Biman B. Nath

Relativistic jets from active galactic nuclei are expected to exhibit strong redshift evolution in their radiative output due to the increasing energy density of the cosmic microwave background (CMB). We investigate the role of inverse Compton (IC) scattering of CMB photons in regulating the radio and X-ray emission from large-scale jets using three-dimensional relativistic magnetohydrodynamic simulations coupled with a hybrid Eulerian–Lagrangian particle framework. By keeping the jet dynamics and ambient medium properties fixed across redshifts, we are able to isolate the impact of the cosmological evolution of the CMB on the jet radiation. From our simulations, we construct synthetic spectral energy distributions and intensity maps considering synchrotron and IC/CMB losses along with particle acceleration from shocks. We are able to reproduce the weak redshift dependence of radio luminosity and the strong enhancement of X-ray emission toward high redshift that is observed in radio-loud quasars. At high redshift, the X-ray luminosity follows the expected (1 + z ) ⁴ scaling, confirming IC/CMB as the dominant mechanism driving the X-ray enhancement. The resulting X-ray-to-radio flux ratio increases systematically with redshift and is consistent with observational constraints. Finally, we show that slower jets exhibit a stronger redshift evolution of the X-ray enhancement than faster jets, highlighting the critical role of jet propagation length scales and particle energy evolution. The simulations also naturally reproduce the steepening of the radio spectral index with redshift—the α – z relation—thus providing a unified framework that allows us to interpret the multiwavelength properties of high-redshift radio sources.

The presence of dust in galaxies at redshifts z > 5 is commonly connected with core collapse supernovae (SN). Galaxies with exceptionally large dust mass, of order of 1-3% of the galaxy stellar mass, have been detected during the last decade. The required SN dust yield is ≳ 1 M _⊙ per supernova, which is comparable to the theoretically predicted maximum. However, the reverse shock (RS) penetrating the SN ejecta significantly destroy the dust particles nucleating there through sputtering. The resulting net dust mass injected into the interstellar gas after processing by the RS turns out to be ≲ 0.1 M _⊙ per SN. This makes the explanation of the existence of z > 5 galaxies with dust masses as high as M _d ≳ (0.01-0.03) M _* a challenging one. In this paper we present arguments in favor of an efficient inhibition of the sputtering behind the RS, caused by a strong radiation cooling from the dust in the supernova ejecta.

The presence of the ≈10⁶ K gas in the circumgalactic medium of the Milky Way (MW) has been well established. However, the location and the origin of the newly discovered hot gas at “supervirial (SV)” temperatures of ≈10⁷ K have been puzzling. This hot gas has been detected in both absorption and emission; here, we focus on the emitting gas only. We show that both the “virial” and the SV temperature gas, as observed in emission, occupy disk-like extraplanar regions, in addition to the diffuse virial temperature gas filling the halo of the MW. We perform idealized hydrodynamical simulations to show that the ≈10⁷ K emitting gas is likely to be produced by stellar feedback in and around the Galactic disk. We further show that the emitting gas at both SV and virial temperatures in the extraplanar regions is metal enriched and is not in hydrostatic equilibrium with the halo but is continuously evolving.

The recent detection of large column density absorption lines from highly ionized gas in a few directions through the circumgalactic medium (CGM) of the Milky Way (MW) has been puzzling. The inferred temperature from these absorption lines far exceeds the virial temperature of the MW, and the column densities are also too large to be easily explained. In this paper, we propose a novel idea to explain these observations and claim that they may not have originated from the CGM, but from a totally different type of source, namely, stellar ejecta from supernovae (SNe) above the Galactic disk that happen to lie in the line of sight to the background quasars. About ∼20% of massive OB stars (progenitors of core-collapse supernovae) are known to be runaway stars that have high ejection velocities near the Galactic plane and can end up exploding as SNe above the Galactic disk. We show that the associated reverse shock in the supernova remnant in the early nonradiative phase can heat the ejecta to temperatures of ≳10⁷ K and can naturally explain the observed high column density of ions in the observed “supervirial” phase along with α-enriched supersolar abundance that is typical of core-collapse supernovae. However, SNe from runaway stars has a covering fraction of ≲0.7% and thus can only explain the observations along limited sightlines.

