We introduce a new model for the structure and evolution of the gas in galactic discs. In the model the gas is in vertical pressure and energy balance. Star formation feedback injects energy and momentum, and non-axisymmetric torques prevent the gas from becoming more than marginally gravitationally unstable. From these assumptions we derive the relationship between galaxies' bulk properties (gas surface density, stellar content, and rotation curve) and their star formation rates, gas velocity dispersions, and rates of radial inflow. We show that the turbulence in discs can be powered primarily by star formation feedback, radial transport, or a combination of the two. In contrast to models that omit either radial transport or star formation feedback, the predictions of this model yield excellent agreement with a wide range of observations, including the star formation law measured in both spatially resolved and unresolved data, the correlation between galaxies' star formation rates and velocity dispersions, and observed rates of radial inflow. The agreement holds across a wide range of galaxy mass and type, from local dwarfs to extreme starbursts to high-redshifts discs. We apply the model to galaxies on the star-forming main sequence, and show that it predicts a transition from mostly gravity-driven turbulence at high redshift to star formation-driven turbulence at low redshift. This transition, and the changes in mass transport rates that it produces, naturally explain why galaxy bulges tend to form at high redshift and discs at lower redshift, and why galaxies tend to quench inside-out.

Recently evidence has emerged for enormous features in the γ-ray sky observed by the Fermi-LAT instrument: bilateral 'bubbles' of emission centered on the core of the Galaxy and extending to around ±10 kpc above and below the Galactic plane. These structures are coincident with a non-thermal microwave 'haze' found in WMAP data and an extended region of X-ray emission detected by ROSAT. The bubbles' γ-ray emission is characterised by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity ∼ 4 × 10 37 erg/s, around one order of magnitude larger than their microwave luminosity while more than order of magnitude less than their X-ray luminosity. Here we show that the bubbles are naturally explained as due to a population of relic cosmic ray protons and heavier ions injected by processes associated with extremely long timescale ( > ∼ 8 Gyr) and high areal density star-formation in the Galactic center.PACS numbers: 98.35. Nq,98.62.Nx,98.70.Rz,98.35.Jk A recent analysis [1,2] of Fermi-LAT [3] γ-ray data has revealed two, enormous bubble-like emission features centered on the core of the Galaxy and extending to around ±10 kpc above and below the Galactic plane. At lower Galactic latitudes these structures are coincident with a non-thermal microwave 'haze' found in WMAP 20-60 GHz data [4,5] and an extended region of diffuse X-ray emission detected by ROSAT [6].A natural explanation of these structures [2] would be that they are due to the same population of highlyrelativistic ( > ∼ 50 GeV) cosmic ray (CR) electrons which synchrotron radiate at multi-GHz frequencies and simultaneously produce > ∼ 1 GeV γ-rays through the inverse Compton (IC) process. However, given the severe radiative energy losses experienced by electrons, the hard spectrum, uniform intensity, vast extension, and energetics of the bubbles render the origin of this particle population extremely mysterious (see fig. 1) [1,2,4,5,7]. In particular, transport of ≥TeV, IC-radiating electrons to the requisite distances from the plane would require velocities of > 0.03 c, too fast for a Galactic wind (this is a conservative lower limit as only electron energy losses on the 2.7 K CMB are accounted for). If diffusive, a diffusion coefficient of ∼ 10 31 cm 2 /s would be required for E e = 1 TeV, 1-2 orders of magnitude larger than the Galactic plane value. One might postulate an in-situ electron acceleration/injection process to surmount these difficulties [1] but here spectral considerations present a severe test. The ∼ E −2 γ γ-ray spectrum might be due to IC emission from a cooled ∼ E −3 e electron population but there is a robustly-detected [1] hardening in the γ-ray spectrum below ∼ 1 GeV. In order to produce such a break either a unique (over the age of the Galaxy) injection event of * Roland.Crocker@mpi-hd.mpg.de † Felix.Aharonian@dias.ir age ∼ 10 6 years or a sharp, ∼1 TeV low-energy hardening or cut-off in the injection spectrum of electrons is required. The former seems unlikely as there are no indications of su...

