We examine the properties of atomic hydrogen (H i) associated with galaxies in the EAGLE simulations of galaxy formation. EAGLE's feedback parameters were calibrated to reproduce the stellar mass function and galaxy sizes at z = 0.1, and we assess whether this calibration also yields realistic H i properties. We estimate the self-shielding density with a fitting function calibrated using radiation transport simulations, and correct for molecular hydrogen with empirical or theoretical relations. The 'standard-resolution' simulations systematically underestimate H i column densities, leading to an H i deficiency in low-mass (M < 10 10 M ) galaxies and poor reproduction of the observed H i mass function. These shortcomings are largely absent from EAGLE simulations featuring a factor of 8 (2) better mass (spatial) resolution, within which the H i mass of galaxies evolves more mildly from z = 1 to 0 than in the standard-resolution simulations. The largest-volume simulation reproduces the observed clustering of H i systems, and its dependence on H i-richness. At fixed M , galaxies acquire more H i in simulations with stronger feedback, as they become associated with more massive haloes and higher infall rates. They acquire less H i in simulations with a greater star formation efficiency, since the star formation and feedback necessary to balance the infall rate is produced by smaller gas reservoirs. The simulations indicate that the H i of present-day galaxies was acquired primarily by the smooth accretion of ionized, intergalactic gas at z 1, which later self-shields, and that only a small fraction is contributed by the reincorporation of gas previously heated strongly by feedback. H i reservoirs are highly dynamic: over 40 percent of H i associated with z = 0.1 galaxies is converted to stars or ejected by z = 0.

We use a model of the Galactic fountain to simulate the neutral-hydrogen emission of the Milky Way Galaxy. The model was developed to account for data on external galaxies with sensitive HI data. For appropriate parameter values, the model reproduces well the HI emission observed at Intermediate Velocities. The optimal parameters imply that cool gas is ionised as it is blasted out of the disc, but becomes neutral when its vertical velocity has been reduced by ~30 per cent. The parameters also imply that cooling of coronal gas in the wakes of fountain clouds transfers gas from the virial-temperature corona to the disc at ~2 Mo/yr. This rate agrees, to within the uncertainties with the accretion rate required to sustain the Galaxy's star formation without depleting the supply of interstellar gas. We predict the radial profile of accretion, which is an important input for models of Galactic chemical evolution. The parameter values required for the model to fit the Galaxy's HI data are in excellent agreement with values estimated from external galaxies and hydrodynamical studies of cloud-corona interaction. Our model does not reproduce the observed HI emission at High Velocities, consistent with High Velocity Clouds being extragalactic in origin. If our model is correct, the structure of the Galaxy's outer HI disc differs materially from that used previously to infer the distribution of dark matter on the Galaxy's outskirts.Comment: 15 pages, 11 figures; accepted for publication in MNRA

We present a systematic study of the extraplanar gas (EPG) in a sample of 15 nearby late-type galaxies at intermediate inclinations using publicly available, deep interferometric H I data from the HALOGAS survey. For each system we mask the H I emission coming from the regularly rotating disc and use synthetic datacubes to model the leftover 'anomalous' H I flux. Our model consists of a smooth, axisymmetric thick component described by 3 structural and 4 kinematical parameters, which are fit to the data via a Bayesian MCMC approach. We find that extraplanar H I is nearly ubiquitous in disc galaxies, as we fail to detect it in only two of the systems with the poorest spatial resolution. The EPG component encloses ∼ 5 − 25% of the total H I mass, with a mean value of 14%, and has a typical thickness of a few kpc, incompatible with expectations based on hydrostatic equilibrium models. The EPG kinematics is remarkably similar throughout the sample, and consists of a lagging rotation with typical vertical gradients of ∼ −10 km s −1 kpc −1 , a velocity dispersion of 15 − 30 km s −1 and, for most galaxies, a global inflow in both the vertical and radial directions with speeds of 20 − 30 km s −1 . The EPG H I masses are in excellent agreement with predictions from simple models of the galactic fountain powered by stellar feedback. The combined effect of photo-ionisation and interaction of the fountain material with the circumgalactic medium can qualitatively explain the kinematics of the EPG, but dynamical models of the galactic fountain are required to fully test this framework.

We use the EAGLE suite of cosmological hydrodynamical simulations to study how the H I content of present-day galaxies depends on their environment. We show that EA-GLE reproduces observed H I mass-environment trends very well, while semi-analytic models typically overpredict the average H I masses in dense environments. The environmental processes act primarily as an on/off switch for the H I content of satellites with M * > 10 9 M . At a fixed M * , the fraction of H I-depleted satellites increases with increasing host halo mass M 200 in response to stronger environmental effects, while at a fixed M 200 it decreases with increasing satellite M * as the gas is confined by deeper gravitational potentials. H I-depleted satellites reside mostly, but not exclusively, within the virial radius r 200 of their host halo. We investigate the origin of these trends by focussing on three environmental mechanisms: ram pressure stripping by the intra-group medium, tidal stripping by the host halo, and satellite-satellite encounters. By tracking back in time the evolution of the H I-depleted satellites, we find that the most common cause of H I removal is satellite encounters. The timescale for H I removal is typically less than 0.5 Gyr. Tidal stripping occurs in halos of M 200 < 10 14 M within 0.5 × r 200 , while the other processes act also in more massive halos, generally within r 200 . Conversely, we find that ram pressure stripping is the most common mechanism that disturbs the H I morphology of galaxies at redshift z = 0. This implies that H I removal due to satellite-satellite interactions occurs on shorter timescales than the other processes.

