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4MOST-StePS: a first look at ultra-deep
Survey Programme Validation (SPV) spectra
SCIENTIFIC CONTEST
4MOST-StePS is an ESO Participating Community Survey for the exploitation of the ESO spectrograph 4MOST@VISTA. The survey aims at observing over the five-year survey period a sample of ~3000 bright galaxies (I_AB < 20.5), in the redshift range 0.3 < z < 0.7 and with 30 hours of exposure time, with the final goal to obtain spectra of exceptional quality that will allow deriving the evolutionary path of the more massive galaxies in the hitherto unexplored intermediate redshift range between LEGA-C at high redshift and SDSS in the local Universe.
Before the official start of the Survey Operations, scheduled for October 2026, there was a dedicated phase known as Survey Programme Validation (SPV), during which targeted observations were carried out to ensure the feasibility of the scientific objectives of all the surveys.
For 4MOST-StePS, the SPV experiment involved observing approximately 150 galaxies for the same total exposure time planned for the survey, namely 30 hours per target. The goal was to assess whether systematic effects could arise when stacking the individual observations collected over multiple epochs.
These spectra are now available for initial tests and scientific analyses.
THESIS OBJECTIVES
The first objective of the thesis will be to analyse the ~150 spectra obtained during the SPV experiment for 4MOST-StePS and to combine the individual exposures into the highest-quality final spectra possible. A key aspect of this work will be to investigate and optimise the stacking procedure, testing different strategies for combining the individual exposures (e.g. weighting them according to their S/N) and assessing the impact of the quality of the individual spectra on the final result. Exposures that are clearly affected by observational problems, such as incorrect fibre positioning or other anomalous behaviour, will be identified and excluded from the stacking when appropriate. The goal will be to define a robust and reproducible procedure for producing optimal stacked spectra from the long series of individual observations.
Once the optimal stacked spectra have been obtained, the student will use them to investigate the stellar population properties of the galaxies by comparing the observed spectra with stellar population models. This analysis will be performed using pPXF (penalised Pixel-Fitting), one of the most widely used full-spectral-fitting codes in the literature. The analysis will allow the student to derive key stellar population properties, such as the stellar ages and metallicities, and to investigate the star-formation histories of the galaxies.
In addition, pPXF can be used to model and measure emission lines when present in the spectra, providing a direct estimate of the current star-formation activity of the galaxies. This will allow the student to explore, within the same framework, both the stellar populations and the ongoing star formation of the observed systems.
Required candidate profile
We are looking for a student with a strong interest in observational astrophysics and galaxy evolution, and in particular in the analysis and interpretation of galaxy optical spectra. Basic programming skills, particularly in Python, would also be an advantage, as the development and optimisation of the spectral stacking procedures will require the student to work with Python-based tools.
Start date: Flexible, from autumn 2026
Duration: 6-8 months, with completion expected according to the student's degree timeline.
Locations: the project will be supervised by researchers at the INAF-Osservatorio Astronomico di Brera (in Milan).
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