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 (HST) narrow-band (F395N) Calcium H & K (CaHK) imaging. Our sample is a factor of $\sim 7$ larger than what is published. Our main findings are: (i) A global metallicity distribution function (MDF) with $⟨[Fe\mathrm{/}H]⟩=-1.55\pm 0.04$ and ${\sigma }_{[Fe\mathrm{/}H]}=0.54\pm 0.03$; (ii) A metallicity gradient of $-0.54\pm 0.07$ dex $R_e^{-1}$ ($-2.1\pm 0.3$ dex kpc${}^{-1}$) over the extent of our imaging ($\sim 2.5R_e$), 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 $⟨[Fe\mathrm{/}H]⟩=-1.78\pm 0.06$ and ${\sigma }_{[Fe\mathrm{/}H]}=0.44_{-0.06}^{+0.07}$, and a red RGB with $⟨[Fe\mathrm{/}H]⟩=-1.08\pm 0.07$ and ${\sigma }_{[Fe\mathrm{/}H]}=0.42\pm 0.06$; (iv) At fixed stellar mass, Tucana is more MR than MW satellites by $\sim 0.4$ 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) $\sim 2$% of Tucana’s stars have $[Fe\mathrm{/}H]<-3$ and 20% $[Fe\mathrm{/}H]>-1$. We provide a catalog for community spectroscopic follow-up.