Intermediate- and high-velocity clouds in the Milky Way - I. Covering factors and vertical heights

Intermediate- and high-velocity clouds (IVCs, HVCs) are a potential source of fuel for star formation in the Milky Way (MW), but their origins and fates depend sensitively on their distances. We search for IVCs and HVCs in HST high-resolution ultraviolet spectra of 55 halo stars at vertical heights $|z|\gtrsim \,1$ kpc. We show that IVCs (40 ≤ |$v$_LSR| < 90 ${\rm km\, s}^{-1}$) have a high detection rate - the covering factor, f_c - that is about constant (f_c = 0.90 ± 0.04) from $z$ = 1.5 to 14 kpc, implying IVCs are essentially confined to |$z$| ≲ 1.5 kpc. For the HVCs (90 ≤ |$v$_LSR| ≲ 170 ${\rm km\, s}^{-1}$), we find f_c increases from f_c ≃ 0.14 ± 0.10 at |$z$| ≲ 2-3 kpc to f_c = 0.60 ± 0.15 at 6 ≲ |$z$| ≲ 14 kpc, the latter being similar to that found towards QSOs. In contrast, the covering factor of very high-velocity clouds (VHVCs; |$v$_LSR| ≳ 170 ${\rm km\, s}^{-1}$) is $f_c \lt 0.04$ in the stellar sample compared to 20 per cent towards QSOs, implying these clouds must be at d ≳ 10-15 kpc (|$z$| ≳ 10 kpc). Gas clouds with |$v$_LSR| > 40 ${\rm km\, s}^{-1}$ at |b| ≳ 15° have therefore |$v$_LSR| decreasing with decreasing |$z$|. Our findings are consistent with a Galactic rain and/or fountain origin for these clouds. In the latter scenario, VHVCs may mostly serve as fuel for the MW halo. In view of their high covering factors and since all the IVCs and some HVCs are found in the thick disc, they appear good candidates as gas reservoirs to help sustain star formation in the MW.