NGC 3259: A signal for an untapped population of slowly accreting intermediate-mass black holes

Low-mass active galactic nuclei (AGNs) can provide important constraints on the formation and evolution of supermassive black holes (SMBHs), which stands as a central challenge in modern cosmology. To date, only small samples of intermediate-mass black holes (IMBHs, M_ BH < 10^5 M_⊙) and "lesser" SMBHs (LSMBHs, M_ BH < 10^6 M_⊙) have been identified. Our present study of NGC 3259 at D=27 Mpc with the Binospec integral field unit (IFU) spectrograph complemented with Keck Echelle Spectrograph and Imager observations demonstrates the strong capability and necessity for spectroscopic follow-up studies. NGC 3259 hosts a black hole (BH) with a mass of M_ BH 10^5 M_⊙, inferred from multi-epoch spectroscopic data, which is accreting at 1% of the Eddington limit, as suggested by the analysis of archival XMM-Newton observations. It is the second-nearest broad-line low-mass AGN after the archetypal galaxy NGC 4395. The spectroscopic data reveal a variable broad Hα profile likely resulting from asymmetrically distributed broad-line region (BLR) clouds or BLR outflow events. X-ray observations and the absence of an optical power-law continuum suggest partial obscuration of the accretion disk and hot corona by a dust torus. We estimated that Sloan Digital Sky Survey (SDSS) observations would only have the capacity to detect similar objects up to D=35 Mpc. A detailed photometric analysis of NGC 3259 using HST images provides a central spheroid stellar mass estimate that is 20 times lower than expected, based on the M_ BH - M^*_ sph relation, making this galaxy a significant outlier. This discrepancy suggests divergent growth pathways for the central BH and spheroid, potentially influenced by the potential presence of a bar in the galaxy. Finally, we demonstrate that the DESI and 4MOST surveys will detect low-accretion-rate IMBHs and LSMBHs, while the sensitivity of future X-ray instruments (such as AXIS and Athena) will secure their classification.