Exploring polarisation and dust grain alignment in a dwarf galaxy

Abstract

The nature and origin of magnetic fields in galaxies remain key questions in galactic evolution. Much of our knowledge of these fields relies on observations of polarised light in Milky-Way-type galaxies. In such systems, the interstellar radiation field, magnetic field, and gas interact with dust grains causing the grains to align with the magnetic field. This alignment gives rise to polarised dust emission, allowing the magnetic field orientation to be mapped. However, theoretical calculations indicate that alignment depends sensitively on the underlying properties of the interstellar medium (ISM). This work investigates dust grain alignment in a system with different characteristics from typical Milky Way type galaxies: the dwarf galaxy IC 10. The dust temperatures for IC 10 is derived by fitting modified blackbody (MBB) models to data from the Herschel Spectral and Photometric Imaging Receiver (SPIRE). Neutral atomic hydrogen (H I) column densities produced from the Very Large Array (VLA) are used as an estimate of the gas column densities in the galaxy, and other relevant ISM properties such as magnetic field strengths were collected from the literature to enter the radiative alignment torque (RAT) calculations. Timescales for dust grain alignment with the magnetic and radiation fields respectively (B-RAT and k-RAT) are calculated, along with the timescale of gas damping effects on the dust grains. Timescales of these processes are calculated for different grain sizes. Additional constraints are calculated with a numerical iteration method on the grain size range that permits alignment by implementing the criterions on dust suprathermal rotation and radiative torque disruption (RAT-D). RAT induced dust grain alignment is found to be possible under the ISM conditions of IC 10 and that B-RAT dominates throughout the investigated regions. Grain alignment in IC 10 operates similarly to how it does in the Milky Way-like galaxy M51. The radiation field plays a key role in determining the radiative torque efficiency (QΓ) and in regulating grain disruption. The absence of reliably detected polarised emission in parts of the galaxy does not therefore necessarily imply the absence of aligned dust grains in those regions. Our calculations predict significant variations in the sizes of aligned grains across IC10, which can be tested by optical polarimetry in the future.