Transient star blue-to-red ratio and star formation history in z  ≳  1 lensed galaxies

¹ Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
² Instituto de Física de Cantabria (CSIC-UC), Avda. Los Castros s/n, 39005 Santander, Spain
³ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA
⁴ Department of Physics, Ben-Gurion University of the Negev, PO Box 653 Be’er-Sheva 8410501, Israel
⁵ Department of Theoretical Physics, University of Basque Country UPV/EHU, Bilbao, Spain
⁶ Ikerbasque, Basque Foundation for Science, Bilbao, Spain
⁷ Donostia International Physics Center, Paseo Manuel de Lardizabal, 4, San Sebastián 20018, Spain

^★ Corresponding author: keihk98@connect.hku.hk

Received: 20 June 2025
Accepted: 6 October 2025

Abstract

The extreme magnification from galaxy clusters and microlenses therein allows for the detection of individual, luminous stars in lensed galaxies as transient events, and hence provides a valuable window into the high-mass stellar population in z > 1 galaxies. As these bright stars can only be formed at specific ages, the relative abundance of transient events at blue (B) and red (R) optical and near-infrared wavelengths (B/R ratio) can provide insights into the recent star formation history of galaxies that are not well constrained by their spectral energy distributions (SEDs). Here, we forward model the transient detection rates in an idealized mock scenario to find that the B/R ratio of strongly lensed z > 1 galaxies decreases quickly with increasing age. This ratio has a moderate sensitivity to metallicity and a comparatively low sensitivity to dust attenuation, with no significant dependency on the stellar initial mass function. Fitting model stellar populations to either the SED or B/R ratio alone of the “Warhol” arc (z = 0.94), we find that neither a simple single starburst nor more complex star formation can simultaneously reproduce both constraints. We then demonstrate that a best-fit model constrained by both the B/R ratio and SED requires a star formation rate that has varied quite dramatically over the past ∼50 Myr, for which the total stellar mass formed over this time is a factor of 10 (with 2 − 3σ significance) different from the best-fit models to the SED alone. Our work shows that the transient B/R ratio can be used as an additional powerful constraint on the recent star formation history of higher-redshift galaxies in future works that are strongly lensed by galaxy clusters.