematically observe the sky for transient phenomena on a timescales from several days to several years (https://iopscience.iop.org/article/10.3847/1538-4357/ab9598).

Young galaxies and quasars that can be classified as Gigahertz-Peaked Spectrum (GPS) sources are among the large group of objects discovered during this project. GPS sources are characterized by a small size and convex spectrum shape peaking at a few gigahertz, and are considered to be one of the first stages of galaxy evolution. In the later stages galaxies expand and transform into large-scale objects.

The discussed group of 12 sources shows significant changes in both radio and optical spectra on the timescale of several years, indicating rapid processes taking place inside these objects.

Comparing CNSS observations with previous radio surveys shows that these objects have now transitioned into a radio-loud state, which indicates either ignition or resumption of their radio activity and the birth of new radio jets. The emergence of new radio emission is most likely related to changes in the accretion rate into the supermassive black hole at the center of each of these galaxies.

The article presents an analysis of a large sample of radio, optical and X-ray data, and addresses an important issue of the early stages of the evolution of radio galaxies, which has not been thoroughly explored so far.

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