How Did the Milky Way’s Halo Form? Designing the HALO7D-X Survey
In this paper, Miranda Apfel and collaborators describe the design and goals of the HALO7D-X survey, which aims to uncover how the Milky Way’s stellar halo -- the sparse, extended population of stars surrounding our galaxy -- was assembled over time. The authors use simulations of galaxy formation to test how well their survey can reveal the masses, orbits, and accretion history of the small galaxies that merged into the Milky Way.
Introduction: Tracing the Milky Way’s Past
The Milky Way’s halo formed in part from smaller galaxies that were torn apart and absorbed into it. Evidence of this process includes streams and clumps of stars -- remnants of those disrupted galaxies -- which have been discovered both in the Milky Way and in other galaxies like Andromeda (M31). Because stars in the halo move slowly and hold on to the memory of where they came from, their positions, motions, and chemical compositions provide clues about their original galaxies and when they were accreted. Previous surveys have revealed many stellar streams and even evidence for a major merger event about 10 billion years ago. However, much of the distant halo remains poorly understood. The HALO7D-X survey was designed to probe this outer region and figure out what types of galaxies built the Milky Way’s halo and when they joined the galaxy.
The HALO7D-X Survey
HALO7D-X is an extension of an earlier project called HALO7D. It focuses on gathering seven pieces of information -- 3D positions, 3D motions, and two chemical abundances -- for distant halo stars. To do this, the team combines data from the Hubble Space Telescope (HST), the Gaia satellite, and the Keck Observatory. These data cover 30 selected regions of the sky, chosen carefully from archival HST images to ensure there are enough halo stars in each field for analysis. Compared to the original HALO7D, the new survey includes many more fields, taking advantage of a method that combines HST and Gaia data to measure the proper motions (sideways motions) of faint stars much more accurately than Gaia alone. The final survey is designed to observe at least 15 halo stars in each line of sight.
Testing the Survey with Simulated Halos
To predict how well HALO7D-X will work, the authors use computer simulations of Milky Way–like halos. These simulations, called the Bullock & Johnston (B&J) halos, model how stars from smaller galaxies are distributed in the halo after accretion, based on their mass, orbit, and the time of their merger. The simulated stars include realistic motions, chemical compositions, and brightnesses, and the team creates “mock” HALO7D-X observations from these simulations.
By comparing these mock observations to the full simulations, the authors test whether their survey can correctly identify which simulated halo the data came from and which features of the accretion history (like the masses of the progenitor galaxies) can be recovered.
How the Data Are Analyzed
To analyze the data, the team builds a detailed statistical model. They divide the observable properties of stars (like their velocity and chemical abundances) into grids, and calculate how likely it is that a given star comes from each of the 11 simulated halos. They then combine these probabilities across all the stars and fields in the mock survey to estimate which simulated halo (or combination of halos) best matches the data. By repeating this process many times and testing different survey designs (with varying numbers of stars and fields), the team evaluates how much data is needed to reliably distinguish between different halo histories.
Results: What Can HALO7D-X Reveal?
The authors find that HALO7D-X is sensitive to two main features of the Milky Way’s accretion history: the distribution of progenitor galaxy masses and the timing of their mergers. Halos that formed from many small, early mergers look very different in the data from those that grew through a few large, late mergers. Interestingly, the survey is not very sensitive to the orbital circularity -- whether the merging galaxies came in on circular or radial orbits -- likely because the survey fields are too small to capture the full shapes of the resulting stellar streams.
By comparing their mock surveys to the simulations, the authors group the 11 simulated halos into three categories based on similarities in their observed properties and underlying accretion histories: (A) halos formed from many small, early mergers, (B) halos built from fewer, more massive and later mergers, and (C) halos dominated by a single massive merger. HALO7D-X is well-suited to distinguish between these categories.
Summary and Outlook
The HALO7D-X survey promises to improve our understanding of how the Milky Way assembled its halo. By combining HST, Gaia, and Keck data over 30 carefully chosen lines of sight, the team will collect detailed seven-dimensional information on distant halo stars. Using simulations, the authors show that the survey can identify whether the halo formed through early, small mergers or later, more massive ones -- shedding light on the Galaxy’s long and complex formation history.
Source: Apfel
