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

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 86.4 pc x 21 pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use Spitzer/IRAC 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 Sersic 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=0.710.10, an effective radius of R-e=(5.1 +/- 1.0) pc, and a Sersic 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 gamma=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.© ESO 2020
The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n degrees[614922]. This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 195.B-0283 and 091.B-0418. We thank the staff of ESO for their great efforts and helpfulness. N.N. acknowledges support by the Sonderforschungsbereich SFB 881 "The Milky Way System" of the German Research Foundation (DFG). F.N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. We acknowledge support by the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (CSIC) (SEV-2017-0709).