A successful last survey of the 'old' Westerbork Synthesis Radio Telescope

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Clik here to view.© Filippo Maccagni, Raffaella Morganti, Tom Oosterloo

Absorption line studies are an efficient way to trace the content of neutral hydrogen (HI) in radio sources and the interplay between the nuclear activity and the cold gas. A survey that was carried out with the WSRT shows HI in almost 30% of the sources, making the planned upcoming surveys with Apertif extremely promising.

Apertif, the new phased array feed of the Westerbork Synthesis Radio Telescope (WSRT), will start surveying the northern sky in search of neutral atomic hydrogen (HI) in radio sources (10^22 W/Hz < P (1.4 GHz) <10^26 W/Hz) out to redshift 0.26. To prepare for this, we have used the telescope until the very last moments before the refurbishment. We started to carry out a survey of 250 radio sources to search for the presence of HI. Such a large sample was never observed before. We observed each source for just a few hours, so the project could be run as "filler" until the very last day before WSRT was switched off.

We detected a great variety of absorption lines, with different shapes, widths and optical depths, shown in the image. Neutral hydrogen with kinematics deviating from regular rotation is traced by asymmetric broad lines with a with a significant blue-shifted component. This is found more often in powerful radio sources, where the radio emission is small, possibly because the radio nuclear activity is 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 HI 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 HI distributed in a rotating disk.

This is a very promising result in view of what we hope to see with the upgraded Apertif. The SHARP survey (Search for HI absorption with Apertif) will expand the results of this pilot project. The SHARP survey will be complemented by the FLASH and MALS HI absorption surveys of ASKAP and MerKat respectively. Together they will probe the presence HI up to a look-back time of 9.2 billion years.

The results are part of the PhD thesis of Filippo Maccagni and are presented in a paper Kinematics and physical conditions of HI in nearby radio sources. The last survey of the old Westerbork Synthesis Radio Telescope by F. M. Maccagni, R. Morganti, T. A. Oosterloo, K. Gereb and N. Maddox accepted for publication in Astronomy & Astrophysics, https://arxiv.org/abs/1705.00492