Papers for Friday, Jun 11 2021

We detect Lyman $\alpha$ absorption from the escaping atmosphere of HD 63433c, a $R=2.67 R_\oplus$, $P=20.5$ d mini Neptune orbiting a young (440 Myr) solar analogue in the Ursa Major Moving Group. Using HST/STIS, we measure a transit depth of $11.1 \pm 1.5$% in the blue wing and $8 \pm 3$% in the red. This signal is unlikely to be due to stellar variability, but should be confirmed by an upcoming second visit with HST. We do not detect Lyman $\alpha$ absorption from the inner planet, a smaller $R=2.15 R_\oplus$ mini Neptune on a 7.1 d orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium absorption for both planets. We measure the host star's X-ray spectrum and FUV flux with XMM-Newton, and model the outflow from both planets using a 3D hydrodynamic code. This model provides a reasonable match to the light curve in the blue wing of the Lyman $\alpha$ line and the helium non-detection for planet c, although it does not explain the tentative red wing absorption or reproduce the excess absorption spectrum in detail. Its predictions of strong Lyman $\alpha$ and helium absorption from b are ruled out by the observations. This model predicts a much shorter mass loss timescale for planet b, suggesting that b and c are fundamentally different: while the latter still retains its hydrogen/helium envelope, the former has likely lost its primordial atmosphere.

The formation of the solar system's giant planets predated the ultimate epoch of massive impacts that concluded the process of terrestrial planet formation. Following their formation, the giant planets' orbits evolved through an episode of dynamical instability. Several qualities of the solar system have recently been interpreted as evidence of this event transpiring within the first ~100 Myr after the Sun's birth; around the same time as the final assembly of the inner planets. In a series of recent papers we argued that such an early instability could resolve several problems revealed in classic numerical studies of terrestrial planet formation; namely the small masses of Mars and the asteroid belt. In this paper, we revisit the early instability scenario with a large suite of simulations specifically designed to understand the degree to which Earth and Mars' formation are sensitive to the specific evolution of Jupiter and Saturn's orbits. By deriving our initial terrestrial disks directly from recent high-resolution simulations of planetesimal accretion, our results largely confirm our previous findings regarding the instability's efficiency of truncating the terrestrial disk outside of the Earth-forming region in simulations that best replicate the outer solar system. Moreover, our work validates the primordial 2:1 Jupiter-Saturn resonance within the early instability framework as a viable evolutionary path for the solar system. While our simulations elucidate the fragility of the terrestrial system during the epoch of giant planet migration, many realizations yield outstanding solar system analogs when scrutinized against a number of observational constraints. Finally, we highlight the inability of models to form adequate Mercury-analogs and the low eccentricities of Earth and Venus as the most significant outstanding problems for future numerical studies to resolve.

We present high-resolution ($\sim$2.4\,kpc) ALMA band 7 observations (rest-frame $\lambda \sim 250\mu$m) of three powerful z$\sim$2.5 quasars ($L_{\rm bol}=10^{47.3}$-$10^{47.5}$ ergs s$^{-1}$). These targets have previously been reported as showing evidence for suppressed star formation based on cavities in the narrow H$\alpha$ emission at the location of outflows traced with [O~{\sc iii}] emission. Here we combine the ALMA observations with a re-analysis of the VLT/SINFONI data to map the rest-frame far-infrared emission, H$\alpha$ emission, and [O~{\sc iii}] emission. In all targets we observe high velocity [O~{\sc iii}] gas (i.e., W80$\sim$1000--2000\,km\,s$^{-1}$) across the whole galaxy. We do not identify any H$\alpha$ emission that is free from contamination from AGN-related processes; however, based on SED analyses, we show that the ALMA data contains a significant dust-obscured star formation component in two out of the three systems. This dust emission is found to be extended over $\approx$1.5--5.5\,kpc in the nuclear regions, overlaps with the previously reported H$\alpha$ cavities and is co-spatial with the peak in surface brightness of the [O~{\sc iii}] outflows. In summary, within the resolution and sensitivity limits of the data, we do not see any evidence for a instantaneous shut down of in-situ star formation caused directly by the outflows. However, similar to the conclusions of previous studies and based on our measured star formation rates, we do not rule out that the global host galaxy star formation could be suppressed on longer timescales by the cumulative effect of quasar episodes during the growth of these massive black holes.

Angular differential imaging (ADI) and spectral differential imaging (SDI) are commonly used for direct detection and characterisation of young, Jovian exoplanets in datasets obtained with the SPHERE/IFS instrument. We compare the performance of ADI, SDI, and three combinations of ADI and SDI to find which technique achieves the highest signal-to-noise ratio (S/N), and we analyse their performance as functions of integration time, field rotation, and wavelength range. We analyse SPHERE/IFS observations of three known exoplanets, namely Beta Pictoris b, 51 Eridani b, and HR 8799 e, with five differential imaging techniques. We split the datasets into subsets to vary each parameter before the data are processed with each technique. The differential imaging techniques are applied using principal component analysis (PCA). The tests show that a combination of SDI and ADI consistently achieves better results than ADI alone, and using SDI and ADI simultaneously (combined differential imaging; CODI) achieved the best results. The integration time test shows that targets with a separation larger than 0.24 arcsec observed with an integration time of more than 10$^3$s were photon-noise limited. Field rotation shows a strong correlation with S/N for field rotations up to 1 full width at half maximum (FWHM), after which no significant increase in S/N with field rotation is observed. Wavelength range variation shows a general increase in S/N for broader wavelength ranges, but no clear correlation is seen. Spectral information is essential to boost S/N compared to regular ADI. Our results suggest that CODI should be the preferred processing technique to search for new exoplanets with SPHERE/IFS. To optimise direct-imaging observations, the field rotation should exceed 1 FWHM to detect exoplanets at small separations.

