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The Magellanic Corona and the formation of the Magellanic Stream

The dominant gaseous structure in the Galactic halo is the Magellanic Stream, an extended network of neutral and ionized filaments surrounding the Large and Small Magellanic Clouds (LMC/SMC), the two most massive satellite galaxies of the Milky Way. Recent observations indicate that the Clouds are on their first passage around our Galaxy, the Stream is made up of gas stripped from both the LMC and the SMC, and the majority of this gas is ionized. While it has long been suspected that tidal forces and ram-pressure stripping contributed to the Stream's formation, a full understanding of its origins has defied modelers for decades. Several recent developments, including the discovery of dwarf galaxies associated with the Magellanic Group, the high mass of the LMC, the detection of highly ionized gas toward stars in the LMC and the predictions of cosmological simulations all support the existence of a halo of warm ionized gas around the LMC at a temperature of $\sim5\times10^{5}\;\mathrm{K}$. Here we show that by including this"Magellanic Corona"in hydrodynamic simulations of the Magellanic Clouds falling onto the Galaxy, we can simultaneously reproduce the Stream and its Leading Arm. Our simulations explain the Stream's filamentary structure, spatial extent, radial velocity gradient, and total ionized gas mass. 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. This prediction is directly testable with the Cosmic Origins Spectrograph on the Hubble Space Telescope.

of highly ionized gas toward stars in the LMC 14,15 suggest the existence of a halo of warm ionized gas around the LMC. Here we show that by including this "Magellanic Corona" in our hydrodynamic simulations of the Magellanic Clouds falling onto the Galaxy, we can simultaneously reproduce the Stream and its Leading Arm. Our simulations explain the Streams filamentary structure, spatial extent, radial velocity gradient, and total ionized gas mass. 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. This prediction is directly testable with the Cosmic Origins Spectrograph on the Hubble Space Telescope.
Our Galaxy is accompanied by two fairly massive dwarf galaxies, the Large and Small Magellanic Clouds, and a massive gaseous tail trailing behind them, the Magellanic Stream. The Stream is an interwoven tail of filaments pulled out of the Magellanic Clouds (MCs) in their orbit around the Milky Way (MW) 1,2,16 . When considered together with its Leading Arm (LA) -the Stream's counterpart in front of the MCs -the Stream stretches over 200 degrees on the sky (see Fig. 1a) 17 .
With a total mass of ∼ 1 − 2 × 10 9 M (consisting of ∼ 2 × 10 8 M neutral hydrogen and the remainder in ionized gas) 6,7 the Magellanic Stream dominates all the other gas clouds in the Galactic halo, both in terms of gas mass and gas inflow rate. Therefore understanding the Stream is essential to a global picture of the Galaxy's circumgalactic medium 12 .
The current paradigm of Stream formation is known as the first-infall model 8,18 . In this scenario, tidal forces from the LMC acting on the SMC when the Clouds are at their first pericentric passage around the MW lead to the formation of the Stream. This model is motivated by the high tangential velocities of the Clouds 3 and the strong morphological disturbances observed in the 2 SMC 8,9,16 . This model successfully reproduces the size and shape of the Stream, but several difficulties remain 12 : (i) the observed Stream is significantly more extended spatially and a factor of up to ten more massive than the simulated Stream, especially when including its ionized component, which dominates the mass budget 6 ; (ii) the fragmented structure of the Stream and Leading Arm indicates that the interaction with the MW gas corona plays a significant role and cannot be ignored; (iii) the Stream is bifurcated, with kinematic and chemical analyses indicating that gas from both the LMC and SMC is present 2,4,5 . This indicates that the Stream has a dual origin, whereas tidal models predict an SMC origin because of the shallower potential well of the SMC.
The discovery of several ultra-faint dwarfs around the LMC 19 indicates that the LMC and SMC likely entered the MW recently as part of a system of dwarf galaxies (the Magellanic Group), with the LMC as its largest member 20,21 . Given the LMC dark matter halo mass of ∼ 2 × 10 11 M 13 , the virial temperature is ∼ 5 × 10 5 K, so the Magellanic Group is expected to contain a warm gas corona at this temperature. Furthermore, cosmological simulations of MW-sized galaxies with LMC-like dwarf satellites 22 predict the existence of ionized gas halos surrounding those satellites 23 .
