COLinesΒΆ

This class implements simulations for Galactic CO emission involving the first 3 CO rotational lines, i.e. \(J=1-0,2-1,3-2\) whose center frequency is respectively at \(\nu_0 = 115.3, 230.5,345.8\) GHz. The CO emission map templates are the CO Planck maps obtained with MILCA component separation algorithm (See Planck paper). The CO maps have been released at the nominal resolution (10 and 5 arcminutes). However, to reduce noise contamination from template maps (especially at intermediate and high Galactic latitudes), we convolved them with a 1 deg gaussian beam.

The Stokes I map is computed from the template one as it follows:

if target \(N_{side}\) <= 512:

  1. The template map at a nside=512 is downgraded at the target \(N_{side}\)

if target \(N_{side}\) > 512 :

  1. The template map at a nside=2048 is downgraded(eventually upgraded) at the target \(N_{side}\)

Q and U maps can be computed from the template CO emission map, \(I_{CO}\), assuming a constant fractional polarization, as:

\[ \begin{align}\begin{aligned}Q = f_{pol} I_{CO} g_d \cos( 2 \psi)\\U = f_{pol} I_{CO} g_d \sin( 2 \psi)\end{aligned}\end{align} \]

with \(g_d\) and \(\psi\) being respectively the depolarization and polarization angle maps estimated from a dust map as :

\[ \begin{align}\begin{aligned}g_d = \frac{ \sqrt{Q^2_{d,353} + U^2_{d,353} } }{f_{pol} I_{d,353} }\\\psi = \frac{1}{2} \arctan {\frac{U_{d,353}}{Q_{d,353}}}\end{aligned}\end{align} \]

Most of the CO emission is expected to be confined in the Galactic midplane. However, there are still regions at high Galactic latitudes where the CO emission has been purely assessed (by current surveys) and where the Planck signal-to-noise was not enough to detect any emission.

The PySM user can include the eventuality of molecular emission (both unpolarized and polarized) at High Gal. Latitudes by co-adding to the emission maps one realization of CO emission simulated with MCMole3D together with the Planck CO map. The polarization is simulated similarly as above.

The MCMole3D input parameters are are obtained from best fit with the Planck CO 1-0 map (see Puglisi et al. 2017 and the documentation). If include_high_galactic_latitude_clouds=True, a mock CO cloud map is simulated with MCMole3D, encoding high Galactic latitudes clouds at latitudes above and below than 20 degrees. The mock emission map is then co-added to the Planck CO emission map. The polarization is simulated similarly as above.

The installation of mcmole3d is not required, HGL clouds can be input to the CO emission by setting run_mcmole3d=False (which is the default). However, if one wants to run several mock CO realizations observing high Galactic latitude patches we encourage to run mcmole3d by changing random_seed in the CO class constructor. The parameter theta_high_galactic_latitude_deg set the latitude above which CO emission from high Galactic latitudes can be included and it has an impact only when run_mcmole3d=True.

The level of polarization in co2 and co3 is 0.1%, which on average is the expected level on 10% of the sky. However, polarization from CO emission have been detected at larger fluxes in Orion and Taurus complexes (Greaves et al.1999 )

See this post for the actual processing of templates.