We study the impact of deviations from the Kennicutt-Schmidt relation (quantified by the `burstiness' parameter $\kappa_s$), gas metallicity ($Z$), and density ($n$) on the observed [OIII]88$\mu$m/[CII]158$\mu$m surface brightness ratios ($\Sigma_{[OIII]}/\Sigma_{[CII]}$) in nine galaxies at $z\approx6-9$. We first discuss possible biases in the measured $\Sigma_{[OIII]}/\Sigma_{[CII]}$ ratios by comparing the data with zoom-in cosmological simulations, and then use a Markov Chain Monte Carlo algorithm to derive the best fit values of ($\kappa_s, Z, n$). We find that (i) the strongest dependence of $\Sigma_{[OIII]}/\Sigma_{[CII]}$ is on $\kappa_s$; (ii) high ratios identify starburst galaxies with short gas depletion times ($t_{dep}=6-49\,\rm Myr$); (iii) a secondary dependence on density is found, with $\Sigma_{[OIII]}/\Sigma_{[CII]}$ anticorrelating with $n$ as a result of the lower [OIII] critical density, (iv) the ratio only weakly depends on $Z$. The nine galaxies are significantly enriched (Z=0.2-0.5 $Z_\odot$), and dense ($n=10^{1-3} {\rm cm}^{-3}$). This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.

The reconstruction of a smooth field onto a fixed grid is a necessary step for direct comparisons to various real-world observations. Projecting SPH data onto a fixed grid becomes challenging in adaptive environments, where some particles may have smoothing lengths far below the grid size, whilst others are resolved by thousands of pixels. In this paper we show how the common approach of treating particles below the grid size as Monte Carlo tracers of the field leads to significant reconstruction errors, and despite good convergence properties is unacceptable for use in synthetic observations in astrophysics. We propose a new method, where particles smaller than the grid size are `blitted' onto the grid using a high-resolution pre-calculated kernel, and those close to the grid size are subsampled, that allows for converged predictions for projected quantities at all grid sizes.

We present a novel approach to disentangle two key contributions to the largest-scale anisotropy of the galaxy distribution: (i) the intrinsic dipole due to clustering and anisotropic geometry, and (ii) the kinematic dipole due to our peculiar velocity. Including the redshift and angular size of galaxies, in addition to their fluxes and positions allows us to measure both the direction and amplitude of our velocity independently of the intrinsic dipole of the source distribution. We find that this new approach applied to future galaxy surveys (LSST and Euclid) and a SKA radio continuum survey allows to measure our velocity ($\beta = v/c$) with a relative error in the amplitude $\sigma(\beta)/\beta \sim 1.3 - 4.5\%$ and in direction, $\theta_{\beta} \sim 0.9^{\circ} - 3.9^{\circ}$, well beyond what can be achieved when analysing only the number count dipole. We also find that galaxy surveys are able to measure the intrinsic large-scale anisotropy with a relative uncertainty of $\lesssim 5\%$ (excluding cosmic variance). Our method enables two simultaneous tests of the Cosmological Principle: comparing the observations of our peculiar velocity with the CMB dipole, and testing for a significant intrinsic anisotropy on large scales which indicates effects beyond the standard cosmological model.

We construct a dynamical model of the Milky Way disk from a data set, which combines Gaia EDR3 and APOGEE data throughout Galactocentric radii between $5.0\leq R\leq19.5$ kpc. We make use of the spherically-aligned Jeans Anisotropic Method to model the stellar velocities and their velocity dispersions. Building upon our previous work, our model now is fitted to kinematic maps that have been extended to larger Galactocentric radii due to the expansion of our data set, probing the outer regions of the Galactic disk. Our best-fitting dynamical model suggests a logarithmic density slope of $\alpha_{\rm DM}=-1.602\pm0.079_{\rm syst}$ for the dark matter halo and a dark matter density of $\rho_{\rm DM}(R_{\odot})=(8.92\pm0.56_{\rm syst})\times 10^{-3}$ M$_{\odot}$ pc$^{-3}$ ($0.339\pm0.022_{\rm syst}$ GeV cm$^{3}$). We estimate a circular velocity at the solar radius of $v_{\rm circ}=(234.7\pm1.7_{\rm syst})$ km s$^{-1}$ with a decline towards larger radii. The total mass density is $\rho_{\rm tot}(R_{\odot})$=$(0.0672\pm0.0015_{\rm syst})$ M$_{\odot}$ pc$^{-3}$ with a slope of $\alpha_{\rm tot}$=$-2.367\pm0.047_{\rm syst}$ for $5\leq R\leq19.5$ kpc and the total surface density is $\Sigma(R{_\odot}, |z|\leq$ 1.1 kpc)=$(55.5\pm1.7_{\rm syst})$ M$_{\odot}$ pc$^{-2}$. While the statistical errors are small, the error budget of the derived quantities is dominated by the 3 to 7 times larger systematic uncertainties. These values are consistent with our previous determination, but systematic uncertainties are reduced due to the extended data set covering a larger spatial extent of the Milky Way disk. Furthermore, we test the influence of non-axisymmetric features on our resulting model and analyze how a flaring disk model would change our findings.

Future data sets will enable cross-correlations between redshift space distortions (RSD) and weak lensing (WL). While photometric lensing and clustering cross-correlations have provided some of the tightest cosmological constraints to date, it is not well understood how to optimally perform similar RSD/WL joint analyses in a lossless way. RSD is typically measured in $3D$ redshift space, but WL is inherently a projected signal, making angular statistics a natural choice for the combined analysis. Thus, we determine the amount of RSD information that can be extracted using projected statistics. Specifically we perform a Fisher analysis to forecast constraints and model bias comparing two different Fingers-of-God (FoG) models using both, the $3D$ power spectrum, $P(k, \mu)$, and tomographic $C(\ell)$. We find that because na\"ive tomographic projection mixes large scales with poorly modelled nonlinear radial modes, it does not provide competitive constraints to the $3D$ RSD power spectrum without the model bias becoming unacceptably large. This is true even in the limit of narrow tomographic bins. In light of this we propose a new radial weighting scheme which unmixes radial RSD scales in projection yielding competitive constraints to the $3D$ RSD power spectrum, while keeping the model bias small. This work lays the groundwork for optimal joint analyses of RSD and cosmic shear.

X-ray pulse profile modeling of PSR J0740+6620, the most massive known pulsar, by the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurement's implications for the neutron star equation of state, employing a nonparametric EoS model based on Gaussian processes and combining information from other X-ray, radio and gravitational-wave observations of neutron stars. Our analysis mildly disfavors equations of state that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of 6.4. For EoSs with multiple stable branches, the transition mass from the hadronic to the hybrid branch is constrained to lie outside ($1, 2$) $M_{\odot}$. We also find that the conformal sound-speed bound is violated inside neutron star cores, implying that core matter is strongly interacting. The squared sound speed reaches a maximum of $0.79^{+0.21}_{-0.26}c^2$ at $3.51^{+2.30}_{-1.76}$ times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740+6620's central density is only $3.0^{+1.6}_{-1.6}$ times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in $\beta$-equilibrium.

