Josh Peek edited section_Introduction_High_velocity_clouds__.tex  almost 8 years ago

Commit id: 3077f5a7a5f516a042493ad6ff0b4ed2f4fe7956

deletions | additions      

       

Complex WD is the largest area positive velocity HVC Complex covering 310 square degrees with a total HI flux of 1.2 $\times 10^7$ K km/s arcmin$^2$. It is by far the largest complexes that exist in the inner two Galactic quadrants, where a very small fraction of HVC flux is detected. With a range of velocities between +90 and +130 km/s, it is consistent with cylindrical rotation on the far side of the inner Galaxy, 20 kpc from the sun with a mass of 6 $\times 10^7 M_\odot$. This would make it very similar in mass, Galactocentric radius, and height to Complex C, the largest area and brightest HVC complex \cite{Thom_2008}. It is still unknown why there is a bias toward the outer disk in the HVC complexes, but determining the distance this outlier object should significantly improve our understanding of the MW's HVC system as a whole.  One major issue in gaining a better physical understanding these enigmatic clouds is their distance. Since there are no objects of standard luminosity in these clouds, there are effectively no distance constraints from HI emission or optical and UV absorption lines toward quasars, which probe only the distance-independent column densities. Distance not only gives us a mass for these structures, but also a context; the spatial  relationship between the cloud and the nearby structure of the  disk gives us insight as to its origin. There are a number of indirect methods for measuring the distance to an HVC complex, including H$\alpha$ emission and kinematic structure \cite{Putman_2003, Peek_2007}, but the only proven direct distance measure is stellar absorption. By observing standard candle stars at high spectral resolution, one can look for absorption lines in Na I, Ca II H & K, Ti II, and numerous ultraviolet absorption lines at the velocity of HI emission from HVCs \cite{1995A&A...302..364S}. By finding detections and non-detections of these absorption lines along lines of sight toward HI emitting HVCs distances can be robustly measured.