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.

💡 Research Summary

This paper presents low‑ and high‑resolution mid‑infrared (mid‑IR) spectroscopy and photometry of eight compact symmetric objects (CSOs) obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. CSOs are young, powerful radio galaxies whose radio structures are confined within ~1 kpc, making them ideal laboratories for studying the early phases of radio‑loud active galactic nuclei (AGN). The authors reduced the IRS data using standard pipelines, performed background subtraction, order stitching, and flux calibration to produce continuous spectra covering 5–38 µm and high‑resolution line profiles between 10–37 µm.

The mid‑IR spectra display a remarkable diversity. All sources exhibit polycyclic aromatic hydrocarbon (PAH) features at 6.2, 7.7, and 11.3 µm, but the strengths vary, indicating moderate star‑formation activity (SFR ≈ 1–10 M⊙ yr⁻¹, generally <10 M⊙ yr⁻¹). Molecular hydrogen rotational lines (H₂ S(1)–S(5)) are detected in every object, revealing warm molecular gas at temperatures of 200–500 K. Silicate features appear both in absorption (9.7 µm) and emission (18 µm), suggesting a clumpy, geometrically thick dust torus that partially obscures the nucleus. Extinction estimates derived from silicate depth and continuum fitting range from AV ≈ 10 to 30 mag.

High‑ionization neon lines,