Galactic Archaeology

Galactic Archaeology reconstructs the formation history of galaxies from the positions, motions, ages, and chemistry of their stars. Its promise is that the physical conditions of the early Universe, at the high redshifts when the Milky Way first began to form, can be inferred from nearby stars in the unrelaxed remnants of its tidally disrupted satellites. This is possible because low-mass stars live for nearly a Hubble time, preserving a fossil record of the systems in which they formed. My work uses large stellar surveys to identify merger debris, map the growth of the Milky Way, and connect present-day Galactic structure to past accretion and star-formation events.

• Assembly history of the Milky Way.
• Stellar halo substructure and merger debris.
• Links between stellar populations, dynamics, and galaxy formation.

The field of Galactic Archaeology has undergone a step change thanks to Gaia's unprecedented, remarkably rich data. In this review, Alis Deason and I summarize the major recent advances that Gaia has made possible.

Gaia

Gaia provides the astrometric backbone for much of modern near-field cosmology. I use Gaia data, often combined with photometric and spectroscopic surveys, to measure stellar motions, discover coherent structures, and build dynamical views of the Milky Way and its satellites.

• Gaia-enabled mapping of the Milky Way disc, halo, and satellites.
• Combination of Gaia with SDSS, DES, APOGEE, and other surveys.
• Discovery and interpretation of phase-space substructure.

But Gaia is much more than this. The extraordinary breadth and richness of its data are showcased in Michael Perryman's series of Gaia science essays.

The New Astronomy Reviews special issue Gaia, the first crop of discoveries also collects in-depth discussions of what Gaia has revealed:

• Milky Way dynamics in light of Gaia
• Stellar streams in the Gaia era
• Galactic Archaeology with Gaia
• The Gaia white dwarf revolution
• How Gaia sheds light on the Milky Way star cluster population
• Gaia's binary star renaissance

Milky Way in Numerical Simulations

Numerical simulations provide a controlled way to connect the present-day Milky Way to the violent events that shaped it. Modern cosmological hydrodynamical simulations now make it possible to follow the coupled evolution of stars, gas, and dark matter, and to interpret the relics seen in the Galactic halo and disc.

• Cosmological context for the Milky Way's merger history.
• Response of the stellar halo, disc, and dark-matter halo to major accretion events.
• Comparison between simulated Milky-Way analogues and Gaia-era observations.

Simulations reveal what may have happened during the disruptive events whose echoes we see today in the Milky Way halo. For example, the last significant merger, Gaia-Sausage/Enceladus, not only reshaped the inner stellar halo of our Galaxy but also wreaked havoc on the disc. Simulations suggest that, during the same period, the Milky Way underwent a profound transformation: its dark-matter halo was reshaped and realigned, while the stellar disc was likely flipped and tilted.

Numerical simulations are also vital for identifying gaps in our understanding of galaxy formation through tensions with the latest observations. For example, the Galactic disc emerged earlier than it does in current simulation suites. This may reflect the unusual formation path of the Milky Way among galaxies of similar mass.

Stellar Streams

Stellar streams are tidal debris from disrupted satellites and clusters. They trace Galactic dynamics, reveal the mass distribution of the Milky Way, and preserve evidence of past interactions.

• Discovery and characterization of tidal streams.
• Streams as probes of the Galactic potential.
• Perturbations caused by satellites, dark matter substructure, and the Magellanic Clouds.

A current snapshot of the Galactic stellar-stream population is given in the review by Ana Bonaca and Adrian Price-Whelan.

Stellar streams are among the most sensitive probes of the dark-matter distribution. In the Milky Way, they can be used not only to measure how the three-dimensional shape of the dark-matter halo changes with radius, but also how the halo evolves with time.

One of the most exciting prospects for stellar-stream characterization and modelling is the possibility of detecting low-mass dark-matter subhaloes through the small-scale perturbations they leave on streams.

Stellar Streams beyond our Galaxy

Low-surface-brightness streams around external galaxies show how galaxies grow through accretion. I am interested in using deep imaging and modern detection methods to find and interpret tidal debris beyond the Milky Way.

