The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images

Abstract

Super massive black holes at the centres of galaxies can cycle through periods of activity and quiescence. Characterising the duty cycle of active galactic nuclei (AGN) is crucial for understanding the impact of the energy they release on the host galaxy. For radio AGN, this can be done by identifying dying (remnant) and restarted radio galaxies from their radio spectral properties. Using the combination of the images at 1400 MHz produced by Apertif, the new phased-array feed receiver installed on the Westerbork Synthesis Radio Telescope, and images at 150 MHz provided by LOFAR, we have derived resolved spectral index images (at a resolution of ~15 arcsec) for all the sources within an approximately 6 deg2 area of the Lockman Hole region. In this way, we were able to select 15 extended radio sources with emission (partly or entirely) characterised by extremely steep spectral indices (steeper than 1.2). These objects represent cases of radio sources in the remnant or the restarted phases of their life cycle. Our findings confirm that these objects are not as rare as previously thought, suggesting a relatively fast cycle. They also show a variety of properties that can be relevant for modelling the evolution of radio galaxies. For example, the restarted activity can occur while the remnant structure from a previous phase of activity is still visible. This provides constraints on the duration of the “off” (dying) phase. In extended remnants with ultra-steep spectra at low frequencies, the activity likely stopped a few hundred megayears ago, and they correspond to the older tail of the age distribution of radio galaxies, in agreement with the results of simulations of radio source evolution. We find remnant radio sources with a variety of structures (from double-lobed to amorphous), possibly suggesting different types of progenitors. The present work sets the stage for exploiting the powerful tool of low-frequency spectral index studies of extended sources by taking advantage of the large areas common to the LOFAR and the Apertif surveys.

Combining LOFAR and Apertif Data for Understanding the Life Cycle of Radio Galaxies

“…This work can now be complemented with the information on the resolved spectral index obtained using the combination of LOFAR images at 150 MHz and Apertif images at 1400 MHz. A first proof-of-concept was presented in [38]. Despite being limited to a relatively small area (about 6 deg 2 of the LH region), and despite using Apertif science verification data, this study did show the potential of adding the spectral information (even if limited to two frequencies only).…”

“…The noise level ranges between 35 and 40 µJy beam −1 in the regions not affected by imaging artefacts. The flux scale of the Apertif mosaic was checked against the NVSS, as was previously done for the image covering a smaller part of the LH area ( [38]). Similarly, a difference of about 10% in the flux scale was found and corrected for.…”

“…The spectral index image was obtained following the procedure described in [38]. The LOFAR image was convolved to the same angular resolution as that of the Apertif mosaic, resulting in an image with average rms noise of ∼130-160 µJy beam −1 , increasing toward the edges of the field.…”

“…Here, we focus on the spectral properties of the restarted and remnant radio galaxies that were already selected and included in the sample of [34]. The selection of new candidates based only on the spectral properties (as done in [38] for a small region of the LH) will be done in a future study. In Figure 3, we show some examples of the resolved spectral index distribution in our target sources and below we describe their properties.…”

The LOFAR LBA Sky Survey

“…Ultra-low frequency data are also crucial for the study of remnants of radio galaxies (Brienza et al 2017;Mahatma et al 2018). Recent investigations have shown that this elusive population of sources exhibits a range of spectral properties and some still show the presence of a faint core (Morganti et al 2020;Jurlin et al 2020). The addition of a very low frequency point for a resolved spectral analysis will constrain the time scale of the 'off' phase.…”