Besançon Galaxy Model

Besançon model of stellar population synthesis of the Galaxy
The model of stellar population synthesis built by our team for three decades is used to elaborate a global view of the Galaxy including dynamical and evolutionary aspects. Scenarii for the formation and evolution produces theoretical distribution functions which are directly compared with survey observations of different types (photometry, kinematics, abundance distributions).
We link the kinematical and dynamical point of view to an evolution scheme through a key parameter: the stellar ages. The age distribution of stars in the solar neighbourhood is derived from a model of galactic evolution. The stellar populations of the galactic disc are self-consistently (locally) constrained by the Boltzmann and Poisson equations through the potential of the mass model. Thus we directly derive observational predictions from an overall description of galactic structure and evolution.
A complete description of the model inputs can be found in Robin, Reylé, Derrière, Picaud (2003) ADS. Since then, substantial changes have been performed. Their description is given below.

The new version

Several important updates have been done recently, concerning
  • the bar population, described in Robin, Marshall, Schultheis, Reylé (2012) ADS.
  • the thick disc and halo populations have been revised in Robin et al, (2014) ADS.
    The thick disc is modelled by 2 isochrones of different ages and metallicities to mimic a long star formation period of 2 Gyr. The bar population is the same as in version 2012, but the extra "thick bulge" population has been removed since the thick-disc short scale length now produces a significant contribution in the bulge region, leading to a negligible contribution from a classical bulge.
  • the thin disc population is the same as in 2012 version with the bar. A revised version described in Czekaj et al, 2014 (ADS), with new evolutionary tracks, star formation history and initial mass function, taking into account binarity, will be released in a near future.
  • this new version implements the computation of parallaxes and adds observational errors on spectroscopic parameters.
  • The new version allows computation on alpha over iron abundance ratios from simple hypotheses in link with metallicity, Galactic position and population
  • The extinction can be modeled by different ways:
    • It can be modeled by a diffuse thin disc. This is not reliable for the galactic plane where a more sophisticated 3D extinction model should be used instead.
    • It can use the 3D map from Marshall et al (2006) ADS
    • The user can include an extinction distribution along the line of sight of his own.
  • Several photometric systems are available for producing the catalogues (UBVRIJHKL, Megacam+Wircam+Spitzer, SDSS+2MASS, GALEX)
  • A new kinematical model has been constructed from the Galactic potential approximated by a Stackel potential Bienaymé, Robin, Famaey (2015). The solar motion and age-velocity dispersion relations have been fitted to RAVE and Gaia DR1. A complete description of this new kinematics is given in Robin, Bienaymé, Fernandez-Trincado, Reylé (2017).
  • The outer disc scale lengths, warp and flare have been revised, according to 2MASS data. The description is given in Amores, Robin, Reylé (2017)

Model use

The model is a powerful tool to constrain either evolutionary scenarii or galactic structure hypothesis through the comparison between model predictions and a large variety of observational constraints such as star counts, photometry or astrometry. Model simulations are produced in the form of:
  • catalogues of pseudo-stars, from Monte-Carlo simulations. For each star, observable parameters as well as intrinsic ones are given.
  • tables of statistical distributions as a function of either observables (magnitudes, colours, proper motions, radial velocities) or intrinsic parameters (distances, spectral type, age, absolute magnitude).
A number of quantitative results have been obtained by comparing model predictions with suitable observations, like multidirectional photometric and astrometric star counts. Simulations may help for preparing observations, for evaluating the galactic stellar contamination in extragalatic studies or in star cluster fields.
The resulting model can be used for simulations of the galactic stellar populations in any galactic field in different photometric systems.

Limitations

There are limitations in the volume of the catalogues simulated (2Gb). Your simulation can be interrupted when it reaches this limit. Hence it is recommended to estimate the volume expected before launching a large simulation. If you require large amount of simulations, you might consider contacting us directly at model@obs-besancon.fr

A web service access to the model is also available (a sample Python client program for using the web service can be downloaded).

The BGM logo is credited Sarah Anderson.
Observatoire Aquitain des Sciences de l'Univers
Observatoire des Sciences de l'Univers THETA de Franche-Comté-Bourgogne
Laboratoire d'Astrophysique de Bordeaux
Institut UTINAM