Panta rhei: the moving, varying sky of Gaia DR4

Motivation

On 2 December 2026 ESA releases Gaia Data Release 4, based on 66 months of observations (25 July 2014– 20 January 2020). It is not an incremental update of DR3. DR4 contains ≈2.8 billion processed sources, more than 130 data products (over half of them new) and ≈400TB of data. For the first time it publishes the epoch data for all sources: 1.3 × 1011 individual astrometric measurements, 1.2 × 1011 epoch photometric measurements and BP/RP spectra, and 6.9 × 109 RVS spectra and radial velocities. It also includes 5.9 million two-body orbital solutions, a catalogue of exoplanet candidates, variability classifications for 315 million sources, 1.97 million variable AGN, 5.4 million quasar candidates and 8.1 million galaxy candidates, and residual images for 7 million bright sources. Cambridge is unusually well placed to exploit this. The photometric processing of Gaia (CU5), including the BP/RP spectra, is led from the IoA, which hosts one of the six DPAC data processing centres (DPCI) and the Gaia Science Alerts pipeline. At the same time, the potential users of DR4 across IoA, KICC, DAMTP and Cavendish work on stellar physics, Galactic dynamics and dark matter, exoplanets, compact dark remnants and gravitational-wave sources, supermassive black holes and AGN, and the time-domain sky of variable and exotic transient objects. Many of them, particularly students and postdocs, have not used Gaia epoch data before. They will have to decide within weeks of the release which of the new products are fit for their science, and what their caveats are. The meeting will bring these two communities together in the first week after the release. About two thirds of the talks will be given by Gaia experts, who will explain what is in DR4, how it was produced, how to access it and where it should not be trusted. Most of these experts are members of the Cambridge DPAC team. Some products lie outside Cambridge’s DPAC expertise, such as the RVS spectra and the quasar and AGN analysis. Those talks will be given by UK DPAC members or other external experts, or by local users with hands-on experience of the product. About one third will be science-case talks: specific, current problems that DR4 can solve. The DR4 performance-verification papers already point to several topics that match KICC themes. Examples are compact dark remnants — black holes and neutron stars found as dark companions in astrometric orbits, and as lenses in photometric and astrometric microlensing events — exotic transients such as tidal disruption events, changing-look quasars and unusual supernovae, caught by the epoch photometry and the Gaia Science Alerts stream; the brown-dwarf desert and giant planets; Cepheid anchors for the distance ladder; time delays in lensed quasars; quasar proper motions; and searches for continuous gravitational waves in astrometric residuals. 1Aims 1. Give local researchers a working, critical understanding of the DR4 data products, their selection functions and systematics, and how to get at the epoch data. 2. Put potential users in direct contact with the people who built the data products, and form small, cross-disciplinary DR4 projects that continue after the meeting. 3. Give early-career researchers a visible role. We will invite PhD students and early-career postdocs (PDRAs) from across Cambridge astronomy — the IoA, KICC, Cavendish Astrophysics and DAMTP — to attend and to take an active part, and we reserve for them the flash talks, the session chairs and the lead roles in the themed discussion groups.

Organisers: 

Vasily Belokurov (IoA/KICC), Nicholas Walton (IoA; lead of the UK Gaia project, ESA Gaia Science Team), Simon Hodgkin (IoA; head of Gaia Science Alerts), N. WynEvans (IoA; Galactic dynamics and Gaia science), Nikhil Sarin (IoA/KICC; Kavli Senior Fellow, gravitational-wave multi-messenger astrophysics and transients), Sera Markoff (IoA; Plumian Professor of Astronomy, accreting black holes and compact objects) and Júlia Laguna i Miralles (IoA; PhD student, early-career researcher representative)

Invited Speakers:

  • L.Wyrzykowski (Astronomical Observatory, University of Warsaw) — microlensing, Gaia alerts, dark remnants; author of the Gaia microlensing catalogue and a member of the Gaia Science Alerts team
  •  Z. Penoyre (Leiden Observatory; IoA PhD) — astrometric binaries and unresolved companions; natural speaker for the non-single-star products
  • O. Panić (University of Leeds) — planet formation, discs and young stars
  • D. Bashi (Israel; formerly Cambridge) — binaries and compact companions in Gaia astrometric orbit
  • N. Ross (Edinburgh) — quasar populations and changing-look quasars; this is the clearest gap in local expertise

Programme outline