Analysing the COBE-DMR data at 31.5, 53 and 90 GHz, Kogut (1999) found a statistically significant correlation with the DIRBE data at 100, 140 and 240µm. He basically discovered the existence of a dust correlated emission for scales larger than 7◦, not explained by a simple thermal dust model. A two- component fit of the correlated COBE data to a model with dust (with spectral index 1.5 < βdust < 2) and radio emission, returned a spectral indexβradio ∼ 2.1+−00..68 for the unknown radio component. This
result induced them to associate the radiation to free-free emission, whose theoretical spectral index is β = −2.15. Later, the same result has been confirmed and extended in frequencies (down to 14 GHz) by Leitch et al. (1997) and de Oliveira-Costa et al. (1997) with the OVRO12 and Saskatoon data
respectively. However, the free-free hypothesis was discarded soon: given the correlation between the free-free emission and the Hα emission, the proposed explanation would require an analogous correlation between the anomalous radio emission and the Hα emission. Instead, the first one was measured to be 5-10 times larger than the second one.
A deep scan around the north celestial pole at 14.5 and 31.7 GHz by Leitch et al. (1997) showed a strong correlation with 100µm dust and free-free like spectrum. However, they also confirmed a lack of associated Hαemission at the expected level. Therefore the emission was proposed to be associated with ionized gas at Te = 106 K. However, again, this hypothesis has been ruled out: such a hot gas would produce X-rays, but no significant detection of them has been measured by ROSAT13 at 1/4
KeV. Furthermore, temperatures of 106 K require energy higher that 102 time the one released by a
Supernova explosion.
All these results substantiated the model of electric dipole emission from small spinning dust grains proposed by Draine & Lazarian (1998a,b). The same PAHs grains which absorb stellar radiation and cool down by radiating in IR bands, undergo collisions with gas-phase species and can become charged. Therefore, they carry a significant permanent dipole moment (of a few tens of a Debye). As a result of the frequent collisions, the spinning PAHs will produce electric dipole emission from rotational levels. The spinning dust paradigm has been increasingly accepted as the explanation of the ‘anomalous component’: this would explain naturally both the correlation with dust, since the same grains are responsible also for the thermal emission, and the absence of correlated Hαradiation.
Draine & Lazarian (1998b) have computed the emissivity jν of the grains, for different size a and
12Owens Valley Radio Observatory 13Rœntgensatellit
electric dipole momentβ, in the 10-100 GHz range. Assuming the angular velocity of the spinning grains to have a Maxwellian distribution:
jν = 8 3π 1/21 c3 Z da f (a) µ 2ω6 < ω2 >3/2exp −3ω2 2< ω2> (1.36) whereµis the intrinsic electric dipole moment of the grain without charge (µ∼ βN1/2, where N is the
total number of atoms of the grain), ω is the angular velocity and f (a) is the size distribution of the grains.
The emissivity was found to be sensitive to the properties of the gas such as the density and the temperature. Specifically, they distinguished the contribution produced by five typical environments in the ISM with various canonical densities and temperatures in order to compute corresponding spinning dust model spectra:
• CNM cold neutral medium
• WNM warm neutral medium
• WIM warm ionized medium
• MC molecular clouds
• DC dark clouds
Depending of the component, the emissivity and the peak frequency change, although the latter lies always between 20 and 30 GHz. Figure 1.12 shows the spectrum expected for these components. Recently Ali-Ha¨ımoud et al. (2009) presented a comprehensive treatment of the spectrum of electric dipole emission from spinning dust grains, updating the commonly used model of Draine & Lazarian.
Following the theoretical expectations, the distinguishing characteristic of spinning dust emission would be the turn-over in the spectrum at lower frequencies, around 10-15 GHz depending on the dimension of the grains. The first detection of this feature came out combining the Tenerife data at 10 and 15 GHz with the COBE-DMR data (de Oliveira-Costa et al., 1997): a peak was measured at∼ 20 GHz in the radio emission, per unit of 100 µm dust emission. This result was confirmed by combining the same data set with the W MAP data, thanks to the larger frequency coverage. Specifically, it was demonstrated that the low-frequency part of the spectrum was inconsistent with the synchrotron emission for intermediate latitudes. A similar analysis was carried out by Banday et al. (2003) with the DMR and the 19 GHz data.
