Cosmology

Abstract:

Massive, starbursting galaxies in the early Universe represent some of the most extreme objects in the study of galaxy evolution. One such source is HFLS3 (z ∼ 6.34), which was originally identified as an extreme starburst galaxy with mild gravitational magnification (μ ∼ 2.2). Here, we present new observations of HFLS3 with the JWST/NIRSpec integral field unit in both low (PRISM/CLEAR; R ∼ 100) and high spectral resolution (G395H/290LP; R ∼ 2700), with high spatial resolution (∼0.1″) and sensitivity. Using a combination of the NIRSpec data and a new lensing model with accurate spectroscopic redshifts, we find that the 3″ × 3″ field is crowded, with a lensed arc (C, z = 6.3425 ± 0.0002), two galaxies to the south (S1 and S2, z = 6.3592 ± 0.0001), two galaxies to the west (W1, z = 6.3550 ± 0.0001; W2, z = 6.3628 ± 0.0001), and two low-redshift interlopers (G1, z = 3.4806 ± 0.0001; G2, z = 2.00 ± 0.01). We present spectral fits and morpho-kinematic maps for each bright emission line (e.g. [OIII]λ5007, Hα, and [NII]λ6584) from the R2700 data for all sources except G2 (whose spectral lines fall outside the observed wavelengths of the R2700 data). From a line ratio analysis, we find that the galaxies in component C are likely powered by star formation, though we cannot rule out or confirm the presence of active galactic nuclei in the other high-redshift sources. We performed gravitational lens modelling, finding evidence for a two-source composition of the lensed central object and a magnification factor (μ = 2.1 − 2.4) comparable to findings of previous work. The projected distances and velocity offsets of each galaxy suggest that they will merge within the next ∼1 Gyr. Finally, we examined the dust extinction-corrected SFRHα of each z > 6 source, finding that the total star formation (510 ± 140 M⊙ yr−1, magnification-corrected) is distributed across the six z ∼ 6.34 − 6.36 objects over a region of diameter ∼11 kpc. Altogether, this suggests that HFLS3 is not a single starburst galaxy, but instead a merging system of star-forming galaxies in the epoch of reionisation.

Abstract:

Thick disks are a prevalent feature observed in numerous disk galaxies, including our own Milky Way. Their significance has been reported to vary widely, ranging from a few percent to 100% of the disk mass, depending on the galaxy and the measurement method. We use the NewHorizon simulation, which has high spatial and stellar mass resolutions, to investigate the issue of the thick-disk mass fraction. We also use the NewHorizon2 simulation, which was run on the same initial conditions, but additionally traced nine chemical elements. Based on a sample of 27 massive disk galaxies with M_* > 10¹⁰M_⊙ in NewHorizon, the contribution of the thick disk was found to be 20% ± 11% in r-band luminosity or 35% ± 15% in mass to the overall galactic disk, which seems in agreement with observational data. The vertical profiles of 0, 22, and 5 galaxies are best fitted by 1, 2, or 3 sech2 components, respectively. The NewHorizon2 data show that the selection of thick-disk stars based on a single [α/Fe] cut is contaminated by stars of different kinematic properties, while missing the bulk of kinematically thick disk stars. Vertical luminosity profile fits recover the key properties of thick disks reasonably well. The majority of stars are born near the galactic midplane with high circularity and get heated with time via fluctuations in the force field. Depending on the star formation and merger histories, galaxies may naturally develop thick disks with significantly different properties.

Abstract:

We study how better resolving the cooling length of galactic outflows affect their energetics. We perform radiativehydrodynamical galaxy formation simulations of an isolated dwarf galaxy (M = 108 M) with the RAMSES-RTZ code, accounting for non-equilibrium cooling and chemistry coupled to radiative transfer. Our simulations reach a spatial resolution of 18 pc in the interstellar medium (ISM) using a traditional quasi-Lagrangian scheme. We further implement a new adaptive mesh refinement strategy to resolve the local gas cooling length, allowing us to gradually increase the resolution in the stellar-feedback-powered outflows, from ≥ 200 pc to 18 pc. The propagation of outflows into the inner circumgalactic medium is significantly modified by this additional resolution, but the ISM, star formation, and feedback remain by and large the same. With increasing resolution in the diffuse gas, the hot outflowing phase (T > 8 × 104 K) systematically reaches overall higher temperatures and stays hotter for longer as it propagates outwards. This leads to two-fold increases in the time-averaged mass and metal outflow loading factors away from the galaxy (r = 5 kpc), a five-fold increase in the average energy loading factor, and a ≈50 per cent increase in the number of sightlines with NO VI ≥ 1013 cm−2. Such a significant boost to the energetics of outflows without new feedback mechanisms or channels strongly motivates future studies quantifying the efficiency with which better-resolved multiphase outflows regulate galactic star formation in a cosmological context.