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

TL;DR

This study uses deep NB395 photometry from Subaru/HSC to investigate the metallicity, mass-function, and tidal-stream structure of NGC 5466, reaching reliable member identification down to $i_{2,0} < 23.5$. A k-nearest neighbor algorithm on the color-color-magnitude diagram is employed to select likely cluster members and derive photometric metallicities that agree with the known cluster value. The analysis finds a tidal-stream surface density that follows a power-law with slope $α = -4.53_{-0.14}^{+0.13}$ beyond the tidal radius, with an additional perpendicular power-law component and a density gap near ~200 pc, consistent with eTidal N-body expectations and recent orbital events. The main-sequence mass function shows a strong negative radial gradient inside the tidal radius and a flatter slope along the outer stream, indicating preferential stripping of low-mass stars, and demonstrating the effectiveness of NB395 photometry for metal-poor populations in wide-field surveys.

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 $α= -4.53_{-0.14}^{+0.13}$. 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.

Figures (2)

• Figure 1: (Upper left) Response curves of the Subaru/HSC filters used in this study. The solid lines show the intrinsic filter response curves, while the dotted lines indicate the total system throughput. (Lower left) Zoomed-in view of the upper-left panel over $3,800 - 4,300$$\AA$, with BOSZ model spectra (2017AJ....153..234B2024AA...688A.197M) overplotted. The model spectra correspond to $\log g=+5.0$, $T_{\rm eff}=3500$ K, and $[\alpha/{\rm Fe}]=+0.25$, for metallicities $[{\rm Fe/H}]=-2.5,~-2.0,~-1.5,~-1.0,~-0.5,$ and $0.0$. The spectra are color-coded by metallicity according to the color bar shown at the bottom. Two vertical black dashed lines indicate the Ca .8 .8 II H&K absorption lines. (Right) A color-color diagram using NB395-based color indices defined in equation (\ref{['eq:NBcolor']}). The color-coded curves represent synthetic model loci computed by convolving BOSZ model spectra with $\log g=+5.0$, $[\alpha/{\rm Fe}]=+0.25$, and $[{\rm Fe/H}]=-2.5,~-2.0,~-1.5,~-1.0,~-0.5,$ and $0.0$ with the HSC filter transmission curves, instrumental sensitivity, and atmospheric absorption. Alt text: Subaru/HSC filter response curves for NB395, $g$, and $i_2$, shown as intrinsic transmissions (solid) and total system throughput (dotted). A zoomed-in view from $3,800$-$4,300$$\AA$ overlays BOSZ spectra ($\log g=5.0$, $T_{\rm eff}=3,500$ K, $[\alpha/{\rm Fe}]=0.25$) color-coded by metallicity ($[{\rm Fe/H}]=-2.5$ to $0.0$). The right panel shows an NB395-based color–color diagram with synthetic model loci for the same metallicities.
• Figure 2: Illustrating the coverage of our HSC imaging. The black circles show our observational footprints, where the solid circle corresponds to the region observed with g, ${{\it i}_{{\it 2},0}}$, NB395, and the dashed circle indicates the region observed with only g-band. The cross mark denotes the center of the NGC 5466 main body, and the red circle indicates the tidal radius estimated in this study (16.1 arcmin). The background color map is the Galactic dust extinction map. Alt text: Map of the Subaru/HSC observational footprint showing regions observed in multiple bands and in g-band only. The plot marks the center of NGC 5466 and highlights the estimated tidal radius of 16.1 arcmin.