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Within the established framework of structure formation, galaxies start as systems of low stellar mass and gradually grow into far more massive galaxies. The existence of massive galaxies in the first billion years of the Universe, as suggested by recent observations, seems to challenge this model, as such galaxies would require highly efficient conversion of baryons into stars. An even greater challenge in this epoch is the existence of massive galaxies that have already ceased forming stars. However, robust detections of early massive quiescent galaxies have been challenging due to the coarse wavelength sampling of photometric surveys. Here we report the spectroscopic confirmation with the James Webb Space Telescope of the quiescent galaxy RUBIES-EGS-QG-1 at redshift z  = 4.90, 1.2 billion years after the Big Bang. Deep stellar absorption features in the spectrum reveal that the stellar mass of the galaxy of 10 11   M ⊙ formed in a short 200 Myr burst of star formation, after which star formation activity dropped rapidly and persistently. According to current galaxy formation models, systems with such rapid stellar mass growth and early quenching are too rare to plausibly occur in the small area probed spectroscopically with JWST. Instead, the discovery of RUBIES-EGS-QG-1 implies that early massive quiescent galaxies can be quenched earlier or exhaust gas available for star formation more efficiently than assumed at present. RUBIES-EGS-QG-1 is an exceptionally massive and mature galaxy discovered just 1.2 billion years after the Big Bang. Its stars formed in an extremely rapid burst, posing a major challenge to all current theoretical models.

Modeling emission lines from the millimeter to the UV and producing synthetic spectra is crucial for a good understanding of observations, yet it is an art filled with hazards. This is the proceedings of “Walking the Line”, a 3-day conference held in 2018 that brought together scientists working on different aspects of emission line simulations, in order to share knowledge and discuss the methodology. Emission lines across the spectrum from the millimeter to the UV were discussed, with most of the focus on the interstellar medium, but also some topics on the circumgalactic medium. The most important quality of a useful model is a good synergy with observations and experiments. Challenges in simulating line emission are identified, some of which are already being worked upon, and others that must be addressed in the future for models to agree with observations. Recent advances in several areas aiming at achieving that synergy are summarized here, from micro-physical to galactic and circum-galactic scale.

This article presents an up-dated analysis of synthetic optical and UV emission lines of simulated galaxies over cosmic time. The strong emission lines are derived from self-consistently coupling novel spectral models accounting for nebular emission from young stars, AGN and Post-AGB stars to cosmological zoom-in as well as large-scale simulations. Investigating the evolution of optical line-ratios in the BPT diagrams, the simulations can successfully reproduce the observed trend of [OIII]/Hβ ratio increasing from low to high redshifts, due to evolving star formation rate and gas metallicity. Standard selection criteria in the BPT diagrams can appropriately distinguish the main ionising source(s) of galaxies at low redshifts, but they are less reliable for metal-poor galaxies, dominating the early Universe. To robustly classify the ionising radiation of such metal-poor galaxies, diagnostic diagrams based on luminosity ratios of UV lines are discussed. The novel interface between simulations and observations is potentially important for the interpretation of high-quality spectra of very distant galaxies to be gathered by next-generation telescopes, such as the James Webb Space Telescope.

We investigate the differential impact of physical mechanisms, mergers and internal energetic phenomena, on the evolution of stellar metallicity gradients in massive, present-day galaxies employing sets of high-resolution, cosmological zoom simulations. We demonstrate that negative metallicity gradients at large radii (>2Reff) originate from the accretion of metal-poor stellar systems. At larger radii, galaxies become typically more dominated by stars accreted from satellite galaxies in major and minor mergers. However, only strong galactic, stellar-driven winds can sufficiently reduce the metallicity content of the accreted stars to realistically steepen the outer metallicity gradients in agreement with observations. In contrast, the gradients of the models without winds are inconsistent with observations. Moreover, we discuss the impact of additional AGN feedback. This analysis greatly highlights the importance of both energetic processes and merger events for stellar population properties of massive galaxies at large radii. Our results are expected to significantly contribute to the interpretation of current and up-coming IFU surveys (e.g. MaNGA, CALIFA).

The James Webb Space Telescope has discovered a surprising population of bright galaxies in the very early Universe (≲500 Myr after the Big Bang) that is hard to explain with conventional galaxy-formation models and whose physical properties are not fully understood. Insight into their internal physics is best captured through nebular lines, but at these early epochs, the brightest of these spectral features are redshifted into the mid-infrared and remain elusive. Using the mid-infrared instrument onboard the James Webb Space Telescope, here we present a detection of Hα and doubly ionized oxygen ([O iii ] 4959,5007 Å) from the bright, ultra-high-redshift galaxy candidate GHZ2/GLASS-z12. Based on these emission lines, we infer a spectroscopic redshift of z  = 12.33 ± 0.04, placing this galaxy just ~400 Myr after the Big Bang. These observations provide key insights into the conditions of this primaeval, luminous galaxy, which shows hard ionizing conditions rarely seen in the local Universe and probably driven by a compact and young burst (≲30 Myr) of star formation. The galaxy’s oxygen-to-hydrogen abundance is close to a tenth of the solar value, indicating a rapid metal enrichment. This study establishes the unique conditions of this notably bright and distant galaxy and the huge potential of mid-infrared observations to characterize these primordial systems. The detection of Hα reported in galaxy candidate GHZ2/GLASS-z12 provides a direct probe of star formation activity and can be used to trace massive stars with ages of ~10 Myr or younger.

