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

Abstract

Context. This is the second of three papers that search for the predicted stellar cusp around the Milky Way’s central black hole, Sagittarius A*, with new data and methods. Aims. We aim to infer the distribution of the faintest stellar population currently accessible through observations around Sagittarius A*. Methods. We used adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through optimised PSF fitting we removed the light from all detected stars above a given magnitude limit. Subsequently we analysed the remaining, diffuse light density. Systematic uncertainties were constrained by the use of data from different observing epochs and obtained with different filters. We show that it is necessary to correct for the diffuse emission from the mini-spiral, which would otherwise lead to a systematically biased light density profile. We used a Paschen α map obtained with the Hubble Space Telescope for this purpose. Results. The azimuthally averaged diffuse surface light density profile within a projected distance of R ≲ 0.5 pc from Sagittarius A* can be described consistently by a single power law with an exponent of Γ = 0.26 ± 0.02stat ± 0.05sys, similar to what has been found for the surface number density of faint stars in Paper I. Conclusions. The analysed diffuse light arises from sub-giant and main-sequence stars with Ks ≈ 19−22 with masses of 0.8−1.5 M⊙. These stars can be old enough to be dynamically relaxed. The observed power-law profile and its slope are consistent with the existence of a relaxed stellar cusp around the Milky Way’s central black hole. We find that a Nuker law provides an adequate description of the nuclear cluster’s intrinsic shape (assuming spherical symmetry). The 3D power-law slope near Sgr A* is γ = 1.13 ± 0.03model ± 0.05sys. The stellar density decreases more steeply beyond a break radius of about 3 pc, which corresponds roughly to the radius of influence of the massive black hole. At a distance of 0.01 pc from the black hole, we estimate a stellar mass density of 2.6 ± 0.3 × 107 M⊙ pc-3 and a total enclosed stellar mass of 180 ± 30 M⊙. These estimates assume a constant mass-to-light ratio and do not take stellar remnants into account. The fact that a flat projected surface density is observed for old giants at projected distances R ≲ 0.3 pc implies that some mechanism may have altered their appearance or distribution.

Spectroscopic Detection of a Cusp of Late-type Stars around the Central Black Hole in the Milky Way

“…Our study provides the first spectroscopic detection of a cusp of giant stars toward the central arcsecond. This finding agrees with the recent studies found a cusp profile of fainter giants outside the central arcsecond out to 2 pc from the SgrA * (Baumgardt et al 2018;Gallego-Cano et al 2018;Schödel et al 2018).…”

The Rate of Stellar Mass Black Hole Scattering in Galactic Nuclei

“…1 However, diffuse light measurements of Schödel et al (2018) give a lower value γ 1 = 1.13 ± 0.08. The difference in the BH number between γ 1 = 1.5 and γ 1 = 1.1 is only ∼ 15%.…”

Can supernova kicks trigger EMRIs in the Galactic Centre?

“…As anticipated above, we selected two different old stellar populations that may dominate the NRR effects (i.e., the computation of t r ) in our model of the GC: (i) light stars with average mass m = 1 M , slope of the density profile γ NRR = 1.25 and number within the inner parsec N NRR (< 1 pc) = 10 6 ('L' population, Schödel et al 2018;Gallego-Cano et al 2018;Baumgardt et al 2018;Habibi et al 2019); (ii) stellar-mass BHs with average mass m = 10 M , with a steeper density profile (γ NRR = 2) and for which N NRR (< 1 pc) = 10 4 ('H' population, Baumgardt et al 2018;Hailey et al 2018).…”