Galaxy formation and evolution

Abstract:

Abstract Recent photometric surveys with JWST have revealed a significant population of mysterious objects with red colours, compact morphologies, frequent signs of active galactic nucleus (AGN) activity, and negligible X-ray emission. These ‘Little Red Dots’ (LRDs) have been explored through spectral and photometric studies, but their nature is still under debate. As part of the BlackTHUNDER survey, we have observed UNCOVER_20466, one of the most distant LRDs known (z = 8.5), with the JWST/NIRSpec IFU. Previous JWST/NIRCam and JWST/NIRSpec MSA observations of this source revealed its LRD nature, as well as the presence of an AGN. Using our NIRSpec IFU data, we confirm that UNCOVER_20466 is an LRD (based on spectral slopes and compactness) that contains an overmassive black hole. However, our observed Balmer decrements do not suggest strong dust attenuation, resulting in a lower $\rm H\beta$-based bolometric luminosity and λEdd ($\sim 10~{{\ \rm per\ cent}}$) than previously found. This source lies on local relations between MBH − σ* and MBH − Mdyn, suggesting that this could be a progenitor of the core of a lower-redshift galaxy. We explore the possible evolution of this source, finding evidence for substantial black hole accretion in the past and a likely origin as a heavy seed at high redshift (∼103 M⊙). $\rm Ly\alpha$ emission is strongly detected, implying $f_{\rm esc}^{\rm Ly\alpha }\sim 30~{{\ \rm per\ cent}}$. The extremely high $\rm [OIII]\lambda 4363$/$\rm H\gamma$ ratio is indicative of not only AGN photoionization and heating, but also extremely high densities ($n_{\rm e}\sim 10^7\, \rm cm^{-3}$), suggesting that this black hole at such high redshift may be forming in an ultra-dense protogalaxy.

Abstract:

Abstract JWST/NIRCam slitless spectroscopy enables dynamical mass measurements for typical star-forming galaxies only a billion years after the Big Bang. We model the Hα morpho-kinematics of 163 galaxies at redshift z ≈ 4-6 from FRESCO and CONGRESS (with JADES imaging), using the $\texttt {geko}$ code, and infer rotational velocities and dispersions within re. Our sample spans log M⋆ ≈ 7-10 and log Mdyn ≈ 9-11. Gas masses are inferred from empirical scaling relations and combined with stellar masses to yield baryonic masses. The resulting median inferred gas-to-baryonic mass fraction is 〈fgas〉 = 0.77. Using these baryonic masses together with the dynamical masses, we derive dark-matter fractions fDM(r < re) within the Hα half-light radius, and find a high median value of 〈fDM〉 = 0.73, where fDM is defined relative to the total (DM+baryonic) mass. About two-thirds of systems are DM-dominated within re ∼ 0.5–1 kpc. We find that fDM decreases with stellar mass, consistent with predictions from simulations. The stellar Tully-Fisher relation shows a tentative offset to higher vcirc at fixed M⋆ and substantial intrinsic scatter, suggesting that the relation is only beginning to emerge at z ∼ 5. We measure a negative correlation between fDM and baryonic surface density Σbar, weaker but broadly consistent with trends at cosmic noon and at z ∼ 0. Qualitatively comparing with modified NFW profiles coupled to an empirical stellar-to-halo mass relation suggests that the lowest fDM (≲ 0.4) require cored inner DM profiles, while the highest fractions favour cuspier profiles, potentially reflecting adiabatic contraction. Overall, the elevated fgas and fDM at z ≳ 4 are compatible with progenitors of baryon-dominated systems at z ∼ 2 and naturally anticipate overmassive black holes at fixed M⋆.