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We present low- and high-resolution mid-infrared (mid-IR) spectra andphotometry for eight compact symmetric objects (CSOs) taken with the InfraredSpectrograph on the Spitzer Space Telescope. The hosts of these young, powerfulradio galaxies show significant diversity in their mid-IR spectra. Thisincludes multiple atomic fine-structure lines, H2 gas, polycyclic aromatichydrocarbon (PAH) emission, warm dust from T = 50 to 150 K, and silicatefeatures in both emission and absorption. There is no evidence in the mid-IR ofa single template for CSO hosts, but 5/8 galaxies show similar moderate levelsof star formation (<10 M_sun/yr from PAH emission) and silicate dust in aclumpy torus. The total amount of extinction ranges from A_V ~ 10 to 30, andthe high-ionization [Ne V] 14.3 and 24.3 um transitions are not detected forany galaxy in the sample. Almost all CSOs show contributions both from starformation and active galactic nuclei (AGNs), suggesting that they occupy acontinuum between pure starbursts and AGNs. This is consistent with thehypothesis that radio galaxies are created following a galactic merger; thetiming of the radio activity onset means that contributions to the IRluminosity from both merger-induced star formation and the central AGN arelikely. Bondi accretion is capable of powering the radio jets for almost allCSOs in the sample; the lack of [Ne V] emission suggests an advection-dominatedaccretion flow mode as a possible candidate. Merging black holes (BHs) withM_BH > 10^8 M_sun likely exist in all of the CSOs in the sample; however, thereis no direct evidence from these data that BH spin energy is being tapped as analternative mode for powering the radio jets.

Spitzer mid-infrared spectroscopy of compact symmetric objects: What powers radio-loud active galactic nuclei?