Galactic centre star formation writ large in gamma-rays

We have modelled the high-energy astrophysics of the inner 200 pc of the Galaxy with a view to explaining the diffuse, broad-band (radio continuum to TeV gamma-ray), non-thermal signal detected from this region. Our modelling pins down the ISM parameters for the environment wherein cosmic ray (CR) electrons and ions reside in the Galactic centre (GC). We find that the magnetic field in this region is 100-300 microG, the gas density < 60 cm^-3, and that a powerful (> 200 km/s) 'super'-wind acts to remove > 95% of the cosmic rays accelerated in the region before they have time to lose their energy in situ. The ~ 10^39 erg/s carried away by the GC cosmic ray protons is precisely enough to energise the ~GeV gamma-ray emission from the Fermi 'bubbles' recently found to extend north and south of the GC out to distances of ~10 kpc, provided that the bubbles constitute thick targets to the GC protons and that the situation has reached steady state. In such a situation of 'saturation' the hard, uniform spectrum of the bubbles are explained and secondary electron synchrotron explains the non-thermal microwave emission found in WMAP data mirroring the bubbles. Given the very low density of the bubble plasma ( 5 Gyr. Our scenario thus has the startling implication that a GC source of nonthermal particles of time-averaged power 10^39 erg/s has persisted since the youth of the Galaxy.Comment: 7 pages, 1 figure. Accepted to the Proceedings of the 25th Texas Symposium on Relativistic Astrophysics (Heidelberg, 2010). References updates and abstract typo corrected: "100-300 mG" -> "100-300 microG

The Galactic Centre - A Laboratory for Starburst Galaxies (?)

The Galactic centre - as the closest galactic nucleus - holds both intrinsic interest and possibly represents a useful analogue to star-burst nuclei which we can observe with orders of magnitude finer detail than these external systems. The environmental conditions in the GC - here taken to mean the inner 200 pc in diameter of the Milky Way - are extreme with respect to those typically encountered in the Galactic disk. The energy densities of the various GC ISM components are typically ~two orders of magnitude larger than those found locally and the star-formation rate density ~three orders of magnitude larger. Unusually within the Galaxy, the Galactic centre exhibits hard-spectrum, diffuse TeV (=10^12 eV) gamma-ray emission spatially coincident with the region's molecular gas. Recently the nuclei of local star-burst galaxies NGC 253 and M82 have also been detected in gamma-rays of such energies. We have embarked on an extended campaign of modelling the broadband (radio continuum to TeV gamma-ray), non- thermal signals received from the inner 200 pc of the Galaxy. On the basis of this modelling we find that star-formation and associated supernova activity is the ultimate driver of the region's non-thermal activity. This activity drives a large-scale wind of hot plasma and cosmic rays out of the GC. The wind advects the locally-accelerated cosmic rays quickly, before they can lose much energy in situ or penetrate into the densest molecular gas cores where star-formation occurs. The cosmic rays can, however, heat/ionize the lower density/warm H2 phase enveloping the cores. On very large scales (~10 kpc) the non-thermal signature of the escaping GC cosmic rays has probably been detected recently as the spectacular 'Fermi bubbles' and corresponding 'WMAP haze'.Comment: Invited talk to appear in Proceedings of IAU Symposium No. 284, 2011 (R.J. Tuffs & C.C. Popescu, eds.) `The Spectral Energy Distribution of Galaxies

Neutrinos from the Galactic Center in the Light of its Gamma-Ray Detection at TeV Energy

We re-evaluate the event rate expected in km^3-scale detectors for neutrinos from the direction of the Galactic Center (GC) in light of recent spectral measurements obtained by the HESS instrument for ~TeV gamma-radiation from this direction. In the most plausible scenario the re-evaluated event rate is smaller than that previously calculated--and here re-calculated--on the basis of EGRET data. However, the GC TeV gamma-ray detections by the Whipple, CANGAROO, and HESS instruments, together with the strong indications for an overabundance of cosmic rays coming from the GC at EeV energies, strengthen the expectation for a detectable, TeV-PeV GC neutrino signal from proton-proton interactions in that region. If the TeV gamma-ray--EeV cosmic ray anisotropy connection is correct, this signal will be detectable within a year and half for km^3-scale neutrino detectors in the Northern Hemisphere at super-TeV energies and, significantly, should also be detectable in 1.6 years by the South Polar IceCube detector at energies > 10^14 eV. The GC neutrino signal should also produce a detectable signal from neutrino showering and resonant W^- production by anti-electron-neutrinos in the volume of a km^3-scale detector.Comment: 12 pages, 1 figure. Version accepted to ApJ Letters. Minor amendment

The Maximum Flux of Star-Forming Galaxies

The importance of radiation pressure feedback in galaxy formation has been extensively debated over the last decade. The regime of greatest uncertainty is in the most actively star-forming galaxies, where large dust columns can potentially produce a dust-reprocessed infrared radiation field with enough pressure to drive turbulence or eject material. Here we derive the conditions under which a self-gravitating, mixed gas-star disc can remain hydrostatic despite trapped radiation pressure. Consistently taking into account the self-gravity of the medium, the star- and dust-to-gas ratios, and the effects of turbulent motions not driven by radiation, we show that galaxies can achieve a maximum Eddington-limited star formation rate per unit area $\dot{\Sigma}_{\rm *,crit} \sim 10^3 M_{\odot}$ pc$^{-2}$ Myr$^{-1}$, corresponding to a critical flux of $F_{\rm *,crit} \sim 10^{13} L_{\odot}$ kpc$^{-2}$ similar to previous estimates; higher fluxes eject mass in bulk, halting further star formation. Conversely, we show that in galaxies below this limit, our one-dimensional models imply simple vertical hydrostatic equilibrium and that radiation pressure is ineffective at driving turbulence or ejecting matter. Because the vast majority of star-forming galaxies lie below the maximum limit for typical dust-to-gas ratios, we conclude that infrared radiation pressure is likely unimportant for all but the most extreme systems on galaxy-wide scales. Thus, while radiation pressure does not explain the Kennicutt-Schmidt relation, it does impose an upper truncation on it. Our predicted truncation is in good agreement with the highest observed gas and star formation rate surface densities found both locally and at high redshift.Comment: Version accepted for publication in MNRAS. 12 pages, 8 figures. New appendix on photon tirin

