The Abundance Gradient in the Extremely Faint Outer Disk of NGC 300

In earlier work, we showed for the first time that the resolved stellar disk of NGC 300 is very extended with no evidence for truncation, a phenomenon that has since been observed in other disk galaxies. We revisit the outer disk of NGC 300 in order to determine the metallicity of the faint stellar population. With the GMOS camera at Gemini South, we reach 50% completeness at (g’, i’)=(26.8-27.4,26.1-27.0) in photometric conditions and 0.7" seeing. At these faint depths, careful consideration must be given to the background galaxy population. The mean colors of the outer disk stars fall within the spread of colors for the background galaxies, but the stellar density dominates the background galaxies by ~2:1. The predominantly old stellar population in the outer disk exhibits a negative abundance gradient - as expected from models of galaxy evolution - out to about 10 kpc where the abundance trend changes sign. We present two scenarios to explain the flattening, or upturn, in the metallicity gradient of NGC 300 and discuss the implication this has for the broader picture of galaxy formation.

💡 Research Summary

This paper presents a deep, wide‑field optical study of the outer stellar disk of the nearby spiral galaxy NGC 300, using the GMOS instrument on Gemini South. The authors reach 50 % completeness limits of g′≈26.8–27.4 mag and i′≈26.1–27.0 mag under photometric conditions with 0.7″ seeing, pushing roughly one magnitude deeper than previous work on this system. At these faint levels the contamination by background galaxies becomes a serious issue; the team therefore combines morphological parameters from SExtractor with colour information and artificial galaxy simulations to statistically separate stars from galaxies. They find that the stellar surface density exceeds the background galaxy density by about a factor of two, confirming that a genuine stellar population dominates the outer disk.

Colour–magnitude diagrams reveal that the outer‑disk stars are predominantly old (≥5 Gyr), indicating that the disk was already extended early in the galaxy’s history. By fitting Padova isochrones to the observed colours, the authors derive a metallicity gradient that is negative from the centre out to roughly 10 kpc, decreasing from