The multi-age stellar populations of Terzan 5 as revealed by JWST

Abstract

The James Webb Space Telescope provides an exciting opportunity to investigate stellar systems located in heavily obscured regions like the Galactic bulge. Possibly, the most enigmatic among them is Terzan 5: long classified as a globular cluster, it is now known to host distinct stellar populations with different iron abundances (ranging approximately from [Fe/H]=-$0.8$ to [Fe/H]=$+0.3$ dex). Indeed the chemical and structural properties collected so far suggest that it is the remnant of one of the primordial clumps that contributed to the early assembly of the bulge, a so-called "Bulge Fossil Fragment". Here we present a new photometric analysis of Terzan 5 based on JWST/NIRCam observations in the F115W and F200W filters, as well as archival HST/ACS optical (F606W and F814W) data. The dataset overcomes the severe and spatially variable extinction along the line of sight and yields the deepest color-magnitude diagram ever obtained for Terzan 5. Proper motion selections and high-resolution differential reddening corrections allow us to isolate bona fide cluster members and to provide an unprecedented view of the main-sequence turn-off region. We clearly identify two main components and determine their respective ages: the old, sub-solar component has an age of 12.5 $\pm$ 0.5 Gyr, while the super-solar component is significantly younger with an age of 4.7 $\pm$ 0.5 Gyr. Interestingly, we also find hints of an even younger main sequence turn-off and sub-giant branch, consistent with the presence of a further stellar component with an age of 3.8 $\pm$ 0.5 Gyr. There is also evidence of a blue plume populated by stars as bright as $m_{\rm F115W}\sim 17.4$, suggesting a prolonged period of star formation extending up to 2.5 Gyr ago.

Figures (12)

• Figure 1: JWST/NIRCam mosaic of Terzan 5 obtained with module B in the F115W filter (composite of all dithered exposures from SW detectors onto a common grid).
• Figure 2: Illustration of the FoV coverage for the dataset of observations of Terzan 5 used in this work. Grey dots represent stars detected in the optical filters. Blue rectangles show the HST/ACS field of view for the first two epochs (SNAP 9799, PI: Rich; GO 12933, PI: Ferraro), while light blue rectangles indicate the third epoch (GO/DD 14061, PI: Ferraro). Red rectangles mark the JWST/NIRCAM module B SW detector boundaries from the first observing cycle (GO 5502, PI: Ferraro, 2024), and orange rectangles show the corresponding fields from the second cycle (GO 5502, PI: Ferraro, 2025). Each of the FoVs shown here corresponds to the pointing of the first exposure in each dataset. All coordinates are expressed as projected offsets in arcseconds ($\Delta x$, $\Delta y$) relative to the center of Terzan 5 ($\alpha_{\mathrm{J2000}} = 17^{\mathrm{h}}48^{\mathrm{m}}4.85^{\mathrm{s}},\quad \delta_{\mathrm{J2000}} = -24^\circ46'44.6"$; Lanzoni2010).
• Figure 3: ($m_{F814W}, m_{F606W}-m_{F814W}$) and ($m_{F115W}, m_{F115W}-m_{F200W}$) CMDs for Terzan 5 obtained from the joint catalog of HST and JWST datasets. This bipanel visualization highlights the complementary wavelength coverage of optical (left panel) and NIR (right panel) data for studying systems like Terzan 5.
• Figure 4: Proper–motion selection of Terzan 5 members. Panels a) and b) display the optical CMDs $(m_{F606W}-m_{F814W}, m_{F814W})$ while panels c) and d) show the NIR ones $(m_{F115W}-m_{F200W},m_{F115W})$ . The left panels in each row show in black all the stars that satisfy the photometric–quality criteria, while panels b) and d) highlight in red only those classified as likely cluster members from the proper–motion analysis. The horizontal dashed lines mark the $m_{F814W}$ magnitude bins adopted in the proper–motion selection. The panel on the upper right shows the corresponding VPDs for each magnitude interval: in each sub–panel, the red dots mark stars retained as members and the black dots indicate rejected (Galactic field) sources. The effectiveness of the proper motion membership selection is clearly demonstrated by the rejection of Galactic field MS stars drawing the evident blue plume (bluer and brighter than Terzan 5's MS-TO) in panels a) and c).
• Figure 5: The figure illustrates the effect of different assumptions of the $R_V$ parameter on the isochrone position in the CMDs. The black dashed line is the 12.5 Gyr isochrone bressan2012 that best-fits the NIR CMD (right panel) under the assumption of the odonnell94 extinction law with the standard value $R_V=3.1$, requiring $E(B-V) =2.08$. Clearly, this isochrone is unable to also match the optical CMD (see the black dashed line in the left panel). The red solid line is the same isochrone plotted by assuming the same extinction law, but adopting $R_V=2.5$: it clearly well reproduces simultaneously both the NIR and the optical CMDs, with $E(B-V)=2.86$.