SQ-A: A Collision-triggered Starburst in the Intragroup Medium of Stephan’s Quintet

Abstract We present new observational evidence supporting the hypothesis that SQ-A, a starburst in the intragroup medium (IGrM) of Stephan’s Quintet (SQ), is triggered by a high-speed collision between two gas systems, one associated with the IGrM ( v ∼ 6900 km s −1 ) and another with the intruder galaxy NGC 7318b ( v ∼ 6000 km s −1 ). The new CO(2–1) data set from the Atacama Large Millimeter/submillimeter Array has angular resolutions between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0</mml:mn> <mml:mover accent="true"> <mml:mi>.</mml:mi> <mml:mi>″</mml:mi> </mml:mover> <mml:mn>2</mml:mn> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>7</mml:mn> <mml:mover accent="true"> <mml:mi>.</mml:mi> <mml:mi>″</mml:mi> </mml:mover> <mml:mn>0</mml:mn> </mml:math> and the new Very Large Array H i data cube has an angular resolution of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>6</mml:mn> <mml:mover accent="true"> <mml:mi>.</mml:mi> <mml:mi>″</mml:mi> </mml:mover> <mml:mn>6</mml:mn> <mml:mo>×</mml:mo> <mml:mn>7</mml:mn> <mml:mover accent="true"> <mml:mi>.</mml:mi> <mml:mi>″</mml:mi> </mml:mover> <mml:mn>9</mml:mn> </mml:math> . The CO maps show that the two gas systems are bridged by another system with an intermediate velocity of ∼6600 km s −1 , whereas the H i data show that the component of v ∼ 6600 km s −1 fits well into a gap in the more extended v ∼ 6000 km s −1 component, albeit with a displacement of ∼5 kpc. Both the bridge and the complementary distributions between different gas systems are common features of starbursts triggered by cloud–cloud collision. An analysis of clumps (sizes of 100–200 pc) reveals very diversified star formation (SF) activity in clumps belonging to different kinematic systems, with the molecular gas depletion time of the v ∼ 6900 km s −1 clumps being more than 10 times longer than that of the v ∼ 6600 km s −1 clumps. The results are consistent with a scenario in which the enhanced SF activity (and the starburst) in the system of v ∼ 6600 km s −1 is due to gas compression generated in cloud–cloud collisions, whereas the suppression of SF in the v ∼ 6900 km s −1 system is due to vortices (i.e. gas rotation) generated in more complex collisions involving dense clouds and diffuse intercloud gas associated with high-speed shocks.