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

I De Looze,T J Parkin,H Roussel,Gianfranco Gentile ,Walter Kieran Gear ,S Madden ,Asantha Cooray ,M Smith ,D Cormier,M Boquien,J Fritz,M Baes,M Sauvage,L Spinoglio,A Boselli,C D Wilson,J Verstappen,V Lebouteiller,G J Bendo,F Galliano,Lisa M Young

doi:10.1111/j.1365-2966.2012.21044.x

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]

Highlights

85 per cent of all galaxies are located outside galaxy clusters, among which half reside in groups (Karachentsev 2005)
Inserting the correct values in equation (3) results in an estimate for the total molecular gas mass MH2 ∼ 1.3 × 105 M. Considering that this value is a factor of ∼5 lower than the MH2 ∼ 6.9 × 105 M inferred from CO (1–0) detections, we argue that most of the H2 in NGC 205 resides in regions of colder temperature (T ≤ Tcrit,CO(3−2) ∼ 33 K) and/or lower density (n ≤ ncrit,CO(3−2) ∼ 2 × 104 cm−3)
Relying on recent results from Martın-Manjon et al (2012) and Boylan-Kolchin et al (2012), finding no evidence for extreme star formation efficiencies in star-forming dwarf galaxies and rather supporting a moderate star formation efficiency (SFE) of 10–20 per cent or less, we argue that non-standard conditions are not likely to occur in NGC 205. and we should invoke efficient mechanisms of gas removal to explain the low observed gas content in NGC 205

Summary

Introduction

85 per cent of all galaxies are located outside galaxy clusters, among which half reside in groups (Karachentsev 2005). Among the low surface brightness galaxies in the Local Group, the dwarf satellite NGC 205 (∼824 kpc; McConnachie et al 2005) of the Andromeda galaxy is of particular importance due to its relatively low metal abundance (Z ∼ 0.13 Z ; Richer & McCall 2008), interesting star formation history and indications of a tidal encounter with its massive companion M31. An old stellar population (10 Gyr; Bica, Alloin & Schmidt 1990) dominates the overall stellar content of the dwarf galaxy, and a plume of bright blue star clusters in the central region of NGC 205 was already identified ∼60 years ago (Baade 1951; Hodge 1973)

Results

Discussion

Conclusion