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Streams & Halo Stars


1. Introduction

The mock data in this section are designed to mimic cold/hot streams in the Milky Way halo and the halo star population. Key questions are:

(1) What can we learn about the gravitational potential from such cold streams or halo stars? (2) Can we find clear evidence for triaxiality? (3) Can we find any evidence for granularity or substructure in the potential?

If posting new tests, please try to approximately follow the template set out for the “spherical collisionless tests” here. Also please add your challenge to the table in the next section.

Key working group coordinators: Robyn Sanderson, Andreas Küpper, Andreea Font

Shaping the Gaia Catalogue (A note from Grainne Costigan)

In order to ensure the Gaia archive meets the need of the community we are looking for feedback from the future archive users. Some feedback has already been collected through the GREAT network and is listed on the following website. Further input is encouraged.

This is part of the GENIUS project set up to maximise the impact of Gaia through providing more advanced tools with the archive i.e. data mining tools and interactive visualization. A small introduction to the project can be found here

For any further information please feel free to send me an email (cosign AT, and I can try and help or pass the information on to the relevant people.

2. Finding streams in the halo

Challenge: How many tidal stellar streams will be detected by Gaia? Use a series of N-body simulations of Milky Way-type galaxies to predict the number and properties of streams to be detected by Gaia in the stellar halo.

Simulations available: Robyn Sanderson's halo of streams, NEW (Aug/2015) Error-convolved Gaia- mock catalogues for K giants and RR Lyrae stars from the Aquarius haloes and haloes from LJMU's Gas-dynamical simulations.

Methods: Great circle method (Cecilia Mateu). Please add your methods here, and add a detailed description on the Methods page.

Realistic Detections of streams

Here we provide some catalogues of streams identified in an automated search of the Aquarius A2 error-convolved mock catalogue using the great circle + proper motion version of the mGC3 method (nGC3, Mateu et al. in prep). These could be used as inputs for the ‘Determining the Potential’ Challenges to explore the effect of errors, biases, incompleteness and contamination.

The catalogues provided correspond to five peaks identified in nGC3 pole count maps, labeled IDpole*. Each file has all the stars associated to each pole detection and so contains stars from the identified stream as well as contaminants from other progenitors. The five detections have different levels of purity ranging from ~70% down to ~10%, and the streams have different morphologies from great-circles to shells. The available files are:

The catalogue files are named AqA2KIII.nGC3.IDpole#.IDstream#.dat, where IDpole refers to the label assigned to each peak detected in the pole count map. IDstream is the progenitor ID for the stream that best corresponds to the IDpole detection. The structure of the catalogues is the following:

  • Av, V, Gmag, Grvs : V-band extinction, V-band, G and Grvs apparent magnitudes
  • xX,xY,xZ,xVX,xVY,xVZ : error-free cartesian galactocentric coordinates (kpc) and velocities (km/s)
  • xl_deg,xb_deg,xRhel_kpc,xmulcosb_masyr,xmub_masyr,xvrad : error-free observables (l,b, heliocentric distance in kpc, proper motions in mas/yr, radial velocities in km/s)
  • gX,gY,gZ,gVX,gVY,gVZ : error-convolved cartesian galactocentric coordinates and velocities.
  • gl_deg,gb_deg,gRhel_kpc,gmulcosb_masyr,gmub_masyr,xvrad : error-convolved observables
  • IDpole: pole detection label in pole-count map.
  • IDstream: progenitor ID. Stars with same IDstream come were accreted in the same progenitor.

For a detailed description of how these catalogues were produced, please go to the Aquarius Error-convolved mocks Section in this Wiki. The file AqA2KIII.nGC3.IDpole.log is a log that contains IDstream, IDpole and purity. Purity is the fraction of stars in IDpole that come from the progenitor IDstream. Frec is the fraction of stars from the progenitor IDstream that lie in the IDpole detection. The full pole-count map showing the IDpole detections is available here.

The figures below show X-Z (left) and X-Y (middle) plots for each of the 5 streams, as well as their corresponding nGC3 pole-count map (right).

These are provisional files provided for Gaia Challenge purposes (potential fitting and stream finding challenges). The final sets for all haloes will be published in Mateu et al. in prep.