SN1991bg-like supernovae are a compelling source of most Galactic antimatter

The Milky Way Galaxy glows with the soft gamma ray emission resulting from the annihilation of $\sim 5 \times 10^{43}$ electron-positron pairs every second. The origin of this vast quantity of antimatter and the peculiar morphology of the 511keV gamma ray line resulting from this annihilation have been the subject of debate for almost half a century. Most obvious positron sources are associated with star forming regions and cannot explain the rate of positron annihilation in the Galactic bulge, which last saw star formation some $10\,\mathrm{Gyr}$ ago, or else violate stringent constraints on the positron injection energy. Radioactive decay of elements formed in core collapse supernovae (CCSNe) and normal Type Ia supernovae (SNe Ia) could supply positrons matching the injection energy constraints but the distribution of such potential sources does not replicate the required morphology. We show that a single class of peculiar thermonuclear supernova - SN1991bg-like supernovae (SNe 91bg) - can supply the number and distribution of positrons we see annihilating in the Galaxy through the decay of $^{44}$Ti synthesised in these events. Such $^{44}$Ti production simultaneously addresses the observed abundance of $^{44}$Ca, the $^{44}$Ti decay product, in solar system material.Comment: Accepted for publication in Proceedings of IAU Symposium 322: The Multimessenger Astrophysics of the Galactic Center 4 page

Constraints on positron annihilation kinematics in the inner Galaxy

Context. The annihilation of cosmic positrons with electrons in the interstellar medium results in the strongest persistent γ-ray line signal in the sky. For the past 50 yr, this 511 keV emission - predominantly from the galactic bulge region and from a low surface-brightness disk - has puzzled observers and theoreticians. A key issue for understanding positron astrophysics is found in cosmic-ray propagation, especially at low kinetic energies (≲ 10 MeV). Aims. We want to shed light on how positrons propagate and the resulting morphology of the annihilation emission. We approach this "positron puzzle" by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and positron source populations in the Milky Way. Methods. By dissecting the positron annihilation emission as measured with INTEGRAL/SPI, we derived spectra for individual and independent regions in the sky. The centroid energies of these spectra around the 511 keV line are converted into Doppler shifts, representing the line-of-sight velocity along different galactic longitudes. This results in a longitude-velocity diagram of positron annihilation. From high-resolution spectra, we also determined Doppler-broadening from γ-ray line shape parameters to study annihilation conditions as they vary with galactic longitude. Results. We found line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements (2-3 km s -1 deg -1), and measurements of radioactive 26Al (7.5-9.5 km s -1 deg -1). The velocity gradient in the inner ±30° is determined to be 4 ± 6 km s -1 deg -1. The width of the 511 keV line is constant as a function of longitude at 2.43 ± 0.14 keV, with possibly different values towards the disk. The positronium fraction is found to be 1.0 along the galactic plane. Conclusions. The weak signals in the disk leave the question open of whether positron annihilation is associated with the high velocities seen in 26Al or rather with ordinarily rotating components of the Milky Way's interstellar medium. We confirm previous results that positrons are slowed down to the 10 eV energy scale before annihilation and constrain bulk Doppler-broadening contributions to ≲ 1.25 keV in the inner radian. Consequently, the true annihilation conditions remain unclear.Peer reviewedFinal Accepted Versio

Strong Evidence that the Galactic Bulge is Shining in Gamma Rays

There is growing evidence that the Galactic Center Excess identified in the $\textit{Fermi}$-LAT gamma-ray data arises from a population of faint astrophysical sources. We provide compelling supporting evidence by showing that the morphology of the excess traces the stellar over-density of the Galactic bulge. By adopting a template of the bulge stars obtained from a triaxial 3D fit to the diffuse near-infrared emission, we show that it is detected at high significance. The significance deteriorates when either the position or the orientation of the template is artificially shifted, supporting the correlation of the gamma-ray data with the Galactic bulge. In deriving these results, we have used more sophisticated templates at low-latitudes for the $\textit{Fermi}$ bubbles compared to previous work and the three-dimensional Inverse Compton (IC) maps recently released by the ${\tt GALPROP}$ team. Our results provide strong constraints on Millisecond Pulsar (MSP) formation scenarios proposed to explain the excess. We find that an $\textit{admixture formation}$ scenario, in which some of the relevant binaries are $\textit{primordial}$ and the rest are formed $\textit{dynamically}$, is preferred over a primordial-only formation scenario at $7.6\sigma$ confidence level. Our detailed morphological analysis also disfavors models of the disrupted globular clusters scenario that predict a spherically symmetric distribution of MSPs in the Galactic bulge. For the first time, we report evidence of a high energy tail in the nuclear bulge spectrum that could be the result of IC emission from electrons and positrons injected by a population of MSPs and star formation activity from the same site.Comment: 21 pages, 13 figures, V2: Minor changes to match submitted version, V3: matches JCAP published versio