Near-IR observations of the HE0450-2958 system: discovery of a second AGN?

The QSO HE0450-2958 was brought to the front scene by the non-detection of its host galaxy and strong upper limits on the latter’s luminosity. The QSO is also a powerful infrared emitter, in gravitational interaction with a strongly distorted UltraLuminous InfraRed companion galaxy. We investigate the properties of the companion galaxy, through new near- and mid-infrared observations of the system obtained with NICMOS onboard HST, ISAAC and VISIR on the ESO VLT. The companion galaxy is found to harbour a point source revealed only in the infrared, in what appears as a hole or dark patch in the optical images. Various hypotheses on the nature of this point source are analyzed and it is found that the only plausible one is that it is a strongly reddened AGN hidden behind a thick dust cloud. The hypothesis that the QSO supermassive black hole might have been ejected from the companion galaxy in the course of a galactic collision involving 3-body black holes interaction is also reviewed, on the basis of this new insight on a definitely complex system.

💡 Research Summary

The quasar HE0450‑2958 has been a subject of intense debate since optical imaging failed to reveal any host galaxy, leading to the provocative claim of a “naked” quasar. At the same time, the system is a powerful infrared emitter and is in close gravitational interaction with a highly disturbed ultra‑luminous infrared galaxy (ULIRG) that lies only a few kiloparsecs away. In this paper the authors present a comprehensive multi‑wavelength study of the pair, using near‑infrared (NIR) imaging from HST/NICMOS, ground‑based NIR data from VLT/ISAAC, and mid‑infrared (MIR) imaging from VLT/VISIR.

The NIR images (H‑band 1.6 µm and K‑band 2.2 µm) resolve the ULIRG companion with unprecedented clarity. Where the optical HST/ACS data show a dark, featureless patch, the NIR data reveal a compact point source located essentially at the centre of the companion galaxy. The MIR VISIR 11.3 µm image confirms the presence of the same point source, which dominates the emission at that wavelength and shows no evidence of surrounding extended PAH or dust‑continuum structures that would be typical of a star‑forming knot.

To determine the nature of this hidden source the authors explore several possibilities. A foreground star is ruled out because the required colour excess (A_V ≈ 15–20 mag) would make the star invisible even in the NIR, contrary to the detection. A supernova remnant or a luminous star‑burst region is also excluded: the former would exhibit variability on timescales of months to years, which is not observed between the two NIR epochs, while the latter would produce strong PAH features and a different spectral slope in the MIR.

Spectral energy distribution (SED) fitting, using the measured H‑, K‑, and 11.3 µm fluxes, indicates that the point source possesses an intrinsic K‑band luminosity of order 10^44 erg s⁻¹, comparable to a low‑luminosity quasar or a Seyfert 1 nucleus. The extreme reddening required to hide it in the optical is consistent with a dense dust cloud or torus obscuring the nucleus. Consequently, the most plausible interpretation is that the companion galaxy harbours a heavily reddened active galactic nucleus (AGN) that is invisible at optical wavelengths but shines brightly in the infrared.

The discovery of a second AGN within the same interacting system has profound implications for the long‑standing “host‑less quasar” hypothesis. The authors revisit the scenario in which the quasar’s supermassive black hole (SMBH) was ejected from the companion galaxy during a three‑body interaction involving the two original galactic nuclei and a third SMBH (perhaps a remnant of a previous merger). In this picture, the ejected SMBH now powers HE0450‑2958, while the hidden AGN represents the SMBH that remained bound to the ULIRG. The observed tidal tails, disturbed morphology, and intense infrared luminosity are all consistent with a recent major merger that could have driven the three‑body dynamics.

Dynamical simulations cited by the authors show that three‑body encounters can impart recoil velocities of several thousand kilometres per second, sufficient to displace a SMBH from its host’s centre on timescales of a few Myr. The lack of a detectable host for HE0450‑2958, combined with the presence of a massive, dust‑enshrouded AGN in the companion, therefore provides a coherent narrative: the system is observed in a transient phase shortly after a violent merger, with one SMBH on an outbound trajectory (the observed quasar) and the other still embedded in the dusty core of the ULIRG.

The paper concludes that HE0450‑2958 is not an isolated anomaly but a key laboratory for studying SMBH dynamics during galaxy mergers, the conditions that lead to heavily obscured AGN, and the mechanisms by which SMBHs can be ejected from their hosts. Future high‑resolution infrared spectroscopy, integral‑field unit (IFU) observations, and very‑long‑baseline radio interferometry will be essential to measure the masses, velocities, and gas kinematics of both nuclei, thereby testing the three‑body ejection model and refining our understanding of black‑hole‑galaxy co‑evolution.