Kavli Affiliate: Roberto Maiolino
| Summary:
JWST has extended the search for accreting black holes into the high-redshift, low-mass regime. We report a low-mass black hole candidate in MARTA 3887, a clumpy, metal-poor galaxy at $z=3.7694$, combining deep JWST/NIRSpec spectroscopy, HST and JWST imaging, and archival VLT/MUSE data. We detect a broad H$α$ component with $L_rm Hα,b=2.35times10^41$ erg s$^-1$ and FWHM $=1270^+260_-250$ km s$^-1$, with no [O III] counterpart of comparable width. Under a broad-line-region interpretation, virial calibrations give $log(M_rm BH/M_odot)=6.52^+0.18_-0.23$ and $λ_rm Edd=0.07$, or up to 0.38 for alternative calibrations. The host comprises at least three compact clumps, with $log(M_star/M_odot)=8.16^+0.09_-0.10$. If stellar photoionization dominates the narrow lines, the gas is highly ionized and metal poor, with $12+log(rm O/H)=7.68pm0.04$ ($simeq0.10,Z_odot$). The inferred $M_rm BH/M_star=2.3%$ lies about 2 dex above the extrapolated local relation, suggesting that black-hole growth has outpaced the assembly of the observed host. If growth began only when the first 10% of the observed stellar mass had formed ($t_10=92$ Myr ago), a $10^2,M_odot$ seed would require a lifetime-mean Eddington ratio $simeq5.7$; earlier growth or a more massive, e.g. direct-collapse, seed would allow sub-Eddington mean accretion. MUSE reveals extended Ly$α$ emission ($simeq6.5$-6.9 proper kpc), offset by $+64^+10_-8$ km s$^-1$ from systemic, with a provisional escape fraction of $simeq0.44$. Together with a Mg II doublet ratio close to the optically thin limit and a high [O III]/[O II] ratio, this makes MARTA 3887 a laboratory to study black-hole growth, stellar assembly, and ionizing-photon escape jointly.
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