The LOFAR view of FR 0 radio galaxies

Abstract

We explore the low-frequency radio properties of the sources in the Fanaroff-Riley class 0 catalog (FR0CAT) as seen by the Low-Frequency ARray (LOFAR) observations at 150 MHz. This sample includes 104 compact radio active galactic nuclei (AGN) associated with nearby (z < 0.05) massive early-type galaxies. Sixty-six FR0CAT sources are in the sky regions observed by LOFAR and all of them are detected, usually showing point-like structures with sizes of ≲3–6 kpc. However, 12 FR 0s present resolved emission of low surface brightness, which contributes between 5% and 40% of the total radio power at 150 MHz, usually with a jetted morphology extending between 15 and 50 kpc. No extended emission is detected around the other FR 0s, with a typical luminosity limit of ≲5 × 1022 W Hz−1 over an area of 100 kpc × 100 kpc. The spectral slopes of FR 0s between 150 MHz and 1.4 GHz span a broad range (−0.7 ≲ α ≲ 0.8) with a median value of ᾱ ∼ 0.1; 20% of them have a steep spectrum (α ≳ 0.5), which is an indication of the presence of substantial extended emission confined within the spatial resolution limit. The fraction of FR 0s showing evidence for the presence of jets, by including both spectral and morphological information, is at least ∼40%. This study confirms that FR 0s and FR Is can be interpreted as two extremes of a continuous population of jetted sources, with the FR 0s representing the low end in size and radio power.

Parsec-scale Radio Structure of 14 Fanaroff–Riley Type 0 Radio Galaxies

“…It is important to note that the VLBI sample we used in this study is at the high flux density end of the overall FR 0 population; since the jets of these sources are found to be mildly relativistic (lower Lorentz factor), it is reasonable to believe that the jets of the majority of FR 0 sources have an even lower Lorentz factor. This is consistent with the conclusion obtained by other groups through other methods (e.g., Sadler et al 2014;Baldi et al 2018b;Capetti et al 2020). These sources do not show any stationary hotspots at the ends of their jets, but the lack of terminal hotspots cannot be used to rule out the possibility that FR 0s represent short-lived episodes of AGN activity that do not last long enough for a galaxy to develop large-scale radio jets.…”

“…The source J1025+1022 is at a redshift of 0.046 (Huchra et al 2012). The source is unresolved at 150-MHz LOFAR observation and 1.4-GHz NVSS observation (Condon et al 1998;Capetti et al 2020). Our observation shows a rich two-sided structure with the jet extending roughly in the northwest-southeast direction on parsec scales.…”

“…Therefore, the extended jets should be dissipative and lie below the surface-brightness threshold of existing large-scale radio surveys (Sadler 2016). Follow-up studies with lower-frequency observations (e.g., Capetti et al 2020) should be performed on larger samples and should also be expanded to lower radio power FR 0s.…”

“…The source J1037+4335 is associated with a quasar at redshift = 0.025 (Falco et al 1999). The 150-MHz LOFAR image and 1.4-GHz NVSS image only detect a compact core with a flux density of ∼ 260.9 mJy and ∼ 132.2 mJy (Condon et al 1998;Capetti et al 2020). J1037+4335 was resolved into two isolated components, separated by 19.7 pc in the previous study (Cheng & An 2018).…”

“…Such jet-ISM interaction could deflect the jet flow and change its direction (Kino et al 2018). The source has a complex radio spectrum: a flat spectrum with a spectral index of = −0.42 between 150 MHz and 1.4 GHz (Capetti et al 2020), an inverted spectrum with = 0.34 between 1.4 GHz and 5 GHz (Capetti et al 2019) Using the core as a reference, a linear regression fit gives the proper motion velocities of three components: N1 (along the jet axis) 0.219±0.017 mas yr −1 , S1 (almost no motion and position angle changing) −0.005±0.054 mas yr −1 , and S2 ( non-radial motion) 0.104±0.037 mas yr −1 . These angular velocities correspond to 0.39 , −0.08 , 0.18 for N1, S1, S2 respectively.…”