• Streams and shells around nearby galaxies.
• Low-surface-brightness structure in wide-field imaging.
• Comparisons between Milky Way archaeology and external-galaxy archaeology.

This field is about to be transformed by the deep, wide data from Rubin's LSST and Euclid. A first glimpse of what is possible can be found in this study.

We searched for clear examples of long stellar streams around galaxies in the Local Universe, taking advantage of the unique imaging products supplied by the DECaLS survey. This work defines the STRRINGS sample. We are now preparing the first results from population-level modelling of STRRINGS extragalactic streams, including constraints on the dark-matter haloes of their host galaxies.

Dwarf Galaxies

Dwarf galaxies are laboratories to study galaxy formation, dark matter, and satellite accretion. My work includes discovery, characterization, and interpretation of faint satellites and diffuse galaxies around the Milky Way and nearby systems.

• Discovery of faint and diffuse satellites.
• Dwarf galaxy structure, dynamics, and tidal evolution.
• Connections between dwarf galaxies, stellar streams, and the Galactic halo.

The Milky Way is special because only here can we identify and study the faintest dwarf galaxies, the so-called ultra-faint satellites, in detail. These systems make up much of the Galactic satellite census and are central to resolving the long-standing missing-satellites problem. Yet we have only begun to chart this population. Ultra-faint dwarfs remain enigmatic, and Rubin's LSST offers the best near-term prospect for discovering more of them and building a more complete picture of their origin and nature.

Globular Clusters

Globular clusters record both in-situ star formation and accretion events. Their orbits, stellar populations, and tidal debris help connect individual clusters to the broader assembly history of the Milky Way.

• Globular cluster orbits and association with Galactic components.
• Cluster debris and tidal tails.
• Globular clusters as tracers of accreted structures.

Aurora, the oldest in-situ Milky Way population, captures the Galaxy before it formed a stable stellar disc. Its metal-poor stars show large chemical scatter and little ordered rotation, pointing to a turbulent, bursty pre-disc phase. The excess of nitrogen-rich stars strengthens this picture: in the nearby Universe, such stars are common as second-generation members of globular clusters but extremely rare in the field, implying that massive bound clusters contributed substantially to the young Galaxy. Similar nitrogen-loud signatures now appear in some high-redshift JWST galaxies, linking Aurora, globular clusters, and the clumpy, bursty conditions under which the first Milky-Way-like systems formed.

Galactic Bar

The Galactic bar shapes the dynamics of the inner Milky Way and leaves signatures across the disc and halo.

• Structure and dynamics of the Milky Way bar.
• Bar-driven resonances and orbital structure.
• Connections between the bar, disc, and inner halo.

The coupling between the bar and the stellar halo remained under-explored until recently. It is now clear that the halo is rich in bar-induced substructure. On the other hand, the bar also helps erase substructure in the inner halo. This is exciting because it reopens the hunt for halo substructure in the near future, for example with Gaia DR4, using techniques that account for bar-driven dispersal.

We have long known that the bar affects stellar motions in the disc, but all-sky spectroscopy from surveys such as APOGEE, combined with Gaia orbital information, is now revealing the scale of that influence. One recent example argues that the bar may help set up the well-known chemical bimodality of the Galactic disc.

Binary Stars

Gaia has made it possible to map binary-star populations with unprecedented scale, precision, and uniformity. Astrometry, photometry, spectroscopy, and time-domain information now give complementary views of how binaries form, evolve, and shape the observed stellar population.

• Astrometric detection and characterization of unresolved binaries.
• Binary fractions and population trends in large homogeneous samples.
• Binary pathways to evolved stars, compact remnants, and other end products of stellar evolution.

We have used Gaia astrometry to detect binaries and measure their incidence across large, homogeneous samples. We also study the end products of stellar evolution in binaries, where binary interaction can leave distinctive signatures in the data.

Gaia also provides repeat radial-velocity measurements, allowing binary properties to be studied more directly than with astrometry alone and without the same distance limitations. We have used RVS data in these recent Gaia RVS studies of binary fractions and multiplicity in halo stars, globular clusters, and wide-binary systems.