Stellar Metallicities and Gradients in the Isolated, Quenched Low-mass Galaxy Tucana
Résumé
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−1e (‑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+0.07−0.06 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.
Domaines
Planète et Univers [physics]Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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