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The $\gamma$-ray emitting compact symmetric objects (CSOs) PKS 1718--649, NGC3894, and TXS 0128+554 are lobe-dominated in the radio emission. In order toinvestigate their $\gamma$-ray radiation properties, we analyze the $\sim$14-yrFermi/LAT observation data of the three CSOs. They all show the low luminosity($10^{41}-10^{43}$ erg s$^{-1}$) and no significant variability in the$\gamma$-ray band. Their $\gamma$-ray average spectra can be well fitted by apower-law function. These properties of $\gamma$-rays are clearly differentfrom the $\gamma$-ray emitting CSOs CTD 135 and PKS 1413+135, for which the$\gamma$-rays are produced by a restarted aligned jet. In the$L_{\gamma}-\Gamma_{\gamma}$ plane, the three CSOs are also located at theregion occupied by radio galaxies (RGs) while CTD 135 and PKS 1413+135 displaythe similar feature to blazars. Together with the similar radio emissionproperty to $\gamma$-ray emitting RGs Cen A and Fornax A, we speculate that the$\gamma$-rays of the three CSOs stem from their extended mini-lobes. Thebroadband spectral energy distributions of the three CSOs can be well explainedby the two-zone leptonic model, where their $\gamma$-rays are produced by theinverse Compton process of the relativistic electrons in extended region. Byextrapolating the observed Fermi/LAT spectra to the very high energy band, wefind that TXS 0128+554 among the three CSOs may be detected by the CherenkovTelescope Array in future.

Lobe-dominated gamma-ray Emission of Compact Symmetric Objects