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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 inz > 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/Rratio) 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 theB/Rratio of strongly lensedz > 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 orB/Rratio 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 theB/Rratio 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 − 3σsignificance) different from the best-fit models to the SED alone. Our work shows that the transientB/Rratio 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.
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Constraining the z ∼ 1 Initial Mass Function with HST and JWST Lensed Stars in MACS J0416.1−2403
Abstract Our understanding of galaxy properties and evolution is contingent on knowing the initial mass function (IMF), and yet to date the IMF is constrained only to local galaxies. Individual stars are now becoming routinely detected at cosmological distances, where luminous stars such as supergiants in background galaxies strongly lensed by galaxy clusters are temporarily further magnified by huge factors (up to 104) by intracluster stars, thus being detected as transients. The detection rate of these events depends on the abundance of luminous stars in the background galaxy and is thus sensitive to the IMF and the star formation history (SFH), especially for the blue supergiants detected as transients in the rest-frame ultraviolet/optical filters. As a proof of concept, we use simple SFH and IMF models constrained by spectral energy distributions (SEDs) to see how well we can predict the Hubble Space Telescope and James Webb Space Telescope transient detection rate in a lensed arc dubbed “Spock” (z= 1.0054). We find that demanding a simultaneous fit of the SED and the transient detection rate places constraints on the IMF, independent of the assumed simple SFH model. We conclude that our likelihood analysis indicates that the data definitively prefers the “Spock” galaxy to have a Salpeter IMF (α = 2.35) rather than a top-heavy IMF (α = 1)—which is thought to be the case in the early universe—with no clear excess of supergiants above the standard IMF.
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- Award ID(s):
- 2308051
- PAR ID:
- 10670748
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- AAS
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 988
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 178
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We present stellar atmosphere modeling of JWST NIRCam photometry of nine highly magnified individual stars in a single galaxy at redshiftz= 0.94 known as the Warhol arc, which is strongly lensed by the galaxy cluster MACS J0416. Seven of these transients were identified by Yan et al. The nine sources are likely red supergiants with temperaturesTeff ≈ 4000 K. We present new long-slit spectroscopy of the Warhol arc acquired with Keck I telescope and the Large Binocular Telescope, and use these data to help constrain the arc’s oxygen abundance to be . A microlensing simulation is performed on synthetic stellar populations using a range of stellar metallicities and initial mass function (IMF) slopes. The temperature distribution of the simulated detectable stars is sensitive to the choice of stellar metallicity, and setting the stellar metallicity equal to the arc’s nebular metallicity ( ) produces a simulated temperature distribution that is consistent with the observations, while lower stellar metallicities ( ) produce simulated temperatures that are inconsistent with the observations. The expected detection rate is strongly anticorrelated with the IMF slope forα > 1.2. For the canonical IMF slopeα = 2.35, the simulation yields expected transient detection rates that agree with the observed detection rates in the Hubble Space Telescope Flashlights filters, but overpredicts the detection rate by a factor of ∼3–12 (<2σtension) in the JWST filters. The simulated detection rate is sensitive to the choice of stellar metallicity, with lower metallicities ( ) yielding a significantly lower simulated detection rate that further reduces the modest tension with the observations.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ade4bd
