ASKAP unveils the secrets of the Small Magellanic Cloud

A team led by Naomi McClure-Griffiths of The Australian National University has used ASKAP to study the mass loss and dynamics of the Small Magellanic Cloud (SMC), one of the Milky Way’s companion dwarf galaxies.

The team’s first image of HI (neutral hydrogen gas) in the SMC was made in a single (not mosaicked) ‘shot’ taken over three nights in November 2017. Its resolution was the highest ever achieved in HI on this field, many times the resolution of the previous best SMC image, which was made with 320 pointings of the ATCA.

The team’s observations, now published in Nature Astronomy, were also the first to quantify the outflow of HI from a dwarf galaxy. They reveal that the SMC is experiencing a large outflow of HI, ∼107 M. If most of this originated in the most recent burst of star formation, 25–60 Myr ago, then the total HI mass flux is ~0.2−1.0 M yr−1.

This HI outflow is up to an order of magnitude greater than the SMC’s star-formation rate and, if sustained, is by itself large enough to quench the SMC’s star formation in 0.6–3 Gyr. (In fact, HI is expected to be only a fraction of the total outflow, 1–50% by mass.)

The SMC and its sibling, the Large Magellanic Cloud (LMC), are trailed by the Magellanic Stream; a second gas stream, the Leading Arm, projects from the LMC. Formation models have found it difficult to account for the mass and shape of these features. The outflow described by McClure-Griffiths et al. may be the source of much of their gas.

A three-colour image of HI emission in the SMC for four velocity ranges (vLSR = 105–113, 129–136, 144–152 and 179–187 km s−1). Red, green and blue are assigned sequentially to adjacent velocities. Credit: McClure-Griffiths et al. and CSIRO

The research team also used its ASKAP observations to study the dynamics of the SMC, presenting its findings in a paper led by Enrico Teodoro (ANU).

Although the SMC is interacting strongly with both the LMC and our Galaxy, its HI rotation curve is like that of many isolated, gas-rich dwarf galaxies. Teodoro et al. decomposed the rotation curve and explored different dynamical models to determine the likely contributions of the SMC’s mass components – gas, stars and dark matter – all of which have unknown 3D distributions.

Prior to the ASKAP observations, the best HI rotation curve for the SMC was one that had been obtained with the ATCA (Stanimirović et al. 2004). Bekki & Stanimirović (2009) suggested that a dominant dark-matter halo was needed to explain this curve. Teodoro et al. updated the rotation curve (achieving the highest linear resolution, ~10 pc, ever obtained for any extragalactic system), and found that they needed a similarly massive dark-matter halo (1.4 x 109 M within a 4 kpc radius) to reproduce it.

Such a halo implies a baryon fraction of 30–40%. This is significantly larger than the baryon fraction of other dwarf galaxies in the local Universe. It seems that the SMC has either accreted baryonic matter or lost part of its dark matter halo through interactions with the LMC and the Milky Way.

In a side note, Teodoro et al. concluded that MOND (modified Newtonian dynamics) cannot explain the dynamics of the SMC.