Abstract
This paper presents a study of the redshift evolution of radio-loud active galactic nuclei (AGN) as a function of the properties of their galaxy hosts in the Bo\"otes field. To achieve this we match low-frequency radio sources from deep $150$-MHz LOFAR observations to an $I$-band-selected catalogue of galaxies, for which we have derived photometric redshifts, stellar masses and rest-frame colours. We present spectral energy distribution (SED) fitting to determine the mid-infrared AGN contribution for the radio sources and use this information to classify them as High- versus Low-Excitation Radio Galaxies (HERGs and LERGs) or Star-Forming galaxies. Based on these classifications we construct luminosity functions for the separate redshift ranges going out to $z = 2$. From the matched radio-optical catalogues, we select a sub-sample of $624$ high power ($P_{150\mathrm{\,MHz}}>10^{25}$ W Hz$^{-1}$) radio sources between $0.5 \leq z < 2$. For this sample, we study the fraction of galaxies hosting HERGs and LERGs as a function of stellar mass and host galaxy colour. The fraction of HERGs increases with redshift, as does the fraction of sources in galaxies with lower stellar masses. We find that the fraction of galaxies that host LERGs is a strong function of stellar mass as it is in the local Universe. This, combined with the strong negative evolution of the LERG luminosity functions over this redshift range, is consistent with LERGs being fuelled by hot gas in quiescent galaxies.
Highlights
The evolution of radio-loud Active Galactic Nuclei (RL active galactic nuclei (AGN)) is closely entwined with that of their host galaxies and the central supermassive black holes that power them
This paper presents a study of the redshift evolution of radio-loud active galactic nuclei (AGN) as a function of the properties of their galaxy hosts in the Bootes field
It should be noted that the SF/low excitation radio galaxies (LERGs)/high excitation radio galaxies (HERGs) classification shown in this plot pertains only to the classification based on the optical spectra of these sources and as such the radio emission may be a result of either star-formation or AGN activity, most of the sources with high fSF values lie on the FIR-radio correlation
Summary
The evolution of radio-loud Active Galactic Nuclei (RL AGN) is closely entwined with that of their host galaxies and the central supermassive black holes that power them. While the most powerful radio sources tend to be high excitation radio galaxies (HERGs), they are found at all radio powers (Best & Heckman 2012) Due to their strong evolution with redshift this mode is likely important in cutting off star formation at high redshifts and setting up the tight black hole vs bulge mass relation that is observed locally (Magorrian et al 1998). In this paper we classify a sample of RL AGN as HERGs and LERGs on the basis of their broad-band spectral energy distributions (SEDs), and study the RL fractions, radio luminosity functions and colour and mass dependencies for the two classes of RL AGN at intermediate redshifts of 0.5 ≤ z < 2. We assume a spectral index of −0.7 unless otherwise stated
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