The CON-quasar stage of IRAS 07251-0248 E

TL;DR

IRAS 07251$-$0248 E hosts a compact obscured nucleus (CON) in which a highly buried AGN emits near quasar luminosity while a more extended, star-forming component surrounds it. The authors combine ALMA 667 μm continuum with JWST mid-IR spectra and Herschel far-IR data to construct a unified continuum model featuring a compact hot core (H_C) and a larger warm disk/torus (W_C) embedded in a foreground envelope (C_C), with a greenhouse effect that raises interior dust temperatures. The best-fit model yields an intrinsic luminosity of $L_{\rm unatt} \sim 10^{12}\ L_{\odot}$ and a bolometric surface density of $\Sigma_{\rm bol} \sim 5\times10^{8}\ L_{\odot}\ \mathrm{pc}^{-2}$, consistent with an AGN-dominated energy source, while JWST detections of deep $\mathrm{H_2O}$, HCN, $\mathrm{C_2H_2}$, CH$_4$, and CO$_2$ mid-IR absorption reveal an expanding $6\ \mu$m photosphere and a high $\mathrm{H_2O}$ abundance of $X_{\mathrm{H_2O}} \approx 8\times10^{-5}$. Far-IR lines probe a surrounding thick disk/torus with $\tau_{100\ \mu m} \sim 10$, indicating strong, optically thick absorption consistent with an AGN-embedded nuclear structure. The inferred outflow energetics support a hot-bubble, partially energy-conserving scenario for nuclear feedback, potentially driving the observed kinematics and associated chemistry, and highlighting the role of CONs in black hole growth and star formation co-evolution in extreme environments.

Abstract

ALMA continuum measurements of the local ULIRG IRAS 07251-0248 E at 667$μ$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 ($τ_{667μm}$ >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 $μ$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 $Σ_{bol}$~ 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 $τ_{100 μm}$~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.

Figures (11)

• Figure 1: ALMA 667 $\mu$m continuum emission of IRAS 07251$-$0248 E and model. Left: Observed emission, with colored scale in units of mJy/beam. Middle: model, composed of an unresolved source ($\mathrm{FWHM}<21$ mas $=34$ pc) and a Gaussian source ($\mathrm{FWHM}=89\times86\,\mathrm{mas}^2$$=146\times141$ pc$^2$). Right: residuals in $\mu$Jy/beam.
• Figure 2: Spectral energy distribution (SED) of IRAS 07251$-$0248 E from $\lambda_{\mathrm{rest}}=2.6$ to 1200 $\mu$m and model. In black, the full JWST NIRSpec & MIRI/MRS spectrum is shown, together with Spitzer, Herschel/PACS (from 60 to 160 $\mu$m, both photometric and spectroscopic) and SPIRE (from 230 to 470 $\mu$m), and ALMA (667 and 1200 $\mu$m) data. The colored circles at 667 $\mu$m indicate the contributions by the unresolved (blue) and resolved (light-blue) components inferred from the model in Fig. \ref{['almacont']}. At 1.2 mm, the open black circle and the cross indicate the flux densities of the unresolved ($<82$ pc) and resolved ($540\times330$ pc$^2$) components, respectively per21. The model includes the $H_C$ in blue (dashed: unattenuated emission; solid: attenuated), the $W_C$ in light-blue (extincted), a cold component ($C_C$) in orange, the stellar component in gray, and the "hidden" component (whose attenuated emission is added to that of the $W_C$, see Section \ref{['sech2oband']}) in dotted-dashed green. Red is total. The insert panels show details of the model results. Upper left: $T_{\mathrm{dust}}$ profile of the $H_C$. Upper right: the optical depths $\tau$ of the $H_C$ and $W_C$ as a function of wavelength. Lower middle: the $T_{\mathrm{dust}}$ profile of the $H_C$ as a function of the optical depth from surface at 7 and 14 $\mu$m.
• Figure 3: H$_2$O $\nu_2=1-0$ band in IRAS 07251$-$0248 observed with JWST MIRI/MRS. Panels a and c show the observed spectrum, with the orange line indicating the adopted continuum level and the light-blue curve showing the contribution from the warm component. Panels b and d show the continuum normalized spectrum, with the blue line showing the model result for the hot component and the dashed line indicating its covering factor ($f_{\mathrm{hot}}$). We label the main transitions that potentially contribute to the H$_2$O band as shown in gon24, but some very highly-excited lines are not detected in IRAS 07251$-$0248. Note that all H$_2$O absorption features are blueshifted relative to the labels.
• Figure 4: Schematic representation of the model source. The extremely compact and expanding core ($H_C$) is shown in blue (the 6 $\mu$m photosphere is its external surface), with gas flowing outward (the surrounding shell outflowing at $\sim-400$ km s$^{-1}$) in green. The starburst torus shown in magenta represents the $W_C$. The torus also appears to be expanding, because the far-IR lines are blueshifted (Appendix \ref{['farir']}, Fig. \ref{['pacs']}). Both the $H_C$ and $W_C$ are embedded in an extended cold component ($C_C$), in orange. The model is axisymmetric about the line of sight.
• Figure 5: ALMA 667 $\mu$m continuum emission of IRAS 07251$-$0248 E and alternative models: a point source (upper) and a Gaussian (lower). Left: Observed emission, with colored scale in units of mJy/beam. Middle: model. Right: residuals in $\mu$Jy/beam. These models give residuals much more prominent than our best point-source $+$ Gaussian model fit in Fig. \ref{['almacont']}.