On the Limitations of the Anomalous Microwave Emission Emissivity

Christopher T Tibbs,Roberta Paladini,Clive Dickinson

doi:10.1155/2012/124931

Christopher T Tibbs, Roberta Paladini + Show 1 more

Open Access

https://doi.org/10.1155/2012/124931

Copy DOI

Abstract

Many studies of anomalous microwave emission (AME) have computed an AME emissivity to compare the strength of the AME detected in different regions. Such a value is usually defined as the ratio between the intensity of the AME at 1 cm and the thermal dust emission at 100 μm. However, as studies of Galactic dust emission have shown, the intensity of the thermal dust emission at 100 μm is strongly dependent on the dust temperature, which has severe implications for the AME emissivity defined in this way. In this work, we illustrate and quantify this effect and find that the AME emissivity decreases by a factor of 11.1 between dust temperatures of 20 and 30 K. We, therefore, conclude that computing the AME emissivity relative to the 100 μm emission does not allow for accurate comparisons between the AME observed in different environments. With this in mind, we investigate the use of other tracers of the dust emission with which to compute the AME emissivity and we ultimately conclude that, despite the difficulty in deriving its value, the column density of the dust would be the most suitable quantity with which to compute the AME emissivity.

Highlights

In recent years there has been growing evidence for the existence of a new component of microwave emission present in the interstellar medium (ISM)
There have been only a handful of detections of this anomalous microwave emission (AME) component originating from both the diffuse ISM at mid-to-high latitudes (e.g., [2,3,4]) and specific Galactic sources such as the Perseus and ρ Oph molecular clouds [5,6,7,8,9], the dark clouds LDN1622 [10, 11], LDN1621 [12], and LDN1111 [13], and a variety of Hii regions [14,15,16]
When computing the AME emissivity relative to NH, we find that region A1 is consistent with the four other regions in the Perseus molecular cloud, which leads to a completely different interpretation

Summary

Introduction

In recent years there has been growing evidence for the existence of a new component of microwave emission present in the interstellar medium (ISM). Observations have shown that the AME occurs in the frequency range ∼10–100 GHz and is highly correlated with the mid-infrared (IR) dust emission It is this correlation with the mid-IR emission that led Draine and Lazarian [19] to postulate their model of spinning dust emission. Given the strong association between the AME and the dust grains in the ISM, the AME emissivity is usually defined as the ratio between the antenna temperature of the AME at wavelengths of 1 cm and the surface brightness of the thermal dust emission at 100 μm This quantity, first calculated as a cross-correlation coefficient over large areas of sky (e.g., [28, 29]), has been computed for a variety of individual regions in which AME has been detected (see Table 1).

The AME Emissivity Defined Relative to the 100 μm Emission

Redefining the AME Emissivity

Conclusions