We present Herschel dust continuum, James Clerk Maxwell Telescope CO (3–2) observations and a search for [C ii] 158 m and [O i] 63 m spectral line emission for the brightest early-type dwarf satellite of Andromeda, NGC 205. While direct gas measurements [Mg∼ 1.5 × 106 M⊙, H i+ CO (1–0)] have proven to be inconsistent with theoretical predictions of the current gas reservoir in NGC 205 (>107 M⊙), we revise the missing interstellar medium mass problem based on new gas mass estimates (CO (3–2), [C ii], [O i]) and indirect measurements of the interstellar medium content through dust continuum emission.
Based on Herschel observations, covering a wide wavelength range from 70 to 500 m, we are able to probe the entire dust content in NGC 205 ( M⊙ at Td∼ 18–22 K) and rule out the presence of a massive cold dust component ( M⊙, Td∼ 12 K), which was suggested based on millimetre observations from the inner 18.4 arcsec. Assuming a reasonable gas-to-dust ratio of ∼400, the dust mass in NGC 205 translates into a gas mass Mg∼ 4–7 × 106 M⊙. The non-detection of [O i] and the low L[C II]-to-LCO(1-0) line intensity ratio (∼1850) imply that the molecular gas phase is well traced by CO molecules in NGC 205. We estimate an atomic gas mass of 1.5 × 104 M⊙ associated with the [C ii] emitting photodissociation regions in NGC 205. From the partial CO (3–2) map of the northern region in NGC 205, we derive a molecular gas mass of M⊙. Upon comparison with the molecular gas mass estimated from CO (1–0) observations ( M⊙), we find most of the H2 gas in NGC 205 to be locked in diffuse regions of low density and/or temperature, characteristic for an interstellar medium with little star formation activity.
New total gas mass estimates from Herschel dust continuum (4–7 × 106 M⊙), Herschel [C ii] line spectroscopic mapping (1.5 × 104 M⊙) and James Clerk Maxwell Telescope CO (3–2) observations (7 × 105 M⊙), including the H i mass ( M⊙) and a correction for heavier elements, confirm the deficiency of the interstellar medium (gas+dust) in the inner regions of NGC 205, which is predicted to contain at least >107 M⊙ of gas if we assume a reasonable star formation efficiency of 10 per cent and account for the mass return from planetary nebulae. In an attempt to explain the missing interstellar medium mass problem, we claim that efficient supernova feedback capable of expelling gas from the inner, star-forming regions to the outer regions and/or tidal interactions with M31 stripping the gas component from the galaxy provide the best explanation for the removal of a significant amount of gas and dust from NGC 205.