ALBERT

Description: Using hydrodynamical simulations, we show for the first time that an episode of star formation in the centre of the Milky Way, with a star formation rate (SFR) ~0.5 M  yr –1 for ~30 Myr, can produce bubbles that resemble the Fermi bubbles (FBs), when viewed from the solar position. The morphology, extent and multiwavelength observations of FBs, especially X-rays, constrain various physical parameters such as SFR, age, and the circumgalactic medium (CGM) density. We show that the interaction of the CGM with the Galactic wind driven by star formation in the central region can explain the observed surface brightness and morphological features of X-rays associated with the FBs. Furthermore, assuming that cosmic ray electrons are accelerated in situ by shocks and/or turbulence, the brightness and morphology of -ray emission and the microwave haze can be explained. The kinematics of the cold and warm clumps in our model also matches with recent observations of absorption lines through the bubbles.

Description: Bondi accretion assumes that there is a sink of mass at the centre – which in the case of a black hole (BH) corresponds to the advection of matter across the event horizon. Other stars, such as a neutron star (NS), have surfaces and hence the infalling matter has to slow down at the surface. We study the initial value problem in which the matter distribution is uniform and at rest at t = 0. We consider different inner boundary conditions for BHs and NSs: outflow boundary condition (mimicking mass sink at the centre) valid for BHs; and reflective and steady-shock (allowing gas to cross the inner boundary at subsonic speeds) boundary conditions for NSs. We also obtain a similarity solution for cold accretion on to BHs and NSs. 1D simulations show the formation of an outward-propagating and a standing shock in NSs for reflective and steady-shock boundary conditions, respectively. Entropy is the highest at the bottom of the subsonic region for reflective boundary conditions. In 2D this profile is convectively unstable. Using steady-shock inner boundary conditions, the flow is unstable to the standing accretion shock instability in 2D, which leads to global shock oscillations and may be responsible for quasi-periodic oscillations seen in the light curves of accreting systems. For steady accretion in the quiescent state, spherical accretion rate on to an NS can be suppressed by orders of magnitude compared to that on to a BH.

Description: Computational modeling is employed to provide a plausible structural explanation for the experimentally-observed differential global genome repair (GGR) propensity of the ALII-N 2 -dG and ALII-N 6 -dA DNA adducts of aristolochic acid II. Our modeling studies suggest that an intrinsic twist at the carcinogen–purine linkage of ALII-N 2 -dG induces lesion site structural perturbations and conformational heterogeneity of damaged DNA. These structural characteristics correlate with the relative repair propensities of AA-adducts, where GGR recognition occurs for ALII-N 2 -dG, but is evaded for intrinsically planar ALII-N 6 -dA that minimally distorts DNA and restricts the conformational flexibility of the damaged duplex. The present analysis on the ALII adduct model systems will inspire future experimental studies on these adducts, and thereby may extend the list of structural factors that directly correlate with the propensity for GGR recognition.

Description: Traditionally the thermodynamic profiles (gas density, temperature, etc.) of galaxy clusters are obtained by assuming spherical symmetry and modelling projected X-ray spectra in each annulus. The outer annuli contribute to the inner ones and their contribution needs to be subtracted to obtain the temperature and density of spherical shells. The usual deprojection methods lead to propagation of errors from outside to in and typically do not model the covariance of parameters in different radial shells. In this paper we describe a method based on a free-form model of clusters with cluster parameters (density, temperature) given in spherical shells, which we jointly forward fit to the X-ray data by constructing a Bayesian posterior probability distribution that we sample using the MCMC technique. By systematically marginalizing over the nuisance outer shells, we estimate the inner entropy profiles of clusters and fit them to various models for a sample of Chandra X-ray observations of 17 clusters. We show that the entropy profiles in almost all of our clusters are best described as cored power laws. A small subsample is found to be either consistent with a power law, or alternatively their cores are not fully resolved (smaller than, or about few kpc). We find marginal evidence for bimodality in the central values of entropy (and cooling time) corresponding to cool-core and non cool-core clusters. The minimum value of the ratio of the cooling time and the free-fall time (min[ t cool / t ff ]; correlation is much weaker with core entropy) is anti-correlated with H α and radio luminosity. H α emitting cold gas is absent in our clusters with min( t cool / t ff ) 10. Our lowest core entropies are systematically and substantially lower than the values quoted by the ACCEPT sample.

