Using 1665/1667 MHz OH line and continuum spectrum observational data from the European VLBI Network (EVN) and the Multi-Element Radio Linked Interferometer Network (MERLIN), Antneh Gashaye, a PhD student from the Xinjiang Astronomical Observatory, Chinese Academy of Sciences, conducted a high-resolution study of the OH MegaMaser emission in the ultraluminous infrared galaxy (ULIRG) IRAS 15250+3609, and revealed the nature and structure of the OH MegaMaser emission in the galaxy. These findings have been published in the Monthly Notices of the Royal Astronomical Society.

Ultra-luminous infrared galaxies (ULIRGs) are extremely active objects in the universe, often associated with galaxy mergers, active galactic nuclei (AGN), and intense starbursts. In recent years, high-resolution radio observations have shown that OH MegaMaser emission can serve as a probe to reveal the gas dynamics and merging mechanisms in the galaxy’s nuclear region. IRAS 15250+3609 is a ULIRGs located at a distance of about 241 Mpc and is considered a post-merger system.

The research results show that the nuclear region is a complex starburst zone measuring 130 × 95 pc, with a central AGN that is obscured by dust. Earlier low-resolution observations only viewed it as a point source, but high-resolution data revealed the extended starburst structure. This structure confirmed the presence of the AGN's radio emission and aligned with its optical Composite classification with a LINER and a starburst.

The OH MegaMaser emission in IRAS 15250+3609 is found to be superposed on and confined within the nuclear source of the host galaxy, which has an active galactic nucleus and an extended starburst region. The emission covers a velocity range from –250 km s⁻¹ to 400 km s⁻¹, with a strong and systematic velocity gradient across the nucleus from west to east.

The study concluded that the excitation for the OH molecules comes from the far-infrared (FIR) radiation field from the starburst nucleus of the host galaxy, which allows for maser amplification of the nuclear radio continuum in the background. The velocity signature and structure of the OH emission indicate that the masering foreground material is part of an ongoing minor merger, with a dwarf galaxy falling nearly radially into the nucleus of IRAS 15250+3609 from the west.

Moreover, three velocity ranges were identified during the merger process: high velocity for infalling material that is still mostly unperturbed, an intermediate velocity range where material first interacts with the host galaxy, and a negative velocity region where material has already crashed into the host.

This research offers new insights into the dynamics of minor mergers, shedding light on how such events influence the kinematics of a galaxy’s nuclear region. The study also emphasizes the importance of high-resolution radio observations in understanding the complex interactions that shape the evolution of galaxies like IRAS 15250+3609.

                                                                                       Position–velocity diagrams of the 18 cm OH MM emission of IRAS 15250+3609.