A lack of classical Cepheids in the inner part of the Galactic disk

TL;DR

This study reports a significant scarcity of classical Cepheids within the inner 2.5 kpc of the Galactic disk, suggesting a gap in young stellar populations in this region, based on infrared survey data and extinction corrections.

Contribution

It provides new observational evidence of the distribution of classical Cepheids in the inner Galaxy, highlighting a lack of these stars in the central 2.5 kpc, and discusses the impact of reddening corrections on distance estimates.

Findings

Most Cepheids are located beyond the Galactic Centre.

A notable absence of Cepheids within 2.5 kpc of the Galactic Centre.

Star-forming regions show a similar distribution pattern.

Abstract

Recent large-scale infrared surveys have been revealing stellar populations in the inner Galaxy seen through strong interstellar extinction in the disk. In particular, classical Cepheids with their period-luminosity and period-age relations are useful tracers of Galactic structure and evolution. Interesting groups of Cepheids reported recently include four Cepheids in the Nuclear Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga et al. and those spread across the inner part of the disk reported by Dekany and collaborators. We here report our discovery of nearly thirty classical Cepheids towards the bulge region, some of which are common with Dekany et al., and discuss the large impact of the reddening correction on distance estimates for these objects. Assuming that the four Cepheids in the NSD are located at the distance of the Galactic Centre and that the near-infrared extinction law, i.e. wavelength dependency of the interstellar extinction, is not systematically different between the NSD and other bulge lines-of-sight, most of the other Cepheids presented here are located significantly further than the Galactic Centre. This suggests a lack of Cepheids in the inner 2.5 kpc region of the Galactic disk except the NSD. Recent radio observations show a similar distribution of star-forming regions.