J/A+A/634/A108 LOFAR view of NGC 3998 (Sridhar+, 2020)
LOFAR view of NGC 3998, a sputtering AGN.
Sridhar S.S., Morganti R., Nyland K., Frank B.S., Harwood J., Oosterloo T.
<Astron. Astrophys. 634, A108 (2020)>
=2020A&A...634A.108S 2020A&A...634A.108S (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; Radio sources
Keywords: galaxies: active - radio continuum: galaxy -
galaxies: individual: NGC 3998 - ISM: jets and outflows
Abstract:
Low-power radio sources dominate the radio sky. They tend to be small
in size and dominated by their cores, but the origin of their
properties and the evolution of their radio plasma are not well
constrained. Interestingly, there is mounting evidence that low-power
radio sources can significantly aect their surrounding gaseous medium
and may therefore be more relevant for galaxy evolution than
previously thought. In this paper, we present low radio frequency
observations obtained with LOFAR at 147MHz of the radio source hosted
by NGC 3998. This is a rare example of a low-power source that is
extremely dominated by its core, but that has two large scale lobes of
low surface brightness. We combine the new 147MHz image with available
1400MHz data to derive the spectral index over the source. Despite
the low surface brightness, reminiscent of remnant structures, the
lobes show an optically thin synchrotron spectral index (∼0.6). We
interpret this as being due to rapid decollimation of the jets close
to the core, to high turbulence of the plasma flow, and to entrainment
of thermal gas. This could be the result of intermittent activity of
the central active galactic nucleus, or, more likely, temporary
disruption of the jet due to the interaction of the jet with the rich
circumnuclear interstellar matter. Both would result in sputtering
energy injection from the core, which would keep the lobes fed, albeit
at a low rate.We discuss these results in connection with the
properties of low-power radio sources in general. Our findings show
that amorphous low surface brightness lobes should not be interpreted
by default as remnant structures. Large deep surveys (in particular
the LOFAR 150MHz LoTSS and the recently started 1400MHz Apertif
survey) will identify a growing number of objects similar to NGC 3998
where these ideas can be further tested.
Description:
NGC 3998 was observed for eight hours with the LOFAR High Band Antenna
(HBA) on 23 March 2015 (Open Time project LC3_025 PI K. Nyland).
Objects:
---------------------------------------------------
RA (2000) DE Designation(s)
---------------------------------------------------
11 57 56.13 +55 27 12.9 NGC 3998 = UGC 6946
---------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 111 1 Informations of fits image
fits/* . 1 fits image
--------------------------------------------------------------------------------
See also:
J/A+A/592/A94 : Radio continuum and gas reservoir in NGC 3998 (Frank+, 2016)
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 48 A21 "datime" Obs.date Observation date
50- 53 I4 Kibyte size Size of FITS file
55- 73 A19 --- FileName Name of FITS file, in subdirectory fits
75-111 A37 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Acknowledgements:
Sarrvesh Sridhar, sarrvesh(at)astron.nl
(End) Patricia Vannier [CDS] 10-Feb-2020