Stellar Streams in the Gaia Era (2405.19410v1)

Abstract: The hierarchical model of galaxy formation predicts that the Milky Way halo is populated by tidal debris of dwarf galaxies and globular clusters. Due to long dynamical times, debris from the lowest mass objects remains coherent as thin and dynamically cold stellar streams for billions of years. The Gaia mission, providing astrometry and spectrophotometry for billions of stars, has brought three fundamental changes to our view of stellar streams in the Milky Way. First, more than a hundred stellar streams have been discovered and characterized using Gaia data. This is an order of magnitude increase in the number of known streams, thanks to Gaia's capacity for identifying comoving groups of stars among the field Milky Way population. Second, Gaia data have revealed that density variations both along and across stellar streams are common. Dark-matter subhalos, as well as baryonic structures were theoretically predicted to form such features, but observational evidence for density variations was uncertain before Gaia. Third, stream kinematics are now widely available and have constrained the streams' orbits and origins. Gaia has not only provided proper motions directly, but also enabled efficient spectroscopic follow-up of the proper-motion selected targets. These discoveries have established stellar streams as a dense web of sensitive gravitational tracers in the Milky Way halo. We expect the coming decade to bring a full mapping of the Galactic population of stellar streams, as well as develop numerical models that accurately capture their evolution within the Milky Way for a variety of cosmological models. Perhaps most excitingly, the comparison between the two will be able to reveal the presence of dark-matter subhalos below the threshold for galaxy formation (~10^6 Msun), and provide the most stringent test of the cold dark matter paradigm on small scales.

• The paper demonstrates Gaia’s transformative role by unveiling over 100 stellar streams and mapping their precise kinematics.
• It leverages comprehensive astrometric data to trace tidal remnants, confirming predictions of hierarchical galaxy formation.
• Findings offer critical constraints on dark matter substructure through observed density variations within stellar streams.

Overview of "Stellar Streams in the Gaia Era"

The paper "Stellar Streams in the Gaia Era," authored by Ana Bonaca and Adrian M. Price-Whelan, explores the transformative impact of the European Space Agency's Gaia mission on our understanding of stellar streams within the Milky Way galaxy. Consistent with the hierarchical model of galaxy formation, which predicts that galaxy halos are populated by the remnants of tidally-disrupted dwarf galaxies and globular clusters, the paper documents the advances made possible by Gaia in tracing these stellar streams.

The hierarchical galaxy formation model posits that large galaxies such as the Milky Way grew through accretion and merging of smaller systems. Stellar streams are the debris of these ancient mergers, providing key insights into the Galaxy's formation history and its dark matter content. The data from the Gaia mission has enabled a dramatic increase in the detection and characterization of these streams. It has provided precise measurements of the position, motion, and brightness of nearly two billion stars, leading to the discovery and description of more than a hundred stellar streams in the Milky Way, significantly increasing the previous count.

Key Advances from Gaia Data

1. Discovery and Characterization: Prior to Gaia, streams were discovered through photometric data, often supplemented by kinematic data from other sources. Gaia’s all-sky astrometric catalog now allows identifying streams based on their kinematic coherence, transforming astrometric data into a primary tool for discovery and characterization.
2. Density Variations: Gaia has revealed common density variations within streams, offering observational evidence for their interaction with subhalo dark matter and Galactic baryonic structures. Observing these variations provides empirical support for theoretical predictions of interactions shaping stellar streams.
3. Stream Kinematics: The availability of proper motions and radial velocities for streams allowed the precise determination of their orbits and constraints on their origins. Gaia has facilitated spectroscopic follow-up, leading to richer kinetic descriptions of stream members.

Implications and Future Prospects

Dark Matter Paradigm

The position and velocity space information on streams acts as a sensitive tracer of the Galactic gravitational field, offering a unique probe of the dark matter distribution within the Milky Way. Gaia data allows astronomers to constrain the presence and properties of dark matter subhalos—structures too small to host their luminous galaxies—providing a precise test of the small-scale predictions of the Cold Dark Matter paradigm.

Galactic Archaeology

With Gaia enabling a near-complete census of stellar streams, theoretical models of galaxy formation and evolution can be rooted in more empirical data. By comparing numerical simulations with observed stream features, the community anticipates unparalleled constraints on models of dark halo substructure and the universe's broader cosmological framework.

Challenges and Speculations

A thorough understanding of the observational biases and the establishment of a uniform, probabilistic framework for stream membership, kinematics, and chemistry are critical to leveraging these data into robust astrophysical insights. The upcoming years are expected to focus on refining techniques to differentiate between various perturbative effects on stellar streams, such as those induced by baryonic components of the Galaxy versus those by dark matter subhalos. As the Gaia mission continues, the comprehensive, precise data it provides will remain a pivotal foundation upon which models of galaxy evolution are expanded and tested.

In summary, the paper solidifies Gaia's role as an unparalleled catalyst in transforming our understanding of stellar streams, emphasizing profound implications for both the structural evolution of the Milky Way and the nature of dark matter.