Detecting hot stars in the Galactic centre with combined near- and mid-infrared photometry

Context. The Galactic centre (GC) is a unique astrophysical laboratory to study the stellar population of galactic nuclei because it is the only galactic nucleus whose stars can be resolved down to milliparsec scales. However, the extreme and spatially highly variable interstellar extinction towards the GC poses a serious obstacle to photometric stellar classification. Aims. Our goal is to identify hot, massive stars in the nuclear stellar disc (NSD) region through combining near-infrared (NIR) and mid-infrared (MIR) photometry, and thus to demonstrate the feasibility of this technique, which may gain great importance with the arrival of the James Webb Space Telescope (JWST). Methods. We combined the GALACTICNUCLEUS NIR survey with the IRAC/Spitzer MIR survey of the GC. We applied the so-called Rayleigh-Jeans colour excess (RJCE) de-reddening method to our combined NIR-MIR data to identify potential hot stars in colour-magnitude diagrams (CMDs). Results. Despite the very low angular resolution of IRAC we find 12 clear candidates for young massive stars among the 1065 sources that meet our selection criteria. Seven out of these 12 stars are previously known hot, massive stars belonging to the Arches and Quintuplet clusters, as well as sources detected by the Hubble Space Telescope/NICMOS Paschen-alpha survey. Five of our massive star candidates have not been previously reported in the literature. Conclusions. We show that the RJCE method is a valuable tool to identify hot stars in the GC using photometry alone. Upcoming instruments with high angular resolution MIR imaging capabilities such as the JWST could surely make more substantial use of this de-reddening method and help establish a far more complete census of hot, young stars in the GC area than what is possible at the moment.