By Julia Roman-Duval, ESA-AURA astronomer at STScI
Dust and metals play a central role in radiation transport, chemistry, heating and cooling in galaxies, thus influencing their chemical evolution, the star formation rate, and shaping the different phases of the interstellar medium (ISM). For decades, astronomers have been studying the processes that shape the evolution of dust and the cycles of metals between the different ISM phases, from hot, ionized, diffuse, to cold, dense, and molecular. These processes are fairly well constrained in the solar neighborhood. For instance, depletion studies in the Milky Way show that gas-phase metals accrete onto dust grains in atomic or molecular clouds, leading to a decrease by a factor 2-3 in the gas-to-dust ratio between the diffuse and dense phases . However, the influence of environmental factors like metallicity and energetics (radiation field, shocks) on the evolution of dust between ISM phases is not understood. A particular consequence of the lack of constraints on the gas-to-dust ratio and its variations with density in low-metallicity galaxies is our inability to estimate accurate molecular gas masses.
We have been investigating the lifecycle of dust and metals in the Magellanic Clouds, which are the two closest low-metallicity galaxies located 50 kpc and 62 kpc away respectively, and which have metallicities Z=0.5 Zo and Z = 0.2 Zo respectively. We have been using FIR emission from dust grains seen in Herschel PACS and SPIRE [2, 3] at 15 pc resolution to trace the solid-phase, and H I 21 cm and CO 1-0 rotational emission to estimate gas surface densities at similar resolution (Figure 1).
Figure 1: Dust surface density maps in the LMC (top) and SMC (bottom) estimated from FIR emission seen in Herschel PACS (100 and 160 mm) and SPIRE (250, 350, 500 mm). The H I surface density, traced by its 21 cm emission seen in ATCA+Parkes observations, is indicated by the black contours (10-60 M¤ pc-2 in steps of 10 M¤ pc-2). The molecular gas surface density is traced by its CO rotational emission seen in the MAGMA survey, as shown by the white contours representing the 1.2 K km/s of the CO integrated intensity.
The shape of the FIR SED suggests that the dust composition in the Magellanic Clouds is different from that in the Milky Way, dust grains being richer in amorphous carbon . Additionally, the gas-to-dust ratio in the Magellanic Clouds does not scale linearly with metallicity, with diffuse phase gas-to-dust ratio values of 370 in the LMC and 1300 in the SMC, or 2.5 and 9 times the Milky Way value respectively [5, Figure 2). If the gas-to-dust ratio scaled linearly with metallicity, we would expect values of 300 and 750 in the LMC and SMC respectively. This non-linear relation between dust abundance and metallicity, which is also seen in dwarf galaxies , could indicate that a smaller fraction of metals is locked up in dust grains in the diffuse phase of low-metallicity galaxies compared to the Milky Way, and that dust grains are destroyed more efficiently in those galaxies. Two arguments concur with this conclusion. First, the filling factor of the dense phase in low-metallicity galaxies is lower than in the Milky Way. Since dust grains are predominantly destroyed by supernova shocks propagating in the diffuse ISM, one would expect a higher dust destruction rate in those galaxies. Secondly, depletion patterns, although at a preliminary stage, seem to confirm the low dust-to-metal ratio in the LMC and SMC.
Our study of the relation between dust and gas across ISM phases also suggests that the gas-to-dust ratio decreases by a factor 2-3 with increasing density, from the diffuse to the dense phase (Figure 3). In principle, this could be evidence of accretion of gas-phase metals onto dust grains in dense molecular clouds. However, although emission-based tracers of the dust and gas phases in external galaxies are valuable to provide context and map the different components of the ISM, they are limited by large (factors 2-3) systematic uncertainties, and degeneracies. First, one has to assume a FIR dust grain emissivity and a model SED to convert the observed FIR fluxes into a dust surface density. However, the emissivity of dust grains depends on their composition and size, and is not constrained to better than a factor of 2 in the Magellanic Clouds. As a result, a gas-to-dust ratio decrease from the diffuse to the dense phase caused by accretion of gas-phase metals on dust grains in the dense ISM would be degenerate with an increase in dust emissivity due to coagulation in the same density range . Dust abundance variations between ISM phases are also degenerate with the presence of CO-dark molecular gas in the translucent envelopes of molecular clouds, where H2 self-shields and can exist, but CO does not and is photo-dissociated. Due to these degeneracies, we still do not know whether the apparent variations in the gas-to-dust ratio is due to CO-dark H2, dust coagulation, or accretion of gas-phase metals onto dust grains in the dense phase, or a combination of those effects. We are in the process of using theoretical models for each of these processes to estimate how big of an effect they can reasonably contribute to the observed, apparent variation in the gas-to-dust ratio.
Figure 2: Relation between dust and atomic gas surface densities in the LMC (top) and SMC (bottom). The grey-scale corresponds to the density of pixels. The red circles show the binned average relation in the diffuse atomic phase, while the blue circles correspond to the molecular phase. The transition between atomic and molecular phases is indicated by the vertical blue dashed line. In the diffuse phase, the slope of the dust-atomic gas relation corresponds to the gas-to-dust ratio, which has a value of 370 in the LMC and 1270 in the SMC.
Figure 3: Gas-to-dust ratio versus dust surface density in the LMC (top) and SMC (bottom). The different ISM phases, atomic, translucent, molecular, are indicated by red, blue, and green colors respectively. The gas-to-dust ratio appears to decrease by a factor 2-3 across ISM phases. This could be evidence for dust growth in the dense ISM, via accretion of gas-phase metals onto dust grains, but could also be a result of dust emissivity variations due to dust grain coagulation, or of an underestimate of the gas surface density in the translucent and dense phases, where we know CO does not track molecular gas accurately in low-metallicity galaxies due to the reduced dust-shielding and increased photo-dissociation.
- Jenkins, E. B. 2009 (ApJ, 700, 1299)
- Meixner, M., et al., 2013 (AJ, 146, 62)
- Gordon, K.D., et al, 2014 (ApJ, Submitted)
- Galliano, F., et al, 2011 (A&A, 536, A88)
- Roman-Duval, J., et al., 2014 (ApJ, submitted)
- Remy-Ruyer, A., et al., 2014 (A&A, 563, A31)