{"work":{"id":"e9acc8dd-fa6a-4867-818b-fa914c8c77d1","openalex_id":"https://openalex.org/W4405173190","doi":"10.48550/arxiv.2412.04557","arxiv_id":"2412.04557","raw_key":null,"title":"An unambiguous AGN and a Balmer break in an Ultraluminous Little Red Dot at z=4.47 from Ultradeep UNCOVER and All the Little Things Spectroscopy","authors":null,"authors_text":"arXiv e-prints , pages =","year":2024,"venue":"astro-ph.GA","abstract":"We present a detailed exploration of the most optically-luminous Little Red Dot ($L_{H\\alpha}=10^{44}$erg/s, $L_V=10^{45}$erg/s, F444W=22AB) found to date. Located in the Abell 2744 field, source A744-45924 was observed by NIRSpec/PRISM with ultradeep spectroscopy reaching SNR$\\sim$100pix$^{-1}$, high-resolution 3-4 micron NIRCam/Grism spectroscopy, and NIRCam Medium Band imaging. The NIRCam spectra reveal high rest-frame EW $W_{H\\alpha,0,broad}>800$\\r{A}, broad H$\\alpha$ emission (FWHM$\\sim$4500 km/s), on top of narrow, complex absorption. NIRSpec data show exceptionally strong rest-frame UV to NIR Fe II emission ($W_{FeII-UV,0}\\sim$340\\r{A}), N IV]$\\lambda\\lambda$1483,1486 and N III]$\\lambda$1750, and broad NIR O I $\\lambda$8446 emission. The spectra unambiguously demonstrate a broad-line region associated with an inferred $M_{BH}\\sim10^9M_\\odot$ supermassive black hole embedded in dense gas, which might explain a non-detection in ultradeep Chandra X-ray data (>$10\\times$ underluminous relative to broad $L_{H\\alpha}$). Strong UV Nitrogen lines suggest supersolar N/O ratios due to rapid star formation or intense radiation near the AGN. The continuum shows a clear Balmer break at rest-frame 3650\\r{A}, which cannot be accounted for by an AGN power-law alone. A stellar population model produces an excellent fit with a reddened Balmer break and implying a massive ($M_*\\sim8\\times10^{10}M_\\odot$), old $\\sim$500 Myr, compact stellar core, among the densest stellar systems known ($\\rho\\sim3\\times10^6M_\\odot$/pc$^2$ for $R_{e,opt}=70\\pm10$ pc), and AGN emission with extreme intrinsic EW $W_{H\\alpha,0}\\gg$1000\\r{A}. However, although high $M_*$ and $M_{BH}$ are supported by evidence of an overdensity containing 40 galaxies at $z=4.41-4.51$, deep high-resolution spectroscopy is required to confirm stellar absorption and rule out that dense gas around the AGN causes the Balmer break instead.","external_url":"https://arxiv.org/abs/2412.04557","cited_by_count":7,"metadata_source":"pith","metadata_fetched_at":"2026-08-05T02:28:24.338817+00:00","pith_arxiv_id":"2412.04557","created_at":"2026-05-09T07:24:10.371640+00:00","updated_at":"2026-08-05T02:28:24.338817+00:00","title_quality_ok":true,"display_title":"E., Matthee, J., et al","render_title":"E., Matthee, J., et al"},"hub":{"state":{"work_id":"e9acc8dd-fa6a-4867-818b-fa914c8c77d1","tier":"hub","tier_reason":"10+ Pith inbound or 1,000+ external citations","pith_inbound_count":26,"external_cited_by_count":7,"distinct_field_count":2,"first_pith_cited_at":"2025-03-20T18:00:00+00:00","last_pith_cited_at":"2026-07-08T17:55:36+00:00","author_build_status":"not_needed","summary_status":"needed","contexts_status":"needed","graph_status":"needed","ask_index_status":"not_needed","reader_status":"not_needed","recognition_status":"not_needed","updated_at":"2026-08-22T05:29:33.710858+00:00","tier_text":"hub"},"tier":"hub","role_counts":[{"context_role":"baseline","n":1}],"polarity_counts":[{"context_polarity":"baseline","n":1}],"runs":{},"summary":{},"graph":{},"authors":[]}}