Does Galactic Dust Twirl and Shine?

May 5, 2015

The question of whether we receive microwave radiation from spinning dust grains in our Galaxy has been debated for 15 years. A collaboration including a KIPAC scientist has provided valuable data indicating that the answer is probably yes.

The ARCADE 2 instrument being launched on a high altitude balloon. Getting above the atmosphere is important in an absolute atrophysical microwave measurement.

Looking at the cosmos in the microwave region of the electromagnetic spectrum has been one of the most exciting avenues of astrophysical research.  The "precision cosmology" enabled by the Cosmic Microwave Background, which has provided some of the most important evidence that our Universe is dominated by dark matter and dark energy, has been most prominent, but we have also learned much about the astrophysics within our own Galaxy. 
 
There are several processes that create microwave radiation.  At the higher microwave frequencies, the "dust" (molecules up to 0.1 mm in size that float between stars) glows with thermal emission.  At the lower frequencies, electrons either spiraling in magnetic fields - synchrotron radiation - or deflecting off of each other - free-free emission, produce Galactic microwave foregrounds that dominate the cosmic background.  Into this trio, more than 15 years ago, two astrophysicists working at Princeton, Bruce Draine and Alex Lazarian, proposed a fourth possible component of Galactic microwave emission, that which would result from the dust grains rotating.
 
This spinning dust spectrum would have a peak intensity at around 35 GHz and fall off relatively rapidly on either side of that peak.  The composition of the dust itself is well characterized from line emission and absorption, and from the thermal spectrum, but the question was whether the rotation is rapid and ubiquitous enough for the rotational emission to be important.  Confirming or denying the existence of spinning dust emission is quite difficult, as microwave photons do not come labeled with their origin mechanism.  Instead, observers must measure the absolute intensity of the emission as a function of frequency, and build models which take into account the frequency dependence of the different components - the microwave background, thermal dust, free-free, synchrotron, and spinning dust.  An additional component of microwave emission, a possible "haze" from the annihilation of dark matter in the center of the Galaxy, has also been recently proposed.
 
Absolute intensity measurements are a special kind of microwave and radio astronomy and require specific engineering considerations.  KIPAC astrophysicist Jack Singal has been involved with colleagues from NASA on such a project, called ARCADE 2, to obtain high accuracy absolute temperature maps of the microwave sky with an instrument flown on a high altitude balloon.  An analysis of the ARCADE 2 data that the team carried out indicates that spinning dust emission is in fact an important component of the microwave sky at the frequencies in question.  In a paper to appear in the Astrophysical Journal, the team concludes that spinning dust emission constitutes 40% of the Galactic Plane emission at 22 GHz.  This data from ARCADE 2 has allowed additional investigations of spinning dust emission with data from WMAP and other instruments. 
 
Understanding whether spinning dust emission is important goes beyond implications for the astrophysics of the interstellar medium.  It also has a significant effect on interpretations of the hypothesized microwave haze toward the center of the Galaxy, which has been attributed to annihilating dark matter.  A fuller understanding of all of the components of the microwave sky, including spinning dust emission, is an essential for investigating outstanding questions in astrophysics and cosmology. 
This work is based in part on a paper submitted to the Astrophysical Journal, and available at arXiv:0901.0562.

Science Contact:
Jack Singal
KIPAC
jacks@slac.stanford.edu