We report the discovery of an active intermediate-mass black hole (IMBH) candidate in the center of nearby barred bulgeless galaxy NGC 3319. The point X-ray source revealed by archival Chandra and XMM−Newton observations is spatially coincident with the optical and UV galactic nuclei from Hubble Space Telescope observations. The spectral energy distribution derived from the unresolved X-ray and UV-optical flux is comparable with active galactic nuclei (AGNs) rather than ultra-luminous X-ray sources, although its bolometric luminosity is only 3.6 × 10 40 erg s −1 . Assuming an Eddington ratio range between 0.001 and 1, the black hole mass (M BH ) will be located at 3 × 10 2 − 3 × 10 5 M ⊙ , placing it in the so-called IMBH regime and could be the one of the lowest reported so far. Estimates from other approaches (e.g., fundamental plane, X-ray variability) also suggest M BH 10 5 M ⊙ . Similar to other BHs in bulgeless galaxies, the discovered IMBH resides in a nuclear star cluster with mass of ∼ 6 × 10 6 M ⊙ . The detection of such a low-mass BH offers us an ideal chance to study the formation and early growth of SMBH seeds, which may result from the bar-driven inflow in late-type galaxies with a prominent bar such as NGC 3319.
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