The nearby elliptical galaxy IC4296 has produced a large (510 kpc) low-luminosity radio source with typical FR I core/jet/lobe morphology. The unprecedented combination of brightness sensitivity, dynamic range, and angular resolution of a new 1.28 GHz MeerKAT continuum image reveals striking new morphological features which we call threads, ribbons, and rings. The threads are faint narrow emission features originating where helical Kelvin-Helmholtz instabilities disrupt the main radio jets. The ribbons are smooth regions between the jets and the lobes, and they appear to be relics of jets powered by earlier activity that have since come into pressure equilibrium. Vortex rings in the outer portions of the lobes and their backflows indicate that the straight outer jets and ribbons are inclined by $i = 60 \pm 5^\circ$ from the line-of-sight, in agreement with photometric, geometric, and gas-dynamical estimates of inclination angles near the nucleus.

The use of Z-pinch facilities makes it possible to carry out well-controlled and diagnosable laboratory experiments to study laboratory jets with scaling parameters close to those of the jets from young stars. This makes it possible to observe processes that are inaccessible to astronomical observations. Such experiments are carried out at the PF-3 facility ("plasma focus", Kurchatov Institute), in which the emitted plasma emission propagates along the drift chamber through the environment at a distance of one meter. The paper presents the results of experiments with helium, in which a successive release of two ejections was observed. An analysis of these results suggests that after the passage of the first supersonic ejection, a region with a low concentration is formed behind it, the so-called vacuum trace, due to which the subsequent ejection practically does not experience environmental resistance and propagates being collimated. The numerical modeling of the propagation of two ejections presented in the paper confirms this point of view. Using scaling laws and appropriate numerical simulations of astrophysical ejections, it is shown that this effect can also be significant for the jets of young stars.

The first binary neutron star merger has already been detected in gravitational waves. The signal was accompanied by an electromagnetic counterpart including a kilonova component powered by the decay of radioactive nuclei, as well as a short $\gamma$-ray burst. In order to understand the radioactively-powered signal, it is necessary to simulate the outflows and their nucleosynthesis from the post-merger disk. Simulating the disk and predicting the composition of the outflows requires general relativistic magnetohydrodynamical (GRMHD) simulations that include a realistic, finite-temperature equation of state (EOS) and self-consistently calculating the impact of neutrinos. In this work, we detail the implementation of a finite-temperature EOS and the treatment of neutrinos in the GRMHD code HARM3D+NUC, based on HARM3D. We include formal tests of both the finite-temperature EOS and the neutrino leakage scheme. We further test the code by showing that, given conditions similar to those of published remnant disks following neutron star mergers, it reproduces both recombination of free nucleons to a neutron-rich composition and excitation of a thermal wind.

In the context of the concordance cosmology, structure formation in the Universe is the result of the amplification, by gravitational effects, of small perturbations in the primeval density field. This results in the formation of structures known as dark matter haloes, where gas collapses and forms stars, giving birth to galaxies. Numerical simulations are an important tool in the theoretical study of galaxy formation and evolution. In the present work, we describe the implementation of a chemical enrichment model in a state-of-the-art cosmological simulation of the Local Group. The simulation includes sub-grid models for the most relevant physical processes. We analyze the chemical and morphological evolution of two galaxies with virial masses similar to that of our Milky Way. For each of the stellar components (disc, bulge and halo), we establish links between their formation history and their chemical evolution. We find that $\alpha$-element (O, Mg, Si) enrichment happens at early stages of evolution, as their main producers are short-lived stars which end their lives as type II supernova explosions. There is also a gradual contamination with the rest of the elements as type Ia supernovae and winds of stars in the asymptotic giant branch occur.

Thermal disc winds occur in many contexts and may be particularly important to the secular evolution and dispersal of protoplanetary discs heated by high energy radiation from their central star. In this paper we generalise previous models of self-similar thermal winds - which have self-consistent morphology and variation of flow variables - to the case of launch from an elevated base and to non-isothermal conditions. These solutions are well-reproduced by hydrodynamic simulations, in which, as in the case of isothermal winds launched from the mid-plane, we find winds launch at the maximum Mach number for which the streamline solutions extend to infinity without encountering a singularity. We explain this behaviour based on the fact that lower Mach number solutions do not fill the spatial domain. We also show that hydrodynamic simulations reflect the corresponding self-similar models across a range of conditions appropriate to photoevaporating protoplanetary discs, even when gravity, centrifugal forces, or changes in the density gradient mean the problem is not inherently scale free. Of all the parameters varied, the elevation of the wind base affected the launch velocity and flow morphology most strongly, with temperature gradients causing only minor differences. We explore how launching from an elevated base affects Ne II line profiles from winds, finding it increases (reduces) the full width at half maximum (FWHM) of the line at low (high) inclination to the line of sight compared with models launched from the disc mid-plane and thus weakens the dependence of the FWHM on inclination.

We present new observations of the odd $z=0.96$ weak-line quasar PG1407+265, and report the discovery of CXOU J140927.9+261813, a $z=0.68$ X-ray cluster. Archival X-ray photometry spanning nearly four decades reveals that PG1407+265 is variable at the 1 dex level on a timescale of years. V-band variability is present with an amplitude less than 0.1 mag. The emission-line properties of PG1407+265 also reveal clear evidence for a powerful inflow or outflow due to near- or super-Eddington accretion, having a mechanical luminosity of order $10^{48}$ erg s$^{-1}$. Our follow-up {\sl Chandra} exposure centered on this object reveal a foreground $z=0.68$ cluster roughly 1' x 1'.5 in extent, offset to the east of PG1407+265, roughly coincident with the $z=0.68$ radio galaxy FIRST J140927.8+261818. This non-cool-core cluster contributes about 10\% of the X-ray flux of PG1407+265, has a mass of $(0.6- 5.5)\times10^{14} M_\odot$, and an X-ray gas temperature of ($2.2-4.3$) keV. Because the projected position of the quasar lies at about twice that of the cluster's inferred Einstein radius, lensing by the cluster is unlikely to explain the quasar's unusual properties. We also discuss the evidence for a second cluster centered on and at the redshift of the quasar.

