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

TL;DR

This study combines Herschel and JCMT observations to reassess the interstellar medium content in the dwarf galaxy NGC 205, revealing a lower dust and gas mass than previously thought and confirming CO as a reliable tracer of molecular gas.

Contribution

It provides new dust and gas mass estimates for NGC 205 using multi-wavelength data, addressing the missing interstellar medium problem in this galaxy.

Findings

Dust mass is estimated at 1.1-1.8e+4 Msun with no evidence for a massive cold dust component.

Gas mass derived from dust and line observations is 4-7e+6 Msun, lower than previous estimates.

CO effectively traces molecular gas, with an estimated M_H2 of 1.3e+5 Msun.

Abstract

We present Herschel dust continuum, James Clerk Maxwell Telescope CO(3-2) observations and a search for [CII] 158 micron and [OI] 63 micron spectral line emission for the brightest early-type dwarf satellite of Andromeda, NGC 205. While direct gas measurements (Mgas ~ 1.5e+6 Msun, HI + CO(1-0)) have proven to be inconsistent with theoretical predictions of the current gas reservoir in NGC 205 (> 1e+7 Msun), we revise the missing interstellar medium mass problem based on new gas mass estimates (CO(3-2), [CII], [OI]) 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 micron, we are able to probe the entire dust content in NGC 205 (Mdust ~ 1.1-1.8e+4 Msun at Tdust ~ 18-22 K) and rule out the presence of a massive cold dust component (Mdust ~ 5e+5 Msun, Tdust ~ 12 K), which was suggested based on millimeter 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 Mgas ~ 4-7e+6 Msun. The non-detection of [OI] and the low L_[CII]-to-L_CO(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 1.5e+4 Msun associated with the [CII] emitting PDR 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_H2 ~ 1.3e+5 Msun. [abridged]