JWST's PEARLS: Temperatures of Nine Highly Magnified Stars in a Galaxy at Redshift 0.94 and Simulated Stellar Population Dependence on Stellar Metallicity and the Initial Mass Function

TL;DR

This paper investigates nine highly magnified stars in the Warhol arc at $z=0.94$ to constrain stellar metallicity and the IMF via microlensing. It combines JWST NIRCam photometry with extensive spectroscopic data (Keck/MOSFIRE, LBT LUCI, VLT/MUSE) and a suite of forward-models including fsps-based galaxy SED fitting and microlensing simulations (GLAFIC, M_SMiLe). The authors find the stars to be red supergiants with $T_{ m eff}\approx 4000$ K, and derive a nebular oxygen abundance $12+\log({\rm O/H})=8.45\pm0.08$, while their stellar-metallicity-inferred temperatures favor $\log(Z_*/Z_\odot)\gtrsim -0.24$; nonetheless, JWST-detectability rates are most consistent with lower $Z_*$, revealing a tension between temperature and rate constraints. The results illustrate how microlensing of individual stars in distant galaxies can probe metallicity and the IMF at $z\approx1$, guiding future JWST campaigns and modeling refinements that may resolve the remaining discrepancies, including the role of substructure and possible $\alpha$-element enhancements.

Abstract

We present stellar atmosphere modeling of JWST NIRCam photometry of nine highly magnified individual stars in a single galaxy at redshift z=0.94 known as the Warhol arc, which is strongly lensed by the galaxy cluster MACSJ0416. Seven of these transients were identified by Yan et al. (2023). The nine sources are all likely red supergiants with temperatures of T~4000K. We present new longslit spectroscopy of the Warhol arc acquired with Keck-I and the Large Binocular Telescope, and use these data to constrain the arc's oxygen abundance to be 12+log(O/H)=8.45+-0.08. We perform a microlensing simulation on synthetic stellar populations using a range of stellar metallicities and initial mass function 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 (log(Z*/Zsun)=-0.24) produces a simulated temperature distribution that is consistent with the observations, while lower stellar metallicities (log(Z*/Zsun)<-0.75) produce simulated temperatures that are inconsistent with the observations. The expected detection rate is strongly anticorrelated with the IMF slope for alpha>1.2. For the canonical IMF slope alpha=2.35, the simulation yields expected transient detection rates that agree with the observed detection rates in the HST Flashlights filters, but over predicts the detection rate by a factor of ~3-12 (<2sigma tension) in the JWST filters. The simulated detection rate is sensitive to the choice of stellar metallicity, with lower metallicities (log(Z*/Zsun)<-0.75) yielding a significantly lower simulated detection rate that further reduces the modest tension with the observations.

Figures (18)

• Figure 1: Top: NIRCam RGB color images of the Warhol arc in all four visits. Blue, F090W + F115W + F150W; Green, F200W + F277W; Red, F356W + F410M + F444W. Bottom: Difference images between the different visits in the F277W filter, with the 9 transients identified. Sources W1$-$W7 were originally identified by Yan_2023. To aid in visual identification of the transients, these difference images have been convolved with a 1-pixel Gaussian kernel. The orientation of these images is North pointing upward and East pointing to the left.
• Figure 2: Left: Slits used for our Keck MOSFIRE observations of the Warhol arc, shown on the JWST NIRCam F115W image of the galaxy. Each slit is 0.7$\arcsec$ wide. Right: H$\alpha$ flux measured from the MOSFIRE spectroscopy in twelve 0.7$\arcsec \times 0.7\arcsec$ boxes. Boxes labeled with "n" are on the negative-parity side of the critical curve, and boxes labeled with "p" are in the symmetrical position on the positive-parity side of the critical curve. The orientation of these images is North pointing upward and East pointing to the left.
• Figure 3: [MUSE O2] $\lambda$3727 flux-intensity map of the Warhol arc. The white box shows the 2.8$\arcsec$ square aperture used to extract the 1D spectrum. The orientation of this image is North pointing upward and East pointing to the left.
• Figure 4: MUSE spectroscopy of the [O2] $\lambda$3727, [Ne3] $\lambda$3869, H$\delta$, and H$\gamma$ emission lines, along with LUCI spectroscopy of the [O3] $\lambda$5007, and H$\alpha$ emission lines, in the Warhol arc. The black line shows the spectrum in a 30 Å window centered on each emission line, and the shaded gray region shows the 1$\sigma$ uncertainty interval. The dashed pink lines show our best-fitting Gaussian profiles.
• Figure 5: Left: Observed photometry and best-fitting stellar model for transient source W3. The shaded diamonds show the observed photometry and the open circles indicate the synthetic photometry from the best-fitting model. Right: The posterior of the logarithm of the stellar temperature for source W3.