Search for: All records

In this work, we present a constraint on the abundance of supergiant (SG) stars at redshiftz ≈ 1, based on recent observations of a strongly lensed arc at this redshift. First we derived a free-form model of MACS J0416.1-2403 using data from the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) program. The new lens model is based on 72 multiply lensed galaxies that produce 214 multiple images, making it the largest sample of spectroscopically confirmed lensed galaxies on this cluster. The larger coverage in BUFFALO allowed us to measure the shear up to the outskirts of the cluster, and extend the range of lensing constraints up to ∼1 Mpc from the central region, providing a mass estimate up to this radius. As an application, we make predictions for the number of high-redshift multiply lensed galaxies detected in future observations with theJames WebbSpace Telescope (JWST). Then we focus on a previously known lensed galaxy atz = 1.0054, nicknamed Spock, which contains four previously reported transients. We interpret these transients as microcaustic crossings of SG stars and explain how we computed the probability of such events. Based on simplifications regarding the stellar evolution, we find that microlensing (by stars in the intracluster medium) of SG stars atz = 1.0054 can fully explain these events. The inferred abundance of SG stars is consistent with either (1) a number density of stars with bolometric luminosities beyond the Humphreys-Davidson (HD) limit (L_max ≈ 6 × 10⁵ L_⊙for red stars), which is below ∼400 stars kpc⁻², or (2) the absence of stars beyond the HD limit but with a SG number density of ∼9000 kpc⁻²for stars with luminosities between 10⁵ L_⊙and 6 × 10⁵ L_⊙. This is equivalent to one SG star per 10 × 10 pc². Finally, we make predictions for future observations with JWST’s NIRcam. We find that in observations made with theF200Wfilter that reach 29 mag AB, if cool red SG stars exist atz ≈ 1 beyond the HD limit, they should be easily detected in this arc.

We report the discovery of an extremely magnified star at redshiftz= 2.65 in the James Webb Space Telescope (JWST) NIRISS pre-imaging of the A2744 galaxy-cluster field. The star’s background host galaxy lies on a fold caustic of the foreground lens, and the cluster creates a pair of images of the region close to the lensed star. We identified the bright transient in one of the merging images at a distance of ∼0.″15 from the critical curve by subtracting the JWST F115W and F150W imaging from coadditions of archival Hubble Space Telescope (HST) F105W and F125W images and F140W and F160W images, respectively. Since the time delay between the two images should be only hours, the transient must be the microlensing event of an individual star, as opposed to a luminous stellar explosion that would persist for days to months. Analysis of individual exposures suggests that the star’s magnification is not changing rapidly during the observations. From photometry of the point source through the F115W, F150W, and F200W filters, we identify a strong Balmer break, and modeling allows us to constrain the star’s temperature to be approximately 7000–12,000 K.