The origin of the extragalactic gamma-ray background is a pressing cosmological mystery. The Fermi Gamma-Ray Space Telescope has recently measured the intensity and spectrum of this background; both are substantially different from previous measurements. We present a novel calculation of the gamma-ray background from normal star-forming galaxies. Contrary to longstanding expectations, we find that numerous but individually faint normal galaxies may comprise the bulk of the Fermi signal, rather than rare but intrinsically bright active galaxies. This result has wide-ranging implications, including: the possibility to probe the cosmic star-formation history with gamma rays; the ability to infer the cosmological evolution of cosmic rays and galactic magnetic fields; and an increased likelihood to identify subdominant components from rare sources (e.g., dark matter clumps) through their large anisotropy.

As the Galaxy evolves, the abundance of deuterium in the interstellar medium (ISM) decreases from its primordial value: deuterium is ‘astrated’. The deuterium astration factor fD, the ratio of the primordial D abundance (the D to H ratio by number) to the ISM D abundance, is determined by the competition between stellar destruction and infall, providing a constraint on models of the chemical evolution of the Galaxy. Although conventional wisdom suggests that the local ISM (i.e. within ∼1–2 kpc of the Sun) should be well mixed and homogenized on time‐scales short compared to the chemical evolution time‐scale, the data reveal gas‐phase variations in the deuterium, iron and other metal abundances as large as factors of ∼4–5 or more, complicating the estimate of the ‘true’ ISM D abundance and of the deuterium astration factor. Here, assuming that the variations in the observationally inferred ISM D abundances result entirely from the depletion of D on to dust, rather than from unmixed accretion of nearly primordial material, a model‐independent, Bayesian approach is used to determine the undepleted abundance of deuterium in the ISM (or a lower limit to it). We find the best estimate for the undepleted, ISM deuterium abundance to be (D/H)ISM≥ (2.0 ± 0.1) × 10−5. This result is used to provide an estimate of (or an upper bound to) the deuterium astration factor, fD≡ (D/H)P/(D/H)ISM≤ 1.4 ± 0.1.

A new measurement of a spatially extended gamma-ray signal from the center of the Andromeda galaxy (M31) has been recently published by the Fermi-LAT collaboration, reporting that the emission broadly resembles the so-called Galactic center excess (GCE) of the Milky Way (MW). Steadily, the weight of the evidence is accumulating on a millisecond pulsar (MSPs) origin for the GCE. These elements prompt us to compare the mentioned observations with what is, perhaps, the simplest model for an MSP population, solely obtained by rescaling of the MSP luminosity function determined in the local MW disk via the respective stellar mass of the systems. Remarkably, we find that without free fitting parameters, this model can account for both the energetics and the morphology of the GCE within uncertainties. For M31, the estimated luminosity due to primordial MSPs is expected to contribute only about a quarter of the detected emission, although a stronger contribution cannot be excluded given the large uncertainties. If correct, the model predicts that the M31 disk emission due to MSPs is not far below the present upper bound. We also discuss additional refinements of this simple model. Using the correlation between globular cluster gamma-ray luminosity and stellar encounter rate, we gauge the dynamical MSP formation in the bulge. This component is expected to contribute to the GCE only at a level 5%, but it could affect the signal's morphology. We also comment on limitations of our model as well as on future perspectives for improved diagnostics.

The rare isotope 6 Li is made only by cosmic rays, dominantly in αα → 6 Li fusion reactions with ISM helium. Consequently, this nuclide provides a unique diagnostic of the history of cosmic rays in our Galaxy. The same hadronic cosmic-ray interactions also produce high-energy γ rays (mostly via pp → π 0 → γγ). Thus, hadronic γ-rays and 6 Li are intimately linked. Specifically, 6 Li directly encodes the local cosmic-ray fluence over cosmic time, while extragalactic hadronic γ rays encode an average cosmicray fluence over lines of sight out to the horizon. We examine this link and show how 6 Li and γ-rays can be used together to place important model-independent limits on the cosmic-ray history of our Galaxy and the universe. We first constrain γ-ray production from ordinary Galactic cosmic rays, using the local 6 Li abundance. We find that the solar 6 Li abundance demands an accompanying extragalactic pionic γ-ray intensity which exceeds that of the entire observed EGRB by a factor of 2 − 6. Possible explanations for this discrepancy are discussed. We then constrain Li production using recent determinations of extragalactic γ-ray background (EGRB). We note that cosmic rays created during cosmic structure formation would lead to pre-Galactic Li production, which would act as a "contaminant" to the primordial 7 Li content of metal-poor halo stars; the EGRB can place an upper limit on this contamination if we attribute the entire EGRB pionic contribution to structure forming cosmic rays. Unfortunately, the uncertainties in the determination of the EGRB are so large that the present γ-ray data cannot guarantee that the pre-Galactic Li is small compared to primordial 7 Li; thus, an improved determination of the EGRB will shed important new light on this issue. Our limits and their more model-dependent extensions will improve significantly with additional observations of 6 Li in halo stars, and with improved measurements of the EGRB spectrum by GLAST.

Pion decay gamma rays have long been recognized as a unique signature of hadronic cosmic rays and their interactions with the interstellar medium. We present a model-independent way of constraining this signal with observations of the Galactic Plane in diffuse gamma rays. We combine detections by the EGRET instrument at GeV energies and the MilagroČerenkov detector at TeV energies with upper limits from KASCADE and CASA-MIA ground arrays at PeV energies. Such a long "lever arm", spanning at least six orders of magnitude in energy, reveals a "TeV excess" in the diffuse Galactic Plane gamma-ray spectrum. While the origin of this excess is unknown, it likely implies also enhanced TeV neutrino fluxes, significantly improving the prospects for their detection. We show that unresolved point sources are a possible source of the TeV excess. In fact, the spectra of the unidentified EGRET sources in the Milagro region must break between ∼ 10 GeV and ∼ 1 TeV to avoid strongly overshooting the Milagro measurement; this may have important implications for cosmic-ray acceleration.Finally, we use our approach to examine the recent suggestion that darkmatter annihilation may account for the observed excess in diffuse Galactic gamma-rays detected by EGRET at energies above 1 GeV. Within our modelindependent approach, current data cannot rule this possibility in or out; however we point out how a long "lever arm" can be used to constrain the pionic gammaray component and in turn limit the "GeV excess" and its possible sources. Experiments such as HESS and MAGIC, and the upcoming VERITAS and GLAST, should be able to finally disentangle the main sources of the Galactic gamma rays.