Galaxy formation and evolution

Abstract:

At high redshift, the cosmic web is widely expected to have a significant impact on the morphologies, dynamics, and star formation rates of the galaxies embedded within it, underscoring the need for a comprehensive study of the properties of such a filamentary network. With this goal in mind, we perform an analysis of high-z gas and dark matter (DM) filaments around a Milky Way-like progenitor simulated with the RAMSES adaptive mesh refinement (AMR) code from cosmic scales (∼1 Mpc) down to the virial radius of its DM halo host (∼20 kpc at z = 4). Radial density profiles of both gas and DM filaments are found to have the same functional form, namely a plummer-like profile modified to take into account the wall within which these filaments are embedded. Measurements of the typical filament core radius r0 from the simulation are consistent with that of isothermal cylinders in hydrostatic equilibrium. Such an analytic model also predicts a redshift evolution for the core radius of filaments in fair agreement with the measured value for DM [r0∝ (1 + z)−3.18 ± 0.28]. Gas filament cores grow as [r0∝ (1 + z)−2.72 ± 0.26]. In both gas and DM, temperature and vorticity sharply drop at the edge of filaments, providing an excellent way to constrain the outer filament radius. When feedback is included, the gas temperature and vorticity fields are strongly perturbed, hindering such a measurement in the vicinity of the galaxy. However, the core radius of the filaments as measured from the gas density field is largely unaffected by feedback; and the median central density is only reduced by about 20 per cent.

Abstract:

Population III (Pop. III) stars, as of yet, have not been detected, however as we move into the era of extremely large telescopes this is likely to change. One likely tracer for Pop. III stars is the He IIλ1640 emission line, which will be detectable by the HARMONI spectrograph on the European Extremely Large Telescope (ELT) over a broad range of redshifts (2 ≤ z ≤ 14). By post-processing galaxies from the cosmological, AMR-hydrodynamical simulation NEWHORIZON with theoretical spectral energy distributions (SED) for Pop. III stars and radiative transfer (i.e. the Yggdrasil Models and CLOUDY look-up tables, respectively) we are able to compute the flux of He IIλ1640 for individual galaxies. From mock 10 h observations of these galaxies we show that HARMONI will be able to detect Pop. III stars in galaxies up to z ∼ 10 provided Pop. III stars have a top heavy initial mass function (IMF). Furthermore, we find that should Pop. III stars instead have an IMF similar to those of the Pop. I stars, the He IIλ1640 line would only be observable for galaxies with Pop. III stellar masses in excess of 107M⊙⁠, average stellar age <1Myr at z = 4. Finally, we are able to determine the minimal intrinsic flux required for HARMONI to detect Pop. III stars in a galaxy up to z = 10.

Abstract:

Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation that captures the properties of the universe on a Gpc scale while achieving a resolution of 1 kpc. Inside the simulation box, we zoom in on a high-resolution cuboid region with a volume of 1049 × 119 × 127 cMpc3. The subgrid physics chosen to model galaxy formation includes radiative heating/cooling, UV background, star formation, supernova feedback, chemical evolution tracking the enrichment of oxygen and iron, the growth of supermassive black holes, and feedback from active galactic nuclei in the form of a dual jet-heating mode. For this simulation, we implemented a hybrid MPI-OpenMP version of RAMSES, specifically targeted for modern many-core many-thread parallel architectures. In addition to the traditional simulation snapshots, lightcone data were generated on the fly. For the post-processing, we extended the friends-of-friend algorithm and developed a new galaxy finder PGalF to analyze the outputs of HR5. The simulation successfully reproduces observations, such as the cosmic star formation history and connectivity of galaxy distribution, We identify cosmological structures at a wide range of scales, from filaments with a length of several cMpc, to voids with a radius of ~ 100 cMpc. The simulation also indicates that hydrodynamical effects on small scales impact galaxy clustering up to very large scales near and beyond the baryonic acoustic oscillation scale. Hence, caution should be taken when using that scale as a cosmic standard ruler: one needs to carefully understand the corresponding biases. The simulation is expected to be an invaluable asset for the interpretation of upcoming deep surveys of the universe.

Abstract:

With the advent of deep optical-to-near-infrared extragalactic imaging on the degree scale, samples of high-redshift sources are being selected that contain both bright star-forming (SF) galaxies and faint active galactic nuclei (AGN). In this study, we investigate the transition between SF- and AGN-dominated systems at z ≃ 4 in the rest-frame ultraviolet (UV). We find a rapid transition to AGN-dominated sources brightward of MUV ≃ −23.2. The effect is observed in the rest-frame UV morphology and size–luminosity relation, where extended clumpy systems become point-source-dominated, and also in the available spectra for the sample. These results allow us to derive the rest-frame UV luminosity function (LF) for the SF- and AGN-dominated subsamples. We find the SF-dominated LF is best fit with a double power law, with a lensed Schechter function being unable to explain the existence of extremely luminous SF galaxies at MUV ≃ −23.5. If we identify AGN-dominated sources according to a point-source morphology criterion, we recover the relatively flat faint-end slope of the AGN LF determined in previous studies. If we instead separate the LF according to the current spectroscopic AGN fraction, we find a steeper faint-end slope of α = −1.83 ± 0.11. Using a simple model to predict the rest-frame AGN LF from the z = 4 galaxy LF, we find that the increasing impact of host galaxy light on the measured morphology of faint AGN can explain our observations.

Abstract:

Context. The stellar initial mass function (IMF) seems to be variable and not universal, contrary to what has been argued in the literature over the last three decades. Several relations of the low-mass end of the IMF slope with other stellar population, photometrical, and kinematical parameters of massive early-type galaxies (ETGs) have been proposed, but consensus on the factual cause of the observed variations has not yet been reached.

Aims. We investigate the relationship between the IMF and other stellar population parameters in NGC 3311, the central galaxy of the Hydra I cluster. NGC 3311 is a unique laboratory, characterized by old and metal-rich stars, that is similar to other massive ETGs for which the IMF slope has been measured to be bottom-heavy (i.e., dwarf-rich); however, it has unusual stellar velocity dispersion and [α/Fe] profiles, both of which increase with radius.

Methods. We use the spatially resolved stellar population parameters (age, total metallicity, and [α/Fe]) that were derived in a forthcoming paper (Barbosa et al. 2020) – via the full-spectrum fitting of high signal-to-noise MUSE observations – to compare the IMF slope in the central part of NGC 3311 (R ≲ 16 kpc) against other stellar parameters, with the goal of assessing their relations and dependencies.

Results. For NGC 3311, we unambiguously invalidate the previously observed direct correlation between the IMF slope and the local stellar velocity dispersion, confirming some doubts that had been raised in the literature. This relation may simply arise as a spatial coincidence between the region with the largest stellar velocity dispersion and the region where the oldest in situ population is found and dominates the light. We also show robust evidence that the proposed IMF−metallicity relation is contaminated by the degeneracy between these two parameters. We do confirm that the stellar content in the innermost region of NGC 3311 follows a bottom-heavy IMF, in line with other literature results. The tightest correlations we found are those between stellar age and the IMF and between the galactocentric radius and the IMF.

Conclusions. The variation of the IMF at its low-mass end is not due to kinematical, dynamical, or global properties in NGC 3311. We speculate instead that the IMF might be dwarf-dominated in the “red nuggets” that formed through a very short and intense star formation episode at high redshifts (z >  2) when the Universe was denser and richer in gas, and which then ended up being the central cores of today’s giant ellipticals.