The Milky Way Stellar Halo Is Actually Tilted And Oblong

By James Anderson •  Updated: 11/20/22 •  7 min read

The actual shape of the diffuse star cloud that envelops the disk of our galaxy has been revealed by a recent study. For many years, astronomers believed that the stellar halo, a cloud of stars, was mostly spherical, like a basketball.

A new model based on recent observations shows that the stellar halo is long and tilted, like a football that has just been kicked. The findings shed light on a variety of astrophysical topics.

The research results, for instance, shed light on our galaxy’s past and its evolution while also providing hints for the ongoing search for the enigmatic substance known as dark matter.

Rewriting The Stellar Halo Textbook

milky way stellar halo

H3 Survey sky coverage as of 2022 May in Galactic coordinates. Fields observed are shown in blue symbols (not drawn to scale).
Credit: Han et al, DOI 10.3847/1538-3881/ac97e9 CC-BY

The shape of the stellar halo is a very important factor that has just been measured with more precision than ever before.

“There are a lot of important implications of the stellar halo not being spherical but instead shaped like a football, rugby ball, or zeppelin, take your pick,”

said lead author Jiwon Jesse Han.

“For decades, the general assumption has been that the stellar halo is more or less spherical and isotropic, or the same in every direction. We now know that the textbook picture of our galaxy embedded within a spherical volume of stars has to be thrown out,”

added co-author Charlie Conroy, professor of astronomy at Harvard University and the Center for Astrophysics.

Galactic Halo Scaffolding

The visible part of what is more generally referred to as the galactic halo is the stellar halo of the Milky Way. Dark matter dominates this galactic halo, and the gravity it produces is the only way to detect its presence. Each galaxy has a unique dark matter halo.

These haloes act as a kind of scaffold on which common, visible matter is suspended. Stars and other observable galactic structure are created as a result of that visible matter. Stellar haloes are therefore valuable astrophysical targets for better understanding how galaxies form and interact, as well as the underlying nature of dark matter.

Understanding the shape of the Milky Way’s stellar halo, on the other hand, has long been a source of consternation for astrophysicists, for the simple reason that we are embedded within it. The stellar halo extends several hundred thousand light years above and below our galaxy’s star-filled plane, where our Solar System is located.

“Unlike with external galaxies, where we just look at them and measure their halos, we lack the same sort of aerial, outside perspective of our own galaxy’s halo,”

explained Han.

Diffuse And Uneven Stars

stellar halo shape

Isodensity surface contours of the fiducial ellipsoid model evaluated at flattened radius rq = 20 kpc. The major, intermediate, and minor axes of the ellipsoid are, respectively, drawn in blue, red, and green. The solar location is marked with a red star, displaced 8 kpc away from the Galactic center. An artist impression of the Galactic disk (NASA/JPL-Caltech/R. Hurt 2008) is overplotted to orient the reader
Credit: Han et al, DOI 10.3847/1538-3881/ac97e9 CC-BY

To make matters worse, the stellar halo has been shown to be quite diffuse, containing only about 1% of the mass of all the stars in the galaxy.

However, astronomers have been successful in identifying many thousands of stars that populate this halo, which are distinguishable from other Milky Way stars due to their distinct chemical makeup (as determined by studies of their starlight) as well as their distances and motions across the sky.

Astronomers have discovered that halo stars are not evenly distributed as a result of such research. The goal has since been to study the patterns of star over-densities — which appear as bunches and streams in space — to determine the ultimate origins of the stellar halo.

The new study by Center for Astrophysics researchers and colleagues makes use of two large datasets gathered in recent years that have delved into the stellar halo in unprecedented depths.

Gaia And Hectochelle in the Halo at High Resolution

The first set comes from the ground-breaking Gaia spacecraft, which the European Space Agency launched in 2013. The positions, motions, and distances of millions of stars in the Milky Way, including some nearby stellar halo stars, have been meticulously measured by Gaia.

The second dataset comes from a ground-based survey called H3 (Hectochelle in the Halo at High Resolution), which was carried out at the Multiple Mirror Telescope observatory at the Fred Lawrence Whipple facility in Arizona and was a joint effort between the CfA and the University of Arizona. Tens of thousands of stellar halo stars that are too far away for Gaia to evaluate have been carefully observed by H3 in great detail.

The decidedly non-spherical halo was produced by combining these data in a flexible model that allowed the stellar halo shape to emerge from all of the observations; the football shape fits in well with previous findings. For example, the shape fits well with a leading theory about how the stellar halo around the Milky Way formed.

According to this model, the stellar halo formed when a lone dwarf galaxy collided with our much larger galaxy 7-10 billion years ago.

The GSE Origin Story Model

The departed dwarf galaxy is humorously referred to as Gaia-Sausage-Enceladus (GSE), where “Gaia” refers to the aforementioned spacecraft, “Sausage” for a pattern observed when plotting Gaia data, and “Enceladus” for the Greek mythological giant who was entombed by a mountain, similar to how GSE was entombed by the Milky Way.

As a result of this galactic collision event, the dwarf galaxy was torn apart and its constituent stars were dispersed throughout a dispersed halo. This origin story explains why stellar halo stars are fundamentally distinct from stars born and raised in the Milky Way.

The results of the study further document how GSE and the Milky Way interacted eons ago. The football-shaped ellipsoid, which is technically known as a triaxial ellipsoid, reflects observations of two star clusters in the stellar halo.

Galactic Collision Evidence

The apparent formation of the pileups occurred when GSE completed two orbits of the Milky Way. During these orbits, GSE would have slowed twice at apocenters, or the furthest points in the dwarf galaxy’s orbit of the greater gravitational attractor, the massive Milky Way; these pauses resulted in the extra shedding of GSE stars.

In the meantime, the inclination of the stellar halo indicates that GSE encountered the Milky Way at an angle rather than head-on.

The stellar halo’s tilt and distribution of stars provide substantial evidence that our galaxy collided with another smaller galaxy 7-10 billion years ago, according to Conroy.

Notably, so much time has passed since the GSE-Milky Way collision that it would have been reasonable to expect the stellar halo stars to dynamically settle into the traditional, long-assumed spherical shape.

Dark Matter Structural Pull

The team asserts that the fact that they haven’t speaks to the larger galactic halo. Due to the gravitational pull of this dark matter-dominated structure, which is likely itself out of alignment, the stellar halo is also kept that way.

“The tilted stellar halo strongly suggests that the underlying dark matter halo is also tilted. A tilt in the dark matter halo could have significant ramifications for our ability to detect dark matter particles in laboratories on Earth,”

said Conroy.

Conroy’s final point makes reference to a number of ongoing and upcoming dark matter detector experiments. If astrophysicists can determine where the substance is more densely concentrated, these detectors may have a better chance of capturing an elusive interaction with dark matter.

As Earth moves through the Milky Way, it will occasionally encounter these regions of dense and faster-moving dark matter particles, increasing the chances of detection.

The identification of the stellar halo’s most likely configuration has the potential to advance numerous astrophysical studies and provide fundamental information about our place in the cosmos.

Reference: Jiwon Jesse Han et al, The Stellar Halo of the Galaxy is Tilted and Doubly Broken. The Astronomical Journal. 164 249DOI 10.3847/1538-3881/ac97e9

Keep Reading