Early results from GLASS-JWST VIII: An Extremely Magnified Blue Supergiant Star at Redshift 2.65 in the Abell 2744 Cluster Field (2207.11658v2)

Abstract: We report the discovery of an extremely magnified star at redshift $z=2.65$ in James Webb Space Telescope (JWST) NIRISS pre-imaging of the Abell 2744 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 $\sim 0.15"$ 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 which 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 7,000--12,000 K.

• The paper identifies an extremely magnified blue supergiant at redshift 2.65 using gravitational lensing and image differencing techniques.
• It employs JWST’s NIRISS imaging alongside archival HST data to extract precise photometric properties and determine stellar temperature.
• The results highlight JWST’s potential in probing distant cosmic phenomena and inform future studies on dark matter and early stellar evolution.

Overview of the Discovery of an Extremely Magnified Blue Supergiant Star at Redshift 2.65

The paper "Early results from GLASS-JWST VIII: An Extremely Magnified Blue Supergiant Star at Redshift 2.65 in the Abell 2744 Cluster Field" presents the discovery and analysis of a highly magnified star behind the galaxy cluster Abell 2744, identified using the James Webb Space Telescope (JWST). The star, located at a redshift of 2.65, exemplifies the significant capabilities of JWST in detecting distant astronomical phenomena through gravitational lensing.

Key Findings

The researchers discovered an extremely magnified star in the galaxy cluster Abell 2744 using JWST's NIRISS pre-imaging and confirmed its characteristics through subtracting JWST images from archival Hubble Space Telescope (HST) images. The star's host galaxy lies on a fold caustic of the galaxy cluster's lens, causing a pair of mirrored images of the star. This discovery was facilitated by microlensing, an event where the background star's brightness increases due to the gravitational influence of a foreground object, which is unlikely to be a supernova due to the short time delay between the mirrored images.

Photometric data from the JWST indicated that the star exhibits a Balmer break, constraining its temperature to be approximately 7,000-12,000 Kelvin. This result, along with the star's extreme magnification exceeding a factor of 1,000, suggests it is a blue supergiant.

Methodology

Data acquisition was executed using the JWST Director’s Discretionary Early Release Science Program. The analyses focused on NIRISS images taken through multiple filters, namely F115W, F150W, and F200W. By comparing these with HST templates in corresponding wavelengths, the team employed image differencing techniques to identify the presence of the transient star. Magnification factors and precise positions relative to the critical curve were derived using magnification maps and existing gravitational lens models.

Additionally, the authors inferred gravitational lens properties using various lens model predictions, assessing the optical depth of microlensing and the required magnification levels for different stellar types. This analysis pointed towards a blue supergiant star requiring high magnification to achieve its observed apparent brightness.

Implications and Future Directions

The detection of this star illustrates JWST's potential in discovering and analyzing individual stars at cosmological distances. It underscores advancements in detecting microlensing events and the role such studies can play in probing the structure of the universe and testing theories related to dark matter and compact astrophysical objects.

This research opens paths for further exploration of galaxy cluster fields and potential microlensing events. Future endeavors may rely on continuous JWST observations to identify more such occurrences and foster a deeper understanding of the universe’s early stages and the intricacies of gravitational lensing. Further detections of microlensed stars may also allow constraints on compact dark matter models and the characteristics of Population III stars.

In conclusion, this paper exemplifies the usage of cutting-edge space telescope technology to advance cosmic frontier exploration, enriching our understanding of stellar formation and evolution at high redshifts.