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A research team from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences has uncovered the origins of double-peaked narrow emission-line characteristics in galactic centers.

Using data from the Sloan Digital Sky Survey's Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) project, the researchers found that these emission lines are not only related to binary black holes but may also originate from mechanisms such as rapidly rotating gas disks and active galactic nucleus (AGN) outflows. The findings are in The Astrophysical Journal.

The team's 2024 statistical analysis showed that double-peaked lines are associated with galactic bars, AGN, and interactions, with the phenomena related to bars being the most common. They have now further revealed that the nuclear rings in barred spiral galaxies can lead to symmetrical double-peaked features. This achievement deepens our understanding of galactic center structures and provides important references for the search for binary black holes.

Astronomers observed AGN in the late 20th century at the center of galaxies and noticed peculiar ionized gas emission lines in the spectra of some targets. They speculated that the double-peaked emission line was formed by the recombination of two AGNs that revolved around each other.

Specifically, in the theory of galaxy formation and evolution, mergers are important channels for galaxy growth. When the central black holes of two galaxies are very close but not merging, the narrow line regions illuminated by their respective accretion disks of AGNs can produce two separate narrow emission lines.

However, due to the close proximity of the two black holes, the spatial resolutions of ground-based telescopes are generally insufficient to directly distinguish them in imaging. Therefore, the double-peaked features in galaxies were once considered the most promising probe for indirectly searching for binary black holes at the center of galaxies.

Decades of research have shown that the characteristics of double-peaked narrow emission lines at the center of galaxies is widespread, and its mechanism of generation is far beyond the dual AGNs rotating with each other; the high-speed rotating gas disk and the gas outflows caused by AGNs may both produce double-peaked features.

In recent years, with the development of new astronomical instruments and technologies, the integrated field spectroscopy (IFS) technology has played a very important role in studying double-peaked narrow emission lines. Due to the fact that the IFS also provides spatially resolved spectral information of galaxies, astronomers can explore the origin of double-peaked features in galaxies by comparing the changes in their characteristics in different positions.

In 2019, a joint team from the National Astronomical Observatory and the Shanghai Astronomical Observatory of CAS, had, for the first time, inferred that its physical origin was a high-speed rotating gas disk. Subsequently, multiple individual cases of other galaxies have drawn different conclusions, including various origin mechanisms such as AGN outflows and projection effects related to interactions.

In 2024, researchers from the Shanghai Astronomical Observatory found that in the local universe, the double-peaked features in galaxies are related to bars, AGNs, and galaxy interactions, and those features related to bars have the highest statistical probability.

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In 2025, using MaNGA data on the basis of a simple galaxy rotation disk model, researchers from the Shanghai Astronomical Observatory proposed the possibility of a symmetrical double-peaked narrow emission-line feature at the center of a galaxy caused by a nuclear ring.

The nuclear ring in a barred spiral galaxy precisely satisfies this condition: the inflow gas driven by the bar gathers into the ring at the inner Lindblad resonance, producing a bright and narrow radiation region with strong rays, while it is located in a region with steep changes in the rotation curve. The scale of the nuclear ring in space is very small (indistinguishable due to PSF effect), but its rotational speed varies greatly.

Therefore, in a "pixel" blurred by PSF, contributions from both the near side (high-speed blue-shifted gas) and the far side (high-speed red-shifted gas) of the ring are mixed, forming a symmetrical "double-peaked profile."

In addition, the rotational motion of the nuclear ring is ordered and symmetrical, which directly leads to the overall symmetry of the spatial variation of the resulting double-peaked emission-line characteristics.

This work provides strong observational evidence for "double-peaked narrow emission lines ≠ binary black holes," deepening our understanding of the central structure of galaxies, and demonstrating how instrumental effects affect scientific discoveries. When searching for elusive binary black holes, it is necessary to carefully eliminate "false positive" signals generated by structures such as nuclear rings.

The researchers note that future searches must use higher spatial resolution observations or combine multi-band information such as X-rays to see if there are two-point sources for confirmation.