Authors: Matsuoka, K. et al.
Measuring the chemical evolution of galaxies can give clues to their star formation histories. This is often done by measuring the metallicity of galaxies at various redshifts. However, as for all things astronomical, this becomes more difficult at high redshift. To alleviate this, studies have used active galactic nuclei (AGN) to measure the metallicities of high-redshift radio galaxies (HzRGs) because of their high luminosities. Specifically, gas clouds photoionized by the active nucleus emit lines in the ultraviolet (among other wavelengths) that can be observed in the optical. Quite convenient.
The currently accepted model of a typical AGN is as follows. Material from an accretion disk falls onto the central black hole, which is encircled by a large torus of gas and dust. Thus, if we observe an AGN from a pole, we see all the way down into the center, where the velocity dispersions of the gas create broad emission lines (this is known as the broad line region, or BLR). However, if we are fortuitous enough to view an AGN edge-on, the torus obscures the BLR, and instead we see emission lines resulting from the slower-moving gas clouds farther away from the black hole. This, naturally, is known as the narrow line region (NLR).
|Figure 1: Line ratios showing possible metallicity evolution.|
Studies using AGN have found no metallicity evolution up to z ~ 6. However, this may be due to the fact that many of these studies focused on the BLR, which could have evolved faster than the rest of the galaxy. Matsuoka et al. (2011), then, concentrate on the NLR of the most distant radio galaxy at z = 5.19, TN J0924-2201 (catchy name). Using the Faint Object Camera and Spectrograph (FOCUS), they detect Lyα and CIV lines, the first time CIV has been detected from a galaxy with z > 5. This indicates that a significant amount of carbon exists even in this high-z galaxy. Additionally, the Lyα/C IV ratio is slightly lower than that from lower-z HzRGs (Figure 1), suggesting possible metallicity evolution because of the higher amounts of carbon. However, this could also be attributed to weaker star-formation activity or Lyα absorption. Upper limits of NV/CIV and CIV/HeII were also measured, but these agree with lower-z HzRG measurements.
The authors also investigate the [C/O] abundance ratio by comparing observational limits of NV/CIV and CIV/HeII to photoionization models, the results of which are shown in Figure 2. Carbon enrichment in these galaxies is delayed compared to α elements, because much of the production of carbon comes from intermediate-mass stars (which have longer lives compared to those stars that create α elements). Thus, [C/O] is a good measure of star formation. The analysis finds a lower limit on [C/O] of -0.5, suggesting that this galaxy has experienced some chemical evolution. Comparison of this limit to previous models suggests an age of TN J0924-2201 of a few hundred million years old.