A few StePS forward in unveiling the complexity of galaxy evolution: light-weighted stellar ages of intermediate-redshift galaxies with WEAVE

¹ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
e-mail: luca.costantin@inaf.it
² Centro de Astrobiología (CSIC-INTA), Ctra de Ajalvir km 4, Torrejón de Ardoz 28850 Madrid, Spain
³ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁵ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁶ Isaac Newton Group of Telescopes, Apartado 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain
⁷ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, Calle Astrofísico Francisco Sánchez s/n, 38205 La Laguna, Tenerife, Spain
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁰ Department of Physics, University of Oxford, Keble Road, OX1 3RH Oxford, UK
¹¹ STFC-RALSpace, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK
¹² Institut de Ciencies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), 02028 Barcelona, Spain
¹³ Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
¹⁴ Instituto de Astrofísica de Andalucía (CSIC), PO Box 3004 18080 Granada, Spain
¹⁵ INAF – IASF Milano, Via Bassini 15, 20133 Milano, Italy
¹⁶ Instituto de Astronomía y Ciencias Planetarias de Atacama, Universidad de Atacama, Copayapu 485, Copiapó, Chile
¹⁷ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 Groningen, The Netherlands
¹⁸ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
²⁰ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²¹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
²² Departamento de Física Teórica, Universidad Autónoma de Madrid, 28049 Cantoblanco, Spain
²³ University of Bologna – Department of Physics and Astronomy, Via Gobetti 93/2, 40129 Bologna, Italy

Received: 22 August 2019
Accepted: 28 September 2019

Abstract

Context. The upcoming new generation of optical spectrographs on four-meter-class telescopes, with their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage, will provide invaluable information for reconstructing the history of star formation in individual galaxies up to redshifts of about 0.7.

Aims. We aim at defining simple but robust and meaningful physical parameters that can be used to trace the coexistence of widely diverse stellar components: younger stellar populations superimposed on the bulk of older ones.

Methods. We produced spectra of galaxies closely mimicking data from the forthcoming Stellar Populations at intermediate redshifts Survey (StePS), a survey that uses the WEAVE spectrograph on the William Herschel Telescope. First, we assessed our ability to reliably measure both ultraviolet and optical spectral indices in galaxies of different spectral types for typically expected signal-to-noise ratios. We then analyzed such mock spectra with a Bayesian approach, deriving the probability density function of r- and u-band light-weighted ages as well as of their difference.

Results. We find that the ultraviolet indices significantly narrow the uncertainties in estimating the r- and u-band light-weighted ages and their difference in individual galaxies. These diagnostics, robustly retrievable for large galaxy samples even when observed at moderate signal-to-noise ratios, allow us to identify secondary episodes of star formation up to an age of ∼0.1 Gyr for stellar populations older than ∼1.5 Gyr, pushing up to an age of ∼1 Gyr for stellar populations older than ∼5 Gyr.

Conclusions. The difference between r-band and u-band light-weighted ages is shown to be a powerful diagnostic to characterize and constrain extended star-formation histories and the presence of young stellar populations on top of older ones. This parameter can be used to explore the interplay between different galaxy star-formation histories and physical parameters such as galaxy mass, size, morphology, and environment.