TheSpitzerSurvey of the Small Magellanic Cloud: Far‐Infrared Emission and Cold Gas in the Small Magellanic Cloud

Abstract

We present new far infrared maps of the Small Magellanic Cloud (SMC) at 24, 70, and 160 µm obtained as part of the Spitzer Survey of the Small Magellanic Cloud (S 3 MC, Bolatto et al. 2006). These maps cover most of the active star formation in the SMC Bar and the more quiescent Wing. We combine our maps with literature data to derive the surface density across the SMC. We find a total dust mass of M dust =3 × 10 5 M ⊙ , implying a dust-to-hydrogen ratio over the region studied of log 10 D/H = −2.86, or 1-to-700, which includes H 2 . Assuming the dust to trace the total gas column, we derive H 2 surface densities across the SMC. We find a total H 2 mass M H2 = 3.2 × 10 7 M ⊙ in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H 2 around six molecular peaks and find that on average H 2 is more extended than CO by a factor of ∼ 1.3. The implied CO-to-H 2 conversion factor over the whole SMC is X CO = 13 ± 1 × 10 21 cm −2 (K km s −1 ) −1 . Over the volume occupied by CO we find a lower conversion factor, X CO = 6 ± 1 × 10 21 cm −2 (K km s −1 ) −1 , which is still a few times larger than that found using virial mass methods. The molecular peaks have H 2 surface densities Σ H2 ≈ 180 ± 30 M ⊙ pc −2 , similar to those in Milky Way GMCs, and correspondingly low extinctions, A V ∼ 1 -2 mag. To reconcile these measurements with predictions by the theory of photoionization-regulated star formation, which requires A V ∼ 6, the GMCs must be ∼ 3 times smaller than our 46 pc resolution element. We find that for a given hydrostatic gas pressure, the SMC has a 2 -3 times lower ratio of molecular to atomic gas than spiral galaxies. Combined with the lower mean densities in the SMC this may explain why this galaxy has only 10% of its gas in the molecular phase.

C+emission from the Magellanic Clouds

“…Thus, the KAO [C II] measurements indicate the presence of significant amounts of hydrogen, most likely in the form of H 2 , not traced by CO. A similar situation has been noted to occur in the magellanic irregular dwarf galaxy IC 10, where [C II] observations likewise points at such an H 2 reservoir (Madden et al 1997). The [C II] results confirm those derived by using thermal continuum emission from dust grains as a tracer for total (atomic + molecular) gas masses in the Magellanic Clouds (Israel 1997;Leroy et al 2007Leroy et al , 2009). …”

The cosmic dust rate across the Universe

“…Data: Green pentagons correspond to irregular galaxies, whereas blue filled circles correspond to spirals. Yellow and green stars represent the dust-to-gas ratio observed in the solar neighborhood (Zubko et al 2004) and in the Small Magellanic Cloud (Leroy et al 2007), respectively. The dashed-black line expresses a linear relation between dust-to-gas and metallity, whereas the dotted straight line is the fit of the whole data sample as done in Rémy-Ruyer et al (2014).…”

Probing the dust properties of galaxies up to submillimetre wavelengths

“…The CO to H 2 conversion factor is usually calibrated with cold (≈12 K) Galactic clouds (solar metallicity) and may not necessarily resemble the physical conditions of low-metallicity objects. Some studies show that this conversion factor can be far higher in low-metallicity environments than in metal-rich ones (Wilson 1995;Madden et al 1997;Leroy et al 2005Leroy et al , 2007. Due to the high excitation and the paucity of the dust in low-metallicity environments, CO is generally a poor tracer of molecular gas.…”