ALMA observations of cool dust in a low-metallicity starburst, SBS 0335−052
Résumé
We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7
observations of an extremely metal-poor dwarf starburst galaxy in the Local Universe,
SBS 0335−052 (12 + log (O/H) ~ 7.2). With these observations, dust is detected at
870 μm (ALMA Band 7), but 87% of the flux in this band is due to
free-free emission from the starburst. We have compiled a spectral energy distribution
(SED) of SBS 0335−052 that spans almost 6 orders of magnitude in wavelength and fit it
with a spherical dust shell heated by a single-age stellar population; the best-fit model
gives a dust mass of (3.8 ± 0.6) × 104 M⊙. We
have also constructed a SED including Herschel archival data for I Zw 18,
another low-metallicity dwarf starburst (12 + log (O/H) ~ 7.17), and fit it with a similar
model to obtain a dust mass of
(3.4 ± 1.0) × 102 M⊙. It appears that for
such low-metallicity dwarf galaxies, the ratio of their stellar mass to their dust mass is
within the range of values found for spirals and other star-forming galaxies. However,
compared with their atomic gas mass, the dust mass of SBS 0335−052 far exceeds the
prediction of a linear trend of dust-to-gas mass ratio with metallicity, while I Zw 18
falls far below. We use gas scaling relations to assess a putative missing gas component
in both galaxies and find that the missing, possibly molecular, gas in SBS 0335−052 is a
factor of 6 times higher than the value inferred from the observed Hi column
density; in I Zw 18 the missing component is much smaller. Finally, we constrain the
H2 surface density conversion factor αCO with our
upper limit for CO J = 3−2 line in SBS 0335−052, and find that this is
consistent with a linear or even super-linear trend of increasing
αCO with decreasing metallicity. Ultimately, despite their
similarly low metallicity, the differences in gas and dust column densities in
SBS 0335−052 and I Zw 18 suggest that metal abundance does not uniquely define
star-formation processes. At some level, self-shielding and the survival of molecules may
depend just as much on gas and dust column density as on metallicity. The effects of low
metallicity may at least be partially compensated for by large column densities in the
interstellar medium.
Origine : Publication financée par une institution
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