We make use of individual (epoch) detection data from the Pan-STARRS 3Pi survey for 2863 optical ICRF3 counterparts in the five wavelength bands g, r, i, z, and y, published as part of the Data Release 2. A dedicated method based on the Functional Principal Component Analysis is developed for these sparse and irregularly sampled data. With certain regularization and normalization constraints, it allows us to obtain uniform and compatible estimates of the variability amplitudes and average magnitudes between the passbands and objects. We find that the starting assumption of affinity of the light curves for a given object at different wavelengths is violated for several percent of the sample. The distributions of root-mean-square variability amplitudes are strongly skewed toward small values, peaking at ~0.1 mag with tails stretching to 2 mag. Statistically, the lowest variability is found for the r band and the largest for the reddest y band. A small "brighter-redder" effect is present, with amplitudes in y greater than amplitudes in g in 57% of the sample. The variability versus redshift dependence shows a strong decline with z toward redshift 3, which we interpret as the time dilation of the dominant time frequencies. The colors of radio-loud ICRF3 quasars are correlated with redshift in a complicated, wavy pattern governed by the emergence of brightest emission lines within the five passbands.

Two competing models, gravitational instability-driven transport and stellar feedback, have been proposed to interpret the high velocity dispersions observed in high-redshift galaxies. We study the major mechanisms to drive the turbulence in star-forming galaxies using a sample of galaxies from the xCOLD GASS survey, selected based on their star-formation rate (SFR) and gas fraction to be in the regime that can best distinguish between the proposed models. We perform Wide Field Spectrograph (WiFeS) integral field spectroscopic (IFS) observations to measure the intrinsic gas velocity dispersions, circular velocities and orbital periods in these galaxies. Comparing the relation between the SFR, velocity dispersion, and gas fraction with predictions of these two theoretical models, we find that our results are most consistent with a model that includes both transport and feedback as drivers of turbulence in the interstellar medium. By contrast, a model where stellar feedback alone drives turbulence under-predicts the observed velocity dispersion in our galaxies, and does not reproduce the observed trend with gas fraction. These observations therefore support the idea that gravitational instability makes a substantial contribution to turbulence in high redshift and high SFR galaxies.

If dark matter decay or annihilate, a large amount of energy and particles would be released into the cosmic plasma. Therefore, they could modify the thermal and ionization history of our universe, then leave footprints on the cosmic microwave background power spectra. In this paper, we take dark matter annihilation as an example and investigate whether different reionization models influence the constraints on dark matter annihilation. We reconstruct the ionization history including both dark matter annihilation and star formation, then put constraints on DM annihilation. Combining the latest Planck data, BAO data, SNIa measurement, star formation rate density from UV and IR data, the upper limit of $\epsilon_0 f_d$ reads $2.7765\times 10^{-24}$ at 95$\%$C.L.. By comparison, we also constrain dark matter annihilation in the instantaneous reionization model from the same data combination except star formation rate density, and the upper limit of $\epsilon_0 f_d$ is $2.8468\times 10^{-24}$ at 95$\%$C.L., which is $2.5\%$ higher than the results in the previous model. This indicates various reionization models have little influence on constraining parameters of dark matter decay or annihilation.

We present the KMOS Galaxy Evolution Survey (KGES), a $K$-band Multi-Object Spectrograph (KMOS) study of the H$\alpha$ and [NII] emission from 288 $K$ band-selected galaxies at $1.2 \lesssim z \lesssim 1.8$, with stellar masses in the range $\log_{10}(M_{*}/\rm{M}_{\odot})\approx$9-11.5. In this paper, we describe the survey design, present the sample, and discuss the key properties of the KGES galaxies. We combine KGES with appropriately matched samples at lower redshifts from the KMOS Redshift One Spectroscopic Survey (KROSS) and the SAMI Galaxy Survey. Accounting for the effects of sample selection, data quality, and analysis techniques between surveys, we examine the kinematic characteristics and angular momentum content of star-forming galaxies at $z\approx1.5$, $\approx1$ and $\approx0$. We find that stellar mass, rather than redshift, most strongly correlates with the disc fraction amongst star-forming galaxies at $z \lesssim 1.5$, observing only a modest increase in the prevalence of discs between $z\approx1.5$ and $z\approx0.04$ at fixed stellar mass. Furthermore, typical star-forming galaxies follow the same median relation between specific angular momentum and stellar mass, regardless of their redshift, with the normalisation of the relation depending more strongly on how disc-like a galaxy's kinematics are. This suggests that massive star-forming discs form in a very similar manner across the $\approx$ 10 Gyr encompassed by our study and that the inferred link between the angular momentum of galaxies and their haloes does not change significantly across the stellar mass and redshift ranges probed in this work.

We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with special focus on the multi-band flux correlations. The dataset has been collected through an extensive multiwavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina and Mets\"ahovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance during a few days without a simultaneous increase in the X-ray flux (i.e. orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. Our multi-band correlation study also hints at an anti-correlation between UV/optical and X-ray at a significance higher than 3 sigmas. A VHE flare observed on 2017 February 4 shows gamma-ray variability on multi-hour timescales, with a factor 10 increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors.

We present a UV study of 3 extended UV (XUV) galaxies that we have observed with the UVIT and the GMRT. XUV galaxies show filamentary or diffuse star formation well beyond their optical disks, in regions where the disk surface density lies below the threshold for star formation. GALEX observations found that surprisingly 30% of all the nearby spiral galaxies have XUV disks. The XUV galaxies can be broadly classified as type 1 and type 2 XUV disks. The type 1 XUV disks have star formation that is linked to that in their main disk, and the UV emission appears as extended, filamentary spiral arms. The UV luminosity is associated with compact star forming regions along the extended spiral arms. The star formation is probably driven by slow gas accretion from nearby galaxies or the intergalactic medium (IGM). But the type 2 XUV disks have star formation associated with an outer low luminosity stellar disk that is often truncated near the optical radius of the galaxy. The nature of the stellar disks in type 2 XUV disks are similar to that of the diffuse stellar disks of low surface brightness galaxies. The star formation in type 2 XUV disks is thought to be due to rapid gas accretion or gas infall from nearby high velocity clouds (HVCs), interacting galaxies or the IGM. In this paper we investigate the star formation properties of the XUV regions of two type 2 galaxies and one mixed XUV type galaxy and compare them with the neutral hydrogen (HI) emission in their disks. We present preliminary results of our UVIT (FUV and NUV) observations of NGC 2541, NGC 5832 and ESO406-042, as well as GMRT observations of their HI emission. We describe the UV emission morphology, estimate the star formation rates and compare it with the HI distribution in these type 2 and mixed XUV galaxies.

