BaStA
Stellar population archaeology of galaxies
in a Bayesian framework

BaStA is a Bayesian framework for inferring stellar population parameters — stellar mass, age, metal content, and star formation history — from the spectral and SED analysis of galaxies. The goal is to decode the archaeological relics represented by stellar population properties so to trace back in time and understand galaxy evolution.

In this website we collect the documentation as well as sharable data products, which are the results of more than two decades of dedicated work.

Website under construction — content being updated
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// method

BaStA methodological approach

The foundations of BaStA trace back to the pioneering work of Kauffmann et al. (2003), who first demonstrated the power of Bayesian inference applied to galaxy spectral diagnostics. Building on this framework, Gallazzi et al. (2005) developed a fully Bayesian method for deriving marginalised posterior PDFs of stellar population parameters from a pre-computed model library, avoiding the computational cost of on-the-fly model generation and making statistical rigour accessible at survey scale. This method, further developed by Zibetti, Gallazzi and collaborators over two decades, has grown into BaStA: the Bayesian Stellar population Analysis algorithm.

core idea
Rather than fitting the full spectrum, BaStA compresses the spectral information into a small set of well calibrated absorption line indices (Worthey et al. 1994; Trager et al. 1998) complemented by broad-band photometry — reducing the dimensionality of the problem while retaining its key diagnostic power, and enabling fast, statistically rigorous Bayesian marginalisation over a library of pre-computed composite stellar population models, which is both vast (up to 500,000 models) and comprehensive.

For each model in the library, the likelihood is computed from the classic χ² statistics, which measure the difference between observed indices and fluxes and the corresponding model predictions, normalised by observational uncertainties. The effects of spectral resolution, velocity dispersion and cosmological band shifts are directly applied to the models — precomputed on a grid of parameters — in order to enable a direct comparison with the observations. The posterior PDF of each physical parameter is obtained by marginalising over all others. Parameter estimates are taken as the median of the posterior, with uncertainties given by the 16th–84th percentile range.

BaStA infers a variety of integral stellar population parameters, both light-weighted and mass-weighted. These include stellar mean ages and metallicities, stellar mass (both formed and present), time markers of the SFH (such as age50, age90 and similar), and dust attenuation. The range and realism of accessible parameters has grown substantially over the years, driven by continuous evolution of the underlying CSP model library:

 
circa 2004
BC03 · exponential SFH · non-evolving metallicity · dust-free

The first BaStA library was utilized in Gallazzi et al. (2005) and was built on the Bruzual & Charlot (2003) stellar population models. Star formation histories were parameterised as simple exponential declines with superimposed bursts, metallicity was held fixed along each SFH, and no explicit dust treatment was included. Despite these simplifications, this library characterised the stellar populations of hundreds of thousands of SDSS galaxies from spectroscopic indices alone.

circa 2008
Dust implementation · TP-AGB prescriptions

Explicit 2-component dust treatment is introduced à la Charlot & Fall (2000), following da Cunha, Charlot & Elbaz (2008). Developments and testing mainly focused on photometry, M/L and the impact of strong TP-AGB prescriptions (CB07 version of BC03 SSP models) on the NIR emission.

2011
SEDlibrary

A brand-new suite of C codes for the massive production of SPS model libraries is initiated by S. Zibetti: SEDlibrary — the theoretical pillar of BaStA. The code not only performs the synthesis of spectra and integral SP parameters from SSPs — given the SFH, chemical enrichment history, and the dust parameters —, but also computes observables in grids of redshift and velocity dispersion.

2013-2017
Delayed SFH · variable metallicity · dust · MILES stellar library · photometry

More physically motivated SFH shapes were introduced — i.e. delayed Gaussian functions (à la Sandage) with a rising phase — alongside a variable chemical enrichment history and explicit dust attenuation. This richer parameterisation both required and enabled the inclusion of broad-band photometry as additional constraints. It also unlocked proper light- and mass-weighted mean metallicity estimates and dust attenuation measurements. The new default SSP models are the so-called CB16, which employ MILES stellar spectral libraries. These new libraries are employed for the first time in Zibetti et al. (2017).

