Properties and formation mechanism of the stellar counter-rotating components in NGC 4191^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Straβe 2, 85748 Garching bei München, Germany
e-mail: lcoccato@gmail.com
² Max Planck Institute for Extraterrestrial Physics, Giessenbachstraβe, 85748 Garching, Germany
³ University Observatory Munich, Scheinerstraβe 1, 81679 Munich, Germany
⁴ Subaru Telescope, 650 North Aohoku Place, Hilo, HI 96720, USA
⁵ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, vicolo dell’ Osservatorio 3, 35122 Padova, Italy
⁶ INAF–Osservatorio Astronomico di Padova, vicolo dell’ Osservatorio 5, 35122 Padova, Italy
⁷ Department of Astronomy, Columbia University, New York, NY 10027, USA

Received: 19 May 2015
Accepted: 21 July 2015

Abstract

Aims. We here distinguish two counter-rotating stellar components in NGC 4191 and characterize their physical properties such as kinematics, morphology, age, and metallicity.

Methods. We obtained integral field spectroscopic observations with VIRUS-W and used a spectroscopic decomposition technique to separate the contribution of two stellar components to the observed galaxy spectrum. We also performed a photometric decomposition, modeling the galaxy with a Sérsic bulge and two exponential disks of different scale length, with the aim of associating these structural components with the kinematic components. We then measured the equivalent width of the absorption line indices on the best-fit models that represent the kinematic components and compared our measurements to the predictions of stellar population models that also account for the variable abundance ratio of α elements.

Results. We have evidence that the line-of-sight velocity distributions (LOSVDs) are bimodal and asymmetric, consistent with the presence of two distinct kinematic components. The combined information of the intensity of the peaks of the LOSVDs and the photometric decomposition allows us to associate the Sérsic bulge and the outer disk with the main kinematic component and to associate the inner disk with the secondary kinematic component. We find that the two kinematic stellar components counter-rotate with respect to each other. The main component is the most luminous and massive; the secondary component rotates along the same direction as the ionized gas. The study of the stellar populations reveals that the two kinematic components have the same solar metallicity and subsolar abundance ratio, without significant radial gradients. On the other hand, their ages show negative gradients and the possible indication that the secondary component is the younger. We interpret our results in light of recent cosmological simulations and suggest gas accretion along two filaments as the formation mechanism of the stellar counter-rotating components in NGC 4191.