In November 2023, a team of astronomers announced in the journal Nature the fabulous discovery of the first known rotating gaseous structure – a planet-forming accretion disk – around an extragalactic massive young stellar object (MYSO) in the remarkable N180B complex (NGC 2122). The discovery was made possible using the high sensitivity and angular resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) in the high Chilean desert (5000 meters).  Lead astronomer Anna McLeod from the Centre for Extragalactic Astronomy at Durham University in the UK, explains the significance of the find –

“When I first saw evidence for a rotating structure in the ALMA data I could not believe that we had detected the first extragalactic accretion disc, it was a special moment,” says Anna McLeod. “We know discs are vital to forming stars and planets in our galaxy, and here, for the first time, we’re seeing direct evidence for this in another galaxy.”

Artist impression of the HH 1177 system with bipolar jets and rotating accretion disk

The lifecycle of massive stars can be summed up as “Live fast and die hard”. The star goes through the early accretion phase quickly – making detection and study difficult – and rapidly burns through its nuclear fuel supply before ending its life in a titanic supernova explosion.  Although protoplanetary disks surrounding MYSOs have been detected around Milky Way stars at radio wavelengths, they are buried within the surrounding natal dust cloud which obscures the disk and star in the optical range.  But despite the added challenge of spatially resolving an object at a distance of 163,000 light-years, two factors about the LMC – its metallicity and dust content – aided in detecting the disk and jets known as HH 1177.

“Lower metal content may have been a boon in finding the circumstellar disk around HH 1177,” McLeod explained. “The lower metal content leads to higher stellar and gas temperatures, and the combination with the lower dust content resulted in the star being visible in the optical,” McLeod says. “Usually, these types of stars are deeply embedded in their natal material, hidden from sight. But because it has formed in an environment that is different from that of the Milky Way, it is not embedded and we can see it.”

The 18th-magnitude central star, which is catalogued as [CGC2008] N180 24 in SIMBAD, is ~12 times as massive as the Sun.  It formed at the tip of a pillar-like molecular cloud structure at the south end of the spectacular emission nebula N180B.  The gas motions found using ALMA show a radial flow of material falling towards from larger distances onto a central disk-like structure, which exhibiting signs of Keplerian rotation (a spinning accretion disk feeding the growth of the central star and likely forming planets).  But this wasn’t a serendipitious discovery…

Accretion disks around nascent stars are associated with parsec-scale Herbig-Haro jets and the HH 1777 jets were also discovered by McCloud’s team in early 2019.  Using the Multi-Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope (VLT), the jet – which spans a total of 10 parsecs (nearly 33 light-years) – is externally ionized and was detected as blue- and redshifted emission peaks in the H-alpha line (indicating motion towards and away from us).  Bow-shocks were also detected just beyond the jets, making HH 1777 the second known extra-galactic HH object and one of the largest known jets in general.  Even more surprising, the jet-driving MYSO was optically visible at the origin of the blue and red jets.

The new observations analyzed the kinematics of the gaseous disk and found the location of the highest velocity gas was near the central star. This verified the molecular lines of the gas traced a rotating gaseous structure (following the Keplerian laws of motion) around the MYSO and revealing an extragalactic accretion disk around the only optically revealed high-mass YSO!

Astronomers have just succeeded in imaging the first close-up view of an extragalactic star!  WOH G64 is a highly massive and luminous Red Supergiant (RSG) experiencing significant mass loss.  The unusual designation is from a 1981 list of supergiant and giant M-type stars in the LMC by Westerlund, Olander, and Hedin.  In 1986, the obscured star was found to be encased in thick dust shell which was assumed to absorb and reradiate ~75% of the star’s luminosity in the near-infrared.

But the spectral energy distribution (SED) couldn’t be explained by a simple spherical dust shell.  In 2008, Keiichi Ohnaka and colleagues presented a model that reproduced the SED using an optically thick silicate torus that was viewed nearly pole-on.  And using the ESO’s Very Large Telescope Interferometer (VLTI) on Paranal Mountain in Chile, they were able to spatially resolve the star and torus, the first for a stellar source in an extragalactic system!  When interviewed, Ohnaka explained,

“Everything is huge about this system. The star itself is so big that it would fill almost all the space between the Sun and the orbit of Saturn and the torus that surrounds it is perhaps a light-year across! Still, because it is so far away, only the power of interferometry with the VLT could give us a glimpse on this object.”

The following year, Emily Levesque, Philip Massey, Bertrand Plez, and Knut Olsen used data gathered with the 6.5-meter Magellan telescopes at Las Campanas, Chile to fit the spectrum of WOH G64 to a M5 star – the latest-type RSG (coolest) in the LMC. Their analysis also pointed to an extremely large star 1540 times the size of the Sun – easily the largest in the LMC!

