Galactic and Extragalactic High Velocity Clouds

High-Velocity Clouds (HVCs), those assemblies of gas around galaxies that do not follow the galactic rotation, have been enigmatic since their discovery almost 60 years ago. Recent observational and theoretical advances have clarified their properties and importance to galaxy formation and evolution. To explore this topic further, a Green Bank Workshop on “Galactic and Extragalactic High-Velocity Clouds” will be held June 26-29, 2022. It will bring together scientists with diverse interests for lively discussions of the latest observations, computational models, and theoretical considerations.

The Workshop will be held at the Green Bank Observatory, where the great natural beauty and intimate setting produce meetings characterized by a high level of interaction. Because of logistical considerations, attendance will be limited to 45 participants.


Lodging will be at the Observatory or nearby locations. All meals will be served in the Observatory Cafeteria. A $550 per participant fee will cover all local expenses, including lodging and meals. It may be possible to waive this cost for some of the participants.

Accommodations for accompanying persons and child care will be available.

Travel Information

The easiest way to get here is to follow the directions on our “Getting Here” page. Please note that some GPS systems do not always give the best route to the Green Bank Observatory, and nothing beats the good old-fashioned paper map for navigation.

We will attempt to coordinate ride-sharing from airports if there is interest.

Workshop Schedule

DateStart TimeDurationTopic/Title
26-June 2022 SundayPeople arrive throughout the afternoon and evening
17:301:00Dinner in the Cafeteria
18:30Informal Reception in the Drake Lounge
27-June  Monday7:451:00Breakfast in the cafeteria
Session 1 — Overview8:450:15Welcome: Jim Jackson  and LOC
Chair: Bob Benjamin9:000:45M. Putman: High Velocity Clouds
9:450:45J. Werk: HVCs and the Circumgalactic Medium
10:500:30N. Lehner:  Discovery and Properties of HVCs Through Absorption Spectroscopy
11:200:45A. Fox: Magellanic HVCs
12:050:20Y. Zheng: The Mocky Way: Investigating Biases in Observing HVCs in the Milky Way
End of session 112:25
12:300:45Lunch in Cafeteria
Session 2 — GBT Tour13:152:15GBT Tour
Session 3 —  Origins and measurements15:300:30F. Fraternali: How to make HVCs via fountain-driven condensation (remote)
Chair: Will Armentrout16:000:30A. Marasco:  Collimated outflows and diffuse inflows traced by ionised HVCs and IVCs at the disc-halo interface
16:300:15Coffee and group photo
16:450:25E. Di Teodoro: HVCs in the nuclear wind of the Milky Way
17:100:25B. Wakker: Metalicities of HVCs
17:350:25T. Tripp: The High-Velocity Clouds Above the Disk of the Outer Milky Way: Misty Precipitating Gas in a Region Roiled by Stellar Streams
End of session 318:00
18:001:00Drinks and Dinner in the Science Center
Session 4: Panel Discussion19:001:30Evening Panel in Science Center Auditorium: The Origin of HVCs — Ionized and Neutral
   Ashley, Howk, Marasco, Tripp
20:30Retire to the Drake Lounge for continued Discussion
28 June Tuesday7:450:45Breakfast in the cafeteria
Session 5 — The Magellenic Streams8:300:25S. Lucchini: A new orbital history for the Magellanic Clouds
8:550:20E. Chwalik: Mapping the Edge of the Magellanic Stream
9:150:20K. Barger: The LMC’s Galactic Feedback
9:350:20D. Nidever: Recent Star Formation in the Magellanic Stream Leading Arm
9:550:20F.J. Lockman: The Smith Cloud
End of session 510:150:20Coffee Break
Session 6 — Theoretical Issues10:350:40A. Gronnow: The effect of magnetic fields on High-Velocity Clouds
11:150:20P. Oh: Thermal Instability and Mixing Layers
11:350:20B. Tan: Radiative Turbulent Mixing Layers in Cold Clouds
11:550:20G. Bryan: The structure of turbulent radiative mixing layers and the cloud/CGM interaction
12:300:45Lunch in the Cafeteria
13:150:20L. Armilotta: The impact of cosmic rays on the dynamics of the extra-planar gas
13:350:20R. Shelton: Simulations of dark-matter Rich HVCs
End of session 613:550:20F. Heitsch:Metallicity Estimates of HVCs: Insights from Simulations
14:150:20Coffee Break
Session 714:350:20L. Jung: Search for magnetic field associated with high-velocity clouds using Faraday rotation
14:550:20A. Hill: Polarized radio emission from the anti-centre shell: Interaction between an HVC and the Galactic magnetic field
15:150:20D. French: A FUSE  HI survey of the Galactic Halo
15:350:30Poster viewing session
16:050:20B. Benjamin: Complexity in the Outer Galaxy
16:250:20T. Minter: No OH in the Smith Cloud (remote)
16:450:20A. Smith: Intermediate Velocity Molecular Clouds
End of session 717:30
17:300:30Drinks in the Lounge
18:001:00Dinner in the cafeteria
Session 819:001:30Panel in the Jansky Lab — Theoretical issues in HVCs:
   Armillotta, Byon, Gronnow, Oh
20:30Discussion continues in the Drake Lounge
29-June Wednesday7:451:00Breakfast in the Cafeteria
Session 9 — HVCs from the nuclear wind0:30R. Bordoloi: The UV perspective on the nuclear wind
9:150:20T. Ashley: Metallicities of Outflowing Fermi Bubble Clouds
9:350:20D. Krishnarao: Anomalous Velocity Gas near Galactic Center
9:550:20F. Cashman: Molecular Gas within the Milky Way’s Nuclear Wind
Session 10 — Extragalactic HVCs10:150:20J.C. Howk: Project AMIGA: H I in a Sea of H II in Andromeda’s Halo
10:350:20Coffee Break
10:550:25DJ Pisano: The HI Clouds surrounding M31
11:200:25H. Gim: Anomalous HI clouds in M100
Session 11: Panel11:450:45HVCs: The Path Forward
Workshop End12:30 Lunch in the Cafeteria

