Previous Article Next Article Table of Contents
The Formation of Heavy Metals in the Early Universe (1Jun94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 38, 1 June 1994) High redshift quasars allow us to study the universe when it was very young. Metal lines in quasar spectra provide both a record of the star formation history of the host galaxy, and in principle a chronometer that can place a lower limit on the age of the universe at the lookback time of the quasar. Unfortunately, most of the strong UV emission lines in quasars are due to alpha-process elements that are rapidly synthesized in type II supernovae explosions. Due to the short lives of their progenitors, the abundance of alpha-process elements cannot place strong constraints on the star formation history. Iron, on the other hand, is mostly enriched through supernovae type Ia explosions. The accepted model for type Ia supernovae involves a merger with a white dwarfs following a common envelope phase, thus delaying the enrichment. Models of the ISM suggest that the Fe enrichment time scale is about 1 Gyr. Thus, a large Fe abundance at high redshift implies significant star formation over the prior 1 Gyr. Strong Fe emission is frequently seen in low redshift AGN in the spectral regions near HB (4861 A) and Mg II (2800 A). Models of such strong Fe II emission imply relative iron abundances at least several times solar. The detection of strong Fe emission thus implies a large enrichment by type Ia supernovae. Recently, infrared spectrometers have become sensitive enough to detect Fe emission at redshifts beyond 2.5, which is where the Fe emission shifts out of the grasp of optical spectrographs. Using the KPNO Cryogenic Spectrometer (CRSP) on the 2.1-m telescope, Richard Elston (CTIO), Gary Hill (U. of Texas, Austin) and Keith Thompson (Naval Research Lab.) observed the spectral regions near HB in two redshift ~ 3.3 quasars, redshifted into the K band (1994 Nature, 367, p.250). They found strong rest-frame Fe emission in both objects (Figure 1). The optical Fe emission is stronger than that seen in 98% of low-redshift quasar samples, thus indicating a high Fe abundance. They argue that star formation must have occurred at z > 6 for qo = 0.1, Ho = 80 cosmology, if the iron was indeed created in type Ia supernovae. Furthermore, if qo = 0.5, one can rule out standard cosmologies with Ho > 80, because the age of the universe would then be less than the required enrichment time of 1 Gyr. [Figure not included] Figure 1: Rest-frame spectra around HB of Q0014+813 and Q0636+680, together with the ultra-strong Fe II emitter IRAS 07598+6508. The various Fe II lines are indicated. Note the good coincidence with features in high redshift quasars, but that the ratios are different from those seen in low redshift QSOs. Since type II and type Ib supernovae synthesize little Fe relative to alpha-process elements, a clear signature that the Fe is synthesized by type Ia supernovae would be a large Fe abundance relative to an alpha-process element. Measuring the strength of Fe II emission near the Mg II line allows this test to be done. Measuring Fe II relative to the alpha-process ion Mg II is a particularly good test, since they have similar ionization potentials and will thus originate in the same regions. In December 1993 Elston, Hill and Thompson observed the region near Mg II, redshifted into the J band, in the same two redshift ~ 3.3 quasars using CRSP with its new 256 X 256 InSb array on the KPNO 4-m telescope (Figure 2). The improved CRSP provided a large increase in performance, allowing the clear detection of the low equivalent-width Fe II complexes near Mg II. This observation demonstrates high Fe abundance relative to Mg II, implying enrichment by type Ia supernovae. [Figure not included] Figure 2: Rest-frame spectra around Mg II of the same quasars showing strong UV Fe II emission, which confirms the high abundance of iron in these objects at z ~ 3.3.
Previous Article Next Article Table of Contents