Volume 592, August 2016
|Number of page(s)||10|
|Published online||04 August 2016|
A rare example of low surface-brightness radio lobes in a gas-rich early-type galaxy: the story of NGC 3998 ⋆
1 Netherlands Institute for Radio
Astronomy (ASTRON), Postbus
AA Dwingeloo, The Netherlands
2 Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa
3 Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
4 National Radio Astronomy Observatory (NRAO), , 520 Edgemont Rd., Charlottesville, VA 22903, USA
5 CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, , Epping, NSW 1710, Australia
Accepted: 5 May 2016
We study the nearby lenticular galaxy NGC 3998. This galaxy is known to host a low-power radio AGN with a kpc-size one-sided jet and a large, nearly polar H i disc. It is therefore a good system to study to understand the relation between the availability of cold-gas and the triggering of AGNs in galaxies. Our new WSRT data reveal two faint, S-shaped radio lobes extending out to ~10 kpc from the galaxy centre. Remarkably, we find that the inner H i disc warps back towards the stellar mid-plane in a way that mirrors the warping of the radio lobes. We suggest that the polar H i disc was accreted through a minor merger, and that the torques causing it to warp in the inner regions are also responsible for feeding the AGN. The “S” shape of the radio lobes would then be due to the radio jets adapting to the changing angular momentum of the accreted gas. The extended radio jets are likely poorly collimated, which would explain their quick fading and, therefore, their rarity in galaxies similar to NGC 3998. The fuelling of the central super-massive black hole is likely occurring via “discrete events”, suggested by the observed variability of the radio core and the extremely high core dominance, which we attribute to the formation and ejection of a new jet resulting from a recent fuelling event.
Key words: galaxies: active / galaxies: individual: NGC 3998 / ISM: jets and outflows / radio continuum: galaxies / radio lines: galaxies / history and philosophy of astronomy
The reduced radiocontinuum image and HI datacube (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/592/A94
© ESO, 2016
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