The Beijing-Arizona Sky Survey (BASS) Data Release 3 (DR3) catalogue was released in 2019, which contains the data from all BASS and the Mosaic z-band Legacy Survey (MzLS) observations during 2015 January and 2019 March, about 200 million sources. We cross-match BASS DR3 with spectral databases from the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) to obtain the spectroscopic classes of known samples. Then, the samples are cross-matched with ALLWISE database. Based on optical and infrared information of the samples, we use the XGBoost algorithm to construct different classifiers, including binary classification and multiclass classification. The accuracy of these classifiers with the best input pattern is larger than 90.0 per cent. Finally, all selected sources in the BASS DR3 catalogue are classified by these classifiers. The classification label and probabilities for individual sources are assigned by different classifiers. When the predicted results by binary classification are the same as multiclass classification with optical and infrared information, the number of star, galaxy and quasar candidates is separately 12 375 838 (P_S>0.95), 18 606 073 (P_G>0.95) and 798 928 (P_Q>0.95). For these sources without infrared information, the predicted results can be as a reference. Those candidates may be taken as input catalogue of LAMOST, DESI or other projects for follow up observation. The classified result will be of great help and reference for future research of the BASS DR3 sources.

We present two further classical novae, V906 Car and V5668 Sgr, that show jets and accretion disc spectral signatures in their H-alpha complexes throughout the first 1000 days following their eruptions. From extensive densely time-sampled spectroscopy, we measure the appearance of the first high-velocity absorption component in V906 Car, and the duration of the commencement of the main H-alpha emission. We constrain the time taken for V5668 Sgr to transition to the nebular phase using [N II] 6584\r{A}. We find these timings to be consistent with the jet and accretion disc model for explaining optical spectral line profile changes in classical novae, and discuss the implications of this model for enrichment of the interstellar medium.

One of the main characteristics of the penumbra of sunspots is the radially outward-directed Evershed flow. Only recently have penumbral regions been reported with similar characteristics to normal penumbral filaments, but with an opposite direction of the flow. Such flows directed towards the umbra are known as counter Evershed flows (CEFs). We aim to determine the frequency of occurrence of CEFs in active regions (ARs) and to characterize their lifetime and the prevailing conditions in the ARs. We analysed the continuum images, Dopplergrams, and magnetograms recorded by SDO/HMI of 97 ARs that appeared from 2011 to 2017. We followed the ARs for $9.6\pm1.4$ days on average. We found 384 CEFs in total, with a median value of 6 CEFs per AR. CEFs are a rather common feature, they occur in 83.5% of all ARs regardless of the magnetic complexity of the AR. However, CEFs were observed on average only during 5.9% of the mean total duration of all the observations analyzed here. The lifetime of CEFs follows a log-normal distribution with a median value of 10.6$_{-6.0}^{+12.4}$ hr. In addition, we report two populations of CEFs depending on whether they are associated with light bridges, or not. We explain that the rarity of reports of CEFs in the literature is a result of highly incomplete coverage of ARs with spectropolarimetric data. By using the continuous observations now routinely available from space, we are able to overcome this limitation.

The Class 0 protostar IRAS 16293$-$2422 Source A is known to be a binary system (A1 and A2) or even a multiple system, which processes a complex outflow structure. We have observed this source in the C$^{34}$S, SO, and OCS lines at 3.1 mm with the Atacama Large Millimeter/submillimeter Array (ALMA). A substructure of this source is traced by our high angular-resolution observation (0\farcs12; 20 au) of the continuum emission. The northwest-southeast (NW-SE) outflow on a 2\arcsec\ scale is detected in the SO ($J_N$ = $2_2$--$1_1$) line. Based on the morphology of the SO distribution, this bipolar outflow structure seems to originate from the protostar A1 and its circumstellar disk, or the circummultiple structure of Source A. The rotation motion of the NW-SE outflow is detected in the SO and OCS emissions. We evaluate the specific angular momentum of the outflowing gas to be $(8.6 - 14.3) \times 10^{-4}$ km s$^{-1}$ pc. If the driving source of this outflow is the protostar A1 and its circumstellar disk, it can be a potential mechanism to extract the specific angular momentum of the disk structure. These results can be a hint for the outflow launching mechanism in this source. Furthermore, they provide us with an important clue to resolve the complicated structure of IRAS 16293$-$2422 Source A.

Hexamethylentetramine has drawn a lot of attention due to its potential to produce prebiotic species. This work aims to gain a better understanding in the chemical processes concerning methylamine under astrophysically relevant conditions. In particular, this work deeps into the formation of N-heterocycles in interstellar ice analogs exposed to UV radiation, which may lead to the formation of prebiotic species. Experimental simulations of interstellar ice analogs were carried out in ISAC. ISAC is an ultra-high vacuum chamber equipped with a cryostat, where gas and vapour species are frozen forming ice samples. Infrared and ultraviolet spectroscopy were used to monitor the solid phase, and quadrupole mass spectrometry served to measure the composition of the gas phase. The variety of species detected after UV irradiation of ices containing methylamine revealed the presence of 12 species which have been already detected in the ISM, being 4 of them typically classified as complex organic molecules: formamide (HCONH2), methyl cyanide (CH$_3$CN), CH$_3$NH and CH$_3$CHNH. Warming up of the irradiated CH$_3$NH$_2$-bearing ice samples lead to the formation of trimethylentriamine (TMT), a N-heterocycle precursor of HMT, and the subsequent synthesis of HMT at temperatures above 230 K.

