Research On The Mass-metallicity Relation Of Mid-redshift Galaxies

The gas-phase metallicity of galaxies reflects the cumulative effects of star forma-tion,holding important information about galactic gas accretion,gas outflow and how galaxies interplay with surrounding environment.The mass-metallicity relation(MZR)is an important scaling relation,which is ubiquitous from local to high-redshift universe.It is related to many parameters including redshift,star formation rate(SFR),gas frac-tion,environmental effects,etc.Among those parameters,the SFR is the most relavant one.The relation between stellar mass,metallicity and SFR is called fundamental metal-licity relation(FMR),which is regarded as a basic relation without redshift evolution.The previous works that study MZR at mid redshift are restricted to relatively small sample,while in this paper we use larger sample to obtain statistically more accurate relation.In this work,we use emission-line galaxies(ELG)sample from the extended Baryon Oscillation Spectroscopic Survey(eBOSS)of the fourth generation of Sloan Digit Sky Survey(SDSS ?).About 180,000 massive star forming galaxies are selected to study the MZR at mid-redshift.The selected sample has a redshift range of 0.6<z<1.05 with median redshift of 0.83,and a mass range of 9<log(M/M?)<12.We di-vide the spectra into different bins according to redshift,stellar mass,SFR,specific star formation rate(sSFR),half-light radius(Rh),mass density and optical color.We stack the spectra in the same bins,which have similar physical properties,to obtain high S/N composite spectra.Then we apply the strong-line method to calculate metallicity.We obtain the MZR at z?0.83.The redshift evolution is confirmed when compar-ing our MZR with previous works at different redshift ranges.At a specific stellar mass,the metallicity decreases as the redshift increase.A similar trend is found when dividing the sample into narrower redshift range,which helps improving the accuracy of MZR.At a specific stellar mass,galaxies with higher SFR/sSFR and larger Rh have systemati-cally lower metallicity,while galaxies with higher mass density and redder optical color have systematically higher metallicity.Among the above physical parameters,the MZR has the most significant dependency on SFR/sSFR.We derive the mid-redshift FMR by introducing ?=log(M)-0.321og(SFR),and find that it is consistent with the local FMR.Given the inhomogeneity of redshift distribution and incompleteness of stellar mass and[O ?]luminosity due to the sample selection bias,we can reasonably explain the differences between our results and previous works.Through this research,we can draw a possible evolution picture of mid-redshift star forming galaxies.At a specific stellar mass,galaxies with lower Rh have higher surface mass density,triggering higher star formation efficiency.A long-term higher star formation efficiency yields higher present-day metallicity.Meanwhile,the long-term gas depletion leads to a lower present-day SFR.