The presence of an LMC corona is also motivated by detections of absorption from highly-ionized carbon in "down-the-barrel" spectroscopic observations of hot stars in the LMC 14,15 . Such coronae are likely kept warm via energy input from stellar feedback and outflows.
Here we show that by including this "Magellanic Corona" in hydrodynamic simulations of Stream formation, the mass budget discrepancy of the Stream can be solved. Crucially, we reproduce the ionized component for the first time. The Magellanic Corona appears to be the key missing ingredient in models of Stream formation. The new simulations shown here have been run with GIZMO Hydrodynamic N-Body code including radiative cooling and heating, star formation, 3 and stellar feedback to model the LMC-SMC-MW dynamics including the Magellanic Corona (see Methods). During the initial stages of the LMC-SMC tidal interaction, the pair lie outside the MW's gravitational influence. The cold gas in the extended disk of the SMC is tidally stripped through repeated encounters with the LMC (as illustrated in Extended Data Figs. 2 and 3b) that occur over a period of 5.7 Gyrs. Because the model includes more massive and more extended disks for the Clouds than previous studies 8 , these repeated orbits of the SMC around the LMC also result in gas extraction from the LMC 9 by dwarf-dwarf galaxy interaction. However, this process acting on both Clouds only contributes 10-20% of the total Stream mass. Corona is therefore a source of pressure, heating, and mass.
Once the Clouds fell into the MW and the MW hot corona, the Stream was amplified by the MW potential until it extended over 200 degrees in the sky, with both leading and trailing components. Fig. 1 shows the Stream displayed at the present time in zenithal equal area (ZEA) projection in the numerical experiment (Fig. 1b) as compared to the observed Stream ( Fig. 1a) 17,24 .
The MW hot corona included in this model has a total mass of ∼ 2 × 10 9 M and does not rotate  (Fig. 1a). Whereas previous models found the gas to be moving ∼100 km s −1 faster than observations in the LA 9 and slower in the Stream 8 , the inclusion of coronal gas decelerates the LA to better match the observed velocity gradient. However the cold gas column density in this region is smoother than in observations, which indicate the LA is clumpy and fragmented 16,17 (see Fig. 2b). This additional source of ionized gas has not been accounted for in previous theoretical work and reconciles the Stream's mass budget.
Another outcome of the model concerns the survivability of the Stream and its Leading Arm in the presence of a MW hot corona. H I studies 16,25 show that the Leading Arm is fragmented, as expected by simulations of its passage through the Galactic halo 26 , but yet it still survives. However, recent hydrodynamic simulations have challenged the overall survivability of the Leading Arm when the MW hot corona is included 27  The inclusion of the Magellanic Corona is further supported by a recent estimate of the ambient gas density near the LA 28 . Following the discovery of stars formed in-situ in the LA 29 , a recent study 28 reports that the density of coronal gas required to separate these young stars from their proposed gaseous nursery (the region known as LA II) is an order of magnitude higher than existing measurements of the MW coronal density 30 . This discrepancy can be resolved by taking the Magellanic Corona into account, because the Magellanic Corona can add to the MW corona to yield the high total density needed to ram-pressure-strip the LA II region away from the nascent stars.
An additional consequence of this model is a possible explanation for the lack of a stellar component of the Stream. In tidal models, stars (in addition to gas) should be stripped from both Clouds due to the gravitational interactions they experience before falling in to the MW. Such a stellar stream awaits discovery, even though sensitive searches have been conducted. However 6 in our model, the Stream is mostly formed by the warm Magellanic Corona, therefore its stellar counterpart is negligible. Some stars were tidally stripped from the SMC when the Clouds were far from the MW, but they are either phase-mixed with the MW stellar halo or extended into a thin and low-density filament of 30 mag arcsec −2 , which is too faint to detect with current telescopes and instrumentation.
The Magellanic Corona will be unambiguously observable via absorption in highly-ionized   Initial set up and simulations.