Primordial black holes (PBHs) produced in the early Universe have attracted wide interest for their ability to constitute dark matter and explain the compact binary coalescence. We propose a new mechanism of PBH production during first-order phase transitions (PTs) and find that PBHs are naturally produced during PTs model-independently. Because of the randomness of the quantum tunneling, there always exists some probability that the vacuum decay is postponed in a whole Hubble volume. Since the vacuum energy density remains constant while radiation is quickly redshifted in the expanding Universe, the postponed vacuum decay then results in overdense regions, which finally collapse into PBHs as indicated by numerical simulations. Utilizing this result one can obtain mutual predictions and constraints between PBHs and GWs from PTs. The predicted mass function of PBHs is nearly monochromatic. We investigate two typical cases and find that 1) PBHs from a PT constitute all dark matter and GWs peak at $1$Hz, 2) PBHs from a PT can explain the coalescence events observed by LIGO-Virgo collaboration, and meanwhile GWs can explain the common-spectrum process detected by NANOGrav collaboration.

Astronomical imaging can be broadly classified into two types. The first type is amplitude interferometry, which includes conventional optical telescopes and Very Large Baseline Interferometry (VLBI). The second type is intensity interferometry, which relies on Hanbury Brown and Twiss-type measurements. At optical frequencies, where direct phase measurements are impossible, amplitude interferometry has an effective numerical aperture that is limited by the distance from which photons can coherently interfere. Intensity interferometry, on the other hand, correlates only photon fluxes and can thus support much larger numerical apertures, but suffers from a reduced signal due to the low average photon number per mode in thermal light. It has hitherto not been clear which method is superior under realistic conditions. Here, we give a comparative analysis of the performance of amplitude and intensity interferometry, and we relate this to the fundamental resolution limit that can be achieved in any physical measurement. Using the benchmark problem of determining the separation between two distant thermal point sources, e.g., two adjacent stars, we give a short tutorial on optimal estimation theory and apply it to stellar interferometry. We find that for very small angular separations the large baseline achievable in intensity interferometry can more than compensate for the reduced signal strength. We also explore options for practical implementations of Very Large Baseline Intensity Interferometry (VLBII).

We present a new model-independent (applicable for a broad range of equations of state) analysis of the neutrino emissivity due to triplet neutron pairing in neutron star cores. We find that the integrated neutrino luminosity of the Cooper Pair Formation (CPF) process can be written as a product of two factors. The first factor depends on the neutron star mass, radius and maximal critical temperature of neutron pairing in the core, $T_{Cn \mathrm{max}}$, but not on the particular superfluidity model; it can be expressed by an analytical formula valid for many nucleon equations of state. The second factor depends on the shape of the critical temperature profile within the star, the ratio of the temperature $T$ to $T_{Cn \mathrm{max}}$, but not on the maximal critical temperature itself. While this second factor depends on the superfluidity model, it obeys several model-independent constraints. This property allows one to analyse the thermal evolution of neutron stars with superfluid cores without relying on a specific model of their interiors. The constructed expressions allow us to perform a self-consistent analysis of spectral data and neutron star cooling theory. We apply these findings to the cooling neutron star in the Cassiopeia A supernova remnant using 14 sets of observations taken over 19 years. We constrain $T_{Cn\mathrm{max}}$ to the range of $ (5-10)\times 10^8$ K. This value depends weakly on the equation of state and superfluidity model, and will not change much if cooling is slower than the current data suggest. We also constrain the overall efficiency of the CPF neutrino luminosity.

Most ultraluminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic Black Hole Binaries, their evidence in the super-Eddington regime remained elusive. However, recent circumstantial evidence had hinted the presence of a recurrent evolutionary cycle in two archetypal ULXs, Holmberg II X-1 and NGC 5204 X-1. Here we build on our previous work and exploit the long-term high-cadence monitoring of Swift-XRT in order to provide evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions. We study the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb-Scargle periodograms and Gaussian processes modelling to look for periodic variability. We show that both sources follow a recurrent evolutionary pattern in the HID that can be characterized by the hard ultraluminous (HUL) and soft ultraluminous (SUL) spectral regimes, and a third state with characteristics similar to the supersoft ultraluminous (SSUL) state. The transitions between the soft states seem aperiodic, as revealed by timing analysis of the light curve of Holmberg II X-1, albeit further investigation is warranted. The light curve of NGC 5204 X-1 shows a periodicity of $\sim$ 200 days, possibly associated with the duration of the evolutionary cycle. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the supercritical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the SUL--SSUL spectral changes.

PSR J$1946+3417$ is a millisecond pulsar (MSP) with a spin period $P\simeq3.17\rm~ms$. Harbored in a binary with an orbital period $P_{\rm b}\simeq27$ days, the MSP is accompanied by a white dwarf (WD). The masses of the MSP and the WD were determined to be $1.83\rm~M_\odot$ and $0.266\rm~M_\odot$, respectively. Specially, its orbital eccentricity is $e\simeq0.134$, which is challenging the recycling model of MSPs. Assuming that the neutron star in a binary may collapse to a strange star when its mass reaches a critical limit, we propose a phase transition (PT) scenario to account for the origin of the system. The sudden mass loss and the kick induced by asymmetric collapse during the PT may result in the orbital eccentricity. If the PT event takes place after the mass transfer ceases, the eccentric orbit can not be re-circularized in the Hubble time. Aiming at the masses of both components, we simulate the evolution of the progenitor of PSR J$1946+3417$ via \texttt{MESA}. The simulations show that a NS / main sequence star binary with initial masses of $1.4+1.6\rm~M_\odot$ in an initial orbit of 2.59 days will evolve into a binary consisting of a $2.0\rm~M_\odot$ MSP and a $0.27\rm~M_\odot$ WD in an orbit of $\sim21.5$ days. Assuming that the gravitational mass loss fraction during PT is $10\%$, we simulate the effect of PT via the kick program of \texttt{BSE} with a velocity of $\sigma_{\rm PT}=60~{\rm km~s}^{-1}$. The results show that the PT scenario can reproduce the observed orbital period and eccentricity with higher probability then other values.

