A statistical study of emission lines from high redshift radio galaxies

We have compiled a sample of 165 radio galaxies from the literature to stud y the properties of the extended emission line regions and their interactio n with the radio source over a large range of redshift 0 < z < 5.2. For eac h source, we have collected radio (size, lobe distance ratio and power) and spectroscopic parameters (luminosity, line width and equivalent width) for the four brightest UV lines. We also introduce a parameter A(Ly alpha) mea suring the asymmetry of the Ly alpha line, assuming the intrinsic redshift of the line is the same as that for the He II lambda 1640 line, and show th at this parameter is a good measure of the amount of absorption in the Ly a lpha line. Using these 18 parameters, we examine the statistical significance of all 1 53 mutual correlations, and find the following significant correlations: (i ) Ly alpha asymmetry ALy alpha With radio size D and redshift z, (ii) line luminosity with radio power, (iii) line luminosities of Ly alpha, C IV, He II and C III] with each other, and (iv) equivalent widths of Lya, C IV, He II and C III] with each other. We interpret the correlation between redshif t and A(Ly alpha) as an increase in the amount of H I around radio galaxies at z > 3. The almost exclusive occurrence of HI absorption in small radio sources could indicate a denser surrounding medium or an un-pressurized, lo w density region, as suggested by Binette et al. (2000). Correlations (ii) to (iv) provide evidence for a common energy source for the radio power and total emission line luminosity, as found in flux density-limited samples o f radio sources. The luminosity of the Ly alpha line relative to the other emission lines an d the continuum shows a strong increase at z greater than or similar to 3, coincident with the increase in the amount of associated Hr absorption. Thi s indicates an increased abundance of hydrogen, both ionized and neutral, w hich may well be the reservoir of primordial hydrogen from which the galaxy is forming. This metallicity evolution is also seen in the nitrogen abunda nce, which shows a variation of more than an order of magnitude, with the z > 3 radio galaxies occupying only the Z < 2Z. region. To examine the ioniz ation mechanism of the extended emission line regions in HzRGs, we plot the UV emission line data in line-ratio diagnostic diagrams. The diagrams invo lving the high ionization C IV, He II and C III] lines seem to confirm prev ious results showing that AGN photo-ionization provides the best fit to the data. However, these models cannot fit the C II]/C III] ratio, which lies closer to the predictions for the highest velocity shock ionization models. We note that the C II] line is five times more sensitive to shock ionizati on than the high ionization UV lines, and show that a combination of shock and photoionization provides a better overall fit to the integrated spectra of HzRGs. A substantial contribution from shock ionization will show up fi rst in shock sensitive lines like C II] or Mg II. We also confirm the findi ngs of Best et al. (2000b) that shock ionization occurs almost exclusively in small radio sources, and show that the angular size distribution can ind eed explain the differences in three HzRG composite spectra. Because most H zRGs have radio sizes <less than or similar to> 150 kpc, their integrated s pectra might well contain a significant contribution from shock ionized emi ssion.