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 selfconsistently 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) A&A 409, 523.

However, important updates have been performed concerning different aspects of the model since this version. They concern :

• the bar population, described in Robin, Marshall, Schultheis, Reylé (2012) A&A 538, A106.
• the thick disc and halo populations, where the thick disc is shown to replace the "thick bulge" population of Robin et al (2012). Moreover the thick disc is modeled with two successive episodes of star formation from 12 to 10 Gyr ago. It is described in : Robin, Reylé et al. (2014) A&A 569, A13.
• A 3D extinction map have been computed by comparing the model with 2MASS photometry in the Galactic plane (at longitudes between -100° and +100° and latitudes between -10° and +10°). This map is explained here : Marshall et al (2006) ADS
• Abundance ratio between alpha elements and metallicity are estimated for each star
• Parallaxes are given according to the distance of each star and the error provided by the user. If the error is not nul, some stars have negative parallaxes, as expected in a real survey.
• New photometric systems can be used: Johnson-Cousins extended to Spitzer [3.8] and [4.5] bands, Megacam-Wircam system for CFHT telescope, SDSS-2MASS, GALEX and ASTROSAT-UVIT in the UV

Hence

• the new model has 5 populations (thin disc, young thick disc, spheroid, bar and old thick disc) Each population is described by a SFR history, an IMF, an age or age-range, a set of evolutionary tracks, kinematics, metallicity characteristics. Notice that the two last populations (bar and old thick disc) still lack a white dwarf population
• Density laws for the thin disc are constrained self-consistently by the potential via the Boltzmann equation and are age-dependent.
• The extinction can be modeled by different ways:
• by a diffuse thin disc. This is not reliable for the galactic plane where a more sophisticated 3D extinction model should be used instead.
• by a 3D map from Marshall et al (2006) ADS
• by an extinction distribution along the line of sight given by the user.

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 random drawing. It includes a pseudo-Poisson noise in the number of stars generated. For each star, observable parameters as well as intrinsic ones are given : absolute visual magnitude, spectral type, effective temperature, gravity, radius, bolometric magnitude, metallicity, alpha over iron abundance ratio, kinematics (radial velocity and proper motions, either in (l,b) or in (Ra, Dec)), parallaxes. We also indicate the (x,y,z) position for each star as well as the assume extinction.
• 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).
• Integrated luminosity in any specified photometric band in the johnson-Cousins photometric system among UBVRIJHKL. Other photometric systems are being studied

A number of quantitative results has 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.


Simulated image of the Galaxy in 3 visible colours, as seen from the sun. Colours are given by the integrated flux in bands BVR.

Simulated image of the Galaxy in 3 infrared colours, as seen from the sun. Colours are given by the integrated flux in bands JHK.

