Revealing the cold dust in low-metallicity environments
2013
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
TKINGFISH ~ 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) ≲
8.3), and the strongest excesses are detected in the most metal-poor galaxies. However, we
also stress the need for observations longwards of the Herschel
wavelengths to detect any submm excess appearing beyond 500 μm.
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