Luminous infrared galaxies (LIRGs) are nearby laboratories for extreme galaxy and black-hole growth, yet their power-houses are buried behind optically thick dust. The HIDDeN project lifted that veil.
Work performed We secured the large ALMA programmes CONquest and follow-up CONfirm, plus complementary APEX, NOEMA, VLA, eMERLIN, VLBI, JWST and VLT/MUSE observations. Vibrationally-excited HCN (HCN-vib) was used as a torch to map 38 U/LIRGs at 0.1–0.3″ (15–50 pc) resolution, with sub-10 pc imaging of IC 860, NGC 1377, Zw049, IRAS17578, ESO-173 and ESO320. A parallel redshift survey extended the method to z ≈ 4.
Main results
• Incidence of buried nuclei – Compact Obscured Nuclei (CONs) are four times more common than assumed; 40 % of the most luminous ULIRGs host a CON.
• Feeding and feedback – CONs show simultaneous radial inflows and radio-quiet, collimated molecular outflows (CMOs). In IC 860 the inflow hides the outflow; in NGC 1377 a 2 pc-wide rotating molecular jet appears without a radio jet, revealing a new feedback channel.
• Launch regions of coutflows: HIDDeN used cutting-edge ALMA capacity to pinpont the nuclear launch regions of the powerful molecular outflows – a key point in determining their nature
• Dense-gas diagnostics – HCN-vib luminosity, coronal free–free emission and mm-continuum slopes were calibrated as quantitative tracers of deeply buried AGN power and used to discover the most distant CON-like object yet (z = 3.9).
• AGN leakage – MUSE [O III] imaging of NGC 4418 uncovered ionised knots beyond the opaque core, showing that even the deepest nuclei can leak ionising photons.
Exploitation and dissemination Although the work is fundamental, the new mm/sub-mm diagnostics, receiver set-ups and analysis pipelines are already being adopted by the ALMA community and folded into JWST planning tools, enhancing future high-redshift surveys. Our coronal model and observational techniques are shared with the community. Results were shared through >30 refereed papers >35 conference talks, several invited keynotes and >six dedicated workshops. Outreach included public lectures, and interviews for Swedish Radio P4, Vetenskapsradion, Sveriges Television and the PI was featured on SRs Söndagsintervjun – a very rare forum for a scientist.
HIDDeN therefore delivers the first statistically robust view of the hidden growth phase linking starbursts and super-massive black-hole accretion and establishes observational benchmarks that will guide ALMA, VLBI, SKA and JWST science in the coming decade.
