11 September 2023 - 13 September 2023

This MW-Gaia COST Action (www.mw-gaia.org) workshop focuses on the central halo of the Milky Way (R<10 kpc). Given its proximity and high stellar density, the inner halo is an ideal target for stellar surveys. However the central halo remains largely unstudied due to overwhelming contamination from the MW disc and high dust extinction. The inner accreted halo is predicted to host the earliest formed and the most primitive building blocks, but also to contain the strongest contribution from various in-situ components. While “unmixing” the inner halo would be ideal, relatively short dynamical timescales and strong perturbations from the Galactic bar make this incredibly difficult. In this workshop, we will discuss the advances that have been made towards characterising the inner halo; focusing on both the accreted and in-situ components, past, present and future tracers of structure and the influence of the Galactic bar in reshaping halo dynamics. We will also discuss what, as a community, we would like to focus on next, through synthesising the perspectives of simulators, theorists and observers. 

This is an interactive meeting. Equal amount of time will be given to the planned presentations and the discussion sessions.

Topics of discussion:

1. Accreted halo. Can we trace the accreted halo component? If so, in what space is it traced best, through chemistry, dynamics or a combination of both? How close to the Galactic Centre does the GS/E debris get? Is there evidence for additional large accretion events like Kraken/Heracles/Koala? Where are the primitive stars from the earliest accreted and smallest dwarfs?
2. In-situ halo. While the exact formation channels contributing to the in-situ halo are still the subject of debate, three different in-situ channels have been proposed to contribute to the inner halo of the Galaxy: the pre-disc Aurora/proto-Galaxy, the splashed disc induced by GS/E and earlier mergers and the star-burst. Can we trace and disentangle all three? What are their properties (total mass, shape, density distributions, metallicity distributions)? What is the contribution from disrupted globular clusters?
3. Bar-halo interactions. How can we use bar-halo interactions to constrain the structure, dynamics and formation of the Galactic bar? When was the bar formed? Is there any connection to the GS/E merger? What are the signatures of the bar interaction in the stellar halo and its sub-structure: stream shepherding, chevron perturbations, spinning up, resonant trapping. 
4. Central MW tracers. RR Lyrae. Red Clump stars. Long Periodic Variables. Metal-poor stars.

Confirmed Invited Speakers:

Jason Sanders (UCL)

Anna Queiroz (AIP)

Anke Ardern-Arentsen (IoA, Cambridge)

Zhen Yuan (Strasbourg Observatory)

Maria Bergemann (MPIA, remote presentation)

Azedah Fattahi (Durham)

Victor Debattista (UCLan)

How to apply

Please note that the attendance is by application only. The participant numbers will be capped.

We will accept applications until August 4th 2023 and will confirm your attendance by August 11th.

SOC & LOC

Stephanie Monty, Anke Ardern-Arentsen, Vasily Belokurov, Elliot Davies, Adam Dillamore, Wyn Evans, GyuChul Myeong, Despina Hatzidimitriou

Logistical details

The meeting will be held at the Institute of Astronomy and the talks will take place in the Lecture Theatre of the Hoyle Building, see the map of the Institute. 

Internet access during the conference will be most easily achieved using eduroam:
https://www.eduroam.org/. For those unable to use eduroam, we provide guest accounts. Please let us know in advance if you need one.

Information on how to reach the city and the Institute of Astronomy may be found here (last updated in 2018). The IoA is a 15-20 minute taxi journey from the railway station, and costs (about £12). To book a taxi at any time try Panther Taxis Ltd (01223 715715) or Camcab (01223 704704). The Whippet Universal bus links West Cambridge to Cambridge (CBG) railway station. The timetable is available at this link. The StageCoach Bus 4 links the city centre to West Cambridge. The timetable is available at this link.

