SDSS IV MaNGA: The impact of the acquisition of gas with opposite angular momentum on the evolution of galaxies

Abstract: A gaseous counter-rotating galaxy is a galaxy containing a gas component with opposite angular momentum to the main stellar disk. The counter-rotating gas provides direct evidence for the accretion of external material, a key aspect in hierarchical galaxy evolution. We identified 303 gaseous counterrotators out of 9992 galaxies in MaNGA. The majority of the counterrotators are early-types. This implies their formation is highly correlated with early-type galaxies although it is still difficult to know if one leads to the other. To disentangle which of the galaxy characteristics within a morphological class were changed by the accretion of counter-rotating gas, we carefully selected a comparison sample with similar fundamental galactic properties, but co-rotation in gas. This comparison shows that gaseous counter-rotation correlates with weak rotation in the stellar component, high central concentration of star forming regions, if present, and a higher fraction of central low ionization emission regions (cLIER) galaxies. The light distributions of the stellar components, dust and HI content (both low), and overall suppressed star formation rates are similar for both samples and seem typical for the morphological class. We claim that elliptical and about half of the lenticular counterrotators, those with weak rotation in the stellar component in the outskirts and central regions, likely have a major merger origin for the gas acquisition, and the other half of lenticulars, with stronger stellar rotation, may have a minor merger or pure gas accretion origin.