The Nuclear Activities And Star Formation In Nearby S0 Galaxies

As a bridge connecting quiescent galaxies with star-forming galaxies,the forma-tion and evolution of SO galaxies can enhance our understanding of a big picture of galaxy formation and evolution.In this paper,we focus on the star formation activity,the interstellar medium(ISM),and the relations between physical properties and envi-ronments for local SO galaxies by using the SDSS survey data,the Herschel data,and the CAHA 3.5m IFU data.Our research work mainly includes the following two parts:Firstly,we study an atypical early-type galaxy,NGC 3665,which has abundant cool gas while less star formation.We present the analysis of physical properties of the ISM in this galaxy,based on the far-infrared(FIR)photometric and spectroscopic data as observed by the Herschel Space Observatory.The fit to the spectral energy distribution reveals a high dust content in the galaxy,with the dust-to-stellar mass ra-tio of Mdust/M*?1.1 × 10-4 that is nearly three times larger than the mean value of local SO+SOa galaxies.For the ionized regions(H ? regions),the electron density(ne)is around 49.5 ± 11.9 cm-3 based on the[N ? 122?m/[N ?]205?m ratio.For the photodissociation regions(PDR),the cooling efficiency is in the range of 1.26 ×x 10-3 and 1.37 × 10-3 based on the([C?]+[O ?]63 ?m)/LrIR,which means the low cooling efficiency in our galaxy;Based on the PDR model,the hydrogen nucleus density and the strength of FUV radiation field are n?104 cn-3 and G0?10-0.25,respectively.Combining with the analysis of CAHA 3.5m IFU data,we find that the above results are consistent with the presence of weak AGN and a low level of star-forming activity in NGC 3665.After discussing about the mechanism for star formation suppression,we find a compact massive bulge in NGC 3665,which stabilizes amount of cool gas against star formation,and can gives strong support to the 'morphological quenching'scenario.This is the reason of the special phenomenon that NGC 3665 is significantly lower by about 1 dex from the Kennicutt-Schmidt(KS)relation.Secondly,we present a study of a complete sample of nearby SO galaxy.In order to derive the properties for large sample of SO galaxies,and to explore the relation with their environment,we construct a sample of 583 SO galaxies with central spectrophoto-metric information from the SDSS.We find that there are various nuclear activities in nearby SO galaxies.For comparing physical properties and environments of these SO galaxies with different activities,we make a classification with their nuclear activities.According to the BPT diagram,we find that 8%of SO galaxies show central star-forming activity,while the fractions of Seyfert,Composite and low-ionization nuclear emission-line regions are 2%,8%,and 21.4%,respectively.Star-forming SOs have the lowest stellar masses,over one magnitude lower than the others,which might be explained by the“down-sizing" effect.After analyzing their environment,we find that the active SOs are mainly located in the sparse environment,while the normal SOs are located in the dense environment,which might suggest that the environment plays an important role in quenching star formation and/or AGN activity in SO galaxies.By performing 2D bulge-disk decomposition for galaxy photometric images,we find that the bulges of approximately 1/3 of star-forming SO galaxies are obviously bluer than their disks,while for majority of other types of SOs,the bulge components show redder color dis-tributions than disk components.Besides,the Sersic index of most star-forming SO-bulges is less than 2,suggesting these bulges might be pseudobulges,while for normal SOs,it is between 2 and 6.Thus,we speculate that the formation of SOs may be due to accreting cool gas from the external by progenitors,leading to a burst of short-lived and centrally-concentrated final star formation,which could be a new formation mechanism of SO galaxies.Through the research of this work,we find that during the routine of the formation and evolution of SO galaxies,on the one hand,the environment plays a very important role.Different environment leads to the different distribution of SO galaxies with differ-ent nuclear activity.The environment can either promotes or triggers star formation in galaxies,or suppresses star formation activity.On the other hand,the compact massive bulges might have important effects in the galaxy evolution.For example,the com-pact bulge can suppress star formation in NGC 3665,further inhibiting the increment of stellar mass.The results of this paper reveal that both the galaxy's self-evolution and its interaction with the environment play a decisive role in the formation and evolution of the SO galaxies.