The W MAP Haze

The first detection of the so called ‘W MAP Haze’ is due to Finkbeiner (2004a). Analysing the W MAP data, he found an approximately circular excess of diffuse emission centered around the Galactic center of radial extent_∼ 20◦ _{and luminosity L}

Finkbeiner (2008a) defined its profile as well described by: h_∝        1 r − 1 r0 for r < r0; 0 for r > r0, (1.37)

where r is the angular distance to the Galactic center and r0is arbitrarily set equal to 45 degrees.

Initially attributed to free-free emission produced by & ₁₀5 _{K gas (Finkbeiner, 2004a), this}

hypothesis was immediately discarded since a counterpart emission in X-ray was not detected (Finkbeiner, 2004b). Finkbeiner (2004b) showed instead that the energetics and profile of the haze emission could be explained by synchrotron emission from the by-products of dark matter annihilation for a generic weakly interacting (_∼ 100 GeV) particle with a standard dark matter halo profile. This idea has been pursued further in several subsequent works (Hooper et al., 2007, 2008; Zhang et al., 2009; Cholis et al., 2009; Cumberbatch et al., 2009).

Other studies carried out different interpretations. The haze has been associated with synchrotron emission produced by pulsars (Zhang et al., 2009; Kaplinghat et al., 2009) or, more recently, by cosmic rays from astrophysical sources (McQuinn & Zaldarriaga, 2010). However, McQuinn & Zaldarriaga (2010) also pointed out that both these interpretations are not able to justify the following observations: 1. Dobler & Finkbeiner (2008a) found the haze spectrum in the microwave to be hard, measuring a power-law electron spectrum with index p_∼2. Such a spectrum is much harder than the galactic synchrotron towards other directions (where p_∼3).

2. The distribution of known galactic cosmic ray sources appears not to be sufficiently concentrated toward the Galactic center in order to create the haze (Finkbeiner, 2004b; Dobler & Finkbeiner, 2008a; Zhang et al., 2009). The distribution of supernovae and pulsars is estimated to peak at_∼4 kpc from the Galactic center, or_∼ 30◦ _{(Lorimer et al., 2006). In contrast, the haze intensity is}

strongly increasing with decreasing angle at<20◦_{from the Galactic center (Dobler & Finkbeiner,}

2008a).

3. A similar excess towards the Galactic center is not present at 408 MHz. If the haze is from synchrotron, this suggests that the source of the haze does not contribute significantly to the population of cosmic ray electrons at_∼ 5 GeV. In addition, the 408 MHz emission appears very disk-like without a significant enhancement in the haze region.

On the other hand, these criteria can be satisfied by dark matter annihilation (Finkbeiner, 2004b), although other authors argued that the annihilation cross-section would need to be significantly boosted (Cumberbatch et al., 2009).

A preliminary analysis (Dobler et al., 2009) of the data produced by the Fermi Space Telescope has shown the existence of aγ-ray ‘haze’ with a similar spatial morphology of the W MAP haze. It has been interpreted as due to the same hard population of electrons that generate the haze in the microwave (Dobler et al., 2009) which interact via inverse Compton effect with the starlight. As the W MAP Haze, also theγ-ray haze could be explained in terms of dark matters annihilation (Cholis et al., 2009).

However, the Fermi collaboration has not claimed any excess in the galactic center region over the standard diffuseγ-ray background (Abdo et al., 2010; Casandjian et al., 2009). Moreover, Linden & Profumo (2010) pointed out that what was measured by Dobler et al. (2009) could be just an artifact of the analysis performed, i.e. an incorrect foreground removal. In fact, they used templates which seem to be inappropriate and to underestimate both theπ0_{decay and inverse Compton scattering (ICS)}

contributions to theγ-ray emission, specifically in the Galactic center region.

The reliability of the models used to trace the different components of the sky is a crucial point also for the detection of the haze in the microwaves. Finkbeiner (2004a) identified the haze as the result of an analysis where the W MAP maps at different frequencies were fitted with a model of the foreground components of the sky, as described by specific templates. In particular, the Haslam map at 408 MHz has been used to trace the synchrotron emission, although the W MAP observations lie at much larger frequencies. Relying on such a crude extrapolation of the morphology of synchrotron emission can thus potentially introduce unphysical residuals. Specifically, Mertsch & Sarkar (2010) have shown by means of simulations, that this leads to residuals of the same order as the claimed haze, along the Galactic disk. Therefore, they support the idea that the W MAP haze is actually an artifact of inappropriate template subtraction, rather than evidence of dark matter annihilation.

Indeed, other authors (Cumberbatch et al., 2009), have noted that the significance of the W MAP haze is substantially reduced by allowing for spatial variation in the frequency-dependence of synchrotron emission in the inner and outer parts of the Galaxy.

Doubts about the actual existence of the haze come also from a recent study by Gold et al. (2010), where they attempted to detect the haze in polarization in the W MAP sky maps, although unsuccessfully. Moreover, the total polarized synchrotron emission within _∼ 10,20 and 30 degrees from the Galactic center is consistent with a spectral index of₋1.2, which is much softer than the value proposed by Dobler & Finkbeiner (2008a): they found that the haze contributes roughly half of the diffuse intensity in synchrotron within 10◦_{(and a decreasing fraction at larger angles). Therefore, Gold}

et al. (2010) seems to suggest that if the haze exists, it must contribute a much smaller fraction to the polarized emission.

However, as pointed out by McQuinn & Zaldarriaga (2010), the Gold et al. (2010) results can be still consistent with the hypothesis of the W MAP haze as a new independent synchrotron emission. First of all, the possibility that the haze emission is not highly polarized is actually plausible, since in the inner regions of the Galaxy the polarized fraction of the synchrotron emission is lower than outer regions, probably due to the strong entanglement of the Galactic magnetic field along the Galactic plane. Furthermore, local structures in the haze region could be dominant in polarization with respect to the haze. More importantly, the structures we know be responsible for the polarized synchrotron emission were not excluded in the analysis performed by Gold et al. (2010), but in the one of Dobler & Finkbeiner (2008a). Finally there is the possibility that the estimation of the spectral index for the electron was wrong and that it is not so hard as it has been claimed. Based on the analysis of Gold et al. (2010), in order to reproduce the amplitude of the haze, a spatial variation in the synchrotron spectral index of only 0.25 between 408 MHz and 23 GHz is required.

confirmation in favour of the W MAP Haze. Indeed, they has demonstrated the existence of two large gamma-ray bubbles, extending 50 degrees above and below the Galactic center, with a width of about 40 degrees in longitude. The hard spectrum emission associated with these bubbles seems to be spatially correlated with the W MAP Haze, and more importantly, the edges of the bubbles line up with features in the ROSAT X-ray maps at 1.5-2 keV. This result would actually confirm the Haze as a new synchrotron emission and would associate it to a large episode of energy injection in the Galactic center, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last_∼ 10 Myr. Besides, the hypothesis of dark matter annihilation or decay would be consequently ruled out.

In the analysis proposed in Chapters 3 and 4, we tried to address the issue of the existence of the W MAP Haze, attempting to confirm the results proposed by Dobler & Finkbeiner (2008a). We refer the reader to these chapters for a more detailed analysis of our results.