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Register a new userThe Galactic center chimneys: The base of the multiphase outflow of the Milky Way
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Outflows and feedback are key ingredients of galaxy evolution. Evidence for an outflow arising from the Galactic center (GC) has recently been discovered at different wavelength. We show that the X-ray, radio, and infrared emissions are deeply interconnected, affecting one another and forming coherent features on scales of hundreds of parsecs, therefore indicating a common physical link associated with the GC outflow. We debate the location of the northern chimney and suggest that it might be located on the front side of the GC because of a significant tilt of the chimneys toward us. We report the presence of strong shocks at the interface between the chimneys and the interstellar medium, which are traced by radio and warm dust emission. We observe entrained molecular gas outflowing within the chimneys, revealing the multiphase nature of the outflow. In particular, the molecular outflow produces a long, strong, and structured shock along the northwestern wall of the chimney. Because of the different dynamical times of the various components of the outflow, the chimneys appear to be shaped by directed large-scale winds launched at different epochs. The data support the idea that the chimneys are embedded in an (often dominant) vertical magnetic field, which likely diverges with increasing latitude. We observe that the thermal pressure associated with the hot plasma appears to be smaller than the ram pressure of the molecular outflow and the magnetic pressure. This leaves open the possibility that either the main driver of the outflow is more powerful than the observed hot plasma, or the chimneys represent a relic of past and more powerful activity. These multiwavelength observations corroborate the idea that the chimneys represent the channel connecting the quasi-continuous, but intermittent, activity at the GC with the base of the Fermi bubbles.
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We summarize basic observational results on Sagittarius~A* obtained from the radio, infrared and X-ray domain. Infrared observations have revealed that a dusty S-cluster object (DSO/G2) passes by SgrA*, the central super-massive black hole of the Milky Way. It is still expected that this event will give rise to exceptionally intense activity in the entire electromagnetic spectrum. Based on February to September 2014 SINFONI observations. The detection of spatially compact and red-shifted hydrogen recombination line emission allows a us to obtain a new estimate of the orbital parameters of the DSO. We have not detected strong pre-pericenter blue-shifted or post-pericenter red-shifted emission above the noise level at the position of SgrA* or upstream the orbit. The periapse position was reached in May 2014. Our 2004-2012 infrared polarization statistics shows that SgrA* must be a very stable system - both in terms of geometrical orientation of a jet or accretion disk and in terms of the variability spectrum which must be linked to the accretion rate. Hence polarization and variability measurements are the ideal tool to probe for any change in the system as a function of the DSO/G2 fly-by. Due to the 2014 fly-by of the DSO, increased accretion activity of SgrA* may still be upcoming. Future observations of bright flares will improve the derivation of the spin and the inclination of the SMBH from NIR/sub-mm observations.
Star formation takes place in the dense gas phase, and therefore a simple dense gas and star formation rate relation has been proposed. With the advent of multi-beam receivers, new observations show that the deviation from linear relations is possible. In addition, different dense gas tracers might also change significantly the measurement of dense gas mass and subsequently the relation between star formation rate and dense gas mass. We report the preliminary results the DEnse GAs in MAssive star-forming regions in the Milky Way (DEGAMA) survey that observed the dense gas toward a suit of well-characterized massive star forming regions in the Milky Way. Using the resulting maps of HCO$^{+}$ 1--0, HCN 1--0, CS 2--1, we discuss the current understanding of the dense gas phase where star formation takes place.
We present new calculations of the mass inflow and outflow rates around the Milky Way, derived from a catalog of ultraviolet metal-line high velocity clouds (HVCs). These calculations are conducted by transforming the HVC velocities into the Galactic Standard of Rest (GSR) reference frame, identifying inflowing (v_GSR < 0 km/s) and outflowing (v_GSR > 0 km/s) populations, and using observational constraints on the distance, metallicity, dust content, covering fractions, and total hydrogen column density of each population. After removing HVCs associated with the Magellanic Stream and the Fermi Bubbles, we find inflow and outflow rates in cool (T~10^4 K) ionized gas of dM_in/dt >~ 0.53+/-0.17 (d/12 kpc) (Z/0.2 Z_sun)^-1 M_sun/yr and dM_out/dt >~ 0.16+/-0.06 (d/12 kpc) (Z/0.5 Z_sun)^-1 M_sun/yr. The excess of inflowing over outflowing gas suggests that the Milky Way is currently in an inflow-dominated phase, but the presence of substantial mass flux in both directions supports a Galactic fountain model, in which gas is constantly recycled between the disk and the halo. We also find that the metal flux in both directions (in and out) is indistinguishable. By comparing the outflow rate to the Galactic star formation rate, we present the first estimate of the mass loading factor (etc_HVC) of the disk-wide Milky Way wind, finding eta_HVC >~ 0.10+/-0.06 (d/12 kpc) (Z/0.5 Z_sun)^-1. Including the contributions from low- and intermediate-velocity clouds and from hot gas would increase these inflow and outflow estimates.
We report the discovery of a $1^circ$ scale X-ray plume in the northern Galactic Center (GC) region observed with Suzaku. The plume is located at ($l$, $b$) $sim$ ($0mbox{$.!!^circ$}2$, $0mbox{$.!!^circ$}6$), east of the radio lobe reported by previous studies. No significant X-ray excesses are found inside or to the west of the radio lobe. The spectrum of the plume exhibits strong emission lines from highly ionized Mg, Si, and S that is reproduced by a thin thermal plasma model with $kT sim 0.7$ keV and solar metallicity. There is no signature of non-equilibrium ionization. The unabsorbed surface brightness is $3times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ arcmin$^{-2}$ in the 1.5-3.0 keV band. Strong interstellar absorption in the soft X-ray band indicates that the plume is not a foreground source but is at the GC distance, giving a physical size of $sim$100 pc, a density of 0.1 cm$^{-3}$, thermal pressure of $1times10^{-10}$ erg cm$^{-3}$, mass of 600 $M_odot$ and thermal energy of $7times10^{50}$ erg. From the apparent association with a polarized radio emission, we propose that the X-ray plume is a magnetized hot gas outflow from the GC.
The hot gaseous halos of galaxies likely contain a large amount of mass and are an integral part of galaxy formation and evolution. The Milky Way has a 2e6 K halo that is detected in emission and by absorption in the OVII resonance line against bright background AGNs, and for which the best current model is an extended spherical distribution. Using XMM-Newton RGS data, we measure the Doppler shifts of the OVII absorption-line centroids toward an ensemble of AGNs. These Doppler shifts constrain the dynamics of the hot halo, ruling out a stationary halo at about 3sigma and a corotating halo at 2sigma, and leading to a best-fit rotational velocity of 183+/-41 km/s for an extended halo model. These results suggest that the hot gas rotates and that it contains an amount of angular momentum comparable to that in the stellar disk. We examined the possibility of a model with a kinematically distinct disk and spherical halo. To be consistent with the emission-line X-ray data the disk must contribute less than 10% of the column density, implying that the Doppler shifts probe motion in the extended hot halo.
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