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Using the Alternative Data Release of the TIFR GMRT Sky Survey (TGSS), westudied the low-frequency properties of FR0 radio galaxies, the largepopulation of compact radio sources associated with red massive early-typegalaxies revealed by surveys at 1.4 GHz. We considered TGSS observations fromFR0CAT, a sample formed by 104 FR0s at z<0.05: all but one of them are coveredby the TGSS, and 43 of them are detected above a 5 sigma limit of 17.5 mJy. Noextended emission has been detected around the FR0s, corresponding to aluminosity limit of < 4 10^23 W/Hz over an area of 100 kpc x 100 kpc. All buteight FR0s have a flat or inverted spectral shape (alpha < 0.5) between 150 MHzand 1.4 GHz: this spectral behavior confirms the general paucity of opticallythin extended emission within the TGSS beam, as is expected for their compact1.4 GHz morphology.  Data at 5 GHz were used to build their radio spectra, which are alsogenerally flat at higher frequencies. By focusing on a sub-sample of FR0s withflux density > 50 mJy at 1.4 GHz, we found that ~75% of them have a convexspectrum, but with a smaller curvature than the more powerful gigahertzpeaked-spectrum sources (GPS). The typical FR0s radio spectrum is betterdescribed by a gradual steepening toward high frequencies, rather than to atransition from an optically-thick to an optically-thin regime, possiblyobserved in only ~15% of the sample.

The low-frequency properties of FR0 radio galaxies