Formation of filaments and feathers in disc galaxies: Is self-gravity enough?

Abstract

Context. Dense filaments, also known as feathers, are kiloparsec-scale dusty features present in nearby main sequence galaxies. Distinct from the spiral arms, filaments constitute a major portion of dense gas concentration. They are expected to play an important role in star formation and are known to harbour embedded star-forming regions and H II regions.Aims. We explore the origin of filaments and feathers in disc galaxies via global gravitational instability.Methods. We conduct a parameter study using three-dimensional hydrodynamical simulations of isolated disc galaxies that are isothermal, self-gravitating and are initialised in equilibrium. Our galaxies are uniquely characterised by two dimensionless parameters, the Toomre Q and the rotational Mach number, ℳ_{c = vc/cs (ratio of circular velocity to sound speed). We carry out simulations covering a wide range in both parameters.Results. We find that galaxies with Q = 1 form filaments within a single rotation, while galaxies with Q ≥ 2 do not, even within a couple of rotations. These filaments are kiloparsec long and are semi-regularly spaced along the azimuth of the galaxy. Their morphology, density contrast and formation timescale vary with ℳc, with filament spacing and instability onset time both inversely proportional to ℳc and the density contrast increasing with ℳc. However, filament growth rates in all Q = 1 galaxies are ∼0.5 Ω, where Ω is the angular frequency. We compare the filament spacing in our simulations with the ones in JWST/Mid-Infrared Instrument and HST observations of nearby galaxies and find them to be in agreement.Conclusions. Our study suggests that self-gravity and rotation alone are sufficient to form filaments and feathers, even in the absence of spiral arms or magnetic fields. The morphologies of the resulting filaments are determined primarily by ℳc, which parametrises the importance of thermal versus rotational support.}