The Milky Way’s nuclear star cluster: Old, metal-rich, and cuspy

Abstract

Context. The environment of Sagittarius A* (Sgr A*), the central black hole of the Milky Way, is the only place in the Universe where we can currently study the interaction between a nuclear star cluster and a massive black hole and infer the properties of a nuclear cluster from observations of individual stars. Aims. This work aims to explore the star formation history of the nuclear cluster and the structure of the innermost stellar cusp around Sgr A*. Methods. We combined and analysed multi epoch high quality AO observations. For the region close to Sgr A* we apply the speckle holography technique to the AO data and obtain images that are ≥50% complete down to Ks ≈ 19 within a projected radius of 5″ around Sgr A*. We used H-band images to derive extinction maps. Results. We provide Ks photometry for roughly 39 000 stars and H-band photometry for ∼11 000 stars within a field of about 40″ × 40″, centred on Sgr A*. In addition, we provide Ks photometry of ∼3000 stars in a very deep central field of 10″ × 10″, centred on Sgr A*. We find that the Ks luminosity function (KLF) is rather homogeneous within the studied field and does not show any significant changes as a function of distance from the central black hole on scales of a few 0.1 pc. By fitting theoretical luminosity functions to the KLF, we derive the star formation history of the nuclear star cluster. We find that about 80% of the original star formation took place 10 Gyr ago or longer, followed by a largely quiescent phase that lasted for more than 5 Gyr. We clearly detect the presence of intermediate-age stars of about 3 Gyr in age. This event makes up about 15% of the originally formed stellar mass of the cluster. A few percent of the stellar mass formed in the past few 100 Myr. Our results appear to be inconsistent with a quasi-continuous star formation history. The mean metallicity of the stars is consistent with being slightly super solar. The stellar density increases exponentially towards Sgr A* at all magnitudes between Ks = 15−19. We also show that the precise properties of the stellar cusp around Sgr A* are hard to determine because the star formation history suggests that the star counts can be significantly contaminated, at all magnitudes, by stars that are too young to be dynamically relaxed. We find that the probability of observing any young (non-millisecond) pulsar in a tight orbit around Sgr A* and beamed towards Earth is very low. We argue that typical globular clusters, such as they are observed in and around the Milky Way today, have probably not contributed to the nuclear cluster’s mass in any significant way. The nuclear cluster may have formed following major merger events in the early history of the Milky Way.

Distance and extinction to the Milky Way spiral arms along the Galactic centre line of sight

“…To obtain the distances to the detected common stars, we used the Gaia parallaxes taking into account that, in general, they cannot be directly inverted to get the distance to a given source (e.g., Luri et al 2018;Bailer-Jones et al 2018). We applied the near-linear trend of distance bias (δ p = −0.054) derived by Schönrich et al (2019). To remove any additional problem related to the distance to individual stars, we computed average distances building histograms of the underlying distributions corresponding to each of the analysed regions (Fig.…”

“…We created de-reddened K s -luminosity functions (KLFs) and fitted them with a linear combination of theoretical luminosity functions (Parsec models) to determine the stellar population and the SFH of the detected spiral arm features. The analysis of the KLFs is based on the relative weight of the main features appearing on the luminosity functions, such as the RC, the red giant branch bump, or the asymptotic giant branch bump (e.g., Nogueras-Lara et al 2020a;Schödel et al 2020). Given that the uncertainties of the luminosity functions are proportionally related to the detected number of stars (∼#stars 1/2 ), a sufficient number of stars is needed to apply the proposed methodology.…”

“…To fit the KLFs, we used a similar approach as in Nogueras-Lara et al (2020a), Schödel et al (2020). We generated 1000 MC samples from the original KLFs, assuming Gaussian uncertainties and fit them with a linear combination of 14 theoretical PARSEC luminosity functions 3 .…”

The nuclear stellar disc of the Milky Way: A dynamically cool and metal-rich component possibly formed from the central molecular zone

“…While in recent years studies were focussed on the NSC itself (see e.g. Neumayer et al 2020;Schödel et al 2020), very little work has been done on the NSD so far. Nuclear stellar discs are also detected in extragalactic systems and are quite common Gonzalez et al (2012).…”

“…These authors find that the bulk of the stars in the NSD is old and formed at least 8 Gyr ago followed by an extended phase of quiescence and recent star formation activity about 1 Gyr ago, where 5% of the mass of the NSD was formed very quickly. With a similar technique, Schödel et al (2020) trace the SFH of the NSC, also finding that ∼80% of its stars are at least about 10 Gyr old. These authors also find signs of an intermediate-age population (∼3 Gyr), but contrary to the NSD the NSC shows no signs of star formation about 1 Gyr ago.…”