| With the integrated field spectroscopy(IFS),we could directly investigate kine-matic properties of stars and gas.Based on this,a new classification of early type galaxies(ETGs)has arisen.Those dominated by stellar velocity are called fast rota-tors,which are speculated to originate from gas rich merging or gas accretion.Whereas those dominated by velocity dispersion are slow rotators,which are produced by dry mergers.However,all ETGs have ceased star formation,and thus we can not find direct evidence to prove these two scenarios.In MaNGA project,we find a low mass early type galaxy(ETGs)that is classified as a fast rotator,which means it is stellar velocity rather than velocity dispersion that dominates internal stellar motion.After fitting the 1D photometric profile of this galaxy,we find that the Sersic index in the outer part is about 2.1-2.4,and there is extra light in the center after we extrapolate the outer Sersic profile to the center.All of these are typical properties of low mass ETGs.Then we find that the inner part of this galaxy is more like a disc,with rotation dominated,while the outer part is more like a bulge as dominated by random motions.Moreover,we identify some tidal features around this galaxy in the deep field optical image.Both this kine-matic misalignment between the inner and outer part and the tidal features indicate this galaxy has experienced a merging event.The inner part has ongoing star formation,in-dicated by its location on the star forming main sequence and on the star forming region in BPT diagram.That demonstrates it is a gas rich merger.We figure that this galaxy is in the final phase of merging and will become a typical low mass ETG after quenching.We thus find a direct evidence to demonstrate that low mass ETGs originates from gas rich merging.Based on the investigation of gas and stellar position angles(PA),former works find that some galaxies have an offset between stellar and gas rotation,known as mis-aligned galaxies.The misaligned gas in this kind of galaxies are speculated from exter-nal processes,such as gas accretion or merging.The study of misaligned galaxies could help us understand the gas acquiring in galaxies.However,former works lack direct evidence to demonstrate this external origin.We use deep images in DESI Legacy Sur-veys to find galaxies with merging features from deep field images as merging sample.We try to find the relationship between merging and misaligned galaxies.Our findings are:(1)The merging sample contains a higher fraction of misaligned galaxies than the whole sample,suggesting that merging is one of the origins of misaligned galaxies.(2)Based on the ?PA distribution of our sample,we find that the number of counter-rotators(with the ?PA?150°)is higher than predicted and the merging fraction of counter-rotators is smaller than that of misaligned galaxies.We argue that this is be-cause misaligned gas can precess and finally settle to a direction in parallel with stellar rotation.Precession enhances the number of counter-rotators and the tidal features dis-appear on this stable phase,resulting in a smaller merging fraction in counter-rotators.(3)co-rotators are more numerous than counter-rotators in the merging sample.This is because that the angular momentum is transferred to stellar and gas spins during merg-ing,which forces the gas to align with stellar motion.This effect is more efficient in star forming galaxies as counter-rotating gas will form stars.The new born stars inherent angular momentum of gas and lead to co-rotators.Finally,we investigate galaxies with irregular velocity fields.We find that(1)this kind of galaxies occupies a higher fraction in merging galaxies than the whole sample,meaning merging is one reason to produce irregular velocity fields.(2)Gas is more susceptible to merging as merging reduces the relative frequency of those with only irregular stellar velocity fields and enhance the frequency of those with irregular gas velocity fields.(3)Non-star forming galaxies contain more galaxies with irregular gas velocity fields,which means they have experienced supernova explosions,AGN feedback,flyby events from a group or cluster,or merging.Such events could perturb gas velocity fields and meanwhile,stripe gas and quench star formation. |
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