Questions and Publication Policy

When making use of data in this part of the challenge, please cite the Gaia Challenge Wiki and Mateu et al. in prep. For any questions please feel free to contact Cecilia Mateu ( For details about the Aquarius and LJMU simulations please contact Andrew Cooper ( and Andreea Font ( respectively.

Detection of halo streams in the local halo

We provide three catalogs based on high-resolution N-body simulations of the interaction between a Milky-Way like disk galaxy within a dark matter halo and orbiting dwarf galaxies with four representative cases of merging histories in term of dynamical friction properties (see figure).

These catalogs include the 3D position and 3D velocities of the accreted particles within 3 kpc of the Sun after 4.63 Gyr:

* Gaia-like errors for FGK dwars

* current ground-based errors

* 'true' values

The format of the ASCII files is:

cols 1-6, x,y,z (kpc), vx,vy,vz (km/s), in the galactic reference frame
col 7, particle id, as follow:
   id=31 counter-rotating satellite, high inclination (60 deg)
   id=32 prograde-rotating satellite, high inclination (60 deg)
   id=33 counter-rotating satellite, low inclination (10 deg) 
   id=34 prograde-rotating satellite, low inclination (10 deg) 
   id=1  local smooth halo (mock catalog)

Further details are given in Re Fiorentin et al. (2015). The N-body simulations are from Murante et al. 2010.

For any questions please feel free to contact Alessandro Spagna ( or Paola Re Fiorentin (

3. Determining the Potential


  1. Compare different codes without any errors
  2. Use python code to simulate Gaia errors (see here)
  3. Check consistency for
    1. spherical halo
    2. axissymmetric potential
    3. triaxial halo
    4. N-body sims
  4. Standardised outputs of tests: mass profiles, acceleration, potential
  5. Test for: changes with assumed distance, orbit, number of streams, number of stars/stream, with or without errors

Index of Challenges

Below are listed a number of different challenges posed by various groups. These explore various ways of complicating the problem. Here we organize the different challenges based on four characteristics: the type of potential (“Easy” = spherical, “Medium” = axisymmetric or triaxial, “Complex” = from a dark matter cosmological simulation), whether Gaia uncertainties are included, whether self-gravity of the disrupting satellites is included, and the number of streams in the sample. Links in the first column take you to the full description further down the page.

Challenge Potential Uncertainties Self-gravity # streams References
Spherical Halo Easy no, yes no 1, 20, 153
Evolving and static streams Easy + time evol. yes no 15; Buist et al. (in prep)
Sagittarius stream Medium no, yes yes 1 Law & Majewski, 2010, ApJ, 714, 229; Price-Whelan et al. (2014)
Pal 5 Medium no, yes yes 1 Küpper et al., in prep.
Via Lactea Complex no yes 4 Diemand et al., 2008, Nature, 454, 735; Bonaca et al. (2014)
Aquarius Complex no, yes yes 10, full halo Cooper et al, 2010; Lowing et al., 2014.; Wang et al, in prep, Mateu et al. in prep
Gas-dynamical sims Complex no, yes yes full halo Font et al, in prep; Wang et al, in prep., Mateu et al. in prep
Progenitor & host DM profile Medium no, yes yes 1 per data set Errani, Penarrubia & Tormen 2015

See the challenges page for a detailed description of the Challenges.

4. Resources

Please, post your coordinate converters, etc. here!

Paul McMillan's coordinate converter is one of the many things in his Torus code. See the executable Coord_converter. It's on Github here:

Andreas Küpper's simple coordinate converter (called coco; in C) can be found here:

Cecilia Mateu's stream finder, mGC3: [NEW features: pole-count map plotting utilities available]

Merce Romero's Gaia Errors Fortran executable and subroutine (July 23rd, 2015): [NEW feature: simulate errors for any Data Release.]

5. Ideas for Publications

From Robyn: Effect of Gaia errors on recovery of a simple potential (“Spherical Halo” challenge): WriteLatex document

From Andreas: For contributing to the paper on the Pal 5 challenge get the repository from

6. Ideas for Gaia Challenge 2015

  1. Doing challenges
  2. Collecting results
  3. Including errors
  4. Including foreground/background model
tests/streams.txt · Last modified: 2022/10/24 12:10 by