Kinematics and physical conditions of H i in nearby radio sources

Abstract

We present an analysis of the properties of neutral hydrogen (H i) in 248 nearby (0.02 < z < 0.25) radio galaxies with S 1.4 GHz > 30 mJy and for which optical spectroscopy is available. The observations were carried out with the Westerbork Synthesis Radio Telescope as the last large project before the upgrade of the telescope with phased array feed receivers (Apertif). The sample covers almost four orders of magnitude in radio power from log P 1.4 GHz = 22.5 W Hz −1 and 26.2 W Hz −1 . We detect H i in absorption in 27% ± 5.5% of the objects. The detections are found over the full range of radio power. However, the distribution and kinematics of the absorbing H i gas appear to depend on radio power, the properties of the radio continuum emission, and the dust content of the sources. Among the sources where H i is detected, gas with kinematics deviating from regular rotation is more likely found as the radio power increases. In the great majority of these cases, the H i profile is asymmetric with a significant blue-shifted component. This is particularly common for sources with log P 1.4 GHz > 24 W Hz −1 , where the radio emission is small, possibly because these radio sources are young. The same is found for sources that are bright in the mid-infrared, i.e. sources rich in heated dust. In these sources, the H i is outflowing likely under the effect of the interaction with the radio emission. Conversely, in dust-poor galaxies, and in sources with extended radio emission, at all radio powers we only detect H i distributed in a rotating disk. Stacking experiments show that in sources for which we do not detect H i in absorption directly, the H i has a column density that is lower than 3.5 × 10 17 (T spin /c f ) cm −2 . We use our results to predict the number and type of H i absorption lines that will be detected by the upcoming surveys of the Square Kilometre Array precursors and pathfinders (Apertif, MeerKAT, and ASKAP).

FLASH early science – discovery of an intervening H i 21-cm absorber from an ASKAP survey of the GAMA 23 field

“…Here we compare our results with that of the radio-selected survey by Maccagni et al (2017). Using the Westerbork Synthesis Radio Telescope (WSRT), they carried out a search for intrinsic H I absorption in 248 nearby (z < 0.25) radio galaxies with core flux densities S 1.4 > 30 mJy, reporting a detection rate of (27 ± 5.5) per cent for their whole sample, with an average peak optical depth sensitivity of approximately τ peak ≈ 0.05.…”

The potential of tracing the star formation history with H I 21-cm in intervening absorption systems

“…6 Since no UV spectrometer is planned for the James Webb Space Telescope, this will be a particular problem for the z H i < ∼ 1 limitation of the SKA pathfinders (e.g. Maccagni et al 2017) upon the demise of the Hubble Space Telescope.…”

“…Given the difficulties in obtaining a large sample of DLAs which exhibit 21-cm absorption 1 , a significantly larger sample may not be available until the science operations of the Square Kilometre Array, or at least its pathfinders (which are generally limited to z abs < ∼ 1, e.g. Allison et al 2016a;Maccagni et al 2017). In the meantime, there are a further 176 intervening absorption systems which have been searched in H i 21-cm absorption.…”

Connecting X-ray absorption and 21 cm neutral hydrogen absorption in obscured radio AGN

“…This has been supplemented by studies showing a higher detection rate of Hi in these kinds of galaxies, particularly in the local Universe (e.g. Gereb et al 2015;Maccagni et al 2017). However, different processes may dominate in particular AGN depending on their accretion rates, jet power, density of the surrounding medium and, importantly, evolutionary age.…”