The CON-quasar stage of IRAS 07251-0248 E

TL;DR

This paper presents detailed multi-wavelength observations of IRAS 07251-0248 E, revealing a highly obscured, compact nucleus with an active galactic nucleus (AGN) emitting quasar-like luminosity, and discusses the nuclear structure, gas dynamics, and potential outflow mechanisms.

Contribution

It introduces a comprehensive model of a deeply buried AGN in IRAS 07251-0248 E, combining ALMA, JWST, and Herschel data to characterize the nuclear environment and outflow properties.

Findings

The nucleus is extremely compact (<27 pc) and hot (T_B >200 K) with high optical depth.

Mid-IR and submillimeter data trace the same nuclear source, revealing a 'greenhouse' effect.

The AGN exhibits quasar luminosity but is heavily obscured, with outflows at ~160 km/s.

Abstract

ALMA continuum measurements of the local ULIRG IRAS 07251-0248 E at 667$\mu$m reveal an extremely compact (R < 27 pc) and bright ($T_B$ >200 K) nucleus with an absorbing foreground envelope and a surrounding (R ~ 75 pc) disk or torus seen nearly face-on. The bright and unresolved nuclear emission implies large optical depths ($\tau_{667\mu m}$ >0.5, corresponding to $N_H > 10^{25}$ cm^-2) of hot dust at >500 K. In addition, JWST observations of the source show strong mid-infrared (mid-IR) absorption in the ro-vibrational bands of H2O nu_2=1-0 (5-7 $\mu$m) and of other species including CO, HCN, C2H2, CH4, and CO2, and Herschel/PACS observations exhibit strong and saturated absorption due to OH, H2O, CH^+, and CH. We propose a model in which the unresolved ALMA submillimeter and JWST mid-IR continua trace the same nuclear source, the former penetrating deep into the nucleus and the latter probing the nuclear photosphere. The continuum model, which includes trapping of photons (the "greenhouse" effect), indicates that the nuclear ($R_h$ ~ 13 pc) luminosity and luminosity surface density are ~10$^{12}$ Lsun and $\Sigma_{bol}$~ 5e8 Lsun pc^-2, arising from an active galactic nucleus (AGN) so buried that high-ionization lines are completely obscured. The observed mid-IR gas-phase molecular bands probe outflowing gas with velocities of ~160 km s^-1 and are reproduced with the predicted $T_{dust}$ profile, while the far-IR molecular absorption lines are generated in the surrounding thick disk or torus with $\tau_{100 \mu m}$~10. We conclude that IRAS 07251-0248 harbors a compact obscured nucleus (CON) that hides an AGN currently emitting at quasar luminosity. While the observed outflow could be driven by radiation pressure, we favor the scenario of a (partially) energy-conserving hot bubble caught in a very early phase of the expulsion of the highly concentrated gas at the galactic nucleus.