Abstract
Several mechanisms have been proposed in recent years to explain kinematic decoupled cores (KDCs) in early type galaxies as well as the large differences in angular momentum between KDCs and host galaxy. Most of the proposed scenarios involve large fractions of merging events, high speed interactions with dwarf spheroidal galaxies, cusp effect of the dark matter density profiles, etc. We here argue that counterrotation as well as fast and slow rotation of disks or spheroids at the center of galaxies can also be explained by a misalignment of the central spheroid equatorial plane with regard to that defined by the observed external stellar rotation. Contrary to what happens at the outer region of disk galaxies, once instability has led to the inner warped core, the perturbed orbits can maintain a common orientation due to the rigid body like rotation at the central region of the galaxy. The spatial configuration that furnishes the smallest angular momentum difference between the KDC and the host galaxy is completely defined by observed parameters in the plane of the sky, namely, the inclination of the inner and outer disks and the angle between the two lines of nodes. As an example we modeled the paradigmatic and extreme case of the 2D radial velocity field of NGC 4382 nucleus. Tilt angles of the KDC not larger than 30 degrees also allow explaining fast and low rotators of the called “Sauron paradigm” in a unified scenario. The maximum for the three parameters, namely, velocity of the inner rotator, difference of position angle and difference with the outer rotation velocity of the whole Sauron sample, are consistently correlated in agreement with the proposed scenario. These quantities do not correlate with the galaxies magnitude, mass (since large and dwarf spheroidals show apparent counterrotation as well) or environment, also suggesting that an internal phenomenon like the central spheroid warping, that we are here proposing, may be at work.
Highlights
More than thirty years ago [1] reported for the first time the discovery of a counterrotating core in the elliptical galaxy NGC 5813
Reference [3] found a misalignment between the inner and the outer rotation axis in NGC 4765 and NGC 4406, concluding that in both galaxies the central core was decoupled from the main body
Studying the kinematics along the minor and major axis of 22 ellipticals [4] have found that 27% of them present a significant amount of minor axis rotation, what is expected if large ellipticals are tri-axial spheroids [5]
Summary
More than thirty years ago [1] reported for the first time the discovery of a counterrotating core in the elliptical galaxy NGC 5813. Two new results further challenged the scenario of small satellites accretion to explain counter rotating cores in elliptical galaxies, namely, the discovery of kinematically distinct cores (KDCs, co- and counterrotating) in an increasing number of dwarf ellipticals with MB ≥ −19 magnitudes [9]-[12] and the striking similarity in the stellar content of the KDCs and the host galaxy [11] [13] [14]. We explore in this letter a rather simple issue, namely, up to which extent projection effects produce a mirage leading to consider as different phenomena those that are strikingly similar It is one of the best know effects in observational kinematics that it is not possible to disentangle how a disk galaxy rotates only by knowing which of the lines of the nodes extremes is receding, it is necessary to know which is the minor axis nearest side. We explore in this letter all the observational consequences of taking this effect into account
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