We study the unresolved X-ray emission in three Local Group dwarf elliptical galaxies (NGC 147, NGC 185 and NGC 205) using XMM-Newton observations, which most likely originates from a collection of weak X-ray sources, mainly cataclysmic variables and coronally active binaries. Precise knowledge of this stellar X-ray emission is crucial not only for understanding the relevant stellar astrophysics but also for disentangling and quantifying the thermal emission from diffuse hot gas in nearby galaxies. We find that the integrated X-ray emissivities of the individual dwarf ellipticals agree well with that of the Solar vicinity, supporting an often assumed but untested view that the X-ray emissivity of old stellar populations is quasi-universal in normal galactic environments, in which dynamical effects on the formation and destruction of binary systems are not important. The average X-ray emissivity of the dwarf ellipticals, including M32 studied in the literature, is measured to be L 0.5−2 keV /M * = (6.0 ± 0.5 ± 1.8) × 10 27 erg s −1 M −1 ⊙ . We also compare this value to the integrated X-ray emissivities of Galactic globular clusters and old open clusters and discuss the role of dynamical effects in these dense stellar systems.

Abstract Previous studies have revealed a population of galaxies in galaxy clusters with ram pressure stripped (RPS) tails of gas and embedded young stars. We observed 1.4 GHz continuum and HI emission with the Very Large Array in its B–configuration in two fields of the Coma cluster to study the radio properties of RPS galaxies. The best continuum sensitivities in the two fields are 6 and 8 μJy per 4″ beam respectively, which are 4 and 3 times deeper than those previously published. Radio continuum tails are found in 10 (8 are new) out of 20 RPS galaxies, unambiguously revealing the presence of relativistic electrons and magnetic fields in the stripped tails. Our results also hint that the tail has a steeper spectrum than the galaxy. The 1.4 GHz continuum in the tails is enhanced relative to their Hα emission by a factor of ∼ 7 compared to the main bodies of the RPS galaxies. The 1.4 GHz continuum of the RPS galaxies is also enhanced relative to their IR emission by a factor of ∼ 2 compared to star-forming galaxies. The enhancement is likely related to ram pressure and turbulence in the tail. We furthermore present HI detections in three RPS galaxies and upper limits for the other RPS galaxies. The cold gas in D100’s stripped tail is dominated by molecular gas, which is likely a consequence of the high ambient pressure. No evidence of radio emission associated with ultra–diffuse galaxies is found in our data.

Various observations are revealing the widespread occurrence of fast and powerful winds in active galactic nuclei (AGNs) that are distinct from relativistic jets, likely launched from accretion disks and interacting strongly with the gas of their host galaxies. During the interaction, strong shocks are expected to form that can accelerate nonthermal particles to high energies. Such winds have been suggested to be responsible for a large fraction of the observed extragalactic gamma-ray background (EGB) and the diffuse neutrino background, via the decay of neutral and charged pions generated in inelastic pp collisions between protons accelerated by the forward shock and the ambient gas. However, previous studies did not properly account for processes such as adiabatic losses that may reduce the gamma-ray and neutrino fluxes significantly. We evaluate the production of gamma rays and neutrinos by AGNdriven winds in detail by modeling their hydrodynamic and thermal evolution, including the effects of their twotemperature structure. We find that they can only account for less than ∼30% of the EGB flux, as otherwise the model would violate the independent upper limit derived from the diffuse isotropic gamma-ray background. If the neutrino spectral index is steep with Γ  2.2, a severe tension with the isotropic gamma-ray background would arise as long as the winds contribute more than 20% of the IceCube neutrino flux in the 10-100 TeV range. At energies  100 TeV, we find that the IceCube neutrino flux may still be accountable by AGN-driven winds if the spectral index is as small as Γ∼2.0-2.1.

Recently, the High Altitude Water Cherenkov (HAWC) collaboration reported the discovery of the TeV halo around the Geminga pulsar. The TeV emission is believed to originate from inverse Compton scattering of pulsar-injected electrons/positrons off cosmic microwave background photons. In the mean time, these electrons should inevitably radiate X-ray photons via the synchrotron radiation, providing a useful constraint on the magnetic field in the TeV halo. In this work, we analyse the data of XMM-Newton and Chandra, and obtain an upper limit for the diffuse X-ray flux in a region of 600 ′′ around the Geminga pulsar, which is at a level of 10 −14 erg cm −2 s −1 . Through a numerical modelling on both the X-ray and the TeV observations assuming isotropic diffusion of injected electrons/positrons, we find the magnetic field inside the TeV halo is required to be < 1µG, which is significantly weaker than the typical magnetic field in the interstellar medium. The weak magnetic field together with the small diffusion coefficient inferred from HAWC's observation implies that the Bohm limit of particle diffusion may probably have been achieved in the TeV halo. We also discuss alternative possibilities for the weak X-ray emission, such as the hadronic origin of the TeV emission or a specific magnetic field topology, in which a weak magnetic field and a very small diffusion coefficient might be avoided. 300 arcsec PWN Geminga Chandra 300 arcsec XMM-Newton PWN Geminga