The LOFAR view of FR0 radio galaxies
  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
smaller than 3-6 kpc. However, 12 FR0s 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 FR0s, with a typical luminosity
limit of 5 x 10$^{22}$ W/Hz over an area of 100 kpc x 100 kpc. The spectral
slopes of FR0s between 150 MHz and 1.4 GHz span a broad range (-0.7 < $\alpha$
< 0.8) with a median value of $\overline\alpha \sim 0.1$; 20% of them have a
steep spectrum ($\alpha$ > 0.5), an indication of the presence of substantial
extended emission confined within the spatial resolution limit. The fraction of
FR0s showing evidence for the presence of jets, by including both spectral and
morphological information, is at least ~40%. This study confirms that FR0s and
FRIs can be interpreted as two extremes of a continuous population of jetted
sources, with the FR0s representing the low end in size and radio power.
