Dr. Feng Haicheng from the Yunnan Observatories of the Chinese Academy of Sciences, alongside his team, has made significant progress in understanding the geometrical and kinematical evolution of the broad-line region (BLR) in active galactic nuclei (AGN). Their study was published in The Astrophysical Journal.

AGNs are some of the most luminous objects in the universe, powered by the accretion of matter into supermassive black holes (SMBHs) at their centers. The BLR, located near the SMBH, consists of high-velocity ionized gas that produces broad emission lines in the ultraviolet and optical AGN spectra.

These emission lines, which can have widths greater than 1,000 km/s, are created through photoionization driven by the radiation from the accretion disk. Gaining a deeper understanding of the geometry and kinematics of the BLR is crucial for examining the physical processes within AGNs and for accurately estimating the masses of their central black holes.

Theoretically, the BLR gas may experience virialized or inflow motions influenced by the SMBH's gravity, or outflows driven by radiation pressure. It is believed that outflows would be more prominent in AGNs with high accretion rates, but observational evidence on this remains inconclusive. One possible reason for this discrepancy is the varying ionization properties of the BLR across different AGNs, which may affect the extent of radiation pressure on the gas.

To explore this further, the researchers focused on studying a single AGN during different brightness states to minimize uncertainties caused by variations between different AGNs. Since 2020, they have conducted a four-year reverberation mapping (RM) campaign on the well-known changing-look AGN NGC 4151 using the 2.4-meter telescope at Lijiang Observatory.

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The study revealed clear stratification in the BLR of NGC 4151, with distinct emission lines originating from specific radial regions within the BLR. The researchers also observed a surprising phenomenon, which they dubbed the "anti-breathing effect" in NGC 4151. Conventional models suggest that an increase in AGN luminosity should cause the BLR radius to expand, resulting in longer time delays for broad emission lines—this is referred to as the "breathing effect."

However, their observations showed the opposite: as the AGN's brightness increased, the time delays for the broad emission lines actually shortened. This anti-breathing effect could be connected to time delays between the ultraviolet and optical continuum variations in the AGN.

Further analysis of velocity-resolved time delays revealed significant changes in the geometry and kinematics of the BLR in NGC 4151 over timescales of less than a year. These rapid changes cannot be fully explained by inhomogeneous gas distributions, radiation pressure variations, or changes in the ionizing radiation field alone.

The study's findings suggest that the physical properties of the BLR are more complex than previously assumed, likely influenced by a combination of different mechanisms working together.