Workshop Schedule (discussion topics subject to change)

Monday, September 11th

 

Azi Fattahi (Durham University) - “Galactic halo and Gaia-Sausage-Enceladus main progenitors in cosmological simulations”

Sergey Khoperskov (AIP, Potsdam) - “Dead man tells tales: MW stellar halo metallicity distribution reveals the past of the GSE-progenitor”

Matthew Orkney (ICCUB, Barcelona) - “The diversity and similarity of Milky Way-like stellar haloes with radially anisotropic massive mergers”

João A. S. Amarante (ICCUB, Barcelona) - “GASTRO library: interpreting substructures of the Milky Way stellar halo with SPH + N-body single merger models”

Danny Horta (CCA, New York) - “The early assembly history of the Milky Way”

Oren Slone (NYU, New York) - “Mechanics of disk tilting as a result of satellite accretion”

Alex Riley (Durham University) - “Mock surveys from cosmological simulations of Milky Way analogues”

1. 9:30 - 11:00 am: Morning Session (Hoyle Lecture Theatre)

• Invited Talk: Azedah Fattahi (~30 mins)
• 2 x contributed talks (Matthew Orkney)
• 30 min discussion. Possible topics: What defines a MW analogue in simulations? Do we have close analogs of MW in any of the simulation suites? Do we need different simulations?

2. 11:00 - 11:30 am: Coffee (Hoyle Lobby)
3. 11:30 - 12:30 pm: Pre-Lunch Session (Hoyle Lecture Theatre)

• 2 x contributed talks (Danny Horta, João A. S. Amarante)
• 30 min discussion. Possible topics: What defines the primordial MW? Is it possible to predict and recover the signature of the primordial MW in simulations and observations? What is in-situ?

4. 12:30 - 2:00 pm: Lunch (Outside Hoyle in the Marquee)
5. 2:00 - 3:30 pm: Afternoon Session (Hoyle Lecture Theatre)

• 2 x contributed talks (Oren Slone, Sergey Khoperskov, Alex Riley)
• ~30 min discussion. Possible topics: What is left to be discovered when it comes to GS/E? Have we pinned down the properties of the GS/E-MW encounter? What role did the encounter play in reshaping the MW?

6. 3:30 - 4:00 pm: Coffee (Hoyle Lobby)
7. Free time to work (Ryle Meeting Room)

Tuesday, September 12th

Maria Bergemann (MPIA, Heidelberg) - “Chemical signatures of in-situ and accreted stellar populations” (remote)

Anna Queiroz (AIP, Potsdam)  - “Chemical tagging of the Galactic components: tracing the stellar populations in the inner Galaxy”

Anke Ardern-Arentsen (IoA, Cambridge) - “Metal-poor stars in the centre of the Milky Way”

Keith Hawkins (University of Texas at Austin) - “Tracing the Old, Metal-Poor Bulge : Milky Way Assembly through Observations of Old Stars”

Nicole Buckley (University of Surrey) - “Dissecting the Stellar Halo Using Chemistry, Extreme Deconvolution and Principal Component Analysis“

Andreia Carrillo (Durham University) - “Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations”

Vedant Chandra (Harvard University) - “Chemodynamically Surveying the Ancient Heart of the Galaxy” (remote)

Kris Youakim (Stockholm University) - “Tidal debris from Omega Centauri discovered with chemo-kinematic tagging”

Boquan Chen (ANU, Sydney) - “Revealing the accretion that fuelled the formation of the Milky Way disk through chemical evolution”

1. 9:30 - 11:00 am: Morning Session (Hoyle Lecture Theatre)

• Invited Talk: Maria Bergemann (~30 mins)
• 2 x contributed talks (Kris Youakim, Erwin Chen)
• 30 min discussion. Possible topics: Best chemical signatures to disentangle in-situ an accreted populations. Globular cluster chemical signatures.

2. 11:00 - 11:30: Coffee (Hoyle Lobby)
3.  11:30 - 12:30 pm: Pre-Lunch Session (Hoyle Lecture Theatre)

• Invited Talk: Anna Queiroz (~30 mins)
• 30 min discussion. Possible topics: When does chemical tagging work best and why? Any new chemical tags that we did not know about?

