Stellar Metallicities and Gradients in the Isolated, Quenched Low-mass Galaxy Tucana

We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope narrowband (F395N) calcium H and K imaging. Our sample is a factor of ∼7 larger than what is available from previous studies. Our main findings are as follows. (i) A global metallicity distribution function (MDF) with 〈 [Fe/H] 〉 = − 1.55 − 0.04 + 0.04 and σ [Fe/H] = 0.54 − 0.03 + 0.03 . (ii) A metallicity gradient of −0.54 ± 0.07 dex R e − 1 (−2.1 ± 0.3 dex kpc⁻¹) over the extent of our imaging (∼2.5 R _e ), which is steeper than literature measurements. Our finding is consistent with predicted gradients from the publicly available FIRE-2 simulations, in which bursty star formation creates stellar population gradients and dark matter cores. (iii) Tucana’s bifurcated RGB has distinct metallicities: a blue RGB with 〈 [Fe/H] 〉 = − 1.78 − 0.06 + 0.06 and σ [Fe/H] = 0.44 − 0.06 + 0.07 and a red RGB with 〈 [Fe/H] 〉 = − 1.08 − 0.07 + 0.07 and σ [Fe/H] = 0.42 − 0.06 + 0.06 . (iv) At fixed stellar mass, Tucana is more metal-rich than Milky Way satellites by ∼0.4 dex, but its blue RGB is chemically comparable to the satellites. Tucana’s MDF appears consistent with star-forming isolated dwarfs, though MDFs of the latter are not as well populated. (v) About 2% of Tucana’s stars have [Fe/H] < −3% and 20% have [Fe/H] > −1. We provide a catalog for community spectroscopic follow-up.