2021/08/31 by Sean Clark, Rongmon Bordoloi, Andrew J. Fox
Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #Galaxy #Milky Way #Order (exchange) #Physics #Quasar #Redshift #Spectral line #Stellar, planetary, and galactic studies #astro-ph.GA
paper · pdf · doi:10.1093/mnras/stac504
25 pages, 11 Figures, Accepted 2022 February 18
arxiv created 2022/02/20 · openalex publication_date 2022/02/23 · arxiv updated 2022/03/09 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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 detection of low column density gas (down to EW > 2 mA). We identify absorption as inflowing or outflowing, by using blue/redshifted high velocity cloud (HVC) absorption components in the Galactocentric rest frame, respectively. The mass surface densities of inflowing and outflowing gas both vary by more than an order of magnitude across the sky, with mean values of ⟨ Σin⟩ \gtrsim 104.6±0.1 M\odot kpc-2 for inflowing gas and ⟨ Σout⟩ \gtrsim 103.5± 0.1 M\odot kpc-2 for outflowing gas, respectively. The mass flow rate surface densities (mass flow rates per unit area) also show large variation across the sky with ⟨ Σ(d)in⟩ \gtrsim (10-3.6±0.1)(d/12 kpc)-1 M\odot kpc-2 yr-1 for inflowing and ⟨ Σ(d)out⟩ \gtrsim (10-4.8±0.1)(d/12 kpc)-1 M\odot kpc-2 yr-1 for outflowing gas, respectively. 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 is also losing a non-negligible mass of gas via outflow.