Planck 2013 results. XIII. Galactic CO emission

TL;DR

The Planck 2013 XIII paper addresses mapping all-sky CO emission to study molecular gas and to monitor CO as a foreground for CMB analyses. It employs Planck HFI data and three CO extraction pipelines (MILCA, Ruler, and Commander-Ruler) to produce Type 1 (native resolution), Type 2 (15′), and Type 3 (~5.5′) maps for $^{12}$CO $(J=1\rightarrow0)$, $(J=2\rightarrow1)$, and $(J=3\rightarrow2)$, with rigorous cross-validation against external CO surveys. It derives both bandpass-based and sky-calibrated CO conversion coefficients, accounts for $^{13}$CO contamination and dust foregrounds, and validates the results against Dame et al., NANTEN, AMANOGAWA, FIRAS, HARP/ACSIS, and high-latitude CO detections. The study reveals that Planck CO maps provide an unprecedented all-sky view of CO emission, including diffuse, subthermally excited gas extending beyond known molecular clouds, and underscores the importance of modeling CO contamination in Planck cosmology analyses.

Abstract

Rotational transition lines of CO play a major role in molecular radio astronomy and in particular in the study of star formation and the Galactic structure. Although a wealth of data exists in the Galactic plane and some well-known molecular clouds, there is no available CO high sensitivity all-sky survey to date. Such all-sky surveys can be constructed using the \Planck\ HFI data because the three lowest CO rotational transition lines at 115, 230 and 345 GHz significantly contribute to the signal of the 100, 217 and 353 GHz HFI channels respectively. Two different component separation methods are used to extract the CO maps from Planck HFI data. The maps obtained are then compared to one another and to existing external CO surveys. From these quality checks the best CO maps in terms of signal to noise and/or residual foreground contamination are selected. Three sets of velocity-integrated CO emission maps are produced: Type 1 maps of the CO (1-0), (2-1), and (3-2) rotational transitions with low foreground contamination but moderate signal-to-noise ratio; Type 2 maps for the (1-0) and (2-1) transitions with a better signal-to-noise ratio; and one Type 3 map, a line composite map with the best signal-to-noise ratio in order to locate the faintest molecular regions. The maps are described in detail. They are shown to be fully compatible with previous surveys of parts of the Galactic Plane and also of fainter regions out of the Galactic plane. The Planck HFI velocity-integrated CO maps for the (1-0), (2-1), and (3-2) rotational transitions provide an unprecedented all-sky CO view of the Galaxy. These maps are also of great interest to monitor potential CO contamination on CMB \Planck\ studies.

Figures (2)

• Figure 1: The average spectral response for each of the HFI frequency bands. The vertical bars represent the CO rotational transitions.
• Figure 2: Comparison of normalised CO conversion coefficients computed from the bandpass measurements (black) or estimated on the sky (blue) for the 100 (top), 217 (middle) and 353 GHz (bottom) channels. See Table \ref{['tab:co_coeff']}.