The Pristine Inner Galaxy Survey (PIGS) IV: A photometric metallicity analysis of the Sagittarius dwarf spheroidal galaxy

Abstract

We present a comprehensive metallicity analysis of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) using Pristine CaHK photometry. We base our member selection on Gaia EDR3 astrometry applying a magnitude limit at 𝐺 0 = 17.3, and our population study on the metallicity-sensitive photometry from the Pristine Inner Galaxy Survey (PIGS). Working with photometric metallicities instead of spectroscopic metallicities allows us to cover an unprecedented large area (∼ 100 square degrees) of the dwarf galaxy, and to study the spatial distribution of its members as function of metallicity with little selection effects. Our study compares the spatial distributions of a metal-poor population of 9719 stars with [Fe/H] < −1.3 and a metal rich one of 30115 stars with [Fe/H] > −1.0. The photometric Sgr sample also allows us to assemble the largest sample of 1150 very metal-poor Sgr candidates ([Fe/H] < −2.0). By investigating and fitting the spatial properties of the metal-rich and metal-poor population, we find a negative metallicity gradient which extends up to 12 degrees from the Sgr center (or ∼ 5.5 kpc at the distance of Sgr), the limit of our footprint. We conclude that the relative number of metal-poor stars increases in the outer areas of the galaxy, while the central region is dominated by metal-rich stars. These finding suggest an outside-in formation process and are an indication of the extended formation history of Sgr, which has been affected by the tidal interaction between Sgr and the Milky Way.

The metallicity distribution in the core of the Sagittarius dwarf spheroidal: Minimising the metallicity biases

“…Given the advanced stage of disruption, the chemical abundance information that low-mass stars recorded in their atmospheres since the epoch of their birth, is a key element to reconstruct the characteristics of the progenitor of the system that we observe today. Indeed, the presence of a metallicity gradient within the Sgr main body (see Vitali et al 2022, for a recent thorough analysis and references) and along the Sgr stream (Bellazzini et al 2006;Chou et al 2007;Monaco et al 2007;Carlin et al 2012;de Boer et al 2014;Gibbons et al 2017;Yang et al 2019;Hayes et al 2020;Ramos et al 2022) is firmly established, suggesting a complex interplay between the metallicity and kinematics that traces the combination of initial conditions in the progenitor and of the disruption process (Gibbons et al 2017;Yang et al 2019;Johnson et al 2020;Ramos et al 2022).…”

“…As briefly reminded above, the Sgr system is very extended, also when considering only the main body (Vasiliev & Belokurov 2020;Majewski et al 2003), and a metallicity gradient with distance from the centre of the galaxy is observed at any scale (see, e.g., Bellazzini et al 1999;Alard 2001;Chou et al 2007;Majewski et al 2013;Vitali et al 2022). This means that while we now can gain a much stronger control on metallicity biases, the MDF will unavoidably depend on the radial range sampled.…”