Description: In this paper we calculate the escape fraction ( f esc ) of ionizing photons from starburst galaxies. Using 2D axisymmetric hydrodynamic simulations, we study superbubbles created by overlapping supernovae in OB associations. We calculate the escape fraction of ionizing photons from the centre of the disc along different angles through the superbubble and the gas disc. After convolving with the luminosity function of OB associations, we show that the ionizing photons escape within a cone of ~40°, consistent with observations of nearby galaxies. The evolution of the escape fraction with time shows that it falls initially as cold gas is accumulated in a dense shell. After the shell crosses a few scaleheights and fragments, the escape fraction through the polar regions rises again. The angle-averaged escape fraction cannot exceed ~[1 – cos (1 rad)] = 0.5 from geometrical considerations (using the emission cone opening angle). We calculate the dependence of the time- and angle-averaged escape fraction on the mid-plane disc gas density (in the range n 0 = 0.15–50 cm –3 ) and the disc scaleheight (between z 0 = 10 and 600 pc). We find that the escape fraction is related to the disc parameters (the mid-plane disc density and scaleheight) roughly so that $f_{\rm esc}^\alpha n_0^2 z_0^3$ (with α 2.2) is a constant. For discs with a given warm neutral medium temperature, massive discs have lower escape fraction than low-mass galaxies. For Milky Way ISM parameters, we find f esc ~ 5 per cent, and it increases to 10 per cent for a galaxy 10 times less massive. We discuss the possible effects of clumpiness of the ISM on the estimate of the escape fraction and the implications of our results for the reionization of the Universe.

Description: Using hydrodynamic simulations, we study the mass-loss due to supernova-driven outflows from Milky Way type disc galaxies, paying particular attention to the effect of the extended hot halo gas. We find that the total mass-loss at inner radii scales roughly linearly with total mass of stars formed, and that the mass loading factor at the virial radius can be several times its value at inner radii because of the swept up hot halo gas. The temperature distribution of the outflowing material in the inner region (~10 kpc) is bimodal in nature, peaking at 10 5  K and 10 6.5  K, responsible for optical and X-ray emission, respectively. The contribution of cold/warm gas with temperature ≤10 5.5 K to the outflow rate within 10 kpc is 0.3–0.5. The warm mass loading factor, 3 e 5 ( T  ≤ 3 10 5  K) is related to the mass loading factor at the virial radius ( v ) as v   25 3 e 5 (SFR/M yr –1 ) –0.15 for a baryon fraction of 0.1 and a starburst period of 50 Myr. We also discuss the effect of multiple bursts that are separated by both short and long periods. The outflow speed at the virial radius is close to the sound speed in the hot halo,  200 km s –1 . We identify two ‘sequences’ of outflowing cold gas at small scales: a fast (500 km s –1 ) sequence, driven by the unshocked free-wind; and a slow sequence (± 100 km s –1 ) at the conical interface of the superwind and the hot halo.

Description: Using hydrodynamical simulations, we show for the first time that an episode of star formation in the centre of the Milky Way, with a star formation rate (SFR) ~0.5 M  yr –1 for ~30 Myr, can produce bubbles that resemble the Fermi bubbles (FBs), when viewed from the solar position. The morphology, extent and multiwavelength observations of FBs, especially X-rays, constrain various physical parameters such as SFR, age, and the circumgalactic medium (CGM) density. We show that the interaction of the CGM with the Galactic wind driven by star formation in the central region can explain the observed surface brightness and morphological features of X-rays associated with the FBs. Furthermore, assuming that cosmic ray electrons are accelerated in situ by shocks and/or turbulence, the brightness and morphology of -ray emission and the microwave haze can be explained. The kinematics of the cold and warm clumps in our model also matches with recent observations of absorption lines through the bubbles.