We created a set of N-body and hydrodynamic simulations of gaseous and stellar exponential disks embedded in a live NFW (NavarroFrenkWhite) dark matter halo of Magellanic-sized galaxies 35 .
The LMC progenitor galaxy has a total dark matter halo mass of 17.75  Runs performed with the LMC outer disk truncated to various radii produce comparable results.
However, for the case reported in this study, the filamentary structure of the Trailing Arm from gas tidally removed from the LMC is present but more tenuous and less pronounced as compared to previous work where the LMC disk was not truncated 9 .
The Magellanic Corona is set up as a halo of warm gas surrounding the LMC, with a mass of ∼ 3×10 9 M (∼1.5% of the LMC total mass) and extends throughout the virial radius of the LMC (∼100 kpc). Even though the LMC is a satellite galaxy, it is still massive enough (with total mass > 10 11 M 39, 40 ) to carry a group of dwarfs that includes the SMC, Carina and Fornax 22 and several additional ultra-faint dwarfs 41 . Hence its hot corona should be at least 10 9 M in mass 23,42 . A less massive LMC (∼ 5 × 10 10 M as inferred from the rotation curve within 8 kpc from the center 3 ) would not harbor a warm corona and such an LMC would not be massive enough to carry the bright dwarfs as the observations suggest. Cosmological simulations confirm these estimates [43][44][45] , and dwarf galaxies in the field have been shown to have circumgalactic gas extending out to a significant fraction of their virial radii [46][47][48] . Furthermore, a conservative observational estimate of the MW suggests that the circumgalactic gas is at least ∼1% of the total Galactic mass. The observed mass in baryons (stars and the interstellar medium) constitutes ∼10% of the total mass, and it is proposed that the other half of the baryons be found in the hot corona 49 . In addition, absorption line studies show that the mass of the circumgalactic gas inside the virial radius is similar to the stellar mass 30,50 . Therefore, the total mass of the Magellanic Corona adopted in this work (1.5% of the LMC mass) should be considered a lower limit. In kT where m is the mean mass per particle, k is Boltzmann's constant, and for T half the virial temperature was assumed.
We note that at T ∼ 5 × 10 5 K, the Magellanic Corona is above the peak range of the cooling curve. Although the gaseous coronae in our models are relatively stable due to the inclusion of radiative heating and cooling, star formation and feedback, there may be additional physical pro-cesses included in cosmological simulations 23,51 , such as AGN feedback, photoionization heating, and cosmic ray heating, that affect the stability and temperature of the circumgalactic gas [54][55][56] .
In addition, a gas corona was set up around the Milky Way assuming an isothermal sphere of gas at T = 1.6 × 10 6 K (the Galactic virial temperature) using the DICE code 57  Subsequently, the same model assumed for the Clouds is carried out with the inclusion of a highdensity (Extended Data Fig. 4b) or low-density MW hot gas corona (Extended Data Fig. 4c,d).
This experiment allowed us to determine that the Leading Arm survives in this model if the MW gas corona has a density of n ∼ 1.7 × 10 −5 cm −3 at a distance of 50 kpc, in agreement with the observational estimates 53 and previous studies 27 . The final run included the model of the Clouds with the inclusion of both the Magellanic Corona and the MW hot halo (Extended Data Fig. 4d).

Analysis.
A particle tracer that allows us to follow each gas particle with its temperature and density was employed to compute the mass of the Magellanic Stream. In these numerical experiments the Stream consists of gas particles stripped from the Clouds that are no longer bound to the main body of their host galaxy. The gravitational potential and its kinetic energy were calculated for each gas particle. Any particle that has a larger kinetic than potential energy was considered unbound. We then projected the locations of gas particles stripped from the Clouds into Magellanic Coordinates and summed up the masses. Based on the locations of the gas particles, they were included either in the Leading or the Trailing Stream. The pygad 64 library was adopted to perform density and temperature calculations and to deposit the particles onto a mesh for visualization. The model does not include the ionization corrections to convert the hydrogen gas into the ionized fraction.
The cold gas stripped from the Clouds is assumed to trace the HI component, whereas the warm coronal gas is assumed to trace the ionized mass.