Spinning nano-carbon grains: Viable origin for anomalous microwave emission

Abstract

Context. Excess microwave emission, commonly known as anomalous microwave emission (AME), is now routinely detected in the Milky Way. Although its link with the rotation of interstellar (carbonaceous) nano-grains seems to be relatively well established at cloud scales, large-scale observations show a lack of correlation between the different tracers of nano-carbons and AME, which has led the community to question the viability of this link. Aims. Using ancillary data and spinning dust models for nano-carbons and nano-silicates, we explore the extent to which the AME that come out of the Galactic Plane might originate with one or another carrier. Methods. In contrast to previous large-scale studies, our method is not built on comparing the correlations of the different dust tracers with each other, but rather on comparing the poor correlations predicted by the models with observed correlations. This is based on estimates that are as realistic as possible of the gas ionisation state and grain charge as a function of the local radiation field and gas density. Results. First, nano-carbon dust can explain all the observations for medium properties, in agreement with the latest findings about the separation of cold and warm neutral medium in the diffuse interstellar medium. The dispersion in the observations can be accounted for with little variations in the dust size distribution, abundance, or electric dipole moment. Second, regardless of the properties and abundance of the nano-silicate dust we considered, spinning nano-silicates are excluded as the sole source of the AME. Third, the best agreement with the observations is obtained when the emission of spinning nano-carbons alone is taken into account. However, a marginal participation of nano-silicates in AME production cannot be excluded as long as their abundance does not exceed YSi ∼ 1%.

Tracing PAH Emission in λ-Orionis Using COBE/DIRBE Data

“…They find that PAH mass is better correlated with AME emission than total dust mass, and argue in support of electric dipole emission as the source of AME. This is in agreement with Ysard et al (2022), who compare dust/PAH correlations derived emission models with all-sky models across the full sky to show that nano-silicates are disfavored compared to a PAHbased origin of AME. Several authors have recently argued that environmental variations, which influence the amount, type, and excitation of PAH species present in a region of space, are critical to take into account in testing the relationship between the presence of PAH emission.…”

“…A potential explanation for this finding within the spinning dust paradigm is that the AME arises from spinning grains that are not PAHs. It has been shown that nano-silicate grains could exist in sufficient abundance to explain the AME Hensley & Draine 2017), although Ysard et al (2022) have recently argued that these grains cannot reproduce the AME spectral energy distribution in detail. Spinning grains with a magnetic dipole moment could also contribute to the observed AME, though they are unable to account for the entirety of the emission (Hoang & Lazarian 2016;Hensley & Draine 2017).…”

“…Spinning grains with a magnetic dipole moment could also contribute to the observed AME, though they are unable to account for the entirety of the emission (Hoang & Lazarian 2016;Hensley & Draine 2017). Another possibility is that PAH emission physics and AME depend differently on local interstellar conditions, eroding the correlation between the two even if they originate from the same grains (Hensley et al 2022;Ysard et al 2022).…”

The Astrodust+PAH Model: A Unified Description of the Extinction, Emission, and Polarization from Dust in the Diffuse Interstellar Medium

“…fitting, and that doing so can obscure true correlations. A correlation between MIR and microwave emission could also be obscured if MIR PAH emission and spinning dust emission have markedly different emissivities in different interstellar environments, even if both are produced by PAHs (Hensley et al 2022a;Ysard et al 2022).…”

“…Finally, the association between the AME and PAHs has been questioned on the basis of a lack of correlation between the AME and MIR PAH emission after removing their mutual correlation with the FIR dust continuum (Hensley et al 2016). In response, some authors have suggested that the AME could arise from spinning nanosilicates instead Hensley & Draine 2017a), though Ysard et al (2022) have recently argued that emission from nanosilicates cannot reproduce the observed AME SED shape in detail. Another possibility advanced by Bell et al (2019) is that comparing broadband MIR and microwave fluxes is not as reliable as inferring parameters like PAH mass from SED The contributions to the model from vibrational and by rotational emission are plotted with green and blue dashed lines, respectively.…”