Studies On Dust Obscuration In Star-forming Galaxies

The physics related to the star-formation in galaxies are the key to understanding the formation and evolution of galaxies.Dust in the interstellar medium(ISM)plays an important role in regulating galaxy formation and evolution,as well as affecting the observational properties of galaxies.This thesis conducts research on the dust obscura-tion with a large sample of local galaxies,including the dependence of dust obscuration on galaxy parameters,the discrepancy between IRX and Ha/H?-and the variation of dust attenuation curves.Unlike the dust extinction,where the point source is absorbed and scattered by foreground dust,the dust attenuation describing the extended sources where stars and dust are mixed together.Our work refers to the latter.We investigate dust obscuration as parameterized by the infrared excess IRX?LIR/LUVin relation to global galaxy properties,using a sample of-32 000 local star-forming galaxies(SFGs)selected from SDSS(Sloan Digital Sky Survey),GALEX(Galaxy Evolution Explorer)and WISE(Wide-field Infrared Survey Explorer).We show that IRX generally correlates with stellar mass(M*),star formation rate,gas-phase metallicity(Z),infrared luminosity(LIR)and the half-light radius(Re).A weak corre-lation of IRX with axial ratio(b/a)is driven by the inclination and thus seen as a projec-tion effect.By examining the tightness and the scatter of these correlations,we find that SFGs obey an empirical relation of the form IRX=10?(LIR)? Re-?(b/a)-? where the power-law indices all increase with metallicity.The best-fitting relation yields a scatter of?0.17 dex and no dependence on stellar mass.Moreover,this empirical relation also holds for distant SFGs out to z=3 in a population-averaged sense,suggesting it to be universal over cosmic time.Our findings reveal that IRX approximately increases with LIR/Rc[1.3-1.5]instead of LIR/Re2(i.e.,surface density).We speculate this may be due to differences in the spatial extent of stars versus star formation and/or complex star-dust geometries.We conclude that not stellar mass but IR luminosity,metallicity and galaxy size are the key parameters jointly determining dust obscuration in SFGs.The gas-phase metallicity turns out to play a fundamental role in shaping the IRX-related relationshipsWe compare the infrared excess(IRX)and Balmer decrement(H?/H?)as dust at-tenuation indicators in relation to other galaxy parameters using a sample of?32000 local star-forming galaxies(SFGs)carefully selected from SDSS,GALEX and WISE.While at fixed H?/H?,IRX turns out to be independent on galaxy stellar mass,the Balmer decrement does show a strong mass dependence at fixed IRX.We find the dis-crepancy.parameterized by the color excess ratio REBV?E(B-V)IRX/E(B-V)H?/H?.is not dependent on the gas-phase metallicity and axial ratio but on the specific star for-mation rate(SSFR)and galaxy size(Re)following REBV=0.79+0.15 log(SSFR/Re2).This finding reveals that the nebular attenuation as probed by the B almer decrement be-comes increasingly larger than the global(stellar)attenuation of SFGs with decreasing SSFR surface density.This can be understood in the context of an enhanced fraction of intermediate-age stellar populations that are less attenuated by dust than the H ? region-traced young population,in conjunction with a decreasing dust opacity of the diffuse ISM when spreading over a larger spatial extent.Once the SSFR surface density of an SFG is known,the conversion between attenuation of nebular and stellar emission can be well estimated using our scaling relation.Based on the?2300 local star-forming galaxies selected from the Galaxy And Mass Assembly(GAMA)survey with reliable FUV to FIR multi-band data,we inves-tigate the variation of the dust attenuation curves.We perform SED fitting on the FUV to FIR multi-band dataset to determine the dust attenuation parameters in galaxies.We show that the slope of the attenuation curve(?)increases(become flatter)with the dust opacity(both AV and AFUv)?We demonstrate that the errors of AV and ? are highly correlated,and their correlation is strongly biased by the degenerate errors.We then re-examine these relationships utilizing simulated data.We show that the relation between attenuation curve slope and dust opacity can be well reproduced by performing same SED fitting to simulated data with ?=-0.5.This suggests the relation between attenu-ation curve slope and dust opacity is not necessarily reflecting the intrinsic connection,but is also possibly dominated by the fitting degeneracies.We stress that this relation should be used with caution.In this thesis,we use the large-sample statistical method to analyze the scaling relations of dust obscuration,the discrepancy between dust attenuation indicators and the variation of dust attenuation curves.Our thesis provides a new understanding of the evolution of dust obscuration,the role of metallicity on dust obscuration,the evolution of dust-star geometry and the relationship between the slope of attenuation curve and dust opacity.