Recent observations and simulations reveal that the circumgalactic medium (CGM) surrounding galaxies is multiphase, with the gas temperatures spanning a wide range at most radii, ∼10⁴ K to the virial temperature (∼10⁶ K for Milky Way). Traditional CGM models using simple density profiles are inadequate at reproducing observations that indicate a broad temperature range. Alternatively, a model based on probability distribution functions (PDFs) with parameters motivated by simulations can better match multi-wavelength observations. In this work, we use log-normal distributions, commonly seen in the simulations of the multiphase interstellar and circumgalactic media, to model the multiphase CGM. We generalize the isothermal background model by Faerman et al. 2017 to include more general CGM profiles. We extend the existing probabilistic models from 1D-PDFs in temperature to 2D-PDFs in density-temperature phase space and constrain its parameters using a Milky Way-like Illustris TNG50-1 halo. We generate various synthetic observables such as column densities of different ions, UV/X-ray spectra, and dispersion and emission measures. X-ray and radio (Fast Radio Burst) observations mainly constrain the hot gas properties. However, interpreting cold/warm phase diagnostics is not straightforward since these phases are patchy, with inherent variability in intercepting these clouds along arbitrary lines of sight. We provide a tabulated comparison of model predictions with observations and plan to expand this into a comprehensive compilation of models and data. Our modeling provides a simple analytic framework that is useful for describing important aspects of the multiphase CGM.

One of the most distant galaxies GN-z11 was formed when the Universe was ≤ 400 Myr old, and it displays a burst-like star formation rate ∼25 M_⊙ yr⁻¹ with a metallicity Z ∼ 0.2 ± 0.1Z_⊙. It resembles z = 2 − 3 galaxies (at “cosmic noon”) except for the fact that the measured reddening E(B − V) = 0.01 ± 0.01 indicates the presence of little or no dust. This marked absence of dust hints towards violent dynamical events that destroy or evacuate dust along with gas out of the galaxy on a relatively short time scale and make it transparent. We apply a 3D numerical model to infer possible physical characteristics of these events. We demonstrate that the energetics of the observed star formation rate is sufficient to tear apart the dusty veil on time scales of 20 − 25 Myr. This can explain the apparent lack of evolution of UV luminosity function of galaxies between and z ≥ 10 and z ∼ 7, by compensating for the lower galaxy masses at higher redshift by the absence of dust. We show, however, that this is a temporary phenomenon and soon after the last of the supernovae explosions have taken place, the expanding shell shrinks and obscures the galaxy on time scales of ≈5 − 8 Myr.

We consider the dynamics of and emission from growing superbubbles in a stratified interstellar gaseous disc driven by energy release from supernovae explosions in stellar clusters with masses M_cl = 10⁵ − 1.6 × 10⁶ M_⊙. Supernovae are spread randomly within a sphere of r_c = 60 pc, and inject energy episodically with a specific rate |$1/130~M_\odot ^{-1}$| proportional to the star formation rate (SFR) in the cluster. Models are run for several values of SFR in the range 0.01 to 0.1 M_⊙ yr⁻¹, with the corresponding average surface energy input rate ∼0.04 − 0.4 erg cm⁻² s⁻¹. We find that the discrete energy injection by isolated SNe are more efficient in blowing superbubbles: asymptotically they reach heights of up to 3 to 16 kpc for M_cl = 10⁵ − 1.6 × 10⁵ M_⊙, correspondingly, and stay filled with a hot and dilute plasma for at least 30 Myr. During this time they emit X-ray, Hα and dust infrared emission. X-ray liminosities L_X∝SFR^3/5 that we derive here are consistent with observations in star-forming galaxies. Even though dust particles of small sizes a ≤ 0.03 μm are sputtered in the interior of bubbles, larger grains still contribute considerably ensuring the bubble luminosity |$L_{\rm IR}/{\rm SFR}\sim 5\times 10^7 L_\odot M_\odot ^{-1} ~{\rm yr}$|⁠. It is shown that the origin of the North Polar Spur in the Milky Way can be connected with activity of a cluster with the stellar mass of ∼10⁵ M_⊙ and the SFR ∼ 0.1 M_⊙ yr⁻¹ some 25–30 Myr ago. Extended luminous haloes observed in edge-on galaxies (NGC 891 as an example) can be maintained by disc spread stellar clusters of smaller masses |$M_\ast {\,\,\lesssim \,\,}10^5~M_\odot$|⁠.