The Galactic Centre is the closest galaxy nucleus and the only one that we can resolve down to milli-parsec scales. It is therefore a unique template to better understand galactic nuclei, whose study is restricted to integrated light due to their great distances. It is also the most prolific star forming environment of the Galaxy averaged over volume. In particular, there are several indicators pointing towards important star formation in the past 10-100 Myr. Nevertheless, the known young clusters and stars only account for ~10% of the total expected mass. Then, where are the missing young stars?

There are two main problems challenging the observation of the Galactic Centre and hampering the determination of its stellar population and, in particular, the detection of young stars. They are the extreme source crowding and the interstellar extinction. In this way, the study of the Galactic Centre is restricted to the infrared regime. Moreover, only small Galactic Centre regions have been observed using spectroscopy, because a complete survey would be prohibitively expensive considering the large number of sources and the required high-angular resolution. Hence, photometric studies are key to address the Galactic Centre open questions, and to detect interesting regions that can be later followed up using spectroscopy. Until very recently, less than 1% of the Galactic Centre had been covered with the sufficient angular resolution to analyse its stellar population. To solve this problem, we developed the GALACTICNUCLEUS survey (PI R. Schödel). This is a near infrared high-resolution photometric catalogue specially designed to study the Galactic Centre. It covers more than 6,000 pc² in the innermost regions of the Galaxy and contains accurate photometry for more than three million of stars.

In our recently accepted Nature Astronomy paper, we used the GALACTICNUCLEUS survey to study a particular region of the Galactic Centre, Sagittarius B1. We find that it hosts several hundreds of thousands of solar masses of young stars. Our results constitute a step forward to better understand recent star formation in the Galactic Centre, and to find a significant amount of its missing young stars.

We chose to study the Sagittarius B1 region due to its strong HII emission that suggest the presence of hot and young stars spread throughout the field. The study started from the impressive GALACTICNUCLEUS  false colour images. Figure 1 shows the false colour image of the Sagittarius B1 region. After visual inspection of all the observed fields, the Sagittarius B1 region appeared as a distinct field with very interesting emission, in particular in Ks, associated with HII regions. This showed us that this field is special, pointing towards the presence of hidden young stars. Moreover, we were particularly lucky regarding the quality of the observations. In this way, the Sagittarius B1 field had the best observing conditions out of the whole survey. This was fundamental for an accurate study of its stellar population, that finally revealed the presence of a significant mass of young stars.

To characterise the stellar population, we used the Ks luminosity function, that contains fundamental information about the stars present in the region. To analyse it, we removed foreground stars and corrected the extinction effect applying an extinction map. Moreover, we also applied a completeness correction to account for stars that are not detected due to crowding and sensitivity effects. We recovered the star formation history by fitting the luminosity function with different theoretical models, and obtained that the region hosts several hundreds of thousands of solar masses of young stars, with ages between 5 and 10 million years.

The presence of such an impressive amount of coeval young stars in this relatively small region, indicates that they were probably formed as a stellar association of young stars that dissolved while orbiting around the Galactic Centre. This picture is also compatible with the upper limit for the formation of gravitationally bound clusters in the Galactic Centre (up to several tens of thousands of solar masses). Thus, we can infer that the stellar mass detected did not form as a unique cluster but, more likely, like an association of young stars. In this way, our results help understand the star formation in the Galactic Centre and probably indicate the fate of the so-far known young clusters, that will eventually dissolve below the high stellar background density. We suggest that there is likely to find more regions like Sagittarius B1 where there is a significant number of dissolved young stars that correspond to star formation events in the past, and account for the missing mass of young stars that has been predicted for the Galactic Centre.

Follow-up work to confirm our results will be carried out using spectroscopy in the Sagittarius B1 region. In this way, it will be possible to precisely determine the presence of young hot massive stars. Moreover, we are also working on a proper-motion approach to find co-moving groups of stars that would indicate the presence of stars with a probable common origin.