Galaxy formation and evolution

Abstract:

We explore the dynamics of stellar discs in the close vicinity of a supermassive black hole (SMBH) by means of direct N-body simulations. We show that an isolated nuclear stellar disc exhibits anisotropic mass segregation meaning that massive stars settle to lower orbital inclinations and more circular orbits than the light stars. However, in systems in which the stellar disc is embedded in a much more massive isotropic stellar cluster, anisotropic mass segregation tends to be suppressed. In both cases, an initially thin stellar disc becomes thicker, especially in the inner parts due to the fluctuating anisotropy in the spherical component. We find that vector resonant relaxation is quenched in the disc by nodal precession, but it is still the most efficient relaxation process around SMBHs of mass 106 M⊙ and above. Two-body relaxation may dominate for less massive SMBHs found in dwarf galaxies. Stellar discs embedded in massive isotropic stellar clusters ultimately tend to become isotropic on the local two-body relaxation time-scale. Our simulations show that the dynamics of young stars at the centre of the Milky Way is mostly driven by vector resonant relaxation leading to an anticorrelation between the scatter of orbital inclinations and distance from the SMBH. If the S-stars formed in a disc less than 10 Myr ago, they may coexist with a cusp of stellar mass black holes or an intermediate mass black hole with mass up to 1000 M⊙ to reproduce the observed scatter of angular momenta.

Abstract:

Context. Compact obscured nuclei (CONs) represent an extreme phase of galaxy evolution where rapid supermassive black hole growth and/or compact star-forming activity is completely obscured by gas and dust. Aims. We investigate the properties of CONs in the mid-infrared and explore techniques aimed at identifying these objects, such as through the equivalent width (EW) ratios of their polycyclic aromatic hydrocarbon (PAH) features. Methods. We modelled Spitzer spectra by decomposing the continua into nuclear and star-forming components, from which we then measured the nuclear optical depth, τN, of the 9.8 μm silicate absorption feature. We also used Spitzer spectral maps to investigate how PAH EW ratios vary with aperture size for objects that host CONs. Results. We find that the nuclear optical depth, τN, strongly correlates with the HCN-vib emission line in the millimetre for CONs, with a Pearson correlation coefficient of 0.91. We find the PAH EW ratio technique to be effective at selecting CONs and robust against highly inclined galaxies, where strong dust lanes may mimic a CON-like spectrum by producing a high τN. Our analysis of the Spitzer spectral maps shows that the efficacy of the PAH EW ratios in isolating CONs is reduced when there is a strong starforming component from the host galaxy. In addition, we find that the use of the inferred nuclear optical depth is a reliable method for identifying CONs in 36+8-7% of ultra-luminous infrared galaxies and 17+3-3% of luminous infrared galaxies, consistent with previous work. Conclusions. We confirm mid-infrared spectra to be a powerful diagnostic of CONs. The increased sensitivity of JWST will allow the identification of CONs at cosmic noon, revealing this extreme but hidden phase of galaxy evolution

Abstract:

Numerical simulations of dwarf galaxies have so far failed to reproduce the observed metallicity-luminosity relation, down to the regime of ultra-faint dwarfs (UFDs). We address this issue by exploring how the first generations of metal-free stars (Pop III) could help increase the mean metallicity ([Fe/H]) of those small and faint galaxies. We ran zoom-in chemo-dynamical simulations of 19 halos extracted from a λ Cold Dark Matter (CDM) cosmological box and followed their evolution down to redshift z = 0. Models were validated not only on the basis of galaxy global properties, but also on the detailed investigation of the stellar abundance ratios ([α/Fe]). We identified the necessary conditions for the formation of the first stars in mini-halos and derived constraints on the metal ejection schemes. The impact of Pop III stars on the final metallicity of UFDs was evaluated by considering different stellar mass ranges for their initial mass function (IMF), the influence of pair-instability supernovae (PISNe), and their energetic feedback, as well as the metallicity threshold that marks the transition from the first massive stars to the formation of low-mass long-lived stars. The inclusion of Pop III stars with masses below 140M⊙, and a standard IMF slope of -1.3 does increase the global metallicity of UFDs, although these are insufficient to resolve the tension with observations. The PISNe with progenitor masses above 140M⊙ do allow the metal content of UFDs to further increase. However, as PISNe are very rare and sometimes absent in the faintest UFDs, they have a limited impact on the global faint end of the metallicity-luminosity relation. Despite a limited number of spectroscopically confirmed members in UFDs, which make the stellar metallicity distribution of some UFDs uncertain, our analysis reveals that this is essentially the metal-rich tail that is missing in the models. The remaining challenges are thus both observational and numerical: (i) to extend high-resolution spectroscopy data samples and confirm the mean metallicity of the faintest UFDs; and (ii) to explain the presence of chemically enriched stars in galaxies with very short star formation histories.