Mid-infrared Diagnostics of the Circumnuclear Environments of the Youngest Radio Galaxies

Abstract

We present a systematic analysis of the mid-infrared (MIR) properties of the youngest radio galaxies, based on low-resolution data provided by the Wide-field Infrared Survey Explorer and IRAS satellites. We restrict our analysis to sources with available X-ray data that constitute the earliest phase of radio galaxy evolution, i.e., those classified as gigahertz-peaked spectrum and/or compact symmetric objects. In our sample of 29 objects, we find that the host galaxies are predominantly red/yellow ellipticals, with some of them displaying distorted morphology. We find a variety of MIR colors and observe that the sources in which the MIR emission is dominated by the ISM component uniformly populate the region occupied by galaxies with a wide range of pronounced (≥0.5M ⊙ yr−1) star formation activity. We compare the MIR color distribution in our sample to that in the general population of local active galactic nuclei (AGNs), in the population of evolved FR II radio galaxies, and also in the population of radio galaxies with recurrent jet activity. We conclude that the triggering of radio jets in AGNs does not differentiate between elliptical hosts with substantially different fractions of young stars; instead, there is a relationship between the jet duty cycle and the ongoing star formation. The distribution of the subsample of our sources with z < 0.4 on the low-resolution MIR versus absorption-corrected X-ray luminosity plane is consistent with the distribution of a sample of local AGNs. Finally, we comment on the star formation rates of the two γ-ray-detected sources in our sample, 1146+596 and 1718–649.

X-ray Emission of the $γ$-ray Loud Young Radio Galaxy NGC 3894

“…the sources for which the intrinsic (i.e., un-absorbed) X-ray fluxes at < 10 keV photon energies are difficult to assess. For those, instead of the X-ray luminosities, Boorman et al used For the WISE 12 µm luminosity of NGC 3894, L 12 µm 1.01×10 42 erg s −1 (Kosmaczewski et al 2020), this correlation would therefore imply EW 1.07 +0.37 −0.28 keV, which is in a (surprisingly) very good agreement with the bestfit value following from our spectral analysis.…”

“…Hot dusty tori in active galaxies are, in general, expected to reprocess about η DT ∼ 10% of the accretion disk bolometric luminosity. With such, the integrated 8 − 1000 µm luminosity of NGC 3894 as estimated by Kosmaczewski et al (2020), would then imply an upper limit on the accretion disk luminosity L bol ∼ 5 × 10 43 erg s −1 , or in terms of the accretion rate λ Edd ≡ L bol /L Edd ∼ 10 −4 . We stress, however, that the above estimate is highly uncertain, because of the uncertain value of the radiative efficiency factor η DT , and also due to the fact that the "callorimetric" properties of circumnuclear dust can only reflect the nuclear power output averaged over some longer period of time, rather than the accretion disk luminosity at a given (present) moment.…”

“…As shown, the radiative output of the system is dominated by the starlight of the host at near-infrared to optical frequencies (H-band absolute magnitude of −21.80; see Perlman et al 2001), with a likely contribution from the circumnuclear hot dust in the mid-IR range. We note in this context that, in the WISE color-color diagram, our target lies outside of the Stern et al (2012) or Mateos et al (2012) "Active Galactic Nucleus (AGN) cuts" (see Kosmaczewski et al 2020), but at the same time that the efficiency of the mid-IR colour selections strongly decreases for lowluminosity AGN (see the related discussion in Stern et al 2012;Boorman et al 2018).…”

“…When comparing NGC 3894 with 1404+286 and 2021+614 (see Table 1 in Kosmaczewski et al 2020, and references therein), we note that all the three sources are similarly compact (linear sizes of ∼ 4 pc, ∼ 10 pc and ∼ 16 pc, respectively), but the latter two are (a) much more X-ray luminous (by approximately two and three orders of magnitude, respectively), and moreover (b) Compton-thick, i.e. characterized by the equivalent hydrogen column densities N H ∼ 10 24 cm −2 , which is about two order of magnitude larger than N H inferred in our analysis for NGC 3894.…”

“…The infrared (IR) studies of NGC 3894 using various telescopes, such as the Infrared Astronomical Satellite (IRAS), the Spitzer Space Telescope, and the Widefield Infrared Survey Explorer (WISE; Willett et al 2010;Kosmaczewski et al 2020), reinforced the 'Lowionization nuclear emission-line region' (LINER) classification of the nucleus (see in this context Condon & Broderick 1988;Kim 1989;Gonçalves & Serote Roos 2004), and indicated a rather low star formation rate in the host, at the level of ∼ 0.5M yr −1 .…”