Physical conditions in Damped Lyman alpha Systems.

摘要

The primary goal of this thesis is to determine physical conditions in high redshift galaxies known as Damped Lymna alpha Systems. To this end, we present several key results. First, we discuss a radio-selected quasar survey that demonstrates that dust bias is likely not an issue affecting surveys for DLAs. In other words, we are seeing the true and complete population of DLAs and not missing some very high column density, dusty DLAs. Second, we use neutral carbon (CI) fine structure states to show the presence of gas with densities and pressures 1--2 orders of magnitude larger than those expected in the global DLA, i.e. the presence of knots of dense cold gas existing in the larger diffuse DLA. To solve the imbalance of pressure between these two phases, we speculate that these regions are created by shocks that provide relatively high densities that in turn allow for the existence of CI. Alternatively, these could be photodissociation regions on the edges of even more dense molecular clouds that could even be hosting star formation. This model would explain the presence of a radiation field generally above the background. We also present unambiguous evidence of the presence of cold gas in DLAs---the discovery of narrow sub-resolution, sub-1 km/s CI components with thermal temperatures < ∼100 K. In the third section we summarize 10+ years of high resolution observations of DLAs that reveal a bimodality in their implied cooling rate, lc. This cooling rate is linked to the star formation rate via the assumption of thermal equilibrium. The bimodality shows that the two populations, the "high-cool" and the "low-cool" DLAs may be representative of two types of DLAs---the high-cool DLAs are heated by a centrally located core of star formation, i.e. a bright, star-forming Lyman Break Galaxy (LBG), while the low-cool DLAs are not hosting a central LBG. Finally, we discuss our discovery of a strong (84 muG) magnetic field at a redshift of z ∼ 0.7. This is unexpected given the current theories of dynamo formation of magnetic fields (for time scale arguments) and also because the general magnetic field permeating the Milky Way is more than order of magnitude smaller at ∼ 6 muG. While this single detection could be a unique event caused by some unique geometry, it certainly inspires further observations of other objects to test whether large magnetic fields could possibly be a general feature of high redshift galaxies. In either case, it spawns the idea that magnetic fields likely play a larger role in galaxy formation and cosmology than has been previously appreciated.