Emission law of the free-free, spinning dust, and main CO molecular line emission (in units of spectral brightness I_ν, e.g., ∝ MJy  sr^-1). The spinning dust emission law is here normalised to unity at ν = 23 GHz and the free-free emission law to I_ν = 2 at the same reference frequency. The integrated amplitude of the first ¹²CO emission lines (transition (J = 1–0)) is normalised to unity, and the other lines (transitions (J = 2–1) and (J = 3–2)) are displayed in proportion to their integrated intensity relative to the first one.