Distances to Galactic high-velocity clouds. Complex C

TL;DR

This study determines the distance to high-velocity cloud complex C, revealing it as a significant inflow of intergalactic gas onto the Milky Way, with implications for galaxy evolution and gas accretion processes.

Contribution

First direct distance bracket established for complex C, linking it to ongoing galactic gas accretion and providing mass and inflow rate estimates.

Findings

Distance to complex C is 3.7-11.2 kpc.

Mass of complex C is 3-14 million solar masses.

Inflow rate of 0.1-0.25 solar masses per year.

Abstract

We report the first determination of a distance bracket for the high-velocity cloud (HVC) complex C. Combined with previous measurements showing that this cloud has a metallicity of 0.15 times solar, these results provide ample evidence that complex C traces the continuing accretion of intergalactic gas falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as well as He, the distance bracket implies a mass of 3-14x10^6 M_sun, and the complex represents a mass inflow of 0.1-0.25 M_sun/yr. We base our distance bracket on the detection of CaII absorption in the spectrum of the blue horizontal branch star SDSS J120404.78+623345.6, in combination with a significant non-detection toward the BHB star BS 16034-0114. These results set a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the BHB star BS 16079-0017.