A 3D view of dwarf galaxies with Gaia and VLT/FLAMES I. The Sculptor dwarf spheroidal

We present a new homogeneous survey of VLT/FLAMES LR8 line-of-sight radial velocities (v(los)) for 1604 resolved red giant branch stars in the Sculptor dwarf spheroidal galaxy. In addition, we provide reliable Ca II triplet metallicities, [Fe/H], for 1339 of these stars. From this combination of new observations (2257 individual spectra) with ESO archival data (2389 spectra), we obtain the largest and most complete sample of v(los) and [Fe/H] measurements for individual stars in any dwarf galaxy. Our sample includes VLT/FLAMES LR8 spectra for similar to 55% of the red giant branch stars at G < 20 from Gaia DR3, and >70% of the brightest stars, G < 18.75. Our spectroscopic velocities are combined with Gaia DR3 proper motions and parallax measurements for a new and more precise membership analysis. We look again at the global characteristics of Sculptor, deriving a mean metallicity of <[Fe/H]> = -1. 82 +/- 0. 45 and a mean line-of-sight velocity of < v(los)> = +111.2 +/- 0.25 km s(-1). There is a clear metallicity gradient in Sculptor, -0.7deg dex(-1), with the most metal-rich population being the most centrally concentrated. Furthermore, the most metal-poor population in Sculptor, [Fe/H] < 2.5, appears to show kinematic properties distinct from the rest of the stellar population. Finally, we combine our results with the exquisite Gaia DR3 multi-colour photometry to further investigate the colour-magnitude diagram of the resolved stellar population in Sculptor. Our detailed analysis shows a similar global picture as previous studies, but with much more precise detail, revealing that Sculptor has more complex properties than previously thought. This survey emphasises the role of the stellar spectroscopy technique and this galaxy as a benchmark system for modelling galaxy formation and evolution on small scales.

Automated summary

We present a new survey of radial velocities and metallicities for red giant stars in the Sculptor dwarf spheroidal galaxy, which includes the largest and most complete sample in any dwarf galaxy. Our analysis reveals a metallicity gradient and a distinct kinematic property for the most metal-poor population in Sculptor. In addition, combining our results with Gaia DR3 photometry provides a more detailed understanding of the resolved stellar population in Sculptor. This survey highlights the importance of stellar spectroscopy in modeling galaxy formation and evolution on small scales.