The low-frequency properties of FR 0 radio galaxies

• Published in: Astronomy and astrophysics (Berlin) Vol. 631; p. A176
• Main Authors: Capetti, A., Baldi, R. D., Brienza, M., Morganti, R., Giovannini, G.
• Format: Journal Article
• Language: English
• Published: Heidelberg EDP Sciences 01.11.2019
• Subjects: Flux density; galaxies: active; galaxies: jets; galaxies: nuclei; Luminosity; Morphology; Radio astronomy; radio continuum: galaxies; Radio galaxies; Radio spectra; Sky surveys (astronomy); Spectra; Spectrum allocation
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201936254

Using the alternative data release of the TIFR GMRT Sky Survey (TGSS), we studied the low-frequency properties of FR 0 radio galaxies, the large population of compact radio sources associated with red massive early-type galaxies revealed by surveys at 1.4 GHz. We considered TGSS observations from FR0CAT, a sample formed by 104 FR 0s at z <  0.05: all but one of them are covered by the TGSS, and 43 of them are detected above a 5σ limit of 17.5 mJy. No extended emission has been detected around the FR 0s, corresponding to a luminosity limit of ≲4 × 1023 W Hz−1 over an area of 100 kpc × 100 kpc. All but eight FR 0s have a flat or inverted spectral shape (α <  0.5) between 150 MHz and 1.4 GHz: this spectral behavior confirms the general paucity of optically thin extended emission within the TGSS beam, as is expected for their compact 1.4 GHz morphology. Data at 5 GHz were used to build their radio spectra, which are also generally flat at higher frequencies. By focusing on a sub-sample of FR 0s with flux density > 50 mJy at 1.4 GHz, we found that ∼75% of them have a convex spectrum, but with a smaller curvature than the more powerful gigahertz peaked-spectrum sources (GPS). The typical FR 0s radio spectrum is better described by a gradual steepening toward high frequencies, rather than to a transition from an optically-thick to an optically-thin regime, possibly observed in only ∼15% of the sample.