Context. We present new photometric data from our Herschel Guaranteed Time Key Programme, the Dwarf
Galaxy Survey (DGS),
dedicated to the observation of the gas and dust in low-metallicity environments. A total of 48
dwarf galaxies were observed with the PACS and SPIRE instruments onboard the Herschel Space
Observatory at 70, 100, 160, 250, 350, and 500 µm.
Aims. The goal of this paper is to provide reliable far infrared (FIR) photometry for the DGS
sample and to analyse the FIR/submillimetre (submm) behaviour of the DGS galaxies. We focus on a
systematic comparison of the derived FIR properties (FIR luminosity, LFIR, dust mass, Mdust , dust
temperature, T, emissivity index, β) with more metal-rich galaxies and investigate the detection of
a potential submm excess.
Methods. The data reduction method is adapted for each galaxy in order to derive the most reliable
photometry from the final maps. The derived PACS flux densities are compared with the Spitzer MIPS
70 and 160 µm bands. We use colour-colour diagrams to analyse the FIR/submm behaviour of the DGS
galaxies and modified blackbody fitting procedures to determine their dust properties. To study the
variation in these dust properties with metallicity, we also include galaxies from the Herschel
KINGFISH sample, which contains more metal-rich environments, totalling 109 galaxies.
Results. The location of the DGS galaxies on Herschel colour-colour diagrams highlights the
differences in dust grain properties and/or global environments of low-metallicity dwarf galaxies.
The dust in DGS galaxies is generally warmer than in KINGFISH galaxies (TDGS ∼ 32 K and TKINGFIS H
∼ 23 K). The emissivity index, β, is ∼ 1.7 in the DGS, however metallicity does not make
a strong effect on β. The proportion of dust mass relative to stellar mass is lower in
low-metallicity galaxies: Mdust /Mstar ∼ 0.02%
for the DGS versus 0.1% for KINGFISH. However, per unit dust mass, dwarf galaxies emit about six
times more in the FIR/submm
than higher metallicity galaxies. Out of the 22 DGS galaxies detected at 500 µm, about 41% present
an excess in the submm beyond the explanation of our dust SED model, and this excess can go up to
150% above the prediction from the model. The excess mainly appears in lower metallicity galaxies
(12+log(O/H) ;S 8.3), and the strongest excesses are detected in the most metal-poor galaxies.
However, we so stress the need for observations longwards of the Herschel wavelengths to detect any
submm excess appearing beyond 500 .