Bar and Spiral Torques in the Triangulum Galaxy M33
The Triangulum Spiral (M33=NGC 598) presents a daunting task to near-infrared observers, due to its huge spatial extent. From the deepest near-infrared (JHKS) images of M33 yet secured, we identify a bar and calculate the gravitational torque, the bar torque as well as the spiral arm torque. Our methodology reveals the presence of a “class 2” bar, meaning that the tangential force field reaches a maximum of ~ 20% relative to the axisymmetric radial force field. Furthermore, we are able for the first time to quantify the Reynolds (1927) spiral arm index for M33 (described as ‘broad’: low arm-interarm contrast), and estimate that the spiral arm torque reaches a maximum of only 15%.
KeywordsGravitational torque bar torque spiral arms Triangulum Spiral M33
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