4. 12:30 - 2:00 pm: Lunch (Outside Hoyle in the Marquee)
5. 2:00 - 3:30 pm: Afternoon Session (Hoyle Lecture Theatre)

• Invited Talk: Anke Ardern-Arentsen
• 2 x contributed talks (Vedant Chandra, Keith Hawkins)
• 30 min discussion. Possible topics: What are the primary drivers of chemical diversity in the low-metallicity regime? What information is encoded in the chemical abundance scatter?

6. 3:30 - 4:00 pm: Coffee (Hoyle Lobby)
7. 4:00 - 5:00 pm: Extended Session (Hoyle Lecture Theatre)

• 2 x contributed talks (Nicole Buckley, Andreia Carrillo)
• 30 min discussion. Possible topics: What chemical information is missing from cosmo sims to help us compare observations with models of MW formation? Or vice versa, what information is missing from observations?

WORKSHOP DINNER AT 7PM AT THE MILLWORKS

Wednesday, September 13th

Jason Sanders (UCL, London) - “Stellar populations of the inner Galaxy: structure, kinematics, age and chemistry”

Marcin Semczuk (University of Barcelona) - “Kinematic fractionation of the Milky Way boxy/peanut bulge seen in variable stars”

Steve Ardern (University of Bath) - “CO in Cepheids – A route to reduce the dispersion on the Cepheid Leavitt Law (LL) for the Milky Way, and to obtain metallicities for large numbers of Cepheids”

Victor Debattista (University of Central Lancashire) - “Bulge/bar formation in numerical simulations”

Elliot Davies (IoA, Cambridge) - “Accelerated phase-mixing in the stellar halo due to a rotating bar”

Adam Dillamore (IoA, Cambridge) - “Stellar halo substructure generated by bar resonances”

Zhen Yuan (Strasbourg Observatory) - “Tracing the low-metallicity planar stars under a rapidly slowing down bar”

Akshara Viswanathan (Kapteyn Institute, Groningen) - “Trapped in the disk: constraining the galactic bar properties using planar metal-poor stars”

1. 9:30 - 11:00 am: Morning Session (Hoyle Lecture Theatre)

• Invited Talk: Jason Sanders (~30 mins)
• 2 x contributed talks (Marcin Semczuk, Steve Ardern)
• 30 min discussion. Possible topics: How does the structure and kinematics of the bar depend on chemistry and age? Is there a classical bulge in the MW?

2. 11:00 - 11:30: Coffee (Hoyle Lobby)
3. 11:30 - 12:30 pm: Pre-Lunch Session (Hoyle Lecture Theatre)

• Invited Talk: Zhen Yuan (~30 mins)
• 1 x contributed talk: (Akshara Viswanathan)
• 15 min discussion. Possible topics: Are there discernible trends in the sub-structure properties on moving from the outer halo to the inner MW and the disk?

4. 12:30 - 2:00 pm: Lunch (Outside Hoyle in the Marquee)
5. 2:00 - 3:30 pm: Afternoon Session (Hoyle Lecture Theatre)

• Invited Talk: Victor Debattista (~30 mins)
• 2 x contributed talks (Elliot Davies, Adam Dillamore)
• 30 min discussion. Possible topics: When and how was the MW’s bar created? Interactions between the bar and the stellar halo.

6. 3:30 - 4:00 pm: Coffee (Hoyle Lobby)
7. 4:00 - 5:00 pm: Final discussion
8. Free time to work (Ryle Meeting Room)

 

Code of conduct

The meeting is held under the IoA code of conduct. Please read this and make sure you understand and agree to it before you come to the meeting. Please contact the organisers if you feel that the code of conduct is not being applied consistently throughout the meeting or in the associated activities.

Acknowledgements

This workshop has been supported by COST Action CA18104: MW-Gaia