Desarrollo con Vuforia

If Stephen Weinberg is right1_{, then the study of microwave foregrounds is certainly one of}

the places where the action is. Despite it being in front of our noses in cosmological terms, there is still a great deal we don’t know about the ISM. Among the surprises that came out of the first sensitive observations of the microwave sky was the anomalous dust-correlated emission. Less surprising is the fact that the different components vary in their spectral behaviour across the sky. This was always going to be a problem, since though extensive surveys of the sky have been done in the radio, the microwave regime is not easily accessible from the ground and is thus even more challenging.

Both of these facts make foreground analysis somewhat messy, as was discussed in detail in Chapters 4, 5, and 7. The work in Chapter 5 might be extended to attempt to map out the spectral variations in each component by performing such fits in small regions that tile the full sky. More complicated methods have also been explored such as fitting a set of spectral models to individual pixels using MCMC methods as in Eriksen et al. (2006a). But in addition to more sophisticated analysis tools, we need a better physical understanding of each source of foreground emission.

178 8. Conclusions Anomalous Dust-correlated Emission

The jury is still out on the origin of the anomalous dust-correlated emission. The hypoth- esis favoured by the WMAP team is that this component is hard synchrotron radiation (Bennett et al., 2003c). The highest-energy cosmic ray electrons lose energy more quickly as they diffuse away from where they were accelerated. Therefore, the spatial distribu- tion of such high-energy electrons should remain fairly concentrated around the supernova remnants, which in turn lie in dusty star forming regions. Therefore a hard synchrotron component might well be spatially correlated with the thermal dust emission. To confirm this possibility requires a better knowledge of the spectrum of cosmic-ray electrons at their acceleration site as well as of the processes by which they lose energy as they diffuse away. Observations of the diffuse γ-ray emission in the Galaxy may shed light on the issue, as the cosmic-ray electrons also produce γ-ray emission through inverse Compton scattering. One obvious criticism of this view is that, considering the strikingly tight spatial cor- relation of these components (Fig. 7.2), it makes sense to assume that a dust-correlated emission comes from the dust itself. A plausible mechanism of dipole radiation from spinning dust grains has been proposed (Draine & Lazarian, 1998a,b) that allows us to distinguish it from a synchrotron component by its spectral turn-over near 10-15 GHz. This component has been detected via this turn-over in different regions of the sky (de Oliveira-Costa et al., 1999, 2000, 2002; Finkbeiner et al., 2002, 2004; Finkbeiner, 2004a; Watson et al., 2005), though a full sky survey at this frequency is not currently available. One further distinction between these hypotheses is in the different predicted polarisation signals, and as mentioned in _§ 5.6, the steep polarisation spectrum at K and Ka is an indication that the anomalous emission is not hard synchrotron. (It is interesting to note that the polarised spectrum appears to harden toward V and W, which is consistent with neither hypothesis, though this is perhaps due to confusion with other components.)

Chapter 5 studies the spectral behaviour of this anomalous component in different regions and finds that it overpredicts the dust-correlated emission that should be present in the Haslam map at 408 MHz if there is no spectral flattening between. This work also finds that the emissivity depends on the dust temperature, such that the coldest regions show the lowest inferred emissivity from the anomalous component. This emission is shown to be the dominant foreground at the lower frequencyWMAP bands and that its emissivity appears to vary roughly by a factor of 2 from region to region. These results may prove important in confirming any hypothesis for the origin of this mysterious foreground.

Polarised Foregrounds

While galactic foregrounds dominate the cosmic temperature signal only near the plane or outside the frequency window around 60 GHz, the polarised emission from the Galaxy dominates the cosmic signal over the full sky and at all frequencies (except for some angular scales). The polarisation components of the Galactic emission are also significantly less well understood than their temperatures, and due to Faraday rotation, cannot easily be extrapolated from observations in the radio.

8.2 Prospects 179

Synchrotron emission is strongly polarised perpendicular to the magnetic field and can have a polarisation fraction up to _∼ 75%. But along a given line of sight, and observed with a given beam, that field is not uniform and the signal is thus reduced significantly by mixing, though it still dominates the cosmic component. A detailed 3-dimensional knowledge of the galactic magnetic field structure is needed in order to accurately model and remove the polarised synchrotron.

At higher frequencies, polarised dust becomes the dominant foreground (the foreground minimum moving from 60 GHz for temperature to _∼ 80 GHz for polarisation). This is simply due to the alignment of asymmetric dust grains perpendicular to the magnetic field. The degree of polarisation depends on the details of the grain sizes and composition, how well they align with the field, and how uniform that field is along the line of sight. The polarisation fraction may be anywhere from 1% to 10%.

The magnetic field can also be studied through the Faraday rotation of extragalactic radio sources as well as galactic pulsars and through the polarisation of starlight due to absorption by aligned dust grains.

Future

Part of what we need to progress on these issues will soon be available: more frequency coverage, such as the nine frequency bands that will be provided by Planck. Those nine bands will make it easier to separate components with different spectral dependencies, particularly when combined with other data.

To study the anomalous dust, observations at 10-15 GHz will be needed in order to see if the spectrum turns over as expected in the spinning dust models. But polarisation, which Planck should measure sensitively over the full sky, will also help answer that question, as different models for the anomalous component predict different polarisation signatures. The full-sky survey by the Akari2 _{(a.k.a. Astro-F or IRIS - InfraRed Imaging Surveyor)}

infrared satellite will be particularly useful for studying the spatial distributions of different dust components. This will shed light on whether the distribution of small spinning dust grains is consistent with this explanation for the anomalous foreground.

Another interesting project in the more distant future is the SKA3 _{survey of Faraday}

rotation to map out the geometry of the Milky Way’s magnetic fields. This will help our understanding of the Galactic synchrotron emission, though SKA is not expected to be on-line until 2019. In the more immediate future is the GALFACTS4 _{continuum survey at}

∼1.4 MHz from Aricebo that will also provide important data for studying the magnetic fields through the synchrotron emission in the radio.

2 http://www.ir.isas.ac.jp/ASTRO-F/ 3 http://www.skatelescope.org/ 4 http://www.ras.ucalgary.ca/GALFACTS/

180 8. Conclusions