Spitzer mid-infrared spectroscopy of compact symmetric objects: What
  powers radio-loud active galactic nuclei?
  We present low- and high-resolution mid-infrared (mid-IR) spectra and
photometry for eight compact symmetric objects (CSOs) taken with the Infrared
Spectrograph on the Spitzer Space Telescope. The hosts of these young, powerful
radio galaxies show significant diversity in their mid-IR spectra. This
includes multiple atomic fine-structure lines, H2 gas, polycyclic aromatic
hydrocarbon (PAH) emission, warm dust from T = 50 to 150 K, and silicate
features in both emission and absorption. There is no evidence in the mid-IR of
a single template for CSO hosts, but 5/8 galaxies show similar moderate levels
of star formation (<10 M_sun/yr from PAH emission) and silicate dust in a
clumpy torus. The total amount of extinction ranges from A_V ~ 10 to 30, and
the high-ionization [Ne V] 14.3 and 24.3 um transitions are not detected for
any galaxy in the sample. Almost all CSOs show contributions both from star
formation and active galactic nuclei (AGNs), suggesting that they occupy a
continuum between pure starbursts and AGNs. This is consistent with the
hypothesis that radio galaxies are created following a galactic merger; the
timing of the radio activity onset means that contributions to the IR
luminosity from both merger-induced star formation and the central AGN are
likely. Bondi accretion is capable of powering the radio jets for almost all
CSOs in the sample; the lack of [Ne V] emission suggests an advection-dominated
accretion flow mode as a possible candidate. Merging black holes (BHs) with
M_BH > 10^8 M_sun likely exist in all of the CSOs in the sample; however, there
is no direct evidence from these data that BH spin energy is being tapped as an
alternative mode for powering the radio jets.
