* Welbeck nets hat-trick in 4-1 win
* Alexis Sanchez also scores in comprehensive victory
* Arsenal keeper Wojciech Szczesny sent off in the second half
By Martyn Herman
LONDON, Oct 1 (Reuters) - Danny Welbeck's hat-trick in a 4-1 thrashing of a feeble Galatasaray side helped ignite Arsenal's Champions League campaign in a lop-sided Group D clash in north London on Wednesday.
The England striker pounced twice before halftime and again after the break to virtually double his career Champions League tally and continue Galatasaray's tale of woe in England where they had not won in nine previous visits.
Alexis Sanchez ended the match as a contest before halftime as Galatasaray's resistance crumbled, although a second-half red card for Arsenal keeper Wojciech Szczesny slightly tarnished the 18th anniversary of manager Arsene Wenger's arrival at the club.
The night belonged to Welbeck, however, as the former Manchester United bit-part forward justified Wenger's faith in signing him in the recent transfer window.
"Hopefully this hat-trick will help him start to gain confidence," Wenger told reporters.
"I didn't know he was so quick, honestly. I knew he was quick but he can be electric, he has great pace."
Welbeck, who was a five-year-old boy when Wenger took charge of Arsenal, helped the Gunners make light of several injuries with a commanding display of front-running that terrorised Galatasaray's overstretched defence.
He had wasted a couple of chances in the 2-0 defeat by Borussia Dortmund a fortnight ago but was far more clinical on home turf, scoring twice in the opening 30 minutes.
Galatasaray's supporters showed plenty of passion at The Emirates, keeping up a cacophony of sound for most of the first half and launching numerous flares onto the pitch.
Their team, however, were harmless, providing the perfect opposition for an Arsenal side outclassed in Germany and needing three points to bolster their hopes of reaching the last 16 -- a feat they have managed every season since 1999-2000.
They did have the first chance of the night, when Brazilian Alex Telles did well down the left and played a clever one-two with Blerim Dzemaili before pulling a shot wide.
It was mainly all Arsenal though and after a brief lull, the home side went in front when the Welbeck curved his run to meet a slide-rule Sanchez pass and plant a shot through the legs of Fernando Muslera.
After 31 minutes Welbeck showed his power, seizing on a bouncing ball, held off a flimsy challenge from Felipe Melo and bounded clear to calmly slot his shot past Muslera.
That sparked the Gala fans into launching their firework display in the corner where they were wedged, at least five red flares landing on the playing surface causing the game to be temporarily halted as skipper Wesley Sneijder appealed for calm.
Once the smoke cleared Arsenal continued to reduce the Turkish defence to ash.
Sanchez was lucky to escape unhurt when Melo launched a dangerous tackle near the touchline and was booked. The Chilean exacted revenge when Mesut Ozil picked a pass and Sanchez beat the offside trap to make it 3-0.
It took Welbeck only seven minutes after the break to complete his first senior club hat-trick, finishing off a flowing passing move with a sublime deft flick after a clever pass by Alex Oxlade-Chamberlain.
"My last hat-trick was for Manchester United's reserves and I'm trying to score goals," he said. "I am getting into positions and it is paying off."
A runaway Arsenal display was checked in unlikely circumstances on the hour when Szczesny rushed off his line and took out Burat Yilmaz, giving referee Gianluca Rocchi little option but to send the Pole off.
"He tried to come out and was fraction late so he conceded the penalty. It can happen," Wenger said.
After Sanchez was substituted to make way for Arsenal's replacement keeper David Ospina, clearly much to his annoyance, Yilmaz converted the penalty.
With 10-man Arsenal saving their legs for Sunday's Premier League trip to Chelsea, Sneijder went close to pulling another goal back for the visitors and Yilmaz forced a flying save from Ospina but it was all too little, too late for Cesare Prandelli's side.
Dortmund top the group with six points, followed by Arsenal (3), Galatasaray (1) and Anderlecht (1). (Reporting by Martyn Herman; editing by Toby Davis)