The X-ray Emission of NGC 5005: An Unobscured Low-Luminosity AGN with a Weakly Accreting Broad-Line Region

Anna Trindade Falcão, R. Middei, G. Fabbiano, M. Elvis, P. Zhu, W. P. Maksym, D. Ł. Król, L. Feuillet

First listed 2025-09-24 | Last updated 2025-09-24

Abstract

We present deep Chandra X-ray observations of NGC 5005, a LINER-dominated galaxy previously reported to host a broad H$α$ emission line. The diffuse soft X-ray emission ($<$3 keV) extends out to $\sim$800 pc, while harder emission ($>$3 keV) is confined to the central $\sim$400 pc. Spatially resolved spectroscopy of the nuclear ($r<150$ pc) and extended ($150<r<500$ pc) regions reveals that these are best described by models including both photoionized and thermal plasma components, consistent with excitation by a low-luminosity AGN and shock-heated gas. Narrow-band imaging and excitation maps from the Hubble Space Telescope (HST) support this interpretation, closely matching the X-ray morphology and ionization structure. The detection of a faint hard X-ray nuclear source with Chandra, combined with stringent upper limits from NuSTAR and Swift, and consistency with the X-ray luminosity predicted from the HST [O III]$λ$5007 emission, indicates that NGC 5005 hosts an intrinsically low-luminosity ($L_{\rm bol} \sim 10^{41}$ erg s$^{-1}$), unobscured AGN. Despite the extremely low Eddington ratio inferred from our measurements ($λ_{\rm Edd} \sim 5 \times 10^{-6}$), the presence of a broad H$α$ line in the optical spectrum suggests the persistence of a thin accretion disk, challenging standard paradigms of accretion flow configurations at such low accretion rates.

Short digest

Deep 255 ks Chandra imaging plus spatially resolved spectroscopy of NGC 5005, tied to HST narrow-band/excitation maps, separates nuclear (r<150 pc) and extended (150–500 pc) components. The soft X-rays (<3 keV) spread to ~800 pc while hard 3–7 keV emission is confined to ~400 pc; both regions require photoionized + thermal plasma, consistent with a LLAGN plus shock-heated gas. A faint hard nuclear source is detected with Chandra, with NuSTAR/Swift setting tight upper limits, and the X-ray power matches that predicted from HST [O III], implying an intrinsically low-luminosity (L_bol ~10^41 erg s^-1) unobscured AGN at λ_Edd ~5×10^-6. The persistence of a broad Hα line at such low accretion suggests a thin disk/BLR survives, challenging standard low-λ_Edd accretion scenarios.

Key figures to inspect

• Figure 1: Inspect the soft vs hard-band morphology—soft emission reaching ~800 pc and a much more compact hard core (~400 pc); note which off-nuclear sources were masked from the analysis.
• Figure 2: Compare the nuclear image to the MARX PSF to verify that the core is broader than a point source at soft energies while the hard core is PSF-like, supporting genuine extended soft emission.
• Figure 3: Use the pie-sector azimuthal profiles to see anisotropy in the circumnuclear emission and how it aligns with disk/NLR structures highlighted by the excitation maps.
• Figure 4: Read the energy-dependent radial and azimuthal profiles to quantify PSF residuals—large excess below 3 keV but minimal above 3 keV—supporting mixed photoionized/thermal gas plus a compact hard nucleus.