Multi-wavelength features of Fermi Bubbles as signatures of a Galactic wind

Using hydrodynamical simulations, we show for the first time that an episode of star formation in the center of the Milky Way, with a star-formation-rate (SFR) $sim 0.5$ M$_odot$ yr$^{-1}$ for $sim 30$ Myr, can produce bubbles that resemble the Fermi Bubbles (FBs), when viewed from the solar position. The morphology, extent and multi-wavelength observations of FBs, especially X-rays, constrain various physical parameters such as SFR, age, and the circum-galactic medium (CGM) density. We show that the interaction of the CGM with the Galactic wind driven by a star formation in the central region can explain the observed surface brightness and morphological features of X-rays associated with the Fermi Bubbles. Furthermore, assuming that cosmic ray electrons are accelerated {it in situ} by shocks and/or turbulence, the brightness and morphology of gamma-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.

Narrow escape: how ionizing photons escape from disc galaxies

In this paper we calculate the escape fraction ($f_{rm esc}$) of ionizing photons from starburst galaxies. Using 2-D axisymmetric hydrodynamic simulations, we study superbubbles created by overlapping supernovae in OB associations. We calculate the escape fraction of ionizing photons from the center of the disk along different angles through the superbubble and the gas disk. After convolving with the luminosity function of OB associations, we show that the ionizing photons escape within a cone of $sim 40 ^circ$, 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 scale heights and fragments, the escape fraction through the polar regions rises again. The angle-averaged escape fraction cannot exceed $sim [1- cos (1 , {rm radian})] = 0.5$ 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 disk gas density (in the range $n_0=0.15-50$ cm $^{-3}$) and the disk scale height (between $z_0=10-600$ pc). We find that the escape fraction is related to the disk parameters (the mid-plane disk density and scale height) roughly so that $f_{rm esc}^alpha n_0^2 z_0^3$ (with $alphaapprox 2.2$) is a constant. For disks with a given WNM temperature, massive disks have lower escape fraction than low mass galaxies. For Milky Way ISM parameters, we find $f_{rm esc}sim 5%$, and it increases to $approx 10%$ for a galaxy ten 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.

Long way to go: how outflows from large galaxies propagate through the hot halo gas

Using hydrodynamic simulations, we study the mass loss due to supernova-driven outflows from Milky Way type disk 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 ($sim $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 $le 10^{5.5}$ K to the outflow rate within 10 kpc is $approx 0.3hbox{--}0.5$. The warm mass loading factor, $eta_{3e5}$ ($Tle 3 times 10^5$ K) is related to the mass loading factor at the virial radius ($eta_{v}$) as $eta_{v} approx 25, eta_{3e5}, left(mbox{SFR}/{rm M}_odot{rm yr}^{-1} right)^{-0.15}$ 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, $lesssim 200$ km s$^{-1}$. We identify two `sequences of outflowing cold gas at small scales: a fast ($approx 500$ km~s$^{-1}$) sequence, driven by the unshocked free-wind; and a slow sequence ($approx pm 100$ km s$^{-1}$) at the conical interface of the superwind and the hot halo.

Superbubble breakout and galactic winds from disk galaxies

We study the conditions for disk galaxies to produce superbubbles that can break out of the disk and produce a galactic wind. We argue that the threshold surface density of supernovae rate for seeding a wind depends on the ability of superbubble energetics to compensate for radiative cooling. We first adapt Kompaneets formalism for expanding bubbles in a stratified medium to the case of continuous energy injection and include the effects of radiative cooling in the shell. With the help of hydrodynamic simulations, we then study the evolution of superbubbles evolving in stratified disks with typical disk parameters. We identify two crucial energy injection rates that differ in their effects, the corresponding breakout ranging from being gentle to a vigorous one. (a) Superbubbles that break out of the disk with a Mach number of order 2-3 correspond to an energy injection rate of order 10^{-4} erg cm^{-2} s^{-1}, which is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (b) A larger energy injection threshold, of order 10^{-3} erg cm^{-2} s^{-1}, or equivalently, a star formation surface density of sim 0.1 solar mass yr^{-1} kpc^{-2}, corresponds to superbubbles with a Mach number sim 5-10. While the milder superbubbles can be produced by large OB associations, the latter kind requires super-starclusters. These derived conditions compare well with observations of disk galaxies with winds and the existence of multiphase halo gas. Furthermore, we find that contrary to the general belief that superbubbles fragment through Rayleigh-Taylor (RT) instability when they reach a vertical height of order the scale height, the superbubbles are first affected by thermal instability for typical disk parameters and that RT instability takes over when the shells reach a distance of approximately twice the scale height.

Time Dependent Models of Flares from Sagittarius A*

The emission from Sgr A*, the supermassive black hole in the Galactic Center, shows order of magnitude variability (flares) a few times a day that is particularly prominent in the near-infrared (NIR) and X-rays. We present a time-dependent model for these flares motivated by the hypothesis that dissipation of magnetic energy powers the flares. We show that episodic magnetic reconnection can occur near the last stable circular orbit in time-dependent magnetohydrodynamic simulations of black hole accretion - the timescales and energetics of these events are broadly consistent with the flares from Sgr A*. Motivated by these results, we present a spatially one-zone time-dependent model for the electron distribution function in flares, including energy loss due to synchrotron cooling and adiabatic expansion. Synchrotron emission from transiently accelerated particles can explain the NIR/X-ray lightcurves and spectra of a luminous flare observed 4 April 2007. A significant decrease in the magnetic field strength during the flare (coincident with the electron acceleration) is required to explain the simultaneity and symmetry of the simultaneous lightcurves. Our models predict that the NIR and X-ray spectral indices differ by 0.5 and that there is only modest variation in the spectral index during flares. We also explore implications of this model for longer wavelength (radio-submm) emission seemingly associated with X-ray and NIR flares; we argue that a few hour decrease in the submm emission is a more generic consequence of large-scale magnetic reconnection than delayed radio emission from adiabatic expansion.