The nuclear X-ray emission of nearby early-type galaxies

Nuclear hard X-ray luminosities (Lx,nuc) for a sample of 112 early type galaxies within a distance of 67 Mpc are used to investigate their relationship with the central galactic black hole mass Mbh, the inner galactic structure (using the parameters describing its cuspiness), the age of the stellar population in the central galactic region, the hot gas content and the core radio luminosity. Lx,nuc ranges from 10^{38} to 10^{42} erg/s, and the Eddington ratio Lx,nuc/Ledd from 10^{-9} to 10^{-4}. Lx,nuc increases on average with the galactic luminosity Lb and Mbh, with a wide variation by up to 4 orders of magnitude at any fixed Lb>6x10^9 Lb,sun or Mbh>10^7 Msun. This large range should reflect a large variation of the mass accretion rate \dotMbh. On the circumnuclear scale, \dotMbh at fixed Lb (or Mbh) could vary due to differences in the fuel production rate from the stellar mass return linked to the inner galactic structure; however, \dotMbh should vary with cuspiness by a factor exceeding a few only in hot gas poor galaxies and for large differences in the core radius. Lx,nuc does not depend on age, but less luminous nuclei are found among galaxies with a younger stellar component. Lx,nuc is detected both in gas poor and gas rich galaxies, on average increases with the total galactic hot gas cooling rate L_{X,ISM}, but again with a large variation. The lack of a tight relationship between Lx,nuc and the circumnuclear and total gas content can be explained if the gas is heated by black hole feedback, and/or the mass effectively accreted can be largely reduced with respect to that entering the circumnuclear region. Differently from Lx,nuc, the 5 GHz VLA luminosity shows a trend with the inner galactic structure similar to that of the total soft X-ray emission; therefore they could both be produced by the hot gas.

💡 Research Summary

The authors present a systematic study of nuclear hard X‑ray emission (2–10 keV) in a well‑defined sample of 112 early‑type galaxies within 67 Mpc. Using Chandra observations they measure nuclear X‑ray luminosities (Lₓ,ₙᵤ) that span four orders of magnitude, from ≈10³⁸ to 10⁴² erg s⁻¹, corresponding to Eddington ratios of 10⁻⁹–10⁻⁴. By compiling black‑hole mass estimates (M_BH) from stellar‑dynamical measurements, photometric parameters describing the inner surface‑brightness profile (cuspiness, core radius), stellar population ages, total hot‑gas X‑ray luminosities (L_X,ISM), and 5 GHz VLA core radio powers, they explore how Lₓ,ₙᵤ correlates with each of these quantities.

The main findings are: (1) Lₓ,ₙᵤ shows a loose positive trend with both galaxy B‑band luminosity (L_B) and M_BH, but at any fixed L_B > 6 × 10⁹ L_⊙ or M_BH > 10⁷ M_⊙ the nuclear X‑ray power can vary by up to four dex. This scatter is interpreted as reflecting a large intrinsic spread in the mass accretion rate onto the black hole (Ṁ_BH). (2) The inner stellar structure, quantified by the cusp slope and core radius, can in principle modulate the stellar mass‑loss rate that fuels the nucleus. However, the data reveal that only in gas‑poor systems and when the core radii differ dramatically does cuspiness translate into a factor‑of‑a‑few change in Ṁ_BH. In most galaxies the structural differences are insufficient to explain the observed Lₓ,ₙᵤ dispersion. (3) No clear dependence of Lₓ,ₙᵤ on the age of the central stellar population is found; nevertheless, galaxies with younger central stars tend to host comparatively faint nuclei, suggesting that recent star formation may be associated with lower available fuel or more efficient feedback. (4) Lₓ,ₙᵤ is detected both in hot‑gas‑rich and hot‑gas‑poor galaxies. On average it rises with the total hot‑gas cooling luminosity L_X,ISM, but the correlation is weak and exhibits large scatter. This lack of a tight relationship can be understood if a substantial fraction of the gas entering the circumnuclear region is either heated by AGN feedback (jets, winds) or prevented from reaching the black hole by outflows, thereby reducing the effective accretion rate relative to the supply rate. (5) In contrast, the 5 GHz VLA core radio luminosity correlates strongly with the inner galaxy structure in a manner similar to the total soft X‑ray emission, implying that both radio and soft X‑ray outputs are largely powered by the hot interstellar medium rather than by the accretion flow itself.

The authors conclude that nuclear hard X‑ray emission in early‑type galaxies is primarily governed by the black‑hole mass and the global hot‑gas reservoir, but the actual accretion rate is modulated by feedback processes that decouple the inflow from the available fuel. The weak link between Lₓ,ₙᵤ and structural or stellar‑age parameters suggests that low‑efficiency accretion models (RIAF/ADAF) must be supplemented by a realistic treatment of heating, outflows, and the multi‑phase nature of the circumnuclear medium. Their work highlights the importance of multi‑wavelength diagnostics—hard X‑rays tracing the accretion flow, soft X‑rays and radio tracing the hot ISM—to disentangle the complex interplay between black‑hole growth and galaxy evolution in the nearby universe.