The Event Horizon Telescope (EHT), now with its first ever image of the photon ring around the supermassive blackhole of M87, provides a unique opportunity to probe the physics of supermassive black holes through Very Long Baseline Interferometry (VLBI), such as the existence of the event horizon, the accretion processes as well as jet formation in Low Luminosity AGN (LLAGN). We build a theoretical model which includes an Advection Dominated Accretion Flow (ADAF) with emission from thermal and non-thermal electrons in the flow and a simple radio jet outflow. The predicted spectral energy distribution (SED) of this model is compared to sub-arcsec resolution observations to get the best estimates of the model parameters. The model-predicted radial emission profiles at different frequency bands are used to predict whether the inflow can be resolved by the EHT or with telescopes such as the Global 3-mm VLBI array (GMVA). In this work the model is initially tested with high resolution SED data of M87 and then applied to our sample of 5 galaxies (Cen A, M84, NGC 4594, NGC 3998 and NGC 4278). The model then allows us to predict if one can resolve the inflow for any of these galaxies using the EHT or GMVA.

The reionization of helium describes the transition from its singly ionized state to a doubly-ionized state in the intergalactic medium (IGM). This process is important for the thermal evolution of the IGM and influences the mean free path of photons with energies above 54.4 eV. While it is well-known that helium reionization is mostly driven by the contribution of energetic quasars at z < 6, we study here how helium reionization proceeds if there is an additional contribution due to the annihilation of dark matter. We explore the effects of different dark matter profiles for the dark matter clumping factor, which can significantly enhance the annihilation rate at late times. We find that the presence of dark matter annihilation enhances the He ++ abundance at early stages where it would be zero within the standard model, and it can further increase during structure formation, reflecting the increase of the dark matter clumping factor. The latter is, however, degenerate with the build-up of the quasar contribution, and we therefore expect no significant changes at late times. We expect that future studies of the He + Lyman α forest may help to assess whether the evolution is consistent with the contribution from quasars alone, or if an additional component may be required.

Fractal dimensions can be used to characterize the clustering and lacunarities in density distributions. We use generalized fractal dimensions to study the neutral hydrogen distribution (HI) during the epoch of reionization. Using a semi-numeric model of ionized bubbles to generate the HI field, we calculate the fractal dimensions for length scales ∼ 10h −1 cMpc. We find that the HI field displays significant multifractal behaviour and is not consistent with homogeneity at these scales when the mass averaged neutral fractionx M HI 0.5. This multifractal nature is driven entirely by the shapes and distribution of the ionized regions. The sensitivity of the fractal dimension to the neutral fraction implies that it can be used for constraining reionization history. We find that the fractal dimension is relatively less sensitive to the value of the minimum mass of ionizing haloes when it is in the range ∼ 10 9 − 10 10 h −1 M . Interestingly, the fractal dimension is very different when the reionization proceeds inside-out compared to when it is outside-in. Thus the multifractal nature of HI density field at high redshifts can be used to study the nature of reionization.

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In 2019, the Event Horizon Telescope Collaboration (EHTC) has published the first image of a supermassive black hole (SMBH) obtained via the Very Large Baseline Interferometry (VLBI) technique. In the future, it is expected that additional and more sensitive VLBI observations will be pursued for other nearby Active Galactic Nuclei (AGN), and it is therefore important to understand which possible features can be expected in such images. In this paper, we post-process General Relativistic Magneto-Hydrodynamical (GR-MHD) simulations which include resistivity, thus providing a self-consistent jet formation model, including resistive mass loading of a wind launched from a disc in Keplerian rotation. The ray-tracing is done using the General Relativistic Ray-Tracing code GRTRANS assuming synchrotron emission. We study the appearance of the black hole environment including the accretion disc, winds and jets under a large range of condition, varying black hole mass, accretion rate, spin, inclination angle, disc parameters and observed frequency. When we adopt M87-like parameters, we show that we can reproduce a ring-like feature (similar as observed by the EHT) for some of our simulations. The latter suggests that such Keplerian disc models thus could be consistent with the observed results. Depending on their masses, accretion rates, spin and the sensitivity of the observation, we note that other SMBHs may show additional features like winds and jets in the observations.