References

• E.B. Amores, A.C. Robin, C. Reylé Evolution over time of the Milky Way’s disc shape Astron. Astrophys.,602, A67, 2017 ADS
• A.C. Robin, O. Bienaymé, J.G. Fernandez-Trincado, C. Reylé Kinematics of the local disk from the RAVE survey and the Gaia first data release Astron. Astrophys., 605, A1, 2017ADS
• O. Bienaymé, A.C. Robin, B. Famaey Quasi-integral of motion for axisymmetric potentials Astron. Astrophys., 581, A123, 2015
• M.A. Czekaj, A.C. Robin, F. Figueras, X. Luri et M. Haywood The Besancon Galaxy Model renewed I. Constraints on the Galactic thin disc evolution from Tycho data Astron. Astrophys., 564, A102, 2014ADS
• A.C. Robin, C. Reylé, J. Fliri, M. Czekaj, C.P. Robert et A.M.M. Martins Constraining the thick disc formation scenario of the Milky Way Astron. Astrophys., 569, A13, 2014ADS
• A.C. Robin, D.J. Marshall, M. Schultheis and C. Reylé. Stellar populations in the Milky Way bulge region: towards solving the Galactic bulge and bar shapes using 2MASS data Astron. Astrophys., 538, A106, 2012 ADS
• C Reylé, D. J. Marshall, A.C. Robin and M. Schultheis The Milky Way's external disc constrained by 2MASS star counts Astron. Astrophys., 495(3), 819-826, 2009 ADS
• D.J. Marshall, A.C. Robin, C. Reylé, M. Schultheis and S. Picaud. Modelling the Galactic interstellar extinction distribution in three dimensions Astron. Astrophys., 453, 635-651, 2006 ADS
• M. Schultheis, A.C. Robin, C. Reylé, H.J. McCracken, E. Bertin, Y. Mellier and 0. Le Fèvre. Stellar populations in the CFHTLS - I. New constraints on the IMF at low mass Astron. Astrophys., 447, 185-198, 2006 ADS
• S. Picaud & A.C. Robin. 3D outer bulge structure from near infrared star counts, Astron. Astrophys., 428, 891-903, 2004 ADS
• A.C. Robin, C. Reylé, S. Derrière and S. Picaud. A synthetic view on structure and evolution of the Milky Way, 2003, Astron. Astrophys., 409, 523 ADS (erratum: 2004, Astron. Astrophys., 416, 157)
• M. Haywood, A. C. Robin, M. Crézé The evolution of the Milky Way disc. II. Constraints from star counts at the galactic poles Astron. Astrophys., 320, 440-459, 1997, ADS
• M. Haywood, A. C. Robin, M. Crézé The evolution of the Milky Way disc. I. Vertical structure and local constraints Astron. Astrophys., 320, 428-439, 1997, ADS
• A.C. Robin M. Haywood, M. Crézé, D.K. Ojha, O. Bienaymé. The thick disc of the Galaxy: sequel of a merging event, 1996, Astron. Astrophys., 305, 125 ADS
• M. Haywood 1995, PhD thesis, Université Paris-VI ADS
• O. Bienaymé, A.C. Robin & M. Crézé The mass density in our Galaxy , 1987, Astron. Astrophys., 180, 94-110 ADS
• A.C. Robin & E. Oblak. A kinematic model of the Galaxy In European Regional Astronomy Meeting of the IAU, Volume 4, 323-326, 1987, ADS
• A. Robin & M. Crézé. Stellar populations in the Milky Way - A synthetic model, 1986, Astron. Astrophys., 157, 71-90 ADS

Disclaimer

Simulations are made at your own risks. The authors are not responsible for wrong applications of their model.

In case you would like complementary informations, you may contact the authors directly (modele[at]obs-besancon.fr).

Changes log

Here are summarized the model changes since the version released on November 25, 2003 (version described in A. C. Robin, C. Reylé, S. Derrière and S. Picaud. A synthetic view on structure and evolution of the Milky Way, 2003, Astron. Astrophys., 409:523 [ADS] and erratum: 2004, Astron. Astrophys., 416:157)

