Abstract
Abstract
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.
Funder
National Science Foundation
NASA ∣ NASA Headquarters
Publisher
American Astronomical Society