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• The spatially resolved star formation history of the dwarf spiral galaxy NGC 5474

  2024. Giacomo Bortolini (et al.). Monthly notices of the Royal Astronomical Society 527 (3), 5339-5355

  Artikel

  We study the resolved stellar populations and derive the star formation history of NGC 5474, a peculiar star-forming dwarf galaxy at a distance of ∼7 Mpc, using Hubble Space Telescope Advanced Camera for Surveys data from the Legacy Extragalactic UV Survey (LEGUS) programme. We apply an improved colour–magnitude diagram fitting technique based on the code SFERA and use the latest PARSEC–COLIBRI stellar models. Our results are the following. The off-centre bulge-like structure, suggested to constitute the bulge of the galaxy, is dominated by star formation (SF) activity initiated 14 Gyr ago and lasted at least up to 1 Gyr ago. Nevertheless, this component shows clear evidence of prolonged SF activity (lasting until ∼10 Myr ago). We estimate the total stellar mass of the bulge-like structure to be (5.0 ± 0.3) × 10⁸ M_⊙. Such a mass is consistent with published suggestions that this structure is in fact an independent system orbiting around and not within NGC 5474’s disc. The stellar overdensity located to the South–West of the bulge-like structure shows a significant SF event older than 1 Gyr, while it is characterized by two recent peaks of SF, around ∼10 and ∼100 Myr ago. In the last Gyr, the behaviour of the stellar disc is consistent with what is known in the literature as ‘gasping’. The synchronized burst at 10–35 Myr in all components might hint to the recent gravitational interaction between the stellar bulge-like structure and the disc of NGC 5474.

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

  2024. Giacomo Bortolini (et al.). Astronomy and Astrophysics 689

  Artikel

  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.

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