Possible violations of Lorentz invariance (LIV) can produce vacuum birefringence, which results in a frequency-dependent rotation of the polarization plane of linearly polarized light from distant sources. In this paper, we try to search for a frequency-dependent change of the linear polarization angle arising from vacuum birefringence in the spectropolarimetric data of astrophysical sources. We collect five blazars with multiwavelength polarization measurements in different optical bands $(UBVRI)$. Taking into account the observed polarization angle contributions from both the intrinsic polarization angle and the rotation angle induced by LIV, and assuming that the intrinsic polarization angle is an unknown constant, we obtain new constraints on LIV by directly fitting the multiwavelength polarimetric data of the five blazars. Here we show that the birefringence parameter $\eta$ quantifying the broken degree of Lorentz invariance is limited to be in the range of $-8.91\times 10^{-7}$ < $\eta$ < $2.93\times10^{-5}$ at the $2\sigma$ confidence level, which is as good as or represents one order of magnitude improvement over the results previously obtained from ultraviolet/optical polarization observations. Much stronger limits can be obtained by future multiwavelength observations in the gamma-ray energy band.

We test the statistical isotropy (SI) of the $E$-mode polarization of the Cosmic Microwave Background (CMB) radiation observed by the Planck satellite using two statistics, namely, the $\alpha$ estimator that is derived from the contour Minkowski Tensor (CMT), and the Directional statistic ($\mathcal{D}$ statistic). The $\alpha$ estimator obtained from the CMT provides information about the alignment of structures and can be used to infer statistical properties such as Gaussianity and SI of random fields. The $\mathcal{D}$ statistic is based on detecting preferred directionality shown by vectors defined by the field. We compute $\alpha$ and $\mathcal{D}$ statistic for the low resolution component separated SMICA $E$-mode map of CMB polarization, and compare with the values calculated using FFP10 SMICA simulations. We find good agreement between the Planck data and SMICA simulations for both $\alpha$ estimator and $\mathcal{D}$ statistic.

Whilst ``slingshot'' prominences have been observed on M-dwarfs, most if not all theoretical studies have focused on solar-like stars. We present an investigation into stellar prominences around rapidly rotating young M-dwarfs. We have extrapolated the magnetic field in the corona from Zeeman-Doppler maps and determined the sites of mechanical stability where prominences may form. We analyse the prominence mass that could be supported and the latitude range over which this material is distributed. We find that for these maps, much of this prominence mass may be invisible to observation - typically <1\% transits the stellar disc. On the rapidly-rotating M-dwarf V374 Peg (P$_{\rm rot}$ = 0.45 days) where prominences have been observed, we find the visible prominence mass to be around only 10\% of the total mass supported. The mass loss rate per unit area for prominences scales with the X-ray surface flux as $\dot{M}/A \propto$ $F_X^{1.32}$ which is very close to the observationally-derived value for stellar winds. This suggests that prominence ejection may contribute significantly to the overall stellar wind loss and spin down. A planet in an equatorial orbit in the habitable zone of these stars may experience intermittent enhancements of the stellar wind due to prominence ejections. On some stars, this may occur throughout 20\% of the orbit.

Cyclotron auto-resonance acceleration has been recently advanced as a potential mechanism for accelerating nuclei to ZeV energies (1 ZeV = $10^{21}$ eV). All results have been based on single-particle calculations employing analytic solutions to the relativistic equations of motion in the combined magnetic and radiation fields. Here, many-particle simulation results are presented which lend support to the single-particle calculations. Each single-particle result is found to lie well within one standard deviation of the ensemble average of the corresponding many-particle simulation.

Helium is the second most common chemical species in the Universe. The study of helium abundance has the potential to unravel the chemical evolution of and within galaxies. In this study, we provide an empirical calibration for the singly ionized helium abundance: $12+\log_{10}({\rm He}^+/{\rm H}^+)$, based on the emission line flux ratio He$_{\lambda5876}$/H$\alpha$ from Galactic and extragalactic HII regions compiled from the literature. Based on this calibrator, we explore for the first time the helium abundance in a large sample of HII regions located in galaxies representative of the nearby Universe from the CALIFA survey. Furthermore, this calibrator allows us to explore the variations of the helium abundance with respect to the oxygen abundance. The observed trends are in agreement with a change in the chemical enrichment with mass/oxygen abundance similar to the one observed due to the inside-out model in a MW-galaxy (highlighting the connection between resolved and global trends in galaxies). Our calibrator provides an empirical proxy to estimate the helium abundance at kpc scales as well as to constrain chemical evolutionary models.

Grain surface chemistry is key to the composition of protoplanetary disks around young stars. The temperature of grains depends on their size. We evaluate the impact of this temperature dependence on the disk chemistry. We model a moderately massive disk with 16 different grain sizes. We use POLARIS to calculate the dust grain temperatures and the local UV flux. We model the chemistry using the 3-phase astrochemical code NAUTILUS. Photoprocesses are handled using frequency-dependent cross-sections, and a new method to account for self and mutual shielding. The multi-grain model outputs are compared to those of single-grain size models (0.1 $\mu$m), with two different assumptions for their equivalent temperature. We find that the Langmuir-Hinshelwood (LH) mechanism at equilibrium temperature is not efficient to form H$_2$ at 3-4 scale heights ($H$), and adopt a parametric fit to a stochastic method to model H$_2$ formation instead. We find the molecular layer composition (1-3 $H$) to depend on the amount of remaining H atoms. Differences in molecular surface densities between single and multi-grain models are mostly due to what occurs above 1.5 $H$. At 100 au, models with colder grains produce H$_2$O and CH$_4$ ices in the midplane, and warmer ones produce more CO$_2$ ices, both allowing efficient depletion of C and O as soon as CO sticks on grain surfaces. Complex organic molecules (COMs) production is enhanced by the presence of warmer grains in the multi-grain models. Using a single grain model mimicking grain growth and dust settling fails to reproduce the complexity of gas-grain chemistry. Chemical models with a single grain size are sensitive to the adopted grain temperature, and cannot account for all expected effects. A spatial spread of the snowlines is expected to result from the ranges in grain temperature. The amplitude of the effects will depend on the dust disk mass.

