Subaru/HSC NB395 view of NGC 5466: metallicity, mass function, and the nature of its tidal stream

TL;DR

This study uses Subaru/HSC imaging with a specialized narrowband filter to analyze the metallicity, mass function, and tidal features of globular cluster NGC 5466, demonstrating effective member selection and insights into its tidal disruption.

Contribution

It introduces an improved member-selection method using a k-nearest neighbor algorithm on CCMDs and applies it to reveal the cluster's metallicity distribution, tidal structure, and mass function gradients.

Findings

Photometric metallicities agree with previous measurements.

The tidal stream's density profile follows a power law with slope -4.53.

A density gap suggests recent tidal interactions or pericentric passage.

Abstract

We present a deep photometric study of the globular cluster NGC 5466 and its tidal stream using Subaru/Hyper Suprime-Cam (HSC) imaging with the metallicity-sensitive narrowband filter NB395. We develop an improved member-selection technique based on a k-nearest neighbor algorithm applied to the color-color-magnitude diagram (CCMD), enabling reliable candidate identification down to $i_{2,0} < 23.5$. Photometric metallicities derived from NB395 colors agree with previous measurements, supporting the robustness of our calibration. While modest residual contamination and possible offsets - potentially driven by variations in light-element abundances - may remain beyond 10 arcmin, the metallicity distribution of high-probability inner members matches the known mean metallicity of NGC 5466, demonstrating the effectiveness of our method. The spatial distribution of NB395-selected stars clearly delineates the tidal stream. Beyond the tidal radius, the azimuthally averaged radial surface density profile follows a power law with slope $\alpha = -4.53_{-0.14}^{+0.13}$. We also detect a power-law component perpendicular to the stream, suggestive of multiple apogalactic passages. A density gap is identified at a projected distance of $\sim200$ pc from the cluster center, consistent with eTidal N-body predictions and possibly associated with a recent pericentric passage or Galactic disk interaction. Analysis of the main-sequence mass function reveals a strong negative radial gradient in the slope within the tidal radius, whereas the slope along the outer stream is relatively flat, consistent with preferential tidal stripping of low-mass stars. These results highlight the power of HSC/NB395 photometry for identifying metal-poor populations and deriving photometric metallicities, underscoring its value for future wide-field surveys.