• 2019 Aug 12, 10h33 (CEST):
  • There was a wrong density of low mass stars in the bar population (at ~Mv = 12.6) which has been corrected today. The change is noticeable only for star count simulations in the bulge region at apparent magnitude fainter than about 24.5 in I. Notice that there are still no stars at mass<0.15 Msun for the bar population (Pop=10) due to lack of low mass Padova isochrones.
• 2019 May 10:
  • Simulations are now limited to 2 million stars
• 2019 January 18:
  • The web site has been moved to https (secured connection)
• 2019 January 16:
  • The present version of the Besançon Galaxy Model is now the default one; it is accessible via address: http://model.obs-besancon.fr/
• 2018 September 21:
  • new photometric systems are available : Gaia G, Spitzer, SDSS, GALEX and UVIT bands
  • The kinematics is from the fit to radial velocities from RAVE and proper motions from Gaia-DR1 presented in Robin et al, 2017
  • The outer disc scale length, warp and flare parameters are from Amores et al, 2017
• 2017 October 26:
  • commissioning of the version based on IVOA UWS web service"
• 2016 May 24:
  • issue of version 1603 of the model (see more details in the model "Description")
• 2013 April 30:
  • corrected spectral types for M dwarfs
  • corrected kinematics of the bulge
  • new thick disc fitted to SDSS and 2MASS data
• 2013 February 12:
  • the distribution in distance inside each volume element has been revised for a smoother distribution. Nowadays the distribution is smooth and can be studied at scales smaller than the distance step used (generally 20pc in the plane and 50 pc far from the plane).
  • White dwarfs: for simulation in the UV for Galex and UVIT satellites, white dwarfs distribution have been refined. Now the model generates two sequences, DA and DB, using Bergeron et al white dwarf models.
  • Halo blue horizontal branch: also for improving simulation in the UV halo BHB have been revised. We now use Basti models for simulating the blue horizontal branch for halo stars. The rest of the isochrones still come from Bergbusch and Vandernberg (1992) ADS.
• 2012 June 1st: version 1204 for Galhis team
• 2009 Octobre 21st: added Galex-UVIT and Megacam+Wircam+Spitzer photometric systems
• 2006 December 7th: This new version 0611 is an update of the Galaxy model from recent  results, concerning the IMF at low masses (see Schultheis et al, Astron. Astrophys, 447:185, 2006 ADS). It also implements a few new features:
  • An option is proposed for the new extinction model (Marshall et al., Astron. Astrophys., 453:635, 2006 ADS), valuable in the plane: -90<l<90, -10<b<10.
  • A new photometric system composed of the visible bands of the megacam camera at CFHT (u* g' r' i' z') and the NIR bands of the Wircam instrument (J H K) is now available.
  • The SDSS photometric system is also implemented but just as a color transformed from the CFHTLS system. The color equations are from Nicolas Regnault (private communication) from a detailed analysis of the SNLS (Supernovae Legacy Survey) conducted at CFHT. Available bands are g,r,i,z,J,H,K. J, H, K are close to the 2MASS system.
  • "On the fly" star count computations are possible for obtaining row counts  without photometric errors.
  • White dwarf models have been improved in the Megacam photometric  system.
  • Stellar radius is added in the catalogue simulations for each star.
  • A small error on the coordinate transformation between equatorial to galactic system has been corrected. In some region on the sky (near the Galactic plane) it implies a small change in the counts, when done in the equatorial system (up to 10% at the very maximum, smaller than a few % in general).
• 2005 Jun. 20: A bug has been discovered in the computation of the kinematics, potentially giving wrong values of the velocities of the thick disc in particular. The bug has been introduced on 2005, Feb. 17. All velocities should be disregarded but number of simulated stars and photometry are correct. The bug has been corrected on 2005, Jun 20^th at 11:56 CEST (GMT+2:00). Simulations made before 2005, Feb. 17 are correct.
• 2005 Feb. 28: correction of an error in the handling of the photometric system in the form ; since December 4 2004, when using the HTML link "new access to the model form " in the summary of the supplied parameters to submit a further simulation, the photometric system used was not properly kept ; this resulted in an error message ("SYSTEME PHOTOMETRIQUE NON DEFINI") in the simulation output.
• 2005 Feb. 22: radial velocities are explicitely constrained within range -999, +999 km/s
• 2004 Dec. 6: Added use of CFHT-Megacam photometric system.
• 2004 Aug. 17: Previous to this date the external disc flare was improperly applied to the thick disc population. It has been removed. The error caused a slight increase (compared to what is expected without a flare) of the number of thick disc stars at galactocentric distances larger than 9500 pc at low galactic latitudes. The change is not more than 1% in counts of thick disc stars in the plane up to magnitude V=20. Nowadays the thick disc population follows the warp but does not flare in the external part. Please refer to the publication for details about the warp and flare values.
• 2004 Feb. 18: In the previous version the V-I colours of thick disc and halo white dwarfs were wrong (too red). Other colours and V magnitude were correct. This has been corrected since Wed Feb 18 21:15 CET (GMT+1:00).
• 2004 Feb. 10: the slope of the flare was incorrect in the parameter file. This error produced extra stars at large distances (> 10kpc) at low galactic latitudes in some user conditions (deep star counts). The proportion of extra stars was estimated to a few percent. It is correct since Tue Feb 10 11:42 CET (GMT+1:00).