MIGHTEE: Discovery of a triple-double radio galaxy

Abstract

Triple-double radio galaxies (TDRGs) are amongst the rarest subpopulations of radio galaxies (RGs). They are characterised by three pairs of radio lobes, where each pair of lobes represents an episode of nuclear activity. Such a feature makes them key objects that can be used to constrain the duty cycle of RGs. In this paper, we report the discovery of J022248\m060934, a new TDRG, hosted by a galaxy at a spectroscopic redshift of $z \approx$ 0.94. We have used the MIGHTEE-DR1 data set and MIGHTEE sub-band images as our main data. In total intensity, J022248\m060934 has a bright core and triple-double, edge-brightened-like peaks of radio emission. The polarimetry of the source reveals an inhomogeneous density of the hosting environment which is consistent with the more pronounced bending in its eastern lobes. The spectral index and curvature maps suggest an inverted core and an ultra-steepening of the spectrum towards the outer lobes which reinforce a recurrent nuclear activity. We perform individual spectral age fitting of the components of the source using the JP model and we found a lower limit total age of $\sim$16 Myr. We also derive a short inactive period between the active phases and a rapid duty cycle of 90 per cent for the first cycle of activity. Our spectral ageing analysis suggests that the triple-double structure in TDRGs is not the product of long quiescent periods, as deduced by previous works based on kinematic ages.

Figures (12)

• Figure 1: Radio continuum image of the TDRG showing its morphology as well as the position of its host galaxy as indicated by the red cross. Background: Pan-STARRS DR1 gri$-$composite optical image; Contour: MIGHTEE-hi continuum (levels: $-$3, 3, 5, 10, 15, 25, 30, 35, 40, 50, 75, 100, 150, 200, 250, 300 $\times \sigma_{local}$ = 5.9 $\mu$Jy/beam). The three pairs of lobes are indicated by I (outermost), II (middle) and III (innermost). The scale indicating 100 kpc (12.4 arcsec) and the synthesized beam of MIGHTEE-hi are in the bottom left corner of the image.
• Figure 2: Spectral index maps above 3$\sigma_{local}$ level of the TDRG. Top panel: map computed between 150 and 1540 MHz with MIGHTEE-1540 contours (levels: $-$ 3, 3, 5, 10, 15, 20, 30, 40, 50, 75, 100, 150, 200 $\times \sigma_{local}$ = 16.1 $\mu$Jy/beam); Middle panel: map computed between 150 and 325 MHz with LOFAR contours (levels: $-$3, 3, 5, 10, 15, 20, 25, 30, 35 $\times \sigma_{local}$ = 366 $\mu$Jy/beam); Bottom panel: map computed between 325 and 1540 MHz with the same MIGHTEE-1540 contours. The solid black circle in the bottom left corner indicates the resolution of each map. The purple and blue colours depict the steepest emissions. The white cross represents the position of the optical host galaxy.
• Figure 3: Fitted radio spectrum of the core from 150 to 3000 MHz assuming an homogeneous SSA (solid green line) and a uniform FFA (dotted black line) models. To limit contamination from extended emissions, the peak flux densities were used. The flux errors were assumed to be 10 per cent for each measurement, except at 3000 MHz where 8 per cent was used instead following the Quick Look users guide.
• Figure 4: Spectral curvature map of the TDRG from 150 to 1540 MHz. The most curved regions are indicated by dark blue colour. The contours on top show the 1540 MHz MIGHTEE radio morphology of the TDRG above 3$\sigma_{local}$ (contour levels: $-$ 3, 3, 5, 10, 15, 20, 30, 40, 50, 75, 100, 150, 200 $\times \sigma_{local}$ = 16.1 $\mu$Jy/beam). The synthesized beam of the map is shown in the bottom left corner.
• Figure 5: Resulting maps of the spectral age fitting using JP model. The outcomes of the fitting for the eastern to the western outer lobes are displayed from top to bottom. The spectral age distributions are shown in the first column, followed by the error maps and the $\chi^2$. The lowest (highest) values are indicated by the blue (red) colours. For the eastern inner lobe and the western middle lobe, the fits were done using the MIGHTEE sub-band data only due to low detection in LOFAR and GMRT.