Molecules at Early Epochs. III. The Lyman-Alpha Disk System toward 1331+170
Abstract
We present high signal-to-noise spectroscopic observations of the z = 1.776 absorption system toward the QSO MC 1331 + 170. The system contains strong H I Lyα absorption characteristic of the so-called "Lyman-α disk" systems from which a neutral hydrogen column density of 1.5 x 10^21^ cm^-2^ is inferred. Data on lines of C, Si, Fe, O, Mg, and Al at the Lyα and 21 cm redshift are presented, and the existence of detectable absorption by C and Si in several states of ionization allows us to construct a range of simple physical models for the absorbing cloud. The most plausible of these suggest matter and energy densities in the z = 1.776 cloud similar to those found in Galactic diffuse interstellar clouds: density n_H_ ~ 10-3000 cm^-3^ and an ultraviolet radiation field 0.1-50 times as intense as mean Galactic starlight. The metal-to-hydrogen ratio in the gas is found to be of the order of 0.05 times the solar value. The lines of the Lyman and Werner systems of molecular hydrogen lie below the atmospheric limit and cannot be observed from the ground. We synthesize the H_2_ spectrum and predict that if its column density exceeds 10^14^ cm^-2^ it should be detectable with the Space Telescope and would provide further constraints on physical conditions in the cloud, especially with respect to the dust-to-gas ratio and the temperature. We search unsuccessfully for absorption by carbon monoxide and infer that its abundance relative to that of neutral, atomic hydrogen is at least one order of magnitude lower than that found in diffuse or thick Galactic interstellar clouds. Because of the complexities of the CO photochemistry, however, this limit does not provide a useful constraint on our models.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 1988
- DOI:
- 10.1086/166950
- Bibcode:
- 1988ApJ...335..584C
- Keywords:
-
- H I Regions;
- Intergalactic Media;
- Lyman Alpha Radiation;
- Quasars;
- Abundance;
- Line Spectra;
- Red Shift;
- Signal To Noise Ratios;
- Astrophysics;
- ABUNDANCES;
- COSMOLOGY;
- GALAXIES: INTERGALACTIC MEDIUM;
- QUASARS