Now, Ohnaka has led another team that just published (October 2024) the first detailed interferometric image of an RSG outside the Milky Way! Using the highly-sensitive, second-generation GRAVITY interferometric instrument of the VLTI, they imaged the innermost circumstellar environment of the dust-enshrouded star in the near-infrared.  The faint ring-like structure in the reconstructed image might be interpreted as the inner rim of the dust disk or torus, though additional confirmation is required. They were also surprised to find the star has dimmed since the observations in 2008, which may be explained by hot dust forming close to the star. Ohnaka summarized the fantastic finding in a press release on November 21, 2024.

“We discovered an egg-shaped cocoon closely surrounding the star and we’re excited because this may be related to the drastic ejection of material from the dying star before a supernova explosion.”

WOH G64 is located at 04 55 10.5 -68 20 30, just 8.4′ south of the bright LMC cluster NGC 1755. It’s not an isolated star, though it doesn’t reside in a Lucke-Hodge OB association. As an RSG, it’s also a long-period variable, with a V magnitude range of just 17.7 to 18.8 due to visual extinction.

The discovery history of the Magellanic Stream starts in 1965 when radio-astronomer Nan Dieter found 21-cm (radio) emitting, high-velocity neutral H I clouds near the South Galactic Pole. In 1972, Peter Wannier and Gregory Wrixon made further observations with increased sensitivity and a broader frequency range. These clouds were immense! A narrow H I filament extended 60° across the sky with radial velocities up to –400 km/second.  But they suggested the emission originated nearby in our local spiral arm.

Just a year later in 1973, Australian astronomer Don Mathewson had a Eureka moment. When he plotted the H I ribbon and extrapolated it beyond the South Galactic Pole, it passed near the Magellanic Clouds!  Furthermore his calculations predicted the filament’s velocity should match the Clouds at that position.  Mathewson was excited by this prediction, but needed verification. He was fortunate to immediately gain access to the Parkes 64-meter (201-ft) radio dish in New South Wales. Although he initially sampled only 20 points, a clear connection was made with the Magellanic Clouds.  After further mapping the filament’s structure with a smaller 6-ft dish at Parkes, the 1974 discovery paper announced, “This filament, which follows very closely a great circle over its entire 180° arc across the sky, is given the name “the Magellanic Stream”.”

The entire structure has three distinct components:

1)  The Magellanic Bridge, a mainly gaseous (H I), but also stellar, bridge connecting the LMC and SMC. It is thought to have formed during an encounter between the Clouds about 200 million years ago.   Most of the stars and gas in the Bridge indicate an origin in the SMC, though some young clusters have formed in situ after its formation.  The initial eastern extension of the SMC towards the LMC is also called the “SMC Wing”.  NGC 796, a young, massive SMC cluster, lies near the interface of the SMC Wing and the Magellanic Bridge.

2)  The ginormous Trailing Stream extends over 100° and is the largest extragalactic structure in the sky!  It consists of two distinct gaseous filaments (a main SMC filament and a LMC filament perhaps related to the 30 Doradus Cluster) that appear intertwined and probably formed from past Cloud tug-of-war tidal interactions.  Despite efforts to detect a corresponding stellar component, none was found for over five decades. Recently (2020), 15 stars were identified near the tip of the Trailing Stream (with distances of 40 to 80 kpc from the center of the Milky Way) that may be debris from past Cloud interactions.

3)  The Leading Arm comprises clumpy and fragmented high-velocity gas clouds ahead of the LMC and SMC in their orbits.  It actually connects to the plane of the Milky Way and extends above it.  Since it leads the orbital motion of the Magellanic Clouds this implies a Cloud tidal origin, rather than tidal stripping by the Milky Way.  In 2018, a team led by Andrew J. Fox made chemical abundance measurements using the Hubble Space Telescope and found similar abundances with the SMC filament of the Trailing Stream.  So, the SMC has lost this tug-of-war! Also in 2019, a young, low-mass stellar association was discovered using GAIA data (Price-Whelan 1). Located at the outskirts of the Milky Way halo but near the edge of the Leading Arm, the cluster may have formed from gas in the Leading Arm that was compressed when encountering the Milky Way’s corona.

For a complete summary of the discovery of the Magellanic Stream, our current state of knowledge, and still open questions, check out Scott Lucchini’s paper posted on the Astrophysics preprint server on November 18th, 2024 at https://arxiv.org/pdf/2410.14772

In a recent study, a team of astronomers discovered that the LMC has lost most of its diffuse circumgalactic medium (CGM) or “corona” during its recent closest approach to the Milky Way. The researchers used twenty-eight line-of-sight quasars to indirectly detect the remaining halo gas through absorption lines of the distant light. Their results indicate that ram pressure by the Milky Way’s halo has stripped away most of the original mass of the LMC’s corona into a trailing gas stream, leaving a truncated halo 50,000 light-years across (10 times smaller than galaxies with a similar mass.) The LMC’s loss is the Milky Way’s gain, as this gas will eventually rain down onto the Milky Way’s halo.

The Hubble Site has more information on this galactic theft, or you can read the preprint of the study here.