The dominant gaseous structure in the Galactic halo is the Magellanic Stream. This extended network of neutral and ionized filaments surrounds the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), the two most massive satellite galaxies of the Milky Way1,2,3,4. Recent observations indicate that the LMC and SMC are on their first passage around the Galaxy5, that the Magellanic Stream is made up of gas stripped from both clouds2,6,7 and that the majority of this gas is ionized8,9. Although it has long been suspected that tidal forces10,11 and ram-pressure stripping12,13 contributed to the formation of the Magellanic Stream, models have not been able to provide a full understanding of its origins3. Several recent developments—including the discovery of dwarf galaxies associated with the Magellanic group14,15,16, determination of the high mass of the LMC17, detection of highly ionized gas near stars in the LMC18,19 and predictions of cosmological simulations20,21—support the existence of a halo of warm (roughly 500,000 kelvin) ionized gas around the LMC (the ‘Magellanic Corona’). Here we report that, by including this Magellanic Corona in hydrodynamic simulations of the Magellanic Clouds falling onto the Milky Way, we can reproduce the Magellanic Stream and its leading arm. Our simulations explain the filamentary structure, spatial extent, radial-velocity gradient and total ionized-gas mass of the Magellanic Stream. We predict that the Magellanic Corona will be unambiguously observable via high-ionization absorption lines in the ultraviolet spectra of background quasars lying near the LMC.
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The simulation data that support our findings are available at https://github.com/DOnghiaGroup/lucchini-2020-sim/. Source data are provided with this paper.
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E.D. acknowledges the hospitality of the Center for Computational Astrophysics at the Flatiron Institute during the completion of this work.
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Extended data figures and tables
The number density n of gas in the models of the Magellanic Corona (dashed red line) and the Milky Way’s (MW) hot corona (solid red line) is shown as a function of radius r (from the centre of the LMC and Milky Way, respectively). Estimates of the Milky Way’s hot coronal density from observations are shown in black. The dotted and dot-dashed lines are fits to data30,52,53. The data points are labelled with the corresponding references28,67,68,69,70,71, and are the same as those included in previous studies27. Downward (upward) pointing triangles indicate upper (lower) limits. Horizontal lines show uncertainty in radii measurements.
a, Time evolution of the distance between the centre of mass of the LMC and the centre of mass of the SMC. The clouds interact gravitationally for a period of 5.7 Gyr (three close encounters) before falling into the Milky Way potential. b–e, Gas column density Σgas at various times during the mutual interactions between the clouds, with the orbital path of the SMC around the LMC shown as a white line (b, at the initial time; c, after 1.4 Gyr; d, after 4.3 Gyr; e, after 5.7 Gyr; marked in a with dotted vertical lines). The gas tidally removed from the LMC and SMC is displayed in addition to the Magellanic coronal gas.
The gas removed from the Magellanic Clouds after about 5.7 Gyr of mutual interactions (before infall into the Milky Way potential) is shown in Cartesian coordinates projected along the z axis onto the x–y plane. The LMC and SMC are at the centre of each panel. a, b, The gas mass surface density of the gas originating in the disks of the clouds. c, d, The gas temperature averaged along the projection axis. Results are shown for models run with (b, d) and without (a, c) the Magellanic Corona included.
The column density (brightness) and line-of-sight velocity (vLOS; colour scale) for four different models for the formation of the Magellanic Stream are shown in zenithal equal-area coordinates. The white lines mark the location of the Galactic disk in the projection. These four models are the same as those shown in Fig. 3. In all four panels, only the gas originating in the gaseous disks of the Magellanic Clouds is displayed. a, Fiducial model, without the Milky Way’s corona or Magellanic Corona (tidal forces only). b, A Milky Way coronal mass of 5 × 109M☉ is included, but the Magellanic Corona is not present. The leading arm does not survive, in agreement with previous studies27. c, Same as in b, with the total mass of the Milky Way’s hot corona reduced to 2 × 109M☉ (see Extended Data Fig. 1), allowing the leading arm to survive. d, Same as in c, but with the addition of the Magellanic Corona. This model provides the best match to observations.
Observed H i data (a) and results of the model (b) of the Magellanic Stream, with line-of-sight velocity displayed by the colour bar (from −350 km s−1 to 400 km s−1) and brightness indicating the relative gas column density. Gas originating in both the LMC and SMC disks from the model including the Milky Way's hot corona and the Magellanic Corona. The video begins 550 million years ago and continues until the present day. The Milky Way disk and background are extracted from real H i images. See Fig. 2 for more information.
Video showing the past 1.34 billion years of the Clouds’ dynamics shown in Magellanic coordinates. a, The gas column density of the simulated Stream composed of the Magellanic Corona gas and cold disk gas stripped from the Clouds. b, Column density only of the simulated cold gas Stream as compared to H i data, with black, gray, white contours corresponding to the observed column density of 1019, 1020, and 1021 cm−2, respectively.
a, Time evolution of the distance between the center of mass of the LMC and the center of mass of the SMC. The clouds interact gravitationally for a period of 5.7 Gyr (three close encounters) before falling into the Milky Way potential. b, Video of gas column density during the Clouds' mutual interactions. Displayed is the gas tidally removed from the LMC and SMC in addition to the Magellanic Coronal gas.
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Lucchini, S., D’Onghia, E., Fox, A.J. et al. The Magellanic Corona as the key to the formation of the Magellanic Stream. Nature 585, 203–206 (2020). https://doi.org/10.1038/s41586-020-2663-4