** Henderson Group rises 16 pct in volume afteragreeing a merger with Janus Capital
** Brokers point to accretiveness of the deal - $110 mln ofsynergies p.a. or 16 pct of the combined group's EBITDA - forshare price reaction
** Best day for the stock since January 2009
** Easily top riser on Stoxx 600 (as well as UK'sFTSE 250 ); peers Aberdeen and Jupiter fourth and fifth top risers (second and third top on FTSE 250);Schroders third top riser on FTSE 100