Planck early results. XXI. Properties of the interstellar mediumin the Galactic plane
2011
Planck has observed the entire sky from 30 GHz to 857 GHz. The observed foreground emission contains contributions from different phases
of the interstellar medium (ISM). We have separated the observed Galactic emission into the different gaseous components (atomic, molecular
and ionised) in each of a number of Galactocentric rings. This technique provides the necessary information to study dust properties (emissivity,
temperature, etc.), as well as other emission mechanisms as a function of Galactic radius. Templates are created for various Galactocentric radii
using velocity information from atomic (neutral hydrogen) and molecular (12CO) observations. The ionised template is assumed to be traced by
free-free emission as observed by WMAP, while 408 MHz emission is used to trace the synchrotron component. Gas emission not traced by the
above templates, namely “dark gas”, as evidenced using Planck data, is included as an additional template, the first time such a component has
been used in this way. These templates are then correlated with each of the Planck frequency bands, as well as with higher frequency data from
IRAS and DIRBE along with radio data at 1.4 GHz. The emission per column density of the gas templates allows us to create distinct spectral
energy distributions (SEDs) per Galactocentric ring and in each of the gaseous tracers from 1.4 GHz to 25 THz (12 μm). The resulting SEDs
allow us to explore the contribution of various emission mechanisms to the Planck signal. Apart from the thermal dust and free-free emission, we
have probed the Galaxy for anomalous (e.g., spinning) dust as well as synchrotron emission. We find the dust opacity in the solar neighbourhood,
τ/NH = 0.92±0.05×10−25 cm2 at 250 μm, with no significant variation with Galactic radius, even though the dust temperature is seen to vary from
over 25 K to under 14 K. Furthermore, we show that anomalous dust emission is present in the atomic, molecular and dark gas phases throughout
the Galactic disk. Anomalous emission is not clearly detected in the ionised phase, as free-free emission is seen to dominate. The derived dust
propeties associated with the dark gas phase are derived but do not allow us to reveal the nature of this phase. For all environments, the anomalous
emission is consistent with rotation from polycyclic aromatic hydrocarbons (PAHs) and, according to our simple model, accounts for (25 ± 5)%
(statistical) of the total emission at 30 GHz.
Keywords:
- Correction
- Cite
- Save
- Machine Reading By IdeaReader
0
References
2
Citations
NaN
KQI