Herschel^★and JCMT observations of the early-type dwarf galaxy NGC 205

Abstract

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.

The interstellar medium in Andromeda's dwarf spheroidal galaxies – I. Content and origin of the interstellar dust

“…Given that the excess in the SPIRE 500 µm waveband appears to be model dependent, we do not consider the excess emission as significant. The absence of a submm excess in NGC 185 is consistent with the lack of any submm/mm excess emission in NGC 205 (De Looze et al 2012).…”

“…With the dust mass surface density peaking in regions with the highest gas densities (De Looze et al 2012, and see Figure 4, right panel, in this paper), our scenario appears at least consistent with efficient metal accretion processes on to the surface of pre-existing grains in the cold neutral medium. Based on semi-analytic chemical evolution models, Schneider et al (2016) show that the ISM density is the dominant factor in setting the efficiency of grain growth.…”

Spatially-resolved dust properties of the GRB 980425 host galaxy

“…an enhancement in the submillimetre emission above the extrapolation from far-IR data (Lisenfeld et al 2002;Galliano et al 2003Galliano et al , 2005Galliano et al , 2011Bendo et al 2006;Galametz et al 2009Galametz et al , 2011Bot et al 2010;Gordon et al 2010;Israel et al 2010;Planck Collaboration 2011a,b;Dale et al 2012;De Looze et al 2012). Apart from the cold dust emission, this excess has also been attributed to different grain properties (e.g.…”

HERschelObservations of Edge-on Spirals (HEROES)

“…With Herschel, we can now take spatially resolved FIR/sub-mm images to map the distribution of cool dust (the bulk of the dusty ISM) in nearby galaxies (e.g. Smith et al 2010Smith et al , 2012Bendo et al 2010Bendo et al , 2012Pohlen et al 2010;Kennicutt et al 2011;Foyle et al 2012;Fritz et al 2012;Boquien et al 2012;Xilouris et al 2012;Aniano et al 2012;Mentuch Cooper et al 2012;De Looze et al 2012c).…”