Lyman α Emission from Disk Absorption Systems in QSOs: Star Formation in Young Galaxy Disks

Extract

The strong self-damped Lyman α absorption systems present in the spectra of high redshift QSOs represent a unique population of absorber which has recently been associated with the precursors of current disk galaxies. In a low resolution survey for what we have come to call “Lyman α disk systems” performed at Lick Observatory (Wolfe, et al. 1986, Ap. J. Suppl. 61, 249) approximately 18 systems with confirmed damped Lyman α profiles and rest frame equivalent widths greater than 5 Å were detected in a sample of 68 high redshift QSOs (Smith, Cohen and Bradley 1986, Ap. J. 310, 583). Subsequent higher resolution study has shown these systems to have the following properties (Turnshek, et al. 1988, Ap. J., in press):

1. 1. Neutral hydrogen column densities, 2 × 10²⁰ ≤ N_HI ≤ 8 × 10²¹ cm⁻².

2. 2. Low-mixed ionization state. Typically the low ionization states dominate the high ionization states (e.g. CII ≫ CIV). Some enrichment has occurred, −2≲[X/H]_⊙ ≲0.

3. 3. Gas density, n ≲ 1 cm⁻³.

4. 4. The gas shows two components, a quiescent (disk) component, σ_ν ≲ 10 km s⁻¹, and a turbulent (halo) component, σ_ν ≲ 20 km s⁻¹. Some systems show only the low velocity dispersion component.

5. 5. At least one system intervening toward a radio QSO (Pks 0458-020) shows 21-cm absorption. The system shows multiple cloud structure with σ_ν ≈ 6 km s⁻¹, T_s ≈ 100 K, and structure extended over several kpc on the sky.

6. 6. There is evidence that these systems may be self gravitating with scale height of the order of 300 pc.

7. 7. These systems represent a unique population of absorber (distinct from the ‘Lyman a forest’ and heavy element systems) covering approximately 20% of the sky to z ≈ 3 and accounting for all of the baryonic matter at that redshift.