Transient star B/R ratio and star formation in $z\gtrsim 1$ lensed galaxies

TL;DR

This work demonstrates that the ratio of lensed-star transients detected in blue versus red filters (the B/R ratio) is a sensitive tracer of the age of stellar populations in $z\gtrsim 1$ galaxies, with modest dependence on metallicity and minimal sensitivity to the IMF or dust. By forward-modeling transient detection rates in the Warhol arc and combining the B/R constraint with the galaxy SED, the authors show that single-burst SFHs cannot reproduce both observables, whereas a non-parametric SFH constrained by both allows a significantly different recent SFH, including much higher star formation in the $10$--$50$ Myr window. Spatially resolved analysis reveals region-specific SFHs, illustrating how B/R can break degeneracies in SED fits and reveal burstiness in star formation. The results imply that the B/R ratio is a powerful, ancillary constraint for understanding the recent growth of stellar mass in high-redshift galaxies, particularly when probed through strongly lensed systems with JWST and HST data across multiple regions.

Abstract

The extreme magnification from galaxy clusters and microlenses therein allows 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 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 moderate sensitivity to metallicity and 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 ``Warhol'' arc ($z = 0.94$), we find that neither a simple single starburst nor a 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 $\sim$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.

Figures (14)

• Figure 1: RGB image of the Warhol arc ($z = 0.94$). We show the transients detected in this arc as coral circles Yan_2023Williams_2025 and green triangles Kelly_2022_Flashlights, respectively. We highlight areas characterized as region 2 by Palencia_2025_microlensing in red.
• Figure 2: Subsets of MIST age isochrone MIST used in our SPISEASPISEA stellar population synthesis featuring a range of ages as shown in the legend. The metallicity of each of the subsets is reflected by their linestyle as shown in the bottom legend. All of them have no dust extinction, where the color represents the age of the isochrone as shown in the top legend. For each age isochrone, we also annotate some characteristic masses to indicate the mass-age dependency.
• Figure 3: Filter response curve of HST F200LP (blue shade) and JWST F200W (coral shade), and the black body spectrum of two stars at $30000\,K$ (blue curve) and $4500\, K$ (red curve) --- characteristic temperature of BSGs and RSGs, respectively --- shown at the rest frame and observed frame ($z = 0.94$) wavelengths. One can see that the two filters best capture the brightest part of the black body of the two classes of stars.
• Figure 4: Transient detection rate per unit mass for stellar populations at different ages with metallicity of $0.5 Z_{\odot}$ and $A_{V} = 0$, calculated with Eq. \ref{['eqn: detection_rate']} under the magnification map of Warhol from Palencia_2025_microlensing. Blue curves show the detection rate in the HST F200LP filter (detection limit of $30\,m_{AB}$); red curves show the detection rate in the JWST F200W filter (detection limit of $29.7\,m_{AB}$). Solid and dotted lines show the case for a Kroupa IMF and a top-heavy IMF, respectively.
• Figure 5: Expected $B/R$ ratio calculated based on the F200LP and F200W transient detection rate in Warhol arc shown in Fig. \ref{['fig: detection_rate_all']}, with a Kroupa IMF. Each of the panels shows the case for one of the dust extinctions (upper row) or one of the metallicities (lower row) as indicated in the subtitles. The color of each curve indicates the metallicities (upper row) or dust extinction (lower row). The gray dashed line denotes $B/R = 1$ for reference. The fluctuation in ages earlier than $\sim 50\,$Myr are significant and arise from intrinsic variation due to stellar evolution, whereas those later than $\sim 50\,$Myr are arising from sampling uncertainty given the low transient detection rate at these ages. We do not show the error bars for visualization purposes.