U.S. stocks closed sharply higher Monday, with the Dow Jones Industrial Average soaring nearly 140 points, as merger and acquisition activity boosted all three major indices. Monday also marked the sixth anniversary of the U.S. bull market following the recession in 2009. 

The Dow, which measures the share prices of 30 large industrial companies, gained 138.94 points, or 0.78 percent, to close at 17,995.72. The S&P 500 stock index added 8.17 points, or 0.39 percent, to end at 2,079.43. The Nasdaq composite rose 15.07 points, or 0.31 percent, to finish at 4,942.44.

Alcoa Inc. announced Monday it will buy RTI International Metals Inc. for $1.5 billion, sending shares of the aluminum maker down more than 5 percent to close at $13.70. Meanwhile, shares of RTI skyrocketed more than 39 percent to close at $38.

Separately, Simon Property Group Inc. launched a takeover bid to buy Macerich Co. for more than $16 billion, a deal that would combine two of the largest U.S. shopping mall owners. Shares of Simon edged down 0.08 percent to close at $180.44, while Macerich's stock price gained nearly 7 percent to end the trading session at $92.76. 

Shares of Apple Inc. edged up 0.43 percent to close at $127.14 after the world's most valuable company kicked off its highly anticipated "Spring Forward" event Monday, with CEO Tim Cook formally unveiling the price and final features of the company’s Apple Watch, its first smartwatch and its first product developed under Cook.

While the consumer adoption curve for the Apple Watch remains a hot button question among tech investors, the Street is anticipating 14 million unit sales in 2015 and some analysts are optimistic. “We believe today’s event could be a major door opener for wearable technology across the board as this next paradigm shift is likely set to take place across the consumer/enterprise landscape over the coming years,” Daniel Ives, an analyst at FBR Capital Markets & Co., said in a research note Monday following Apple’s event.

In after-hours trading, shares of Qualcomm Inc. jumped more than 2 percent to $74.50 after the chipmaker announced a $15 billion buyback program and raised its dividend 14 percent.

Economists are looking ahead to Tuesday’s economic calendar, which includes the JOLTS jobs report and the National Federation of Independent Business’ (NFIB) monthly small business index for February. The index, which measures optimism among small businesses, declined to 97.9 in January after it increased to 100.4 in December, which was the index's largest gain since October 2006.