The Near-Infrared Spectrum of the Nuclear Star Cluster: Looking Below the Tip of the Iceberg and Comparisons with Extragalactic Nuclei

TL;DR

This study analyzes near-infrared spectra of the Galactic nuclear star cluster, revealing non-solar chemical compositions near the center and evidence of recent star formation, with implications for understanding nuclear star clusters in galaxies.

Contribution

It provides detailed NIR spectral analysis of the Galactic NSC, highlighting chemical abundance variations and recent star formation activity, comparing these features with extragalactic nuclei.

Findings

Deeper NaI and CaI lines near the NSC center indicate non-solar chemical mixtures.

Equivalent widths of emission features suggest ongoing star formation for several million years.

Spectral similarities with NGC 253 imply comparable star formation episodes in different galactic nuclei.

Abstract

Long-slit near-infrared (NIR) spectra of the Galactic nuclear star cluster (NSC) are discussed. The spectra sample the major axis of the NSC out to its half light radius. The equivalent widths of NaI2.21um and CaI2.26um outside of the center of the NSC, where light from nuclear bulge stars contributes a large fraction to the total flux, are consistent with solar chemical mixtures. In contrast, the equivalent widths of NaI2.21um and CaI2.26um near the center of the NSC are larger than expected from models with solar chemical mixtures, even after light from the brightest evolved stars is removed. That NaI2.21um and CaI2.26um are deeper than predicted for solar chemical mixtures over a range of evolutionary states is consistent with previous studies that find that the majority of stars near the center of the NSC formed from material that had non-solar chemical mixtures. A spectrum of nebular emission and the youngest stars near the GC is also extracted. The equivalent widths of emission features in the extracted NIR spectrum are similar to those in the nuclear spectrum of NGC 253, and it is argued that this agreement is best achieved if the current episode of star formation near the center of the NSC has been in progress for at least a few Myr.