Description: We perform global linear stability analysis and idealized numerical simulations in global thermal balance to understand the condensation of cold gas from hot/virial atmospheres (coronae), in particular the intracluster medium (ICM). We pay particular attention to geometry (e.g. spherical versus plane-parallel) and the nature of the gravitational potential. Global linear analysis gives a similar value for the fastest growing thermal instability modes in spherical and Cartesian geometries. Simulations and observations suggest that cooling in haloes critically depends on the ratio of the cooling time to the free-fall time ( t cool / t ff ). Extended cold gas condenses out of the ICM only if this ratio is smaller than a threshold value close to 10. Previous works highlighted the difference between the nature of cold gas condensation in spherical and plane-parallel atmospheres; namely, cold gas condensation appeared easier in spherical atmospheres. This apparent difference due to geometry arises because the previous plane-parallel simulations focused on in situ condensation of multiphase gas but spherical simulations studied condensation anywhere in the box. Unlike previous claims, our non-linear simulations show that there are only minor differences in cold gas condensation, either in situ or anywhere, for different geometries. The amount of cold gas depends on the shape of t cool / t ff ; gas has more time to condense if gravitational acceleration decreases towards the centre. In our idealized plane-parallel simulations with heating balancing cooling in each layer, there can be significant mass/energy/momentum transfer across layers that can trigger condensation and drive t cool / t ff far beyond the critical value close to 10.

Description: Energetic winds and radiation from massive star clusters push the surrounding gas and blow superbubbles in the interstellar medium (ISM). Using 1D hydrodynamic simulations, we study the role of radiation in the dynamics of superbubbles driven by a young star cluster of mass 10 6 M . We have considered a realistic time evolution of the mechanical power as well as radiation power of the star cluster, and detailed heating and cooling processes. We find that the ratio of the radiation pressure on the shell (shocked ISM) to the thermal pressure (~10 7  K) of the shocked-wind region is almost independent of the ambient density, and it is greater than unity before 1 Myr. We explore the parameter space of density and dust opacity of the ambient medium, and find that the size of the hot gas (~10 7  K) cavity is insensitive to the dust opacity [ d (0.1–1.5) x 10 –21  cm 2 ], but the structure of the photoionized (~10 4  K) gas depends on it. Most of the radiative losses occur at ~10 4  K, with sub-dominant losses at 10 3  K and ~10 6 –10 8  K. The superbubbles can retain as high as ~10 per cent of its input energy, for an ambient density of 10 3 m H cm –3 . We discuss the role of ionization parameter and recombination-averaged density in understanding the dominant feedback mechanism. Finally, we compare our results with the observations of 30 Doradus.

Description: We explore the formation of superbubbles through energy deposition by multiple supernovae (SNe) in a uniform medium. We use the total energy conserving, 3D hydrodynamic simulations to study how SNe correlated in space and time create superbubbles. While isolated SNe fizzle out completely by ~1 Myr due to radiative losses, for a realistic cluster size it is likely that subsequent SNe go off within the hot/dilute bubble and sustain the shock till the cluster lifetime. For realistic cluster sizes, we find that the bubble remains overpressured only if, for a given n g 0 , N OB is sufficiently large. While most of the input energy is still lost radiatively, superbubbles can retain up to ~5–10 per cent of the input energy in the form of kinetic+thermal energy till 10 Myr for interstellar medium density n g 0 1 cm –3 . We find that the mechanical efficiency decreases for higher densities ( $\eta _{\rm mech} \propto n_{g0}^{-2/3}$ ). We compare the radii and velocities of simulated supershells with observations and the classical adiabatic model. Our simulations show that the superbubbles retain only 10 per cent of the injected energy, thereby explaining the observed smaller size and slower expansion of supershells. We also confirm that a sufficiently large ( 10 4 ) number of SNe are required to go off in order to create a steady wind with a stable termination shock within the superbubble. We show that the mechanical efficiency increases with increasing resolution, and that explicit diffusion is required to obtain converged results.