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Abstract:

We report on a direct search for elastic photon-photon scattering using x-ray and 𝛾 photons from a laser-plasma based experiment. A 𝛾 photon beam produced by a laser wak­field accelerator provided a broadband 𝛾 spectrum extending to above 𝐸𝛾 = 200 MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at 𝐸𝑥 ≈ 1.4 keV, corresponding to an invariant mass of √𝑠 = 1.22 ± 0.22 MeV. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy 𝛾 photon and outputs two lower energy 𝛾 photons. No changes in the 𝛾 photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of 1.5 × 1015 μb. Although far from the QED prediction, this represents the lowest upper limit obtained so far for √𝑠 ≲ 1 MeV.

Abstract:

The structure and chemistry of the dusty interstellar medium (ISM) are shaped by complex processes that depend on the local radiation field, gas composition, and dust grain properties. Of particular importance are polycyclic aromatic hydrocarbons (PAHs), which emit strong vibrational bands in the mid-infrared, and play a key role in the ISM energy balance. We recently identified global correlations between PAH band and optical line ratios across three nearby galaxies, suggesting a connection between PAH heating and gas ionization throughout the ISM. In this work, we perform a census of the PAH heating–gas ionization connection using ∼700,000 independent pixels that probe scales of 40–150 pc in 19 nearby star-forming galaxies from the PHANGS survey. We find a universal relation between log PAH(11.3 μm/7.7 μm) and log ([S ii]/Hα) with a slope of ∼0.2 and a scatter of ∼0.025 dex. The only exception is a group of anomalous pixels that show unusually high (11.3 μm/7.7 μm) PAH ratios in regions with old stellar populations and high starlight-to-dust emission ratios. Their mid-infrared spectra resemble those of elliptical galaxies. Active galactic nucleus hosts show modestly steeper slopes, with a ∼​​​​​​10% increase in PAH(11.3 μm/7.7 μm) in the diffuse gas on kiloparsec scales. This universal relation implies an emerging simplicity in the complex ISM, with a sequence that is driven by a single varying property: the spectral shape of the interstellar radiation field. This suggests that other properties, such as gas-phase abundances, gas ionization parameter, and grain charge distribution, are relatively uniform in all but specific cases.

Abstract:

We measured the harmonic-space power spectrum of galaxy clustering auto-correlation from the Evolutionary Map of the Universe Pilot Survey 1 data (EMU PS1) and its cross-correlation with the lensing convergence map of cosmic microwave background (CMB) from Planck Public Release 4 at the linear scale range from ℓ = 2 to 500. We applied two flux density cuts at 0.18 and 0.4mJy on the radio galaxies observed at 944MHz and considered two source detection algorithms. We found the auto-correlation measurements from the two algorithms at the 0.18mJy cut to deviate for ℓ ≥ 250 due to the different criteria assumed on the source detection and decided to ignore data above this scale. We report a cross-correlation detection of EMU PS1 with CMB lensing at ∼5.5σ, irrespective of flux density cut. In our theoretical modelling we considered the SKADS and T-RECS redshift distribution simulation models that yield consistent results, a linear and a non-linear matter power spectrum, and two linear galaxy bias models. That is a constant redshift-independent galaxy bias b(z) = bg and a constant amplitude galaxy bias b(z) = bg/D(z). By fixing a cosmology model and considering a non-linear matter power spectrum with SKADS, we measured a constant galaxy bias at 0.18mJy (0.4mJy) with bg = 2.32-0.33^+0.41 (2.18-0.25^+0.17) and a constant amplitude bias with bg = 1.72-0.21^+0.31 (1.78-0.15^+0.22). When σ8 is a free parameter for the same models at 0.18mJy (0.4mJy) with the constant model we found σ8 = 0.68-0.14^+0.16 (0.82 ±0.10), while with the constant amplitude model we measured σ8 = 0.61-0.20^+0.18 (0.78-0.09^+0.11), respectively. Our results agree at 1σ with the measurements from Planck CMB and the weak lensing surveys and also show the potential of cosmology studies with future radio continuum survey data.