A New Distance Constraint on Complex C Using SEGUE Spectra of Blue Horizontal Branch Stars

Abstract

Using low-resolution Sloan Digital Sky Survey (SDSS) spectra of blue horizontal branch (BHB) stars at various distances, we present the detection of Complex C, a large inflowing high-velocity cloud (HVC). We carefully isolate excess Ca ii absorption due to Complex C from that of the stellar and interstellar components. Using a distance bracketing technique, our analysis of the BHB spectra allows us to place Complex C between 7.3 < Dkpc < 11.7. At this distance, we calculate a cloud mass between 2.2 and 5.6 × 106 M⊙ and an infall time ranging from 4.0 × 107 to 8.8 × 107 yr, depending on the local velocity of each region. This detection is right on the boundary of what is possible with low-resolution SDSS spectra, but marks a significant step forward toward broadly constraining physical properties of HVCs, all of which depend on their distance.