Metallicity distribution of red giants in the Inner Galaxy from Near Infrared spectra

TL;DR

This study analyzes near-infrared spectra of red giants in the Inner Galaxy to map their metallicity distribution, revealing a continuous transition between bulge and disc populations, suggesting a possible common component like the Galactic bar.

Contribution

It provides the first large-scale near-infrared spectroscopic survey of inner Galaxy red giants to study their metallicity distribution and Galactic structure.

Findings

Identified two stellar populations with different metallicities.

Observed a continuous variation in stellar populations from bulge to disc.

Suggested the presence of a single component, possibly the Galactic bar.

Abstract

In this paper, we present low resolution (R=500) near-infrared spectra for selected and serendipitous sources in six inner in-plane Galactic fields, with the aim of analysing the stellar content present. From the equivalent widths of the main features of the K band spectra (the NaI, CaI and CO bandheads) we have derived the metallicities of the sources by means of the empirical scale obtained by Ram\'irez et al. (2000) and Frogel et al. (2001) for luminous red giants. Our results show how the mean metallicity of the sample varies with Galactic longitude. We find two groups of stars, one whose [Fe/H] is similar to the values obtained for the bulge in other studies (Molla et al. 2000; Schultehis et al 2003), and a second one with a metallicity similar to that of the inner parts of the disc (Rocha-Pinto et al. 2006). The relative density of both groups of stars in our sample varies in a continuous way from the bulge to the disc. This could hint at the existence of a single component apart from the disc and bulge, running from l=7 to l=27 and able to transport disc stars inwards and bulge stars outwards, which could be the Galactic bar that has been detected in previous works as an overdensity of stars located at those galactic coordinates (Hammersley et al. 1994, 2000; Picaud et al. 2003).