Title: Characterizing The Star Cluster Populations in Stephan’s Quintet Using HST and JWST Observations
Authors: P. Aromal, S. C. Gallagher, K. Fedotov, N. Bastian, U. Lisenfeld, J. C. Charlton, P. N. Appleton, J. Braine, K. E. Johnson, P. Tzanavaris, B. H. C. Emonts, A. Togi, C. K. Xu, P. Guillard, L. Barcos-Muñoz, L. J. Smith, and I. S. Konstantopoulos
First Author’s Institution: Physics and Astronomy Department, University of Western Ontario, London, ON, Canada
Status: Accepted to ApJ, available on arXiv
On July 12th, 2022 the first images taken with the James Webb Space Telescope (JWST) were released to the public. All of the astronomers in my department gathered together to watch the images be revealed in real time. It was exciting for everyone; from graduate students getting to see a glimpse into the future possibilities of their fields, to retired professors getting to see the fruits of their decades-long labour in advocating for the telescope to be built.
One image that was showcased was of Stephan’s Quintet (SQ), an actively interacting galaxy group. We were all immediately impressed by the clarity of the star-forming regions in the dense gas between the galaxies in the image. Now, over three years later, the authors of this paper lay out a comprehensive study of the star clusters in those same regions, taking advantage of JWST’s multi-wavelength imaging capabilities.
HST + JWST = OMG!
While JWST’s depth and resolution is exciting for those studying high-redshift galaxies, for local universe astronomers JWST really shines when used in combination with the Hubble Space Telescope (HST). The star clusters in SQ were already cataloged in 2015 using HST, but the filters used in the imaging only captured optical and very near infrared (IR) wavelengths. If you want to get enough data to be able to accurately estimate the ages of these star clusters, especially accounting for reddening caused by surrounding dust, you need to push your imaging further into the IR.
Unlike HST, JWST can take images in two wavelength filters at once, meaning you can get more data with roughly the same observing time. Here they imaged the same star clusters that HST looked at with five new JWST filters, all at longer IR wavelengths than HST. Together, they had flux measurements at ten different wavelengths across the optical and IR spectrum for each star cluster, meaning they could perform spectral energy distribution (SED) fitting.
Go Ahead, Guess My Age
The best way to estimate an unresolved star cluster’s age is to get its full spectrum of light and fit spectroscopic models with varying ages to it until the model matches the observations. But, if you have individual brightness measurements at many different, spread-out wavelengths you can still fit spectra models to them and make a best guess despite the gaps in your full spectrum. Here the authors compare their multi-wavelength data to CIGALE SED models, allowing both the cluster ages and amount of dust extinction to vary.
This is where the long-wavelength JWST imaging is most necessary, because it is sometimes difficult to determine a cluster’s age with this method. For instance, is the light a cluster is giving off mostly red because its stars are very old, or is it because its stars are actually bluer and younger and the cluster just appears red due to foreground dust?
IR imaging can “see through” any dust and break this age-extinction degeneracy. For most of the clusters, the authors found that the original HST-only SED fitting age estimates were accurate, but for 121 clusters in the sample (about 8% of the total), the JWST imaging made a significant change in the age estimates, shifting them to younger ages.
Where Do The Hip, Young Star Clusters Hang Out?
Once they has their updated ages for the clusters they then mapped out where they were located in SQ, and how they traced the known tidal structures in the group. These are structures, usually consisting of gas, dust, and stars, which are formed from the tidal forces galaxies exert on one another as they interact and merge. They found that throughout all tidal regions of SQ that there was a large number of low-mass, young star clusters, all about 3-5 Myr old. This timescale lines up with previous studies’ estimate of when NGC 7318b was throught to first fall into SQ, compressing the gas in the group and creating tidal shocks that would trigger star formation.
Additionally, their star cluster age distribution for the group (see Figure 2) has a second, broader peak of higher-mass clusters with ages areound 200 Myr. This would corresspond to when the most recent encounter between NGC 7320c (which has now passed through the group and is out of frame in Figure 1) and NGC 7319 was estimated to have occurred.
Going even older, they found that the star clusters in the group that are more than 1 Gyr old are also the most massive, and are predominantly concentrated around NGC 7318a. This is the most massive elliptical galaxy in the group, and the most likely to have a rich, old star cluster population, formed prior to any tidal interactions.
Taken together, the updated age measurements of the star clusters in SQ is not only a study of star formation history, but also of the galaxy-galaxy interaction history in the system!
Looking Ahead
While the suthors have improved the age estimates of the clusters in SQ, there are still limitations to their analysis. Their estimated star formation rate for the group is much lower than that estimated with other tracers, such as H alpha emission, meaning this study may still be missing a fraction of very young star clusters. The most likely culprits are embedded clusters which haven’t had enough time to expel the surrounding gas from the massive clouds they formed within.
Future work will attempt to identify these embedded clusters using their near IR JWST imaging, since in this study they focused only on the previously HST-identified clusters. In addition, combining their JWST data with high-resolution radio observations taken with ALMA will allow them to study the gas in the group more closely and how it influences the star formation.
This work highlights JWST’s excellent application to star cluster observations, building on the data we already have from decades of HST use, and it looks like we’re only just getting started!
Astrobite edited by Skylar Grayson
Featured image credit: ESA, Wikimedia Commons
