MARTA: A "little" black hole hidden in a clumpy, metal-poor galaxy at z~3.8

Abstract

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$\alpha$ component with $L_{\mathrm{H}\alpha ,\mathrm{b}}=2.35\times 10^{41}$ erg s${}^{-1}$ and FWHM $=1270_{-250}^{+260}$ km s${}^{-1}$, with no [O III] counterpart of comparable width. Under a broad-line-region interpretation, virial calibrations give $\log (M_{\mathrm{BH}}/M_{\odot })=6.52_{-0.23}^{+0.18}$ and ${\lambda }_{\mathrm{Edd}}=0.07$, or up to 0.38 for alternative calibrations. The host comprises at least three compact clumps, with $\log (M_{\star }/M_{\odot })=8.16_{-0.10}^{+0.09}$. If stellar photoionization dominates the narrow lines, the gas is highly ionized and metal poor, with $12+\log (O/H)=7.68\pm 0.04$ ($\simeq 0.10Z_{\odot }$). The inferred $M_{\mathrm{BH}}/M_{\star }=2.3\%$ 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 $\simeq 5.7$; earlier growth or a more massive, e.g. direct-collapse, seed would allow sub-Eddington mean accretion. MUSE reveals extended Ly$\alpha$ emission ($\simeq 6.5$-6.9 proper kpc), offset by $+64_{-8}^{+10}$ km s${}^{-1}$ from systemic, with a provisional escape fraction of $\simeq 0.44$. 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.

arXiv commentMain body : 19 pages, 12 figures. Appendix : 9 pages, 7 figures, 1 table. Comments are welcome