Revealing the cold dust in low-metallicity environments
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abstract
We present new photometric data from our Herschel Key Programme, the Dwarf
Galaxy Survey (DGS), dedicated to the observation of the gas and dust in 48
low-metallicity environments. They were observed with PACS and SPIRE onboard
Herschel at 70,100,160,250,350, and 500 microns. We focus on a systematic
comparison of the derived FIR properties (FIR luminosity, dust mass, dust
temperature and emissivity index) with more metal-rich galaxies and investigate
the detection of a potential submm excess. The data reduction method is adapted
for each galaxy to derive the most reliable photometry from the final maps.
PACS flux densities are compared with the MIPS 70 and 160 microns bands. We use
colour-colour diagrams and modified blackbody fitting procedures to determine
the dust properties of the DGS galaxies. We also include galaxies from the
Herschel KINGFISH sample, containing more metal-rich environments, totalling
109 galaxies. The location of the DGS galaxies on Herschel colour-colour
diagrams highlights the differences in global environments of low-metallicity
galaxies. The dust in DGS galaxies is generally warmer than in KINGFISH
galaxies (T_DGS~32 K, T_KINGFISH~23 K). The emissivity index, beta, is ~1.7 in
the DGS, but metallicity does not make a strong effect on beta. The
dust-to-stellar mass ratio is lower in low-metallicity galaxies:
M_dust/M_star~0.02% for the DGS vs 0.1% for KINGFISH. Per unit dust mass, dwarf
galaxies emit ~6 times more in the FIR than higher metallicity galaxies. Out of
the 22 DGS galaxies detected at 500 micron, 41% present an excess in the submm
not explained by our dust SED 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. We stress the need for observations
longwards of the Herschel wavelengths to detect any submm excess appearing
beyond 500 micron.
