Steep-spectrum sources and the duty cycle of the radio emission

TL;DR

This paper investigates the evolution and duty cycle of young radio sources, revealing that many may fade quickly or never grow into large radio galaxies, challenging traditional evolutionary models.

Contribution

It introduces evidence that a significant fraction of young radio sources may have short lifespans or intermittent activity, affecting their evolutionary pathways.

Findings

Many young radio sources fade rapidly after onset.

Some sources show evidence of intermittent radio activity.

Fossils indicate short-lived or recurrent radio emission episodes.

Abstract

It is currently accepted that intrinsically compact and bright radio sources characterized by a convex spectrum peaking at frequencies ranging from 100 MHz to a few GHz are young objects. Following the evolutionary models, these objects would evolve into the population of classical radio galaxies. However, the fraction of young radio sources in flux density-limited samples is much larger than what expected from the number counts of large radio sources. This may suggest that for some reason a significant fraction of young objects would never become large radio galaxies with sizes up to a few Mpc. The discovery of the young radio source PKS 1518+047 characterized by an uncommonly steep spectrum confirms that the radio emission may switch off shortly after its onset. Then the source spectrum steepens and evolves due to energy losses. If the interruption is not temporary, the fate of the fading sources is to disappear at frequencies lower than those explored by current radio telescopes. Fossils of past activities have been recently found at pc-scale distances from newly born radio sources, suggesting the presence of short-lived objects with an intermittent radio emission.