Galactic winds are observed in many spiral galaxies with sizes from dwarfs up to the Milky Way, and they sometimes carry a mass in excess of that of newly formed stars by up to a factor of 10. Multiple driving processes of such winds have been proposed, including thermal pressure due to supernova heating, ultraviolet radiation pressure on dust grains or cosmic ray (CR) pressure.We here study wind formation due to CR physics using a numerical model that accounts for CR acceleration by supernovae, CR thermalization by Coulomb and hadronic interactions, and advective CR transport. In addition, we introduce a novel implementation of CR streaming relative to the rest frame of the gas. Streaming CRs excite Alfv´en waves on which they scatter, thereby limiting the CRs’ effective bulk velocity.We find that CR streaming drives powerful and sustained winds in galaxies with virial massesM200 1011M . In dwarf galaxies (M200 ∼ 109M ) the winds reach a mass loading factor of ∼5, expel ∼60 per cent of the initial baryonic mass contained inside the halo’s virial radius and suppress the star formation rate by a factor of ∼5. In dwarfs, the winds are spherically symmetric while in larger galaxies the outflows transition to biconical morphologies that are aligned with the disc’s angular momentum axis. We show that damping of Alfv´en waves excited by streaming CRs provides a means of heating the outflows to temperatures that scale with the square of the escape speed, kT ∝ υ2 esc. In larger haloes (M200 1011M ), CR streaming is able to drive fountain flows that excite turbulence, providing another means of heating the halo gas. For halo masses M200 1010M , we predict an observable level of Hα and X-ray emission from the heated halo gas. We conclude that CR-driven winds should be crucial in suppressing and regulating the first epoch of galaxy formation, expelling a large fraction of baryons, and – by extension – aid in shaping the faint end of the galaxy luminosity function. They should then also be responsible for much of the metal enrichment of the intergalactic medium

Hydrogen in the Universe was (re)ionised between redshifts z ≈ 10 and z ≈ 6. The nature of the sources of the ionising radiation is hotly debated, with faint galaxies below current detection limits regarded as prime candidates. Here we consider a scenario in which ionising photons escape through channels punctured in the interstellar medium by outflows powered by starbursts. We take account of the observation that strong outflows occur only when the star formation density is sufficiently high, and estimate the galaxy-averaged escape fraction as a function of redshift and luminosity from the resolved star formation surface densities in the EAGLE cosmological hydrodynamical simulation. We find that the fraction of ionising photons that escape from galaxies increases rapidly with redshift, reaching values of 5-20 per cent at z > 6, with the brighter galaxies having higher escape fractions. Combining the dependence of escape fraction on luminosity and redshift with the observed luminosity function, we demonstrate that galaxies emit enough ionising photons to match the existing constraints on reionisation while also matching the observed UV-background post-reionisation. Our findings suggest that galaxies above the current Hubble Space Telescope detection limit emit half of the ionising radiation required to reionise the Universe.

Understanding the evolution of carbon and iron in the Milky Way's halo is of importance because these two elements play crucial roles in constraining star formation, Galactic assembly, and chemical evolution in the early universe. Here we explore the spatial distributions of the carbonicity, [C/Fe], and metallicity, [Fe/H], of the halo system based on medium-resolution (R∼1300) spectroscopy of ∼58,000 stars in the southern hemisphere from the AAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicity map exhibits a positive gradient with distance, as similarly found for the Sloan Digital Sky Survey carbonicity map of Lee et al. The metallicity map confirms that [Fe/H] decreases with distance from the inner halo to the outer halo. We also explore the formation and chemical evolution history of the halo by considering the populations of carbon-enhanced metalpoor (CEMP) stars present in the AEGIS sample. The cumulative and differential frequency of CEMP-no stars (as classified by their characteristically lower levels of absolute carbon abundance, A(C)7.1, for subgiants and giants) increases with decreasing metallicity and is substantially higher than previous determinations for CEMP stars as a whole. In contrast, that of CEMP-s stars (with higher A(C)) remains almost flat, at a value of ∼10% in the range −4.0[Fe/H]−2.0. The distinctly different behaviors of the CEMP-no and CEMP-s stars relieve the tension with population synthesis models assuming a binary mass-transfer origin, which previously struggled to account for the higher reported frequencies of CEMP stars, taken as a whole, at low metallicity.

We investigate the properties of the galaxies that reionized the Universe and the history of cosmic reionization using the "Evolution and Assembly of GaLaxies and their Environments" (eagle) cosmological hydrodynamical simulations. We obtain the evolution of the escape fraction of ionising photons in galaxies assuming that galactic winds create channels through which 20 percent of photons escape when the local surface density of star formation is greater than 0.1 M yr −1 kpc −2 . Such threshold behaviour for the generation of winds is observed, and the rare local objects which have such high star formation surface densities exhibit high escape fractions of ∼ 10 percent. In our model the luminosity-weighted mean escape fraction increases with redshift as fesc = 0.045 ((1 + z)/4) 1.1 at z > 3, and the galaxy number weighted mean as f esc = 2.2 × 10 −3 ((1 + z)/4) 4 , and becomes constant ≈ 0.2 at redshift z > 10. The escape fraction evolves as an increasingly large fraction of stars forms above the critical surface density of star formation at earlier times. This evolution of the escape fraction, combined with that of the star formation rate density from eagle, reproduces the inferred evolution of the filling factor of ionised regions during the reionization epoch (6 < z < 8), the evolution of the post-reionization (0 z < 6) hydrogen photo-ionisation rate, and the optical depth due to Thomson scattering of the cosmic microwave background photons measured by the Planck satellite.

