Planck Intermediate Results. IX. Detection of the Galactic haze with Planck

TL;DR

Using Planck and WMAP data, the study detects and characterizes the Galactic microwave haze, revealing its spectrum and morphology, and suggesting a new cosmic-ray acceleration mechanism near the Galactic center.

Contribution

First detection and detailed spectral analysis of the Galactic haze at microwave frequencies, linking it to the gamma-ray haze and proposing a novel cosmic-ray acceleration process.

Findings

Haze spectrum is a power-law with index -2.55

Haze morphology matches Fermi gamma-ray bubbles

Electrons likely accelerated by a new mechanism, not supernova shocks

Abstract

Using precise full-sky observations from Planck, and applying several methods of component separation, we identify and characterize the emission from the Galactic "haze" at microwave wavelengths. The haze is a distinct component of diffuse Galactic emission, roughly centered on the Galactic centre, and extends to |b| ~35 deg in Galactic latitude and |l| ~15 deg in longitude. By combining the Planck data with observations from the WMAP we are able to determine the spectrum of this emission to high accuracy, unhindered by the large systematic biases present in previous analyses. The derived spectrum is consistent with power-law emission with a spectral index of -2.55 +/- 0.05, thus excluding free-free emission as the source and instead favouring hard-spectrum synchrotron radiation from an electron population with a spectrum (number density per energy) dN/dE ~ E^-2.1. At Galactic latitudes |b|<30 deg, the microwave haze morphology is consistent with that of the Fermi gamma-ray "haze" or "bubbles," indicating that we have a multi-wavelength view of a distinct component of our Galaxy. Given both the very hard spectrum and the extended nature of the emission, it is highly unlikely that the haze electrons result from supernova shocks in the Galactic disk. Instead, a new mechanism for cosmic-ray acceleration in the centre of our Galaxy is implied.