Virial clouds and rotational asymmetry in galactic haloes

In 1995, it was suggested that some of the baryonic dark matter in galaxies may be in the form of molecular hydrogen clouds, and a mechanism for observing them had been given. In the same year, a novel method of seeing the clouds was proposed, that is to look for a temperature asymmetry in the cosmic microwave background towards the M31 galaxy, due to a “Doppler effect” induced by the M31 halo rotation. This temperature asymmetry has since been seen and confirmed in M31 and other galaxies, and used to study the rotation of galactic haloes and map their dynamics. It had been questioned whether such clouds could actually exist, and in response, the clouds were modeled and shown to be possible. It then becomes necessary to trace the evolution of those clouds from their formation to the modern day. Here, the development of the ideas is reviewed.

Modelling intergalactic low ionization metal absorption line systems near the epoch of reionization

ABSTRACT We interpret observations of intergalactic low ionization metal absorption systems at redshifts z ≳ 5 in terms of pressure-confined clouds. We find clouds confined by the expected pressure of galactic haloes with masses $11\lt \log M_h/h^{-1}\, \mathrm{M}_\odot \lt 12$ provide a good description of the column density ratios between low ionization metal absorbers. Some of the ratios, however, require extending conventional radiative transfer models of irradiated slabs to spherical (or cylindrical) clouds to allow for lines of sight passing outside the cores of the clouds. Moderate depletion of silicon on to dust grains is also indicated in some systems. The chemical abundances inferred span the range between solar and massive-star-dominated stellar populations as may arise in starburst galaxies. The typical H i column densities matching the data correspond to damped Ly α absorbers (DLAs) or sub-DLAs, with sizes of 40 pc to 3 kpc, gas masses 3.5 < log Mc/M⊙ < 8 and metallicites $0.001\!-\!0.01\, \mathrm{Z}_\odot$. Such systems continue to pose a challenge for galaxy-scale numerical simulations to reproduce.

The Hierarchical Structure of Galactic Haloes: Classification and characterisation with Halo-OPTICS

We build upon Ordering Points To Identify Clustering Structure (OPTICS), a hierarchical clustering algorithm well-known to be a robust data-miner, in order to produce Halo-OPTICS, an algorithm designed for the automatic detection and extraction of all meaningful clusters between any two arbitrary sizes. We then apply Halo-OPTICS to the 3D spatial positions of halo particles within four separate synthetic Milky Way type galaxies, classifying the stellar and dark matter structural hierarchies. Through visualisation of the Halo-OPTICS output, we compare its structure identica- tion to the state-of-the-art galaxy/(sub)halo nder VELOCIraptor, nding excellent agreement even though Halo-OPTICS does not consider kinematic information in this current implementation. We conclude that Halo-OPTICS is a robust hierarchical halo nder, although its determination of lower spatial-density features such as the tails of streams could be improved with the inclusion of extra localised information such as particle kinematics and stellar metallicity into its distance metric.

How unusual is the Milky Way’s assembly history?

ABSTRACT In the lambda cold dark matter (ΛCDM) model of structure formation galactic haloes build-up by accretion of mass and mergers of smaller haloes. The most recent massive merger event experienced by the Milky Way (MW) halo was the accretion of the Large Magellanic Cloud (LMC; which has a stellar mass of ∼109M⊙). Recent analyses of galactic stellar data from the Gaia satellite have uncovered an earlier massive accretion event, the Gaia-Enceladus Sausage (GES), which merged with the MW around 10 Gyr ago. Here, we use the EAGLE cosmological hydrodynamics simulation to study properties of simulated MW-mass haloes constrained to have accretion histories similar to that of the MW, specifically the recent accretion of an ‘LMC’ galaxy and a ‘GES’ merger, with a quiescent period between the GES merger and the infall of the LMC (the ‘LMC and GES’ category). We find that ∼16 per cent of MW-mass haloes have an LMC; ∼5 per cent have a GES event and no further merger with an equally massive object since z = 1; and only 0.65 per cent belong to the LMC and GES category. The progenitors of the MWs in this last category are much less massive than average at early times but eventually catch up with the mean. The LMC and GES category of galaxies naturally end up in the ‘blue cloud’ in the colour–magnitude diagram at z = 0, tend to have a disc morphology and have a larger than average number of satellite galaxies.

