Finding fire-breathing dragons in the Milky Way

Title: The Spur and the Gap in GD-1: Dynamical Evidence for a Dark Substructure in the Milky Way Halo

Authors: Ana Bonaca, David W. Hogg, Adrian M. Price-Whelan, and Charlie Conroy

First Author’s Institution: Harvard-Smithsonian Center for Astrophysics

Status: Published in AAS Journals

Suppose I told you that a fire-breathing dragon lives in my garage, but I cannot show you the dragon because it is floating, invisible, and spits heatless fire. You would not believe that I had any dragons at all. Carl Sagan, the creator of this analogy, argues that my claim of the dragon only makes sense if there is some experiment that could disprove it. In other words, scientific claims have to be testable. Now take a look at the theory of dark matter: astronomers say that the Milky Way is full of invisible blobs of dark matter called subhalos, which only interact with normal matter through gravity. However, this claim sounds a lot like the invisible dragon in my garage unless there is some way to observe the effects of those subhalos.

You would expect the dragon in my garage to leave evidence of footprints or breathing fire.  One way to detect the invisible subhalos, used by the astronomers of today’s paper, is by observing stellar streams. Stellar streams are groups of stars that have been stretched out on their orbit in the outer region of the Milky Way. If a subhalo flies into a stellar stream, the gravitational interactions can rip a hole in the stellar stream. The disrupted stars fly away from the hole and observers on Earth would see them piled up into a spur (see Figure 1). This method was previously used on the Pal 5 stellar stream, but the evidence was not conclusive to prove that the stream was disturbed by a subhalo. The authors today have the advantage of a clear view of the GD-1 stream from the Gaia space telescope, as described in this Astrobite. In the Gaia data, a spur and a gap are clearly visible in the stream, which points to possible interactions with a dark matter subhalo.

The top panel shows the observed positions of stars in the GD-1 stream, where a spur and gaps are visible. The bottom panel shows a model of the stream's history which includes perturbations from a dark matter subhalo.

Figure 1. Top: Positions of stars in the GD-1 stream, observed by the Gaia space telescope. The spur and gaps are labeled with arrows. Both axes indicate the projected sky position of stars along and perpendicular to the stream orbit. Bottom: Positions of stars in a model where GD-1 was perturbed by a dark matter subhalo 495 million years ago (subhalo parameters shown in the legend). These two panels are in excellent agreement. From Fig. 1 in today’s paper.

The high spatial resolution and precision of the data allows the authors to create a model of the orbital history of GD-1. The motion of the stars is determined by the gravitational field throughout the orbit, which, in this case, is the well-studied Milky Way gravitational field plus any potential perturbers, such as dark matter subhalos, molecular clouds, and globular clusters. Thus, the map of the stellar stream encodes useful information about past interactions. The authors ran a suite of simulations, changing the mass and velocity of the perturber, how far away it was at closest approach, and the time of the encounter. The code used to calculate the orbit of stars is publicly available for interested readers.

The best-fit parameters used to construct the final model are shown in the bottom panel of Fig. 1. In this scenario, a dark matter subhalo with 5 million solar masses came within 15 pc of the stellar stream, at a velocity of 250 km/s, and this event happened 495 million years ago. This dense, massive, high-velocity flyby gave the stars a velocity kick, which made a gap. The perturber also kicked the stars perpendicular to the stream motion and set some stars on a loop around the original unperturbed orbit, producing the spur when viewed in projection. Is this excellent agreement with observational data a sign of the elusive dark matter dragon? 

The authors ruled out the possibility that the perturber is a known object. They traced the known orbits back in time for Milky Way globular clusters, satellite dwarf galaxies, and the Milky Way disk. No known object came close enough to GD-1 to produce the observed spur and gap. Thus, the authors conclude that a dark matter subhalo is the most probable perturber that caused the spur and the gap.  

Whilst this evidence is compelling, the authors want other independent ways of confirming the nature of the perturber. They highlight that this hypothesis is testable via measuring the radial velocities of the stars. The authors matched their models to observations using spatial position alone, which means the accepted models can have the stars at the same location but moving with different radial velocities. Future data from the Hubble Space Telescope can observe the radial velocity of stars in this stream, and that provides a test for the different perturber models.

This paper used simulations to show that the observed spur and gap in GD-1 are most likely caused by dark matter subhalos. The authors demonstrated an exciting avenue to find the invisible subhalos, and future research may find out more properties of these subhalos and compare them to the predictions of dark matter theory. Perhaps the dark matter dragon isn’t so elusive after all.

Astrobite edited by Catherine Manea and Keir Birchall

Featured image credit: Ana Bonaca

About Zili Shen

Hi! I am a Ph.D. student in Astronomy at Yale University. My research focuses on ultra-diffuse galaxies and their globular cluster populations. Since I came to Yale, I have worked on two "dark-matter-free" galaxies NGC1052-DF2 and DF4. I have been coping with the pandemic and working from home by making sourdough bread and baking various cookies and cakes, reading books ranging from philosophy to virology, going on daily hikes or runs, and watching too many TV shows.

1 Comment

  1. Notice that the perturber seems to have a lesser than expected radius for the predicted mass. And as the article says: “the high inferred density might point to dark matter physics beyond CDM.” That’s cool!

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