The distribution of stars around the Milky Way’s central black hole

Abstract

Context. The existence of dynamically relaxed stellar density cusp in dense clusters around massive black holes is a long-standing prediction of stellar dynamics, but it has so far escaped unambiguous observational confirmation. Aims. In this paper we aim to revisit the problem of inferring the innermost structure of the Milky Way's nuclear star cluster via star counts, to clarify whether it displays a core or a cusp around the central black hole. Methods. We used judiciously selected adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through image stacking and improved PSF fitting we pushed the completeness limit about one magnitude deeper than in previous, comparable work. Crowding and extinction corrections were derived and applied to the surface density estimates. Known young, and therefore dynamically not relaxed stars, are excluded from the analysis. Contrary to previous work, we analyse the stellar density in well-defined magnitude ranges in order to be able to constrain stellar masses and ages. Results. We focus on giant stars, with observed magnitudes K = 12.5 − 16, and on stars with observed magnitudes K ≈ 18, which may have similar mean ages and masses than the former. The giants display a core-like surface density profile within a projected radius R ≤ 0.3 pc of the central black hole, in agreement with previous studies, but their 3D density distribution is not inconsistent with a shallow cusp if we take into account the extent of the entire cluster, beyond the radius of influence of the central black hole. The surface density of the fainter stars can be described well by a single power-law at R < 2 pc. The cusp-like profile of the faint stars persists even if we take into account the possible contamination of stars in this brightness range by young pre-main sequence stars. The data are inconsistent with a core-profile for the faint stars. Finally, we show that a 3D Nuker law provides a good description of the cluster structure. Conclusions. We conclude that the observed density of the faintest stars detectable with reasonable completeness at the Galactic Centre, is consistent with the existence of a stellar cusp around the Milky Way's central black hole, Sagittarius A*. This cusp is well developed inside the influence radius of Sagittarius A* and can be described by a single three-dimensional power-law with an exponent γ = 1.43 ± 0.02 ± 0.1 sys . This corroborates existing conclusions from Nbody simulations performed in a companion paper. An important caveat is that the faint stars analysed here may be contaminated significantly by dynamically unrelaxed stars that formed about 100 Myr ago. The apparent lack of giants at projected distances of R 0.3 pc (R 8") of the massive black hole may indicate that some mechanism may have altered their distribution or intrinsic luminosity. We roughly estimate the number of possibly missing giants to about 100.

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

“…To avoid biased results in the Sérsic model, which has a central peak, we mask the inner 0.6 pc. This is due to the fact that in our study, the number counts are dominated by bright stars with K s < 16.0 that show a flat profile, different from the cusp profile, which is shown by faint stars with K s > 16.0 (see Buchholz et al 2009;Bartko et al 2010;Yusef-Zadeh et al 2012;Do et al 2013;Gallego-Cano et al 2018;Schödel et al 2018). 3.…”

“…The data and their reduction are described in detail in Gallego-Cano et al (2018); they provide an angular resolution of about 0.070 FWHM over a field of about 1.5 x1.5 . In particular, we use K s-band data from 11 May 2011 obtained with the S27 camera (0.027 pixel scale) of NACO installed at the ESO VLT.…”

“…We do not apply a completeness correction for NACO data because the high-angular-resolution data used here are complete in the range of magnitudes that we study (K s ≤ 16.0, see Gallego-Cano et al 2018). We do not apply a completeness correction for HAWK-I data either but we apply masking generously to those regions with very low density, as we described above.…”

“…Their results were corroborated theoretically by Baumgardt et al (2018), who evolved a star cluster surrounding a central massive black hole over a Hubble time under the combined influence of two-body relaxation and continuous star formation. In order to describe the 3D shape of the cluster in detail, Gallego-Cano et al (2018) and Schödel et al (2018) needed to convert the measured 2D profile into a 3D density law and deal with projection effects. For this reason, they added data at projected radii R ≥ 2 pc from the literature (Schödel et al 2014a;Fritz et al 2016).…”

Nuclear Star Clusters

“…The presence of a central massive black hole within an NSC should lead to the formation of a cusp surrounding the massive black hole with a density slope of ρ ∝ r −γ with γ = 7/4 in singlemass models (Bahcall and Wolf 1976). The presence or absence of this Bahcall-Wolf cusp in NSCs, especially the Milky Way NSC, is debated in both the theoretical (e.g., Merritt 2010;Vasiliev 2017) and observational literature (e.g., Buchholz et al 2009;Schödel et al 2014a;Gallego-Cano et al 2018). The current theoretical literature (Vasiliev 2017;Baumgardt et al 2018;Panamarev et al 2019) suggests that the mass-segregation of stellar-mass black holes towards the center leads to a steep cusp (γ > 1.5) in their distribution, while the stellar distribution remains somewhat flatter.…”