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

2024/12/05 by Ivo Labbe, Labbe, Ivo, Jenny E. Greene +77 · 1 voice · 3 citations
Physics and Astronomy · #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #astro-ph.GA

paper · pdf · doi:10.48550/arxiv.2412.04557

Abstract

We present a detailed exploration of the most optically-luminous Little Red Dot (L_Hα=10⁴⁴erg/s, L_V=10⁴⁵erg/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∼100pix^-1, high-resolution 3-4 micron NIRCam/Grism spectroscopy, and NIRCam Medium Band imaging. The NIRCam spectra reveal high rest-frame EW W_Hα,0,broad>800Å, broad Hα emission (FWHM∼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∼340Å), N IV]λλ1483,1486 and N III]λ1750, and broad NIR O I λ8446 emission. The spectra unambiguously demonstrate a broad-line region associated with an inferred M_BH∼10⁹M_\odot supermassive black hole embedded in dense gas, which might explain a non-detection in ultradeep Chandra X-ray data (>10× underluminous relative to broad L_Hα). 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 3650Å, 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_*∼8×10¹⁰M_\odot), old ∼500 Myr, compact stellar core, among the densest stellar systems known (ρ∼3×10⁶M_\odot/pc² for R_e,opt=70±10 pc), and AGN emission with extreme intrinsic EW W_Hα,0≫1000Å. 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.

Cited by

• JWST provides a new view of cosmic dawn: latest developments in studies of early galaxies
• Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot
• Glimmers in the Cosmic Dawn. III. On the Photometrically Determined Black Hole Mass to Stellar Mass Relation Across Cosmic Time

Discussions

• More exciting measurements on the most luminous little red dot that I know of (F444W=22 at z=4.5!): an unambiguous AGN and a Balmer break in deep UNCOVER and ALT spectroscopy, led by Ivo Labbe arxiv. [bsky, 3 points, 1 comments]

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