AGN activity and the misaligned hot ISM in the compact radio elliptical NGC4278

The analysis of a deep (579 ks) Chandra ACIS pointing of the elliptical galaxy NGC4278, which hosts a low luminosity AGN and compact radio emission, allowed us to detect extended emission from hot gas out to a radius of \sim 5 kpc, with a 0.5–8 keV luminosity of 2.4x10^{39} erg/s. The emission is elongated in the NE-SW direction, misaligned with respect to the stellar body, and aligned with the ionized gas, and with the Spitzer IRAC 8\mum non-stellar emission. The nuclear X-ray luminosity decreased by a factor of \sim 18 since the first Chandra observation in 2005, a dimming that enabled the detection of hot gas even at the position of the nucleus. Both in the projected and deprojected profiles, the gas shows a significantly larger temperature (kT=0.75 keV) in the inner \sim 300 pc than in the surrounding region, where it stays at \sim 0.3 keV, a value lower than expected from standard gas heating assumptions. The nuclear X-ray emission is consistent with that of a low radiative efficiency accretion flow, accreting mass at a rate close to the Bondi one; estimates of the power of the nuclear jets require that the accretion rate is not largely reduced with respect to the Bondi rate. Among possibile origins for the central large hot gas temperature, such as gravitational heating from the central massive black hole and a recent AGN outburst, the interaction with the nuclear jets seems more likely, especially if the latter remain confined, and heat the nuclear region frequently. The unusual hot gas distribution on the galactic scale could be due to the accreting cold gas triggering the cooling of the hot phase, a process also contributing to the observed line emission from ionize gas, and to the hot gas temperature being lower than expected; alternatively, the latter could be due to an efficiency of the type Ia supernova energy mixing lower than usually adopted.

💡 Research Summary

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This paper presents a deep (579 ks) Chandra ACIS observation of the elliptical galaxy NGC 4278, which hosts a low‑luminosity active galactic nucleus (LLAGN) and compact radio emission. The authors detect extended hot gas out to a projected radius of ≈ 5 kpc, with an X‑ray luminosity of 2.4 × 10³⁹ erg s⁻¹ in the 0.5–8 keV band. The diffuse emission is elongated along a NE–SW axis that is misaligned by ~30° with respect to the stellar light distribution but aligns closely with the ionized‑gas filaments and the non‑stellar 8 µm emission seen by Spitzer. This spatial coincidence suggests a strong coupling between the hot phase and the cooler, multi‑phase interstellar medium (ISM).

A key observational advantage is the dramatic decline of the nuclear X‑ray flux by a factor of ~18 since the first Chandra snapshot in 2005. This dimming reduces the point‑source glare and allows the hot gas to be measured even at the nucleus. Spectral analysis reveals a temperature gradient: within the innermost ~300 pc the gas temperature rises to kT ≈ 0.75 keV, whereas the surrounding region maintains a cooler kT ≈ 0.3 keV. The outer temperature is lower than predicted by standard heating models that assume efficient thermalisation of Type Ia supernova (SN Ia) energy and gravitational heating by the central supermassive black hole (SMBH).

The nuclear X‑ray spectrum is consistent with a radiatively inefficient accretion flow (RIAF) accreting at a rate comparable to the Bondi inflow (≈ 10⁻² M⊙ yr⁻¹). However, estimates of the kinetic power carried by the compact radio jets imply that the actual accretion rate cannot be strongly suppressed relative to the Bondi value; otherwise the jets would lack sufficient power. The authors therefore argue that the jets remain confined within the central few hundred parsecs, repeatedly shocking and heating the surrounding hot plasma. This jet‑driven heating provides a natural explanation for the elevated central temperature, especially given the observed variability of the nuclear X‑ray output.

Beyond the nuclear region, the authors discuss two plausible mechanisms for the unusual large‑scale hot‑gas morphology. First, the accretion of cold gas (e.g., HI or molecular clouds) could promote localized cooling of the hot phase, thereby lowering the average temperature to the observed ~0.3 keV and simultaneously fueling the ionized‑gas emission and the 8 µm PAH features. In this scenario, the misaligned hot gas would trace the inflow path of the cold material, explaining its alignment with the ionized filaments. Second, the efficiency with which SN Ia energy mixes into the ISM may be overestimated in conventional models; a lower mixing efficiency would reduce the net heating, again yielding a cooler hot halo.

The paper concludes that the most likely driver of the central temperature excess is the interaction between the confined jets and the ambient hot gas, while the global, misaligned hot‑gas distribution likely results from a combination of cold‑gas accretion and reduced SN Ia heating efficiency. These findings highlight the complex, multi‑phase interplay that can occur in galaxies hosting LLAGNs: low‑radiative‑efficiency accretion, jet feedback, and external gas supply all contribute to shaping the thermal structure of the ISM. The authors suggest that future high‑resolution radio, millimetre, and integral‑field spectroscopic observations, coupled with tailored hydrodynamic simulations, will be essential to disentangle the relative roles of jet confinement, cold‑gas inflow, and supernova heating in governing the evolution of such systems.