The JWST Ultramassive Galaxy Sample -- I. Overview, Photometry, and Stellar Kinematics: Selecting Optimal Template Subsets via Non-Negative Matrix Factorization

Abstract

We introduce the JWST Ultramassive Galaxy Sample, comprising 8 extreme early-type galaxies ($M_\star \gtrsim 2\times10^{12}\,\mathrm{M}_\odot$) drawn as a representative subset from a comprehensive full-sky census of over 100 such objects. This program aims to dynamically measure their central supermassive black holes (BHs) to probe the extreme high-mass end of BH--galaxy scaling relations, a critical regime where the physical mechanisms governing BH growth and host-galaxy co-evolution are expected to fundamentally change. In this first paper, we present the foundational data products: detailed photometric models and spatially resolved nuclear stellar kinematics. To accurately map the stellar mass distribution from the BH sphere of influence out to the extended halo, we combine diffraction-limited JWST/NIRCam imaging with wide-field DESI Legacy Surveys data, parametrizing the surface brightness using Multi-Gaussian Expansion (MGE). We extract stellar kinematics from JWST/NIRSpec integral-field observations using the penalized pixel-fitting (\textsc{pPXF}) method. To fully exploit the continuous $1.66-3.17\,μ\mathrm{m}$ wavelength coverage---avoiding the telluric gaps of ground-based empirical libraries and the computational burden of massive theoretical grids---we introduce the use of near-separable Non-Negative Matrix Factorization (NMF). This algorithm identifies an optimal, highly compressed subset of synthetic spectra from the updated BOSZ library, perfectly capturing its spanning power at a fraction of the computational cost. The resulting kinematic maps reveal that these ultramassive systems are overwhelmingly slow rotators, frequently harboring kinematically decoupled cores. These high-quality photometric and kinematic data provide the inputs required for the robust Jeans Anisotropic Modelling (JAM) of the central BH masses in Paper II.