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.

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.