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Looking for broad HI absorption with Apertif

Submitter: Apertif Commissioning Team
Description: While we are waiting to start commissioning Apertif using the full field of view (i.e. using 39 all beams), we can test other important characteristics of the system to see whether they match the needs of the astronomers so that new and exciting science can be done.

A key parameter is the stability of the spectral response of Apertif (the bandpass). This is of particular importance for the study of the gas in radio sources where we want to trace the presence of diffuse and faint neutral hydrogen that has extreme kinematics (i.e. very large outflow velocities). Such gas is one of the signatures of the feedback effects of the active super massive black hole on the rest of the galaxy.

The good performance of the old WSRT system - where we could reach a level of 1/10000 in the stability of the bandpass - did allow to discover the existence of such gas, a finding completely unexpected and still not fully explained!
Plans are to use Apertif to expand the heritage left by the WSRT and to search for many more cases of gas with extreme kinematics. Although test observations show that the present bandpass stability of Apertif is not quite yet at the level of the old WSRT, still the first results are showing that we are on the right track.

The figure shows one of the radio galaxies (contours of the continuum emission on the left) and (on the right) the HI absorption observed with Apertif (black line). The Apertif data are superposed to the profile obtained years ago with the old MFFE receiver of the WSRT (red line).

Despite the higher noise (due to the limitations of the present Apertif system), the shape of the absorption is well reproduced and a hint of the presence of gas with extreme kinematics (the blueshifted, shallow wing) is also appearing. This is very promising for the future Apertif surveys and, in particular for the Search for HI absorption with APERTIF (SHARP) project.

Copyright: astron
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