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| Paper IPM / Astronomy / 18377 |
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The evolution and star formation history (SFH) of NGC 6822, an isolated dwarf irregular galaxy in the local group, has been studied utilizing evolved asymptotic giant branch (AGB) stars with a period of longer than 100 days, known as long-period variables (LPV). As LPVs pulsate, their maximum luminosity can be related to their birth mass. Padova isochrones are used to obtain the stars' birth mass, age, and pulsation duration (the period that the stars are in the LPV stage) to estimate the star formation rate (SFR) in NGC 6822. The photometric data of 329 variable stars, including LPV and spectroscopy-confirmed C-rich AGB stars, were collected from several published surveys. The SFH has been studied in two regions, the bar region and the outer region. In addition to obtaining the galaxy's SFH assuming a constant metallicity, an age-metallicity relation (AMR) is also adopted to account for the variation in the galaxy's chemical content throughout its lifetime. It is found that the onset of star formation in NGC 6822 traces back to â¼ 12.5 and â¼ 13 Gyr ago in the bar and outer regions, respectively. Furthermore, there are 3 main epochs of star formation peaking at log(t) â¼ 9.66 (4.6 Gyr), log(t) â¼ 9.21 (1.6 Gyr), and log(t) â¼ 7.68 (50 Myr), where t is the look-back time. The significant peak around log(t) â¼ 9.66 is consistent with the previous research suggesting that NGC 6822 passed through the virial radius of the Milky Way around 3-4 Gyr ago.
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