We present analytical solutions for winds from galaxies with NFW dark matter halo. We consider winds driven by energy and mass injection from multiple supernovae (SNe), as well as momentum injection due to radiation from a central black hole. We find that the wind dynamics depends on three velocity scales: (a) v ⋆ ∼ (Ė/2Ṁ ) 1/2 describes the effect of starburst activity, withĖ,Ṁ as energy and mass injection rate in a central region of radius R; (b) v • ∼ (GM • /2R) 1/2 for the effect of a central black hole of mass M • on gas at distance R and (c) v s = (GM h /2Cr s ) 1/2 which is closely related to the circular speed (v c ) for NFW halo, with r s as the halo scale radius and C is a function of halo concentration parameter. Our generalized formalism, in which we treat both energy and momentum injection from starbursts and radiation from central active galactic nucleus (AGN), allows us to estimate the wind terminal speed to be (4v 2 ⋆ + 6(Γ − 1)v 2 • − 4v 2 s ) 1/2 , where Γ is the ratio of force due to radiation pressure to gravity of the central black hole. Our dynamical model also predicts the following: (a) winds from quiescent star forming galaxies cannot escape from 10 11.5 ≤ M h ≤ 10 12.5 M ⊙ galaxies, (b) circumgalactic gas at large distances from galaxies should be present for galaxies in this mass range, (c) for an escaping wind, the wind speed in low to intermediate mass galaxies is ∼ 400-1000 km/s, consistent with observed X-ray temperatures; (d) winds from massive galaxies with AGN at Eddington limit have speeds 1000 km/s. We also find that the ratio [2v 2dictates the amount of gas lost through winds. Used in conjunction with an appropriate relation between M • and M h , and an appropriate opacity of dust grains in infrared (K band), this ratio has the attractive property of being minimum at a certain halo mass scale (M h ∼ 10 12-12.5 M ⊙ ) that signifies the cross-over of AGN domination in outflow properties from starburst activity at lower masses. We find that stellar mass for massive galaxies scales as M ⋆ ∝ M 0.26 h , and for low mass galaxies, M ⋆ ∝ M 5/3 h .

We study gaseous outflows from disk galaxies driven by radiation pressure on dust grains. We include the effect of bulge and dark matter halo and show that the existence of such an outflow implies a maximum value of disk mass-to-light ratio. We show that the terminal wind speed is proportional to the disk rotation speed in the limit of a cold gaseous outflow, and that in general there is a contribution from the gas sound speed. Using the mean opacity of dust grains and the evolution of the luminosity of a simple stellar population, we then show that the ratio of the wind terminal speed (v ∞ ) to the galaxy rotation speed (v c ) ranges between 2-3 for a period of ∼ 10 Myr after a burst of star formation, after which it rapidly decays. This result is independent of any free parameter and depends only on the luminosity of the stellar population and on the relation between disk and dark matter halo parameters. We briefly discuss the possible implications of our results.

We study gaseous outflows from disk galaxies driven by the combined effects of ram pressure on cold gas clouds and radiation pressure on dust grains. Taking into account the gravity due to disk, bulge and dark matter halo, and assuming continuous star formation in the disk, we show that radiation or ram pressure alone is not sufficient to drive escaping winds from disk galaxies, and that both processes contribute. We show that in the parameter space of star formation rate (SFR) and rotation speed of galaxies, the wind speed in galaxies with rotation speed v c ≤ 200 km s −1 and SFR ≤ 100 M ⊙ yr −1 , has a larger contribution from ram pressure, and that in high mass galaxies with large SFR, radiation from the disk has a greater role in driving galactic winds. The ratio of wind speed to circular speed can be approximated as vw vc ∼ 10 0.7 SFR 50 M⊙ yr −1 0.4 vc 120 km/s −1.25. We show that this conclusion is borne out by observations of galactic winds at low and high redshift and also of circumgalactic gas. We also estimate the mass loading factors under the combined effect of ram and radiation pressure, and show that the ratio of mass loss rate to SFR scales roughly as v −1 c Σ −1 g , where Σ g is the gas column density in the disk.

We present the Iκ α model of galaxy formation, in which a galaxy's star formation rate is set by the balance between energy injected by feedback from massive stars and energy lost by the deepening of the potential of its host dark matter halo due to cosmological accretion. Such a balance is secularly stable provided that the star formation rate increases with the pressure in the star forming gas. The Iκ α model has four parameters that together control the feedback from star formation and the cosmological accretion rate onto a halo. Iκ α reproduces accurately the star formation rate as a function of halo mass and redshift in the eagle hydrodynamical simulation, even when all four parameters are held constant. It predicts the emergence of a star forming main sequence along which the specific star formation rate depends weakly on stellar mass with an amplitude that increases rapidly with redshift. We briefly discuss the emerging mass-metallicity relation, the evolution of the galaxy stellar mass function, and an extension of the model that includes feedback from active galactic nuclei (AGN). These self-regulation results are independent of the star formation law and the galaxy's gas content. Instead, star forming galaxies are shaped by the balance between stellar feedback and cosmological accretion, with accurately accounting for energy losses associated with feedback a crucial ingredient.