2020
SFH descriptors

A new set of lookback times for different stellar mass fractions are introduced as parameters to estimate, to enable a better characterization of the SFH (Edoardo Rossi's MSc, 2021).

2021
CB19 with PARSEC evolutionary tracks implemented

New standard base SSP, with updated tracks, which also extend the parameter space to higher metallicities. Used in SDSS/2025 analysis by Daniele Mattolini et al. (2025).

2025-in progress
Variable elemental abundance ratios [α/Fe]

We are currently working on extending BaStA to account for variable elemental abundance ratios [α/Fe], in close collaboration with Alex Vazdekis at the Instituto de Astrofísica de Canarias (Tenerife, Spain). Part of Daniele Mattolini's PhD project.

//BaStA at work

BaStA applications and key results

BaStA and the libraries produced with SEDlibrary have been applied to a variety of datasets, from the local Universe to high z, both in single-aperture spectra and in spatially resolved IFS.

2004-2008
SDSS-I

First full Bayesian application to SDSS galaxies, deriving stellar mass, age, and metallicity for ~175,000 galaxies, and their scaling relations. These results constitute Anna Gallazzi's PhD thesis and are published in a series of papers (Gallazzi et al. (2005, 2006, 2008).

2008-2009
Multiband imaging

Libraries including dust are used to derive M/L and stellar mass maps for a handful of nearby galaxies with multiband imaging (Zibetti, Charlot & Rix 2009).

2010–2021
SDSS group catalogs

BaStA-derived SP properties of SDSS-I galaxies are used to investigate the links between them and the environment in a central-satelite galaxy formalism (Pasquali et al. 2010, 2019; Gallazzi et al. 2021).

2010-2014
z~0.7 with IMACS @ Magellan

First all-type galaxy SP scaling relations at z~0.7 (70 galaxies; Gallazzi et al. 2014)

2010-2020
z>2 lensed galaxies @VLT

SP characterization of magnified z>2 galaxies with XShooter@VLT (Toft et al. 2012, 2017; Stockmann et al. 2020)

2014-2022
CALIFA IFS survey

Spatially SP scaling relations for ~400 galaxies observed in IFS by CALIFA — glocal mass dependence; link with dynamical mass. See Zibetti et al. (2017, 2020); Zibetti & Gallazzi (2022); Scholz-Diaz et al. (2026))

2018-2026
z~0.7 with LEGA-C @ ESO-VLT
       

All-type galaxy SP scaling relations at z~0.7 for 552 galaxies, with much more robust statistics (Gallazzi et al. 2026a, b). Environmental investigations (Scholz-Diaz et al. in prep.).

2022-2026
SDSS-I 20 years later

Revision of SDSS-I scaling relations, based on final DR7, improved statistical weights, fiber-aperture corrections and state-of-the-art models (PhD Thesis of Daniele Mattolini; Mattolini et al. 2025, Zibetti et al. 2026)

present
High z (>2): DEEP-Dive

Galaxy SP scaling relations at high z~0.7 from JWST NIRSPEC data (DeepDive survey, PI F. Valentino)

(almost) present
StePS surveys in WEAVE & 4MOST

SP scaling relations at z~0.55-0.7

Future
MOONS@VLT · MOSAIC@ELT · WST
// bibliography

Publications

→ Full ADS library with papers based on or directly related to BaStA.

Key selected papers describing the BaStA method and its applications.

