Imaging of I Zw 18 by JWST II. Spatially resolved star formation history

Abstract

Context. The blue compact dwarf (BCD) galaxy I Zw 18 is one of the most metal-poor (Z ∼ 3% Z⊙) star-forming galaxies known in the local Universe. Since its discovery, the evolutionary status of this system has been at the center of numerous debates within the astronomical community. Aims. We aim to probe and resolve the stellar populations of I Zw 18 in the near-IR using JWST/NIRCam's unprecedented imaging spatial resolution and sensitivity. Additionally, our goal is to derive the spatially resolved star formation history (SFH) of the galaxy within the last 1 Gyr, and to provide constraints for older epochs. Methods. We used the point spread function fitting photometry package DOLPHOT to measure positions and fluxes of point sources in the F115W and F200W filters' images of I Zw 18, acquired as part of the JWST GTO ID 1233 (PI: Meixner). Furthermore, to derive I Zw 18's SFH, we applied a state-of-the-art color-magnitude diagram (CMD) fitting technique (SFERA 2.0), using two independent sets of stellar models: PARSEC-COLIBRI and MIST. Results. Our analysis of I Zw 18's CMD reveal three main stellar populations: one younger than ∼30 Myr, mainly associated with the northwest star-forming (SF) region; an intermediate-age population (∼100-800 Myr), associated with the southeast SF region; and a red and faint population, linked to the underlying halo of the galaxy, older than 1 Gyr and possibly as old as 13.8 Gyr. The main body of the galaxy shows a very low star formation rate (SFR) of ∼10-4 M⊙ yr-1 between 1 and 13.8 Gyr ago. In the last billion years, I Zw 18 shows an increasing trend, culminating in two strong bursts of SF around ∼10 and ∼100 Myr ago. Notably, I Zw 18 Component C mimics the evolution of the main body, but with lower SFRs on average. Conclusions. Our results confirm that I Zw 18 is populated by stars of all ages, without any major gaps. Thus, I Zw 18 is not a truly young galaxy, but rather a system characterized by an old underlying stellar halo, in agreement with what has been found in other BCDs by similar studies. The low SF activity exhibited at epochs older than 1 Gyr is in agreement with the "slow cooking"dwarf scenario proposed in the literature, and could have contributed to its low metal content. The galaxy is now experiencing its strongest episode of star formation (∼0.6 M⊙ yr-1 ) mainly located in the northwest region. A recent gravitational interaction between the main body and Component C is the most likely explanation for this starburst.