Indian astronomers delved into the intriguing world of large-scale turbulence within the interstellar medium of NGC 6946, a spiral galaxy. The research, available on the arXiv pre-print server since October 4, offers valuable insights into this cosmic phenomenon.
The interstellar medium (ISM) in NGC 6946 is known to exhibit turbulence on scales spanning thousands of light years. This turbulence plays a pivotal role in multiple cosmic processes, including star formation, molecular cloud stability, and the behavior of cosmic rays. Despite numerous prior studies, the exact origins of this turbulence have remained an enigma.
NGC 6946, a prominent late-type spiral galaxy situated about 19.2 million light years away, boasts a sizable optical radius of approximately 32,000 light years, a dynamic mass of roughly 730 billion solar masses, and an annual star formation rate of 2.5 solar masses. Previous observations had already identified signs of turbulence within its disk.
In this study, Meera Nandakumar and Prasun Dutta from the Indian Institute of Technology (BHU) Varanasi meticulously examined the turbulence in NGC 6946. They scrutinized data gathered from the Karl G. Jansky Very Large Array (VLA) and The HI Nearby Galaxy Survey (THINGS), deploying the visibility moment estimator (VME) for quantifying the neutral atomic hydrogen (HI) column density and the power spectrum of line-of-sight turbulent velocity from VLA and THINGS.
The findings are compelling, revealing the existence of a substantial energy cascade within NGC 6946’s disk. Notably, the study disclosed a density power spectrum with a slope of -1.81, while the column density exhibited a slope of -0.96. These results suggest that the driving forces behind the turbulence in NGC 6946 likely combine compressive and solenoidal components.
The researchers also noticed the presence of inter-arm magnetic fields, referred to as magnetic arms, with a field strength of around 20 µG in NGC 6946. This leads them to speculate that the magnetic field within the galaxy might play a central role in generating large-scale turbulence, with these magnetic structures being an outcome of the disk’s turbulent dynamics.
The study’s authors further elaborated that the strong regular magnetic fields originating from the magnetic spiral arms in NGC 6946 are possibly contributing to the solenoidal aspect, while self-gravity or gravitational instability may primarily drive the compressive component of the turbulence.
Meera Nandakumar and Prasun Dutta have future plans to conduct additional research into the nature of turbulence within the interstellar medium. They emphasize the need for measuring the turbulence cascade in a diverse range of galaxies and interpreting these results in conjunction with various numerical simulations to advance our comprehension of this fascinating cosmic phenomenon.