We present high-resolution hydrodynamical simulations of isolated dwarf galaxies including self-gravity, non-equilibrium cooling and chemistry, interstellar radiation fields (IRSF) and shielding, star formation, and stellar feedback. This includes spatially and temporally varying photoelectric (PE) heating, photoionization, resolved supernova (SN) blast waves and metal enrichment. A new flexible method to sample the stellar initial mass function allows us to follow the contribution to the ISRF, the metal output and the SN delay times of individual massive stars. We find that SNe play the dominant role in regulating the global star formation rate, shaping the multi-phase interstellar medium (ISM) and driving galactic outflows. Outflow rates (with massloading factors of a few) and hot gas fractions of the ISM increase with the number of SNe exploding in low-density environments where radiative energy losses are low. While PE heating alone can suppress star formation slightly more (a factor of a few) than SNe alone can do, it is unable to drive outflows and reproduce the multi-phase ISM that emerges naturally when SNe are included. These results are in conflict with recent results of Forbes et al. who concluded that PE heating is the dominant process suppressing star formation in dwarfs, about an order of magnitude more efficient than SNe. Potential origins for this discrepancy are discussed. In the absence of SNe and photoionization (mechanisms to disperse dense clouds), the impact of PE heating is highly overestimated owing to the (unrealistic) proximity of dense gas to the radiation sources. This leads to a substantial boost of the infrared continuum emission from the UV-irradiated dust and a far infrared line-to-continuum ratio too low compared to observations. Though sub-dominant in regulating star formation, the ISRF controls the abundance of molecular hydrogen via photodissociation.

We investigate the early impact of single and binary supernova (SN) explosions on dense gas clouds with three-dimensional, high-resolution, hydrodynamic simulations. The effect of cloud structure, radiative cooling and ionising radiation from the progenitor stars on the net input of kinetic energy, f kin = E kin /E SN , thermal energy, f therm = E therm /E SN , and gas momentum, f P = P/P SN , into the interstellar medium (ISM) is tested. For clouds withn = 100 cm −3 , the momentum generating Sedov and pressure-driven snowplough phases are terminated early (∼ 0.01 Myr) and radiative cooling limits the coupling to f therm ∼ 0.01, f kin ∼ 0.05, and f P ∼ 9, significantly lower than without cooling. For pre-ionised clouds the efficiencies are only increased by ∼ 50%, independent of the cloud structure. This only suffices to accelerate ∼ 5% of the cloud to radial velocities 30 km s −1 . A second SN might further enhance the coupling efficiencies if delayed past the Sedov phase of the first explosion. Such very low coupling efficiencies cast doubts on many galaxy-scale sub-resolution models for supernova feedback, most of which are validated a posteriori by qualitative agreement of galaxy properties with observations. Ionising radiation appears not to significantly enhance the immediate coupling of SNe to the surrounding gas as it drives the ISM into inert dense shells and cold clumps, a process which is unresolved in galaxy scale simulations. Our results support previous conclusions that supernovae might only drive a wind if a significant fraction explodes in low-density environments or if they are supported by processes other than ionising radiation.

The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more self-consistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H + , H, H 2 , C + , and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions (σ HI ≈ 0.8σ 300−8000 K ∼ 10−20 km s −1 , σ Hα ≈ 0.6σ 8000−3×10 5 K ∼ 20 − 30 km s −1 ), and strong outflows with mass loading factors (ratio of outflow to star formation rate) of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense (ρ ∼ 10 −25 − 10 −24 g cm −3 ) and slow (v ∼ 20 − 40 km s −1 ) but there is a high-velocity tail of up to v ∼ 500 km s −1 with ρ ∼ 10 −28 − 10 −27 g cm −3 .

We present a hydrodynamical simulation of the turbulent, magnetized, supernova (SN)-driven interstellar medium (ISM) in a stratified box that dynamically couples the injection and evolution of cosmic rays (CRs) and a self-consistent evolution of the chemical composition. CRs are treated as a relativistic fluid in the advection-diffusion approximation. The thermodynamic evolution of the gas is computed using a chemical network that follows the abundances of H + , H, H 2 , CO, C + , and free electrons and includes (self-)shielding of the gas and dust. We find that CRs perceptibly thicken the disk with the heights of 90% (70%) enclosed mass reaching 1.5 kpc ( 0.2 kpc). The simulations indicate that CRs alone can launch and sustain strong outflows of atomic and ionized gas with mass loading factors of order unity, even in solar neighborhood conditions and with a CR energy injection per SN of 10 50 erg, 10% of the fiducial thermal energy of an SN. The CR-driven outflows have moderate launching velocities close to the midplane ( 100 km s −1 ) and are denser (ρ ∼ 10 −24 − 10 −26 g cm −3 ), smoother, and colder than the (thermal) SN-driven winds. The simulations support the importance of CRs for setting the vertical structure of the disk as well as the driving of winds.

Feedback from massive stars is believed to be a key element in the evolution of molecular clouds. We use high‐resolution 3D smoothed particle hydrodynamics simulations to explore the dynamical effects of a single O7 star‐emitting ionizing photons at 1049 s−1 and located at the centre of a molecular cloud with mass 104 M⊙ and radius 6.4 pc; we also perform comparison simulations in which the ionizing star is removed. The initial internal structure of the cloud is characterized by its fractal dimension, which we vary between D=2.0 and 2.8, and the standard deviation of the approximately log‐normal initial density PDF, which is σ10 = 0.38 for all clouds. (i) As regards star formation, in the short term ionizing feedback is positive, in the sense that star formation occurs much more quickly (than in the comparison simulations), in gas that is compressed by the high pressure of the ionized gas. However, in the long term ionizing feedback is negative, in the sense that most of the cloud is dispersed with an outflow rate of up to ∼10−2 M⊙yr−1, on a time‐scale comparable with the sound‐crossing time for the ionized gas (∼1−2 Myr ), and triggered star formation is therefore limited to a few per cent of the cloud's mass. We will describe in greater detail the statistics of the triggered star formation in a companion paper. (ii) As regards the morphology of the ionization fronts (IFs) bounding the H ii region and the systematics of outflowing gas, we distinguish two regimes. For low D≲2.2, the initial cloud is dominated by large‐scale structures, so the neutral gas tends to be swept up into a few extended coherent shells, and the ionized gas blows out through a few large holes between these shells; we term these H ii regions shell dominated. Conversely, for high D≳2.6, the initial cloud is dominated by small‐scale structures, and these are quickly overrun by the advancing IF, thereby producing neutral pillars protruding into the H ii region, whilst the ionized gas blows out through a large number of small holes between the pillars; we term these H ii regions pillar dominated. (iii) As regards the injection of bulk kinetic energy, by ∼1 Myr, the expansion of the H ii region has delivered a mass‐weighted rms velocity of ∼6 km s−1; this represents less than 0.1 per cent of the total energy radiated by the O7 star.