The extended structure of the dwarf irregular galaxy Sagittarius

TL;DR

This study reveals that the dwarf irregular galaxy Sagittarius has a more extended stellar structure than previously known, with a smooth, flattened old star distribution and a complex, asymmetric HI gas morphology, lacking systemic rotation.

Contribution

It provides the first detailed mapping of Sagittarius's stellar and HI structures, showing an extended, exponential stellar profile and complex gas morphology without rotation.

Findings

Stellar body extends up to 1600 pc, more than previously thought.

Old and intermediate stars are smoothly distributed and flattened.

HI gas shows asymmetry, multiple maxima, and no systemic rotation.

Abstract

We present a detailed study of the stellar and HI structure of the dwarf irregular galaxy Sagittarius. We use new deep and wide field photometry to trace the surface brightness profile of the galaxy out to ~5.0' (corresponding to ~1600 pc) and down to $\mu_V\simeq 30.0$ mag/arcsec$^2$, thus showing that the stellar body of the galaxy is much more extended than previously believed, and it is similarly (or more) extended than the overall HI distribution. The whole major-axis profile is consistent with a pure exponential, with a scale radius of $\simeq 340$ pc. The surface density maps reveal that the distribution of old and intermediate-age stars is smooth and remarkably flattened out to its edges, while the associated HI has a much rounder shape, is off-centred and presents multiple density maxima and a significant hole. No clear sign of systemic rotation is detectable in the complex HI velocity field. No metallicity gradient is detected in the old and intermediate age population of the galaxy, and we confirm that this population has a much more extended distribution than young stars (age$\lt 1$ Gyr).