Title: The Chemical Evolution of Star-Forming Galaxies Over the Last 11 Billion Years
Authors: H. Jabran Zahid, Margaret J. Geller et al
First Author’s Institution: University of Hawaii at Manoa, Institute for Astronomy, Honolulu, HI
Next the authors determined the metallicity of the galaxies in their sample from measuring the ratios of the strength of multiple Oxygen, Hydrogen, and Nitrogen emission lines. The figure above shows their results. The survey data, used for z<1, are sorted into equally populated bins of stellar mass and the data points represent the median stellar mass and metallicity for each bin. The data points for the z>1 sample is determined by stacked spectra sorted by stellar mass. This figure shows that at fixed stellar mass the average metallicity increases as a function of cosmic time since z~2.3 and also that the shape of the MZ relation evolves with time which suggests that the evolution is mass-dependent. In other words, at high stellar mass you see that the MZ relationship starts to flatten out for z<0.8. Furthermore, at fixed stellar mass the more massive galaxies enrich more rapidly as compared to their low-mass counterparts for z≥0.8.In order to see how stellar mass effects the metallicity evolution of their sample the authors also plot the scatter in the MZ relation as a function of stellar mass, shown in the figure below. This scatter is defined as the difference between the upper and lower limits of the 85% and 50% contour obtained from their metallicity measurements. The top panels of the figure show the upper and lower limits containing the central 85% (A) and 50% (B) of the metallicity distribution as a function of mass for the z<1 galaxies. That is, they divided their samples in two for this analysis. The bottom panels show the difference between these limits (85% – C and 50% – D).
The top panels show that the flattening (i.e., decreasing slope) of the MZ relationship, at high stellar masses, is mass-dependent for the z<1 sample and also that the metallicity distribution reaches a constant value (i.e., saturates) for the high-mass galaxies. Furthermore, the fraction of saturated galaxies at a fixed stellar mass increases at late times. This “metallicity downsizing” agrees with galaxy downsizing in which massive galaxies tend to form their stars earlier than low-mass galaxies. Hence, at late times the star formation rate of low-mass galaxies increases allowing for continued metal enrichment whereas it decreases for massive galaxies causing the metallicity to saturate. This is easily seen in the lower panels in which the scatter of the difference between the upper and lower limits of their samples decreases with increasing mass at high masses. These results show that the MZ relation flattens with cosmic time.