The physics of the non-thermal Universe provides information on the acceleration mechanisms in extreme environments, such as black holes and relativistic jets, neutron stars, supernovae or clusters of galaxies. In the presence of magnetic fields, particles can be accelerated towards relativistic energies. As a consequence, radiation along the entire electromagnetic spectrum can be observed, and extreme environments are also the most likely sources of multi-messenger emission. The most energetic part of the electromagnetic spectrum corresponds to the very-high-energy (VHE, E>100 GeV) gamma-ray regime, which can be extensively studied with ground based Imaging Atmospheric Cherenkov Telescopes (IACTs). The results obtained by the current generation of IACTs, such as H.E.S.S., MAGIC, and VERITAS, demonstrate the crucial importance of the VHE band in understanding the non-thermal emission of extreme environments in our Universe. In some objects, the energy output in gamma rays can even outshine the rest of the broadband spectrum. The Cherenkov Telescope Array (CTA) is the next generation of IACTs, which, with cutting edge technology and a strategic configuration of ~100 telescopes distributed in two observing sites, in the northern and southern hemispheres, will reach better sensitivity, angular and energy resolution, and broader energy coverage than currently operational IACTs. With CTA we can probe the most extreme environments and considerably boost our knowledge of the non-thermal Universe.

We consider the inverse Seesaw scenario for neutrino masses with the approximate Lepton number symmetry broken dynamically by a scalar with Lepton number two. We show that the Majoron associated to the spontaneous symmetry breaking can alleviate the Hubble tension through its contribution to $\Delta N_\text{eff}$ and late decays to neutrinos. Among the additional fermionic states required for realizing the inverse Seesaw mechanism, sterile neutrinos at the keV-MeV scale can account for all the dark matter component of the Universe if produced via freeze-in from the decays of heavier degrees of freedom.

We calculate the contribution to the baryon asymmetry of the Universe (BAU) from a $\mathcal{CP}$-violating source of the light quarks (charm, strange, down, up) and the electron, resulting from a dimension-six effective field theory term. We derive relevant bounds from the electric dipole moments of the electron and neutron to estimate the maximal contribution from each single flavor modification. Current bounds show that the charm quark can generate at most $\mathcal{O}(1\%)$ of the BAU, while the lighter quarks and the electron contribute at much lower levels.

We show that photon spheres of supermassive black holes generate high-frequency stochastic gravitational waves through the photon-graviton conversion. Remarkably, the frequency is universally determined as $m_e\sqrt{m_e /m_p} \simeq 10^{20} \text{Hz}$ in terms of the proton mass $m_p$ and the electron mass $m_e$. It turns out that the density parameter of the stochastic gravitational waves $ \Omega_{ \text{gw}}$ could be $ 10^{-12}$. Since the existence of the gravitational waves from photon spheres is robust, it is worth seeking methods of detecting high-frequency gravitational waves around $10^{20}$Hz.

Compressional plasma perturbations may cause thermal misbalance between plasma heating and cooling processes. This misbalance significantly affects the dispersion properties of compressional waves providing a feedback between the perturbations and plasmas. It has been shown that Alfv\'en waves may induce longitudinal (compressional) plasma motions. In the present study, we analyze the effects of thermal misbalance caused by longitudinal plasma motions induced by shear Alfv\'en waves. We show that thermal misbalance leads to appearance of exponential bulk flows, which itself modifies the Alfv\'en induced plasma motions. In the case of sinusoidal Alfv\'en waves, we show how the amplitude and phase shift of induced longitudinal motions gain dependence on the Alfv\'en wave frequency while shedding light on its functionality. This feature has been investigated analytically in application to coronal conditions. We also consider the evolution of longitudinal plasma motions induced by the shear sinusoidal Alfv\'en wave by numerical methods before comparing the results obtained with our presented analytical predictions to justify the model under consideration in the present study.

The axion-like particles with ultralight mass ($\sim10^{-22}$eV) can be a possibile candidate of dark matter, known as the fuzzy dark matter (FDM). These particles form Bose-Einstein condensate in the early universe which can explain the dark matter density distribution in galaxies at the present time. We study the time evolution of ultralight axion-like field in the near region of a strong gravitational wave (GW) source, such as binary black hole merger. We show that GWs can lead to the generation of field excitations in a spherical shell about the source that eventually propagate out of the shell to minimize the energy density of the field configuration. These excitations are generated towards the end of the merger and in some cases even in the ringdown phase of the merger, therefore it can provide a qualitatively distinct prediction for changes in the GW waveform observed on Earth. This would be helpful in investigating the existence of FDM in galaxies.

The fast developments of radio astronomy open a new window to explore the properties of Dark Matter (DM). The recent direct imaging of the supermassive black hole (SMBH) at the center of M87 radio galaxy by the Event Horizon Telescope (EHT) collaboration is expected to be very useful to search for possible new physics. In this work, we illustrate that such results can be used to detect the possible synchrotron radiation signature produced by DM annihilation from the innermost region of the SMBH. Assuming the existence of a spiky DM density profile, we obtain the flux density due to DM annihilation induced electrons and positrons, and derive new limits on the DM annihilation cross section via the comparison with the EHT integral flux density at 230 GHz. Our results show that the parameter space can be probed by the EHT observations is largely complementary to other experiments. For DM with typical mass regions of being weakly interacting massive particles, the annihilation cross section several orders of magnitude below the thermal production level can be excluded by the EHT observations under the density spike assumption. Future EHT observations may further improve the sensitivity on the DM searches, and may also provide a unique opportunity to test the interplay between DM and the SMBH.

Neutrinos, being the only fermions in the Standard Model of Particle Physics that do not possess electromagnetic or color charges, have the unique opportunity to communicate with fermions outside the Standard Model through mass mixing. Such Standard Model-singlet fermions are generally referred to as "sterile neutrinos''. In this review article, we discuss the theoretical and experimental motivation for sterile neutrinos, as well as their phenomenological consequences. With the benefit of hindsight in 2020, we point out potentially viable and interesting ideas. We focus in particular on sterile neutrinos that are light enough to participate in neutrino oscillations, but we also comment on the benefits of introducing heavier sterile states. We discuss the phenomenology of eV-scale sterile neutrinos in terrestrial experiments and in cosmology, we survey the global data, and we highlight various intriguing anomalies. We also expose the severe tension that exists between different data sets and prevents a consistent interpretation of the global data in at least the simplest sterile neutrino models. We discuss non-minimal scenarios that may alleviate some of this tension. We briefly review the status of keV-scale sterile neutrinos as dark matter and the possibility of explaining the matter-antimatter asymmetry of the Universe through leptogenesis driven by yet heavier sterile neutrinos.