New constraints on the structure of the nuclear stellar cluster of the Milky Way from star counts and MIR imaging

Abstract

Context. The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now. Aims. We aim to study the size and spatial structure of the MWNSC. Methods. This study uses data and methods that address potential shortcomings of previous studies on the topic. We use 0.2 angular resolution K s data to create a stellar density map in the central 86.4pc x 21pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use IRAC/Spitzer mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sérsic model to the MWNSC and investigate its symmetry. Results. Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of q = 0.71 ± 0.10, an effective radius of R e = (5.1 ± 1.0) pc, and a Sérsic index of n = 2.2 ± 0.7. Its major axis may be tilted out of the Galactic plane by up to −10 degrees. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of γ = 1.38 ± 0.06 sys ± 0.01 stat . Conclusions. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.

GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre

“…This points towards physically different components with different SFHs, in agreement with previous work (e.g. Launhardt et al 2002;Nishiyama et al 2013;Nogueras-Lara et al 2018b;Gallego-Cano et al 2020;Nogueras-Lara et al 2020a). In particular, the RC feature appears to be thicker for the NSD regions (excluding the NSD W, whose lower completeness avoids a good coverage for faint K s magnitudes), which is probably the result of the presence of a secondary RC feature due to a star formation event ∼ 1 Gyr ago that was responsible for the formation of ∼ 5 % of the stellar mass (Nogueras-Lara et al 2020a).…”

“…Gravity Collaboration et al 2018;Do et al 2019), with a standard distribution of 150 pc, corresponding to the effective radius of the NSD (e.g. Nishiyama et al 2013;Gallego-Cano et al 2020 We kept all the stars with K s < 18.3 mag, corresponding to the 50 % completeness limit of the GNS survey (Nogueras-Lara et al 2020b)…”

GALACTICNUCLEUS: A high angular-resolution JHKs imaging survey of the Galactic centre

“…We estimated that a change in ∼200 pc (∼ diameter of the NSD, e.g. Nishiyama et al 2013;Gallego-Cano et al 2020) can produce a variation of α HK s around 0.05. This effect is less important for α JH , where it accounts for ∼0.02.…”

Simulations of the Milky Way’s Central Molecular Zone – II. Star formation

“…As noted in Section 3.2, such star formation might also be related to the formation of the nuclear stellar cluster (NSC; see e.g. Genzel et al 2010;Schödel et al 2014;Gallego-Cano et al 2020).…”

“…Such star formation might also be related to the formation of the nuclear stellar cluster (NSC; see e.g. Genzel, Eisenhauer & Gillessen 2010;Schödel et al 2014;Gallego-Cano et al 2020) by providing in situ newly born stars and, since such stars are rotating, it might contribute to its observed rotation (Feldmeier et al 2014;Chatzopoulos et al 2015;Feldmeier-Krause et al 2015;Tsatsi et al 2017;Neumayer, Seth & Boeker 2020). Fig.…”