Probing the dust grain alignment mechanisms in spiral galaxies with M51 as the case study
arxiv(2024)
摘要
Magnetic fields (B-fields) in galaxies have recently been traced using
far-infrared and sub-mm polarimetric observations with SOFIA, JCMT, and ALMA.
The main assumption is that dust grains are magnetically aligned with the local
B-field in the interstellar medium (ISM). However, the range of physical
conditions of the ISM, dust grain sizes, and B-field strengths in galaxies
where this assumption is valid has not been characterized yet. Here, we use the
well-studied spiral galaxy M51 as a case study. We find that the timescale for
the alignment mechanism arising from magnetically aligned dust grains (B-RAT)
dominates over other alignment mechanisms, including radiative precession
(k-RAT) and mechanical alignment (v-MAT), as well as the randomization effect
(gas damping). We estimate the sizes of the aligned dust grain to be in the
range of 0.009-0.182 μm and 0.019-0.452 μm for arms and inter-arms,
respectively. We show that the difference in the polarization fraction between
arms and interarms can arise from the enhancement of small dust grain sizes in
the arms as an effect of the grain alignment disruption (RAT-D). We argue that
the RAT-D mechanism needs to have additional effects, e.g., intrinsic
variations of the B-field structure and turbulence, in the galaxy's components
to fully explain the polarization fraction variations within the arms and
inter-arms.
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