Spatially resolved gas flows around the Milky Way

Abstract

ABSTRACT We present spatially resolved measurements of cool gas flowing into and out of the Milky Way (MW), using archival ultraviolet spectra of background quasars from the Hubble Space Telescope/Cosmic Origins Spectrograph. We co-add spectra of different background sources at close projected angular separation on the sky. This novel stacking technique dramatically increases the signal-to-noise ratio of the spectra, allowing the detection of low-column-density gas (down to EW > 2 mÅ). We identify absorption as inflowing or outflowing, by using blue/redshifted high-velocity cloud absorption components in the Galactocentric rest frame, respectively. The mass surface densities of both inflowing and outflowing gases vary by more than an order of magnitude across the sky, with mean values of 〈Σin〉 ≳ 104.6 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for inflowing gas and 〈Σout〉 ≳ 103.5 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for outflowing gas. The mass flow rate surface densities (mass flow rates per unit area) also show large variation across the sky, with $\langle \dot{\Sigma }(d)_{\rm in}\rangle \gtrsim (10^{-3.6\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for inflowing gas and $\langle \dot{\Sigma }(d)_{\rm out}\rangle \gtrsim (10^{-4.8\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for outflowing gas. The regions with highest surface mass density of inflowing gas are clustered at smaller angular scales (θ < 40°). This indicates that most of the mass in inflowing gas is confined to small, well-defined structures, whereas the distribution of outflowing gas is spread more uniformly throughout the sky. Our study confirms that the MW is predominantly accreting gas, but it is also losing a non-negligible mass of gas via outflow.