Topics to be considered include:

  • The origin of HVCs
  • New observations
  • Magnetic, ionized, metal, and dark matter components
  • The Magellanic Stream
  • HVCs in other galaxies
  • HVCs effect on our Galaxy and its CGM
  • HVCs as probes of their environments
  • The fates of HVCs
  • The connection between nuclear winds and HVCs
  • Intermediate Velocity Clouds and their Connections to HVCs

Scientific Organizing Committee

  • Kat Barger (TCU)
  • Joss Bland-Hawthorn (Univ. Sydney)
  • Andy Fox (STScI)
  • Filippo Fraternali (Univ. Groningen)
  • Jay Lockman (Green Bank Observatory)
  • Naomi McClure-Griffiths (Australian National Univ.)
  • Mary Putman (Columbia Univ.)
  • Robin Shelton (Univ. Georgia)

Registered Participants

Will ArmentroutGreen Bank Observatory
Lucia ArmillottaPrinceton University
Dr. Trisha AshleySpace Telescope Science Institute
Kat BargerTexas Christian University
Bob BenjaminUniversity of Wisconsin
Rongmon BordoloiNorth Carolina State University
Greg BryanColumbia University
Francie CashmanSpace Telescope Science Institute
Erica ChwalikMontana State University
Enrico Di TeodoroJohns Hopkins University
Andy FoxSTScI
David M. FrenchSTScI
Hansung GimMontana State University
Eric GoetzUniversity of Georgia
Asger GronnowKapteyn Institute, University of Groningen
Fabian HeitschUNC Chapel Hill
Alex S. HillUBC Okanagan / DRAO
Chris HowkUniversity of Notre Dame
Jim JacksonGreen Bank Observatory
Lyla JungAustralian National University
Dhanesh Krishnarao (DK)Johns Hopkins University / Colorado College
Glen LangstonNational Science Foundation
Nicolas LehnerUniversity of Notre Dame
Jay LockmanGreen Bank Observatory
Scott LucchiniUniversity of Wisconsin – Madison
Antonino MarascoINAF – Padova Observatory
Jennifer Donovan MeyerNRAO
Toney MinterGreen Bank Observatory
David NideverMontana State University
D.J. PisanoUCT/WVU
Mary PutmanColumbia University
Evan SchneiderUniversity of Pittsburgh
Nigel SharpNational Science Foundation
Dr. Robin SheltonUniversity of Georgia
Allison SmithArecibo Observatory
Brent TanUCSB
Todd TrippUniversity of Massachusetts – Amherst
Ananya TuliPhD candidate at the University of Notre Dame
Jo VazquezTexas Christian University
Bart WakkerUniversity of Wisconsin-Madison
Chen WangUniversity of Georgia
Jessica WerkUniversity of Washington
Ellie WhiteGreen Bank Observatory
Yong ZhengUC Berkeley

Local Organizing Committee

  • Brenne Gregory
  • Sue Ann Heatherly
  • Jay Lockman
  • Pedro Salas
  • Ellie White