A new model for including galactic winds in simulations of galaxy formation – I. Introducing the Physically Evolved Winds (PhEW) model

ABSTRACT The propagation and evolution of cold galactic winds in galactic haloes is crucial to galaxy formation models. However, modelling of this process in hydrodynamic simulations of galaxy formation is oversimplified owing to a lack of numerical resolution and often neglects critical physical processes such as hydrodynamic instabilities and thermal conduction. We propose an analytic model, Physically Evolved Winds, that calculates the evolution of individual clouds moving supersonically through a uniform ambient medium. Our model reproduces predictions from very high resolution cloud-crushing simulations that include isotropic thermal conduction over a wide range of physical conditions. We discuss the implementation of this model into cosmological hydrodynamic simulations of galaxy formation as a subgrid prescription to model galactic winds more robustly both physically and numerically.

Galactic ionizing photon budget during the epoch of reionization in the Cosmic Dawn II simulation

ABSTRACT Cosmic Dawn II yields the first statistically meaningful determination of the relative contribution to reionization by galaxies of different halo mass, from a fully coupled radiation-hydrodynamics simulation of the epoch of reionization large enough (∼100 Mpc) to model global reionization while resolving the formation of all galactic haloes above ${\sim}10^8 \, {\rm M}_{\odot }$. Cell transmission inside haloes is bi-modal – ionized cells are transparent, while neutral cells absorb the photons their stars produce – and the halo escape fraction fesc reflects the balance of star formation rate (SFR) between these modes. The latter is increasingly prevalent at higher halo mass, driving down fesc (we provide analytical fits to our results), whereas halo escape luminosity, proportional to fesc × SFR, increases with mass. Haloes with dark matter masses within $6\times 10^{8} \, {\rm M}_{\odot }\lt M_{\rm halo}\lt 3 \times 10^{10} \, {\rm M}_{\odot }$ produce ∼80 per cent of the escaping photons at z = 7, when the universe is 50 per cent ionized, making them the main drivers of cosmic reionization. Less massive haloes, though more numerous, have low SFRs and contribute less than 10 per cent of the photon budget then, despite their high fesc. High-mass haloes are too few and too opaque, contributing <10 per cent despite their high SFRs. The dominant mass range is lower (higher) at higher (lower) redshift, as mass function and reionization advance together (e.g. at z = 8.5, xH i = 0.9, $M_{\rm halo}\lt 5\times 10^9 \, {\rm M}_{\odot }$ haloes contributed ∼80 per cent). Galaxies with UV magnitudes MAB1600 between −12 and −19 dominated reionization between z = 6 and 8.

The little things matter: relating the abundance of ultrafaint satellites to the hosts’ assembly history

ABSTRACT Ultrafaint dwarf galaxies ($M_\star \le 10^{5}\, {\rm M}_\odot$) are relics of an early phase of galaxy formation. They contain some of the oldest and most metal-poor stars in the Universe which likely formed before the epoch of hydrogen reionization. These galaxies are so faint that they can only be detected as satellites of the Milky Way. They are so small that they are not resolved in current cosmological hydrodynamic simulations. Here, we combine very high-resolution cosmological N-body simulations with a semi-analytic model of galaxy formation to study the demographics and spatial distribution of ultrafaint satellites in Milky Way-mass haloes. We show that the abundance of these galaxies is correlated with the assembly history of the host halo: at fixed mass, haloes assembled earlier contain, on average, more ultrafaint satellites today than haloes assembled later. We identify simulated galactic haloes that experience an ancient Gaia-Enceladus-Sausage-like and a recent LMC-like accretion event and find that the former occurs in 33 per cent of the sample and the latter in 9 per cent. Only 3 per cent experience both events and these are especially rich in ultrafaint satellites, most acquired during the ancient accretion event. Our models predict that the radial distribution of satellites is more centrally concentrated in early-forming haloes. Accounting for the depletion of satellites by tidal interactions with the central disc, we find a very good match to the observed radial distribution of satellites in the Milky Way over the entire radial range. This agreement is mainly due to the ability of our model to track ‘orphan’ galaxies after their subhaloes fall below the resolution limit of the simulation.

MEGA: Merger graphs of structure formation

ABSTRACT When following the growth of structure in the Universe, we propose replacing merger trees with merger graphs, in which haloes can both merge and split into separate pieces. We show that this leads to smoother mass growth and eliminates catastrophic failures in which massive haloes have no progenitors or descendants. For those who prefer to stick with merger trees, we find that trees derived from our merger graphs have similar mass growth properties to previous methods, but again without catastrophic failures. For future galaxy formation modelling, two different density thresholds can be used to distinguish host haloes (extended galactic haloes, groups, and clusters) from higher density subhaloes: sites of galaxy formation.