The existence of this component has been confirmed more recently by the analysis of the W MAP data performed by Bonaldi et al. (2007), Davies et al. (2006), and Miville-Deschˆenes et al. (2008). Moreover, Ysard et al. (2010) have carried out a correlation analysis of the IRAS data with a full-sky spinning dust template at 23 GHz produced by Miville-Deschˆenes et al. (2008). They found a strong correlation between the template and the 100 µm IRAS band and, more important, with the 12 µm band. This has been interpreted as the confirmation of the spinning dust model, since small dust grains (PAHs) carry the 12µm flux.
Figure 1.12: The spectrum of the spinning dust. The radio emissivity is given in terms of Jy/sr per unit hydrogen column density. The dotted curves show the spectra for the different components. The solid lines are the spectra when the corresponding thermal emission is added. Observational data from COBE-DMR (Kogut et al., 1996a), Saskatoon (de Oliveira-Costa et al., 1997) and OVRO (Leitch et al., 1997) are also shown. The plot is taken from Draine & Lazarian (1998b).
Beside the spinning dust grains, other scenarios have been proposed as possible explanations for the anomalous component. Fullerenes of 60-200 carbon atoms have been suggested to be responsible for the anomalous component (Iglesias-Groth, 2006). Large grains of submicron radius may have a significant contribution in the 10 to 100 GHz range. Draine & Lazarian also proposed that magnetic dipole emission may be emitted by large grains over a broad range between 10 and 100 GHz. Spontaneous magnetization is required in order to have a peak at the observed frequency: this could be possible thanks to the presence of magnetite and metallic iron. Currently, magnetic dipole emission can not be ruled out as a possible explanation for the anomalous component of dust at microwave frequencies.
Some hints about the processes that occur in the anomalous emission of dust can be provided by observation of individual compact dust clouds. Watson et al. (2005) discovered very strong anomalous emission in the Perseus molecular cloud (G 159.6-18.5) with the COSMOSOMAS14 experiment at
11-17 GHz: the derived spectrum is consistent with the spinning dust model. Furthermore, a strong correlation has been found with the IRAS 25 µm band suggesting that the anomalous emission is produced by very small spinning dust grains. More recently, a morphological study of the Lynds Dark
Source Dust emissivity Reference µK (MJy/sr)−1
HII regions
6 HII regions (mean) 3.3±1.7 Dickinson et al. (2007)
LPH96 5.8±2.3 Dickinson et al. (2006)
Cool dust clouds
15 regions WMAP 11.2±1.5 Davies et al. (2006) All-sky WMAP 10.9±1.1 Davies et al. (2006) LDN1622 24.1±0.7 Casassus et al. (2006) G159.618.5 17.8±0.3 Watson et al. (2005)
Table 1.1: Comparison of 100µm dust emissivities for HII regions and cooler dust clouds, from data at or near 30 GHz (Dickinson et al., 2007). Emissivities, in unitsµK (MJy/sr)−1, have been normalised to 31 GHz.
Nebula LDN1622 was carried out by Casassus et al. (2006) at 31 GHz with CBI. Combining these observations with the W MAP data, a well defined spectrum consistent with the spinning dust emission is produced, with a peak around 30 GHz. They also showed a strong correlation with all the IRAS data, in particular with the 25µm band, which again supports the hypothesis of spinning dust emission from very small grains.
In order to confirm this idea, it is important to study the anomalous emission from compact HII regions: in fact very small dust grains are not expected to exist in those regions. Therefore, any detection of anomalous emission would discard the hypothesis of very small grains and would leave space to other interpretations. Dickinson et al. (2006) observed the LPH96 (G201.66+1.64) with the CBI at 31 GHz and found that the spectrum is basically consistent with simple free-free. A more extensive study of the anomalous emission in HII regions in the southern hemisphere, has been undertaken by Dickinson et al. (2007) (see table 1.1). Again the derived spectrum was found to be consistent with free-free emission, although some excess of emission has been measured in six regions. This excess could be associated with spinning dust emission or other emission mechanisms.
We would clearly need an accurate template for this anomalous emission to model the foreground amplitudes at W MAP frequencies, but this remains elusive. Finkbeiner (2004a) and Davies et al. (2006) have proposed that the FDS template modulated by some power-law of the dust temperature provides a better fit than the unperturbed sky map. However, Bonaldi et al. (2007) suggest that although the anomalous emission is tightly correlated with thermal dust, the correlation is not perfect. Nevertheless, we expect that the FDS model provides a representative template and we adopted it for our analysis.