Remnant radio galaxies in the LOFAR Lockman Hole
I will present recent 150-MHz deep observations performed with the Low-frequency Array (LOFAR) of the well-known extragalactic region of the Lockman Hole. Thanks to its high sensitivity and resolution this data allows us to perform new studies of the radio loud AGN population at low radio frequencies. In particular, we conducted a systematic search of remnant radio galaxies, which represent the final “dying” phase of the radio galaxy evolution, when the jets have switched off. This class of sources is best to investigate the life-cycle of radio loud AGN as well as to quantify the role of radio AGN feedback. Indeed, the modelling of their radio spectrum provides constraints on the time-scales of activity and quiescence of the radio source and on its energy output. For a long time there have been claims that deep low-frequency surveys would have enhanced the detection of this class of sources, which are usually rare in flux limited samples.
With our search, we thus intend to provide good statistics on the detection and properties of remnant radio galaxies. To avoid selection biases towards any specific class of objects we used both morphological and spectral selection criteria. To do this we combined the LOFAR data with publicly available surveys at other frequencies as well as dedicated deep observations. We find that the fraction of candidate remnant sources is < 6-8% of the entire radio source population and is dominated by steep spectrum sources. To better understand the observed fraction we developed mock catalogues of the radio sky population based on radio galaxy evolution models. These models are used to constrain the main mechanisms contributing to the source luminosity evolution i.e. adiabatic expansion, radiative losses, as well as to make predictions on their fraction in flux limited samples.