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Kinematic decoupling between the gaseous and stellar components has been observed to be common in local ETGs (Bertola et al., 1999; Sarzi et al., 2006; Davis et al., 2011), with hints of this behaviour

being already present at high redshift (Wisnioski et al., 2015). The distribution of misalignments between the gas and stars is a direct probe of the origin of the gas in ETG. Theoretically, a newly formed gas disc is predicted to precess due to the e↵ect of the gravitational potential of the stars, with gas precessing faster at smaller radii, where the torque is larger. Cloud-cloud collisions will also work to realign the di↵erently precessing gas rings, and the system will eventually settle into a configuration where the gas and stellar components are either aligned or misaligned by 180 . The relaxation time for this process was initially predicted to be of order of the dynamical time (Tohline & Durisen, 1982; Lake & Norman, 1983), however recent work based on hydrodynamics in the cosmological framework has highlighted that misalignments might persist on much longer (several Gyr) timescales (van de Voort et al., 2015), mostly because of the e↵ect of continuous cosmological accretion. This longer relaxation timescale also provides a better fit to the misalignment distributions presented in the literature (Davis & Bureau, 2016). For weakly triaxial systems stable gaseous orbits are allowed both in the plane containing the long and intermediate axes and the plane of the short and intermediate axis (Franx et al., 1991). If the origin of the gas is external, therefore, the distribution of the gas-stars kinematic misalignment should display three peaks, respectively at 0, 90, 180 , with the peak around 90 being weaker and scaling with the degree of triaxiality.

In this section we derive the kinematic position angle (PA) of both gas and stars following the procedure in Krajnovi´c et al. (2006). In short, having fixed the kinematical centre to the photometric one, the algorithm constructs a bi-antisymmetric version of the velocity field and compares it with the observed velocity field. The resulting PA is thus only representative of the axis of symmetry of the velocity field and the measurement does not imply that the galaxy is best fitted with a thin disc model. In our sample four eLIER galaxies present particularly complex gaseous kinematics with no obvi- ous symmetry axis and have been excluded from the following analysis. Two of these galaxies show approaching/trailing features while other two show evidence for extreme changes in the direction of rotation of the gas (MaNGA-ID 1-38167, 1-274663). The remaining eLIER galaxies display a wide range of gas velocity field patterns. While some are consistent with disc rotation, in other systems the velocity field appears disturbed. Overall, the large majority of galaxies in this class show coherent velocity shear patterns, showing both red- and blue-shifted emission. These gas flows are not always associated with large scale rotation. In Cheung et al. (2016), for example, we propose a biconical out- flow model for an eLIER galaxy (MaNGA-ID 1-217022), which has recently been subject to a minor merger event, as demonstrated by the presence of a nearby companion and the detection of neutral gas via the NaD absorption feature.

The observed distribution of stars-gas misalignment for the eLIER galaxies in our sample is shown in Fig. 4.8, with 30 out of 49 galaxies having misalignments larger than 30 . Making the appropriate volume corrections, necessary in order to take into account the MaNGA selection function, we infer

that 65 ± 7% of eLIERs are misaligned with | stars gas| > 30 . This number is significantly larger

from that inferred by the SAURON and ATLAS3D _{surveys (Sarzi et al., 2006; Davis et al., 2011).}

However, both surveys adopt a pure morphological selection, thus including galaxies with residual star formation into their sample, which would not be classified as eLIERs. Galaxies with detectable star formation (SF and cLIERs) have a misalignment distribution strongly peaked at zero (Fig. 4.8,

bottom panel, only 11 ± 2% with | stars gas| > 30 ). Thus a small contamination from SF or cLIER

galaxies can bring our result in line with previous work. Moreover our results confirm the theoretical prediction for the case of external accretion, as the observed distribution of misalignments for eLIERs

Figure 4.8:Histograms of the distribution of kinematic misalignments between the position angle of the major axis of the stellar component and of the ionised gas for the eLIER galaxies (left) and SF and cLIER galaxies (right) in the current MaNGA sample.

is peaked at 0, 90 and 180 .

In absence of internal processes (i.e. stellar mass loss), the misalignment histogram is predicted to be symmetric around 90 (Davis & Bureau, 2016). Making the assumption of isotropic accretion and long relaxation times, one would naively expect 150/180⇠83% of galaxies to have misalignments larger than 30 . The presence of a stronger peak at zero and the fact that the number of misaligned galaxies is lower than a naive estimate for isotropic accretion implies that internal processes are also likely to play a role, although secondary, in shaping the observed misalignment distribution. In partic- ular, stellar mass loss will not only inject into the ISM some amount of gas sharing the same kinematic properties as the parent stars. This pre-existing disc would also create additional torque on the accreted misaligned gas and contribute to realign it with the stellar kinematic field (Sarzi et al., 2013; Davis & Bureau, 2016). Both e↵ects would contribute to increase the fraction of aligned galaxies.