Gallazzi, A., Charlot, S., Brinchmann, J., White, S. D. M., Tremonti, C. A.
The ages and metallicities of galaxies in the local universe
MNRAS 362, 41 (2005) — founding paper
→ ADS  ·  DOI  ·  arXiv
Zibetti, S., Charlot, S., Rix, H.
Resolved stellar mass maps of galaxies - I. Method and implications for global mass estimates
MNRAS 400, 1181 (2009)
→ ADS  ·  DOI  ·  arXiv
Gallazzi, A., Bell, E. F., Zibetti, S., Brinchmann, J., Kelson, D. D.
Charting the Evolution of the Ages and Metallicities of Massive Galaxies since z = 0.7
ApJ 788, 72 (2014)
→ ADS  ·  DOI  ·  arXiv
Toft, S., Zabl, J., Richard, J., Gallazzi, A., Zibetti, S. et al.
A massive, dead disk galaxy in the early Universe
Nature 546, 510 (2017)
→ ADS  ·  DOI  ·  arXiv
Zibetti, S., Gallazzi, A. R., Ascasibar, Y., Charlot, S., Galbany, L. et al.
Resolving the age bimodality of galaxy stellar populations on kpc scales
MNRAS 468, 1902 (2017)
→ ADS  ·  DOI  ·  arXiv
Gallazzi, A. R., Pasquali, A., Zibetti, S., La Barbera, F.
Galaxy evolution across environments as probed by the ages, stellar metallicities, and [α /Fe] of central and satellite galaxies
MNRAS 502, 4457 (2021)
→ ADS  ·  DOI  ·  arXiv
Zibetti, S., Gallazzi, A. R.
Stellar mass as the glocal driver of galaxies stellar population properties
MNRAS 512, 1415 (2022)
→ ADS  ·  DOI  ·  arXiv
Zibetti, S., Rossi, E., Gallazzi, A. R.
On the maximum age resolution achievable through stellar population synthesis models
MNRAS 528, 2790 (2024)
→ ADS  ·  DOI  ·  arXiv
Mattolini, D., Zibetti, S., Gallazzi, A. R., Scholz-Díaz, L., Pratesi, J.
Re-assessing the stellar population scaling relations of the galaxies in the Local Universe
A&A 703, A5 (2025)
→ ADS  ·  DOI  ·  arXiv
Gallazzi, A. R., Zibetti, S., van der Wel, A., Nersesian, A., Kaushal, Y. et al.
LEGA-C stellar populations scaling relations. I: Chemo-archaeological downsizing trends at z~0.7
arXiv:2512.07952 (2025)
→ ADS  ·  arXiv
Gallazzi, A. R., Zibetti, S., van der Wel, A., Nersesian, A., Kaushal, Y. et al.
LEGA-C stellar populations scaling relations. II: Dissecting mass-complete archaeological trends and their evolution since z~0.7 with LEGA-C and SDSS
arXiv:2511.11805 (2025)
→ ADS  ·  arXiv
// catalogs & repositories

Data access

Published catalogs of BaStA-derived stellar population parameters.
Interactive database interfaces coming soon.

Website under construction — datasets being uploaded - links will be provided soon
SDSS

SDSS Galaxy Catalogs

Stellar masses, ages, and metallicities for SDSS DR7 galaxies

→ View data repository !!! TO BE ACTIVATED SOON !!!
TBD

??? release

??? Description of dataset, survey, and parameters included.???

→ ??
coming soon

Interactive Query Interface

Web-based access and TAP to BaStA catalogs

// people

Team & Contact

Core Team

BaStA is developed and maintained by:

ARG
Anna R. Gallazzi
INAF – Osservatorio Astrofisico di Arcetri, Firenze (Italy)
SZ
Stefano Zibetti
INAF – Osservatorio Astrofisico di Arcetri, Firenze (Italy)

For questions, collaborations, or data requests, contact us at our institutional email addresses: firstname.lastname@inaf.it

Current team members

Main & historical collaborators

BaStA has its roots in the PhD work of Anna Gallazzi at MPA-Garching under the supervision of Stéphane Charlot (Institut d'Astrophysique de Paris), whose foundational ideas on stellar population synthesis and Bayesian inference shaped the entire framework.

// Credits

The BaStA website is developed and maintained by S. Zibetti and powered by Wordpress on INAF ia2 servers. Built with assistance from Claude AI by Anthropic.