CO-Dark Star Formation and Black Hole Activity in 3C 368 at z = 1.131: Coeval Growth of Stellar and Supermassive Black Hole Masses^{∗ †}

Abstract

We present the detection of four far-infrared fine-structure oxygen lines, as well as strong upper limits for the CO(2-1) and [N ii] 205 µm lines, in 3C 368, a well-studied radio-loud galaxy at z = 1.131. These new oxygen lines, taken in conjunction with previously observed neon and carbon fine-structure lines, suggest a powerful active galactic nucleus (AGN), accompanied by vigorous and extended star formation. A starburst dominated by O8 stars, with an age of ∼ 6.5 Myr, provides a good fit to the fine-structure line data. This estimated age of the starburst makes it nearly concurrent with the latest episode of AGN activity, suggesting a link between the growth of the supermassive black hole and stellar population in this source. We do not detect the CO(2-1) line, down to a level twelve times lower than the expected value for star forming galaxies. This lack of CO line emission is consistent with recent star formation activity if the star-forming molecular gas has low metallicity, is highly fractionated (such that CO is photodissociated through much of the clouds), or is chemically very young (such that CO has not yet had time to form). It is also possible, though we argue unlikely, that the ensemble of fine structure lines are emitted from the region heated by the AGN.

CO and Fine-structure Lines Reveal Low Metallicity in a Stellar-mass-rich Galaxy at z ∼ 1?

“…Unfortunately, our 230 GHz rest-frame ALMA observations (Lamarche et al 2017), with a synthesized beam of size 3. 12 × 1.…”

“…Using the ALMA continuum observations centered at 480.5 GHz (rest-frame) presented here, together with our previous ALMA continuum measurements at 205 µm (rest-frame; Lamarche et al 2017), we can calculate the slope of the Rayleigh-Jeans power-law for the thermal dust continuum of 3C 368. This is particularly interesting in this case, since the core component of 3C 368, the star-forming galaxy, makes up the smallest contribution to the continuum flux at rest-frame 480.5 GHz, the flux being dominated instead by the radio lobes.…”

“…where h is the Planck constant, A ul is the Einstein A coefficient for the line transition at frequency ν ul , 9.8 × 10 −5 s −1 and 5785.9 GHz, respectively, for the [O iii] 52 µm line (Carilli & Walter 2013), and the ratio of the level population in the upper state (n u ) to the total (n i ) is determined by detailed balance. Adopting the col- Rubin (1985), we predicted that ∼ 84% of the oxygen within the H ii regions of 3C 368 is in the O ++ state (Lamarche et al 2017). Using that ionization fraction here, we obtain an estimated [O/H] ratio of 1.2 × 10 −4 , or ∼ 0.3 Z (Asplund et al 2009), within about a factor of two uncertainty, where this uncertainty is dominated by both the uncertainty in determining the free-free contribution to the radio SED and the uncer- An alternative explanation for the radio emission emanating from the core component of 3C 368 is that it originates from the AGN itself.…”

“…Radio SED of the core component of 3C 368, decomposed into thermal (gold) and non-thermal (red) contributions. Data fromBest et al (1998a) andLamarche et al (2017).…”

“…;Lamarche et al 2017) with the estimated free-free flux density at 230 GHz (rest-frame), and using Equation 2, we calculate [O ++ /H] = 1 × 10 −4 . Then, to determine the [O ++ /O] ratio, and hence scale our ionic oxygen abundance to total oxygen abundance, an estimate for the hardness of the ambient radiation field within 3C 368 is required.…”