Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

摘要: Knowledge about the outermost portions of galaxies is limited owing to small amount light coming from them. It known that in many cases atomic hydrogen (H i) extends well beyond optical radius1. In centres galaxies, however, molecular (H2) usually dominates by a large factor2,3,4, raising question whether H2 also abundant outer regions. Here we report detection emission carbon monoxide (CO), most tracer H2, radius nearby galaxy NGC 4414. The host clouds probably formed regions relatively high H i column density and absence spiral waves. relative strength lines two lowest rotational levels indicates both temperature are quite low compared conditions closer centre. inferred surface material continues monotonic decrease inner We conclude although can form region this galaxy, there little mass associated with