The nucleus of the Milky Way is known to harbour regions of intense star formation activ-

Employing data collected during the first 25 months of observations by the Fermi-LAT, we describe and subsequently seek to model the very high energy (>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the morphological, spectral, and temporal characteristics of the central source, 1FGL J1745.6−2900. The data show a clear, statistically significant signal at energies above 10 GeV, where the Fermi-LAT has angular resolution comparable to that of HESS at TeV energies. This makes a meaningful joint analysis of the data possible. Our analysis of the Fermi data (alone) does not uncover any statistically significant variability of 1FGL J1745.6−2900 at GeV energies on the month timescale. Using the combination of Fermi data on 1FGL J1745.6−2900 and HESS data on the coincident, TeV source HESS J1745−290, we show that the spectrum of the central gamma-ray source is inflected with a relatively steep spectral region matching between the flatter spectrum found at both low and high energies. We model the gamma-ray production in the inner 10 pc of the Galaxy and examine cosmic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies can explain well the spectral phenomenology. We find considerable degeneracy between different parameter choices which will only be broken with the addition of morphological information that gamma-ray telescopes cannot deliver given current angular resolution limits. We argue that a future analysis performed in combination with higher-resolution radio continuum data holds out the promise of breaking this degeneracy.

An anomalous gamma-ray excess emission has been found in Fermi Large Area Telescope data 1 covering the centre of the Galaxy 2, 3 . Several theories have been proposed for this 'Galactic Centre Excess'. They include self-annihilation of dark matter particles 4 , an unresolved population of millisecond pulsars 5 , an unresolved population of young pulsars 6 , or a series of burst events 7 . Here we report on a new analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic Centre Excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic Centre Excess is not a dark matter phenomenon but rather associated with the stellar population of the Galactic bulge and nuclear bulge.The main challenge in pinning down the properties of the Galactic Centre Excess (GCE) is the modelling of diffuse Galactic emission from the interaction of cosmic rays with interstellar gas and radiation fields, by far the dominant source of gamma-rays in this region. The Fermi-Large Area Telescope (LAT) Collaboration designed a diffuse Galactic emission model based on a template 8 approach that is optimized to single-out gamma-ray point sources. This approach presupposes that the diffuse Galactic emission can be modelled as a linear combination of interstellar gas, inverse Compton maps, and several other diffuse components. Due to the limited kinematic resolution of gas tracers towards the Galactic Centre (GC), interstellar gas correlated gamma rays from the GC direction are difficult to disentangle. Previous studies 3, 4, 9 utilized interstellar gas maps that were constructed with an interpolation approach that assumed circular motion of interstellar gas. This kinematic assumption provides for an estimate of the distance to a part of the interstellar gas. However, it is well established that the Galaxy contains a central bar which causes non-circular motion of interstellar gas in its inner regions, so assuming circularity introduces a significant and avoidable bias to gamma-ray analyses of the GC region 10 .We use Fermi-LAT data accumulated between August 4, 2008 and September 4, 2015 in the 15 • ×15 • region around the GC. Hydrodynamical simulations 10 that account for the effects of the Galactic bar were used to better determine the diffuse Galactic gamma-ray emission. To evaluate the impact that the choice of interstellar gas models have on our results, we also constructed atomic and molecular hydrogen gas maps using an interpolation approach that reproduced those used in most previous gamma-ray analyses of the GC. We split each into 4 concentric rings, each with its own normalization parameter. Details of the model components and approach are provided in the Methods section.Interstellar gas map templates constructed using the results of hydrodynamical simulati...

Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic center may differ substantially compared to spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253 +0.016, its turbulence, magnetic field, and filamentary structure. Using column density maps based on dust

We consider the high‐energy astrophysics of the inner ∼200 pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to (i) sustain the steady‐state, in situ population of cosmic rays (CRs) required to generate the region’s non‐thermal radio and TeV γ‐ray emission; (ii) drive a powerful wind that advects non‐thermal particles out of the inner Galactic Centre; (iii) supply the low‐energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm envelope detected throughout the Central Molecular Zone; (iv) accelerate the primary electrons which provide the extended, non‐thermal radio emission seen over ∼150 pc scales above and below the plane (the Galactic Centre lobe); and (v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼10 kpc), generate the recently identified Wilkinson Microwave Anisotropy Probe (WMAP) haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/μ G < 40 0 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region’s dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs – in this starburst‐like environment – act to substantially modify the conditions of star formation. We speculate that the wind we identify plays a crucial role in advecting low‐energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511 keV line emission. We present extensive appendices reviewing the environmental conditions in the Galactic Centre, deriving the star formation and supernova rates there, and setting out the extensive prior evidence that exists, supporting the notion of a fast outflow from the region.

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