Counterrotating Stellar Disks in Early-Type Spirals: NGC 3593

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Abstract

We report the discovery of two counterrotating stellar disks in the early-type spiral galaxy NGC 3593. The major axis kinematics shows the presence of two dynamically cold counterrotating components. The surface brightness profile is well reproduced by the sum of the contributions of two exponential disks of different scale lengths (r_1 = 40"; r_2 = 10") and different central surface brightnesses ( mu_{r,1} = 19.9; mu_{r,2} = 18.5 mag/arcsec^2). The v and sigma radial profiles are easily reproduced by the means of a kinematical model adopting the above photometric parameters. An ionized gas disk is present. It corotates with the smaller scale length and less massive (M_2 = 2.7 x 10^9 M_sun) disk, and counterrotates with the larger and more massive (M_1 = 1.2 x 10^10 M_sun) one. We conclude that the smaller stellar disk is the result of a slow adiabatic acquisition of a conspicuous amount of counterrotating gas (M_infall ~ 4.3 x 10^9 M_sun) by the preexisting galaxy, originally constituted mainly by a gas-free stellar disk (disk 1). The counterrotating gas settled into the equatorial plane and then formed the inner stellar disk (disk 2).

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