Centaurus A: morphology and kinematics of the atomic hydrogen

Abstract

We present new ATCA 21-cm line observations of the neutral hydrogen in the nearby radio galaxy Centaurus A. We image in detail (with a resolution down to 7 , ∼100 pc) the distribution of H i along the dust lane. Our data have better velocity resolution and better sensitivity than previous observations. The H i extends for a total of ∼15 kpc. The data, combined with a titled-ring model of the disk, allow to conclude that the kinematics of the H i is that of a regularly rotating, highly warped structure down to the nuclear scale.The parameters (in particular the inclination) of our model are somewhat different from some of the previously proposed models but consistent with what was recently derived from stellar light in a central ring. The model nicely describes also the morphology of the dust lane as observed with Spitzer. There are no indications that large-scale anomalies in the kinematics exist that could be related to supplying material for the AGN. Large-scale radial motions do exist, but these are only present at larger radii (r > 6 kpc). This unsettled gas is mainly part of a tail/arm like structure. The relatively regular kinematics of the gas in this structure suggests that it is in the process of settling down into the main disk. The presence of this structure further supports the merger/interaction origin of the H i in Cen A. From the structure and kinematics we estimate a timescale of 1.6−3.2 × 10 8 yr since the merging event. No bar structure is needed to describe the kinematics of the H i. The comparison of the timescale derived from the large-scale H i structure and those of the radio structure together with the relative regularity of the H i down to the sub-kpc regions does not suggest a oneto-one correspondence between the merger and the phase of radio activity. Interestingly, the radial motions of the outer regions are such that the projected velocities are redshifted compared to the regular orbits. This means that the blueshifted absorption discovered earlier and discussed in our previous paper cannot be caused by out-moving gas at large radius projected onto the centre. Therefore, the interpretation of the blueshifted absorption, together with at least a fraction of the redshifted nuclear absorption, as evidence for a regular inner disk, still holds. Finally, we also report the discovery of two unresolved clouds detected at 5.2 and 11 kpc away (in projection) from the H i disk. They are likely an other example of a left-over of the merger that brought the H i gas.

Recurrent radio emission and gas supply: the radio galaxy B2 0258+35

“…Even invoking an extreme time-delay between these events (longer than in other studied cases, see Emonts et al 2006), the fact that more than one stage of activity is seen cannot be unambiguously linked to clear events in the process of building of the gas disk. This is reminiscent of what was already found in the case of Centaurus A (Struve et al 2010b). Therefore, despite the large reservoir of H I in the disk, the actual fuel of the AGN may have a different origin.…”

Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A

“…2.3.1). Various authors (Stickel et al 2004;Struve et al 2010;Auld et al 2012) have reported the existence of cold clouds along the jet axis but offer no direct evidence for jet-cloud interaction. We choose to also neglect the contribution from cold gas to the total entrainment.…”

H i debris in the IC 1459 galaxy group

“…Though galaxy transformations can happen in lower density environments (Cappellari et al 2011), they occur more often in dense groups and clusters. Substantial H i debris are typically detected near massive, often early-type galaxies (Schneider 1989;Ryder et al 2001;Bekki et al 2005a,b;Chung et al 2009;English et al 2010;Struve et al 2010). In particular, we highlight the ATLAS-3D H i results for a large sample of nearby early-type galaxies, which show a 40% H i detection rate for the non-cluster targets…”