It is commonly believed that galaxies use, throughout the Hubble time, a very small fraction of the baryons associated to their dark matter halos to form stars. This so-called low "star formation/Ω c is the cosmological baryon fraction, is expected to reach its peak at nearly L * (at efficiency ≈ 20%) and decline steeply at lower and higher masses. We have tested this using a sample of nearby star-forming galaxies, from dwarfs (M 10 7 M ) to high-mass spirals (M 10 11 M ) with Hi rotation curves and 3.6µm photometry. We fit the observed rotation curves with a Bayesian approach by varying three parameters, stellar mass-to-light ratio Υ , halo concentration c and mass M halo . We found two surprising results: 1) the star formation efficiency is a monotonically increasing function of M with no sign of a decline at high masses, and 2) the most massive spirals (M 1 − 3 × 10 11 M ) have f ≈ 0.3 − 1, i.e. they have turned nearly all the baryons associated to their haloes into stars. These results imply that the most efficient galaxies at forming stars are massive spirals (not L * galaxies), they reach nearly 100% efficiency and thus, once both their cold and hot gas is considered into the baryon budget, they have virtually no missing baryons. Moreover, there is no evidence of mass quenching of the star formation occurring in galaxies up to halo masses of M halo ≈ a few × 10 12 M .

We use mock interferometric H I measurements and a conventional tilted-ring modelling procedure to estimate circular velocity curves of dwarf galaxy discs from the APOS-TLE suite of ΛCDM cosmological hydrodynamical simulations. The modelling yields a large diversity of rotation curves for an individual galaxy at fixed inclination, depending on the line-of-sight orientation. The diversity is driven by non-circular motions in the gas; in particular, by strong bisymmetric fluctuations in the azimuthal velocities that the tilted-ring model is ill-suited to account for and that are difficult to detect in model residuals. Large misestimates of the circular velocity arise when the kinematic major axis coincides with the extrema of the fluctuation pattern, in some cases mimicking the presence of kiloparsec-scale density 'cores', when none are actually present. The thickness of APOSTLE discs compounds this effect: more slowly-rotating extra-planar gas systematically reduces the average line-of-sight speeds. The recovered rotation curves thus tend to underestimate the true circular velocity of APOSTLE galaxies in the inner regions. Non-circular motions provide an appealing explanation for the large apparent cores observed in galaxies such as DDO 47 and DDO 87, where the model residuals suggest that such motions might have affected estimates of the inner circular velocities. Although residuals from tilted ring models in the simulations appear larger than in observed galaxies, our results suggest that non-circular motions should be carefully taken into account when considering the evidence for dark matter cores in individual galaxies.

Aims. We studied the global distribution and kinematics of the extra-planar neutral gas in the Milky Way.Methods. We built 3D models for a series of Galactic H i layers, projected them for an inside view, and compared them with the Leiden-Argentina-Bonn 21-cm observations. Results. We show that the Milky Way disk is surrounded by an extended halo of neutral gas with a vertical scale-height of 1.6 +0.6 −0.4 kpc and an H i mass of 3.2 +1.0 −0.9 × 10 8 M , which is ∼5−10% of the total Galactic H i. This H i halo rotates more slowly than the disk with a vertical velocity gradient of −15 ± 4 km s −1 kpc −1 . We found evidence for a global infall motion in the halo, both vertical (20 +5 −7 km s −1 ) and radial (30 +7 −5 km s −1 ). Conclusions.The Milky Way H i extra-planar layer shows properties similar to the halos of external galaxies, which is compatible with it being predominantly produced by supernova explosions in the disk. It is most likely composed of distinct gas complexes with masses of ∼10 4−5 M , of which the intermediate velocity clouds are the local manifestations. The classical high-velocity clouds appear to be a separate population.

High-velocity clouds consist of cold gas that appears to be raining down from the halo to the disc of the Milky Way. Over the past fifty years, two competing scenarios have attributed their origin either to gas accretion from outside the Galaxy or to circulation of gas from the Galactic disc powered by supernova feedback (galactic fountain). Here we show that both mechanisms are simultaneously at work. We use a new galactic fountain model combined with high-resolution hydrodynamical simulations. We focus on the prototypical cloud complex C and show that it was produced by an explosion that occurred in the Cygnus-Outer spiral arm about 150 million years ago. The ejected material has triggered the condensation of a large portion of the circumgalactic medium and caused its subsequent accretion onto the disc. This fountain-driven cooling of the lower Galactic corona provides the low-metallicity gas required by chemical evolution models of the Milky Way's disc.