[Paper Review] Star counts in the Galaxy. Simulating from very deep to very shallow photometric surveys with the TRILEGAL code

L. Girardi, M. A. T. Groenewegen+2 more|ArXiv.org|Apr 2, 2005

Stellar, planetary, and galactic studies54 references3 citations

TL;DR

This paper presents TRILEGAL, a flexible population synthesis code that simulates stellar photometry across the Galaxy using complete evolutionary tracks and a spectral library to model photometric systems. It successfully reproduces star counts from very deep (CDFS, R=16–23) to shallow (Hipparcos, V<8) surveys with a single calibration, except near the Galactic Center and Plane.

ABSTRACT

We describe TRILEGAL, a new populations synthesis code for simulating the stellar photometry of any Galaxy field. The code attempts to improve upon several technical aspects of star count models, by: dealing with very complete input libraries of evolutionary tracks; using a stellar spectral library to simulate the photometry in any broad-band system; being very versatile allowing easy changes in the input libraries and in the description of all of its ingredients -- like the SFR, AMR, IMF, and geometry of Galaxy components. In a previous paper (Groenewegen et al. 2002), the code was first applied to describe the very deep star counts of the CDFS stellar catalogue. Here, we briefly describe its initial calibration using EIS-deep and DMS star counts, which are adequate samples to probe both the halo and the disc components of largest scale heights (oldest ages). We then present the changes in the calibration that were necessary to cope with some improvements in the model input data, and the use of more extensive photometry datasets: the relatively shallower 2MASS catalogue, which probes mostly the disc at intermediate ages, and the immediate solar neighbourhood as sampled by Hipparcos, which contains a somewhat larger fraction of younger stars than deeper surveys. Remarkably, the same model calibration can reproduce well the star counts in all the above-mentioned data sets, that span from the very deep magnitudes of CDFS (16

Motivation & Objective

• To develop a self-consistent, flexible population synthesis code capable of simulating stellar photometry across a wide range of Galactic fields and photometric systems.
• To improve upon traditional star count models by replacing ad hoc assumptions about scale heights and luminosity functions with physically motivated, continuous evolutionary tracks.
• To calibrate the model using multi-depth, multi-field data (EIS-deep, DMS, 2MASS, Hipparcos) to ensure consistency across different stellar populations and survey depths.
• To enable future use of TRILEGAL for upcoming wide-field optical/infrared surveys by ensuring compatibility with evolving stellar models and photometric systems.

Proposed method

• TRILEGAL uses a modular framework integrating stellar evolutionary tracks, atmospheric models, and a self-consistent spectral library to compute photometry in any broad-band system.
• It applies the fundamental equation of stellar statistics (Eq. 1) to compute number counts as a function of magnitude, galactic coordinates, and distance, incorporating stellar density profiles (disc, halo, bulge) and extinction.
• The code employs a flexible input system allowing users to modify the star formation rate (SFR), age-metallicity relation (AMR), initial mass function (IMF), and geometry of Galactic components.
• It ensures physical consistency by enforcing continuity in core mass, envelope mass, and surface composition across isochrones, avoiding ad hoc adjustments.
• The model is calibrated using high-quality, multi-wavelength data from EIS-deep, DMS, 2MASS, and Hipparcos, with iterative refinement to match observed number counts.
• It supports virtually any photometric system (e.g., UBVRI, JHK, SDSS, 2MASS, HST instruments) via a unified spectral library and extinction coefficient calculation.

Experimental results

Research questions

• RQ1 Can a single, physically consistent population synthesis model reproduce star counts across vastly different survey depths, from very deep (CDFS, R=16–23) to shallow (Hipparcos, V<8)?
• RQ2 To what extent can a model based on evolutionary tracks and spectral synthesis replace ad hoc assumptions about scale heights and luminosity functions in star count modeling?
• RQ3 How well does TRILEGAL reproduce the absolute magnitude versus color diagram of the Hipparcos catalogue, which includes a higher fraction of young stars?
• RQ4 Why do significant deviations occur only near the Galactic Center and Plane, and in one set of South Galactic Pole data?
• RQ5 Can the same calibration be used across diverse photometric systems and survey depths without re-tuning parameters?

Key findings

• TRILEGAL successfully reproduces star counts across a dynamic range from R=16 to V=8, spanning deep surveys (CDFS) to shallow ones (Hipparcos), with deviations exceeding 50% only near the Galactic Center and Plane.
• The model calibration successfully reproduces the absolute magnitude versus color diagram of the Hipparcos catalogue, including its higher fraction of young stars.
• The same model calibration works across multiple photometric systems, including 2MASS, SDSS, EIS, and HST instruments, demonstrating broad applicability.
• The model identifies that assigning large scale heights to all evolved stars (e.g., giants) is physically incorrect, as many are young (less than 2 Gyr), contradicting earlier assumptions.
• Significant deviations occur only in fields with strong bulge or disk contamination (Galactic Center/Plane) and one anomalous South Galactic Pole dataset, indicating model limitations in high-density regions.
• The code’s flexibility allows rapid integration of improved stellar evolutionary tracks (e.g., Marigo et al. 2003) and new photometric systems, ensuring long-term relevance for upcoming wide-field surveys.