Recent observations have established that dwarf galaxies can host black holes of intermediate mass (IMBH, 100Mȯ < MIMBH ≲ 105 Mȯ). With modern numerical models, we can test the growth of IMBHs as well as their evolutionary impact on the host galaxy. Our novel subsolar-mass (0.8 solar mass) resolution simulations of dwarf galaxies (M* = 2 × 107 Mȯ) have a resolved three-phase interstellar medium and account for non-equilibrium heating, cooling, and chemistry processes. The stellar initial mass function is fully sampled between 0.08–150 Mȯ while massive stars can form HII regions and explode as resolved supernovae. The stellar dynamics around the IMBH is integrated accurately with a regularization scheme. We present a viscous accretion disk model for the IMBH with momentum, energy, and mass conserving wind feedback. We demonstrate how the IMBH can grow from accretion of the cold and warm gas phase and how the presence of the IMBH and its feedback impacts the gas phase structure.

One major deficiency of state-of-the-art galaxy formation models consists in their inability of capturing the observed galaxy downsizing trend significantly over-estimating the number density of low-mass galaxies, in particular at high redshifts. Employing an enhanced galaxy formation model with a full chemical enrichment scheme (DeLucia et al., 2014), we present an improved model for stellar feedback (based on parametrizations from cosmological zoom simulations), in which strong gas outflows occur due to bursty star formation at high z, while star formation is mainly “quiescent” not causing any significant outflows anymore at low z. Due to the stronger gas outflows at high z, early star formation is strongly delayed towards later times. This helps to sufficiently detach the evolution of galaxy growth from the hiearchical dark matter assembly resulting in a fairly good agreement with the evolution of the observed stellar mass function (SMF, see Fig. 1). With our new feedback scheme, we can also successfully reproduce many other observational constraints, such as the metallicity content, the cold gas fractions or the quiescent galaxy fractions at both low and high redshifts. The resulting new-generation galaxy catalogues (Hirschmann et al., in prep) based on that model are expected to significantly contribute to the interpretation of current and up-coming large-scale surveys (HST, JWST, Euclid). This will, in turn, provide a rapid verification and refinement of our modeling.

The James Webb Space Telescope has discovered a surprising population of bright galaxies in the very early Universe (≲500 Myr after the Big Bang) that is hard to explain with conventional galaxy-formation models and whose physical properties are not fully understood. Insight into their internal physics is best captured through nebular lines, but at these early epochs, the brightest of these spectral features are redshifted into the mid-infrared and remain elusive. Using the mid-infrared instrument onboard the James Webb Space Telescope, here we present a detection of Hα and doubly ionized oxygen ([O iii] 4959,5007 Å) from the bright, ultra-high-redshift galaxy candidate GHZ2/GLASS-z12. Based on these emission lines, we infer a spectroscopic redshift of z = 12.33 ± 0.04, placing this galaxy just ~400 Myr after the Big Bang. These observations provide key insights into the conditions of this primaeval, luminous galaxy, which shows hard ionizing conditions rarely seen in the local Universe and probably driven by a compact and young burst (≲30 Myr) of star formation. The galaxy's oxygen-to-hydrogen abundance is close to a tenth of the solar value, indicating a rapid metal enrichment. This study establishes the unique conditions of this notably bright and distant galaxy and the huge potential of mid-infrared observations to characterize these primordial systems.The James Webb Space Telescope has discovered a surprising population of bright galaxies in the very early Universe (≲500 Myr after the Big Bang) that is hard to explain with conventional galaxy-formation models and whose physical properties are not fully understood. Insight into their internal physics is best captured through nebular lines, but at these early epochs, the brightest of these spectral features are redshifted into the mid-infrared and remain elusive. Using the mid-infrared instrument onboard the James Webb Space Telescope, here we present a detection of Hα and doubly ionized oxygen ([O iii] 4959,5007 Å) from the bright, ultra-high-redshift galaxy candidate GHZ2/GLASS-z12. Based on these emission lines, we infer a spectroscopic redshift of z = 12.33 ± 0.04, placing this galaxy just ~400 Myr after the Big Bang. These observations provide key insights into the conditions of this primaeval, luminous galaxy, which shows hard ionizing conditions rarely seen in the local Universe and probably driven by a compact and young burst (≲30 Myr) of star formation. The galaxy's oxygen-to-hydrogen abundance is close to a tenth of the solar value, indicating a rapid metal enrichment. This study establishes the unique conditions of this notably bright and distant galaxy and the huge potential of mid-infrared observations to characterize these primordial systems.

We investigate the differential impact of physical mechanisms, mergers (stellar accretion) and internal energetic phenomena, on the evolution of stellar population gradients in massive, present-day galaxies employing a set of high-resolution, cosmological zoom simulations. We demonstrate that negative metallicity and color gradients at large radii (>2Reff) originate from the accretion of metal-poor stellar systems. At larger radii, galaxies become typically more dominated by stars accreted from satellite galaxies in major and minor mergers. However, only strong galactic winds can sufficiently reduce the metallicity content of the accreted stars to realistically steepen the outer metallicity and colour gradients in agreement with present-day observations. In contrast, the gradients of the models without winds are inconsistent with observations (too flat). In the wind model, colour and metallicity gradients are significantly steeper for systems which have accreted stars in minor mergers, while galaxies with major mergers have relatively flat gradients, confirming previous results. This analysis greatly highlights the importance of both energetic processes and merger events for stellar population properties of massive galaxies at large radii. Our results are expected to significantly contribute to the interpretation of current and up-coming IFU surveys (like MaNGA and Califa), which in turn can help to better constrain still uncertain models for energetic processes in simulations.