XRISM Spies the Remains of a Galactic Donut

New X-ray observations reveal the potential remnant of a dusty torus in the galaxy Messier 81. These observations help to advance our knowledge of low-luminosity active galactic nuclei, a population of accreting black holes that is still poorly understood.

Understanding Accretion

Nearly all galaxies in our universe harbor a supermassive black hole. Many of these black holes are actively accreting matter from their surroundings, and these active galactic nuclei are powerful producers of radiation, winds, and jets. Roughly 40% of galaxies host a black hole that accretes matter at only a modest rate, which translates into a low-luminosity active galactic nucleus, or LLAGN.

Given how common it is for black holes to occupy this low-luminosity state, it’s critical to understand whether these systems are structured according to the typical picture of an active galactic nucleus, with a compact accretion disk surrounded by a dusty, donut-shaped torus. To probe the details of the LLAGN state, a team led by Jon Miller (University of Michigan) has turned to one of the best-studied — but still poorly understood — objects in this class.

Meet Messier 81

Just 12 million light-years away, the spiral galaxy Messier 81 is home to the nearest and brightest LLAGN. Messier 81’s central black hole was selected for observation during the 6-month Performance Verification phase of the X-ray Imaging and Spectroscopy Mission (XRISM), which launched in 2023. Messier 81 is the first LLAGN to be observed with a microcalorimeter, a type of instrument that detects photons by measuring minuscule changes in the temperature of the detector.

Miller and collaborators focused on XRISM’s observations of several spectral lines from iron: the iron Kα emission line at 6.4 keV, which arises when neutral iron atoms in the vicinity of the active galactic nucleus reflect X-rays generated through accretion, and several emission lines from iron atoms stripped of all but one or two electrons.

Investigating a Diagnostic Line

The iron Kα line appears in the spectra of many active galactic nuclei, and it’s key to understanding whether LLAGNs like this one have the same accretion disk and torus structure as more vigorously accreting active galactic nuclei.

Modeling of the XRISM data suggests that the iron Kα line arises from material no closer than 27,000 gravitational radii from the black hole — a large distance compared to a typical compact accretion disk that closely rings the black hole. Miller and collaborators suggest that the likeliest explanation for the origin of the iron Kα emission is the remnant of a dusty torus that is only lightly obscuring the central accretion disk. This is in line with the hypothesis that LLAGNs are winding down from a higher-luminosity state, and that they maintain certain structures (i.e., a torus) from that highly active state as their accretion rate slows.

The team found that it’s also possible for the emission to arise from an accretion disk whose inner edge lies far from the black hole, in agreement with the predictions of an accretion model called radiatively inefficient accretion flow. The XRISM data didn’t place strong constraints on another possible accretion model, the magnetically arrested disk model. Future observations that either probe more deeply or attempt to detect variability could better illuminate the structure surrounding the black hole.

Finally, a note about the emission lines from ionized iron: these might be due to a wind blowing outward from the accreting black hole, but Miller’s team suggests that they could also indicate the beginning of a feature similar to the so-called Fermi bubbles seen to extend on large scales from the center of the Milky Way.

Citation

“XRISM Reveals a Remnant Torus in the Low-Luminosity AGN M81*,” Jon M. Miller et al 2025 ApJL 985 L41. doi:10.3847/2041-8213/add262