Far-Infrared and Submillimeter Observations of High Redshift Galaxies

Observations at far-infrared and submillimeter wavelengths promise to revolutionize the study of high redshift galaxies and AGN by providing a unique probe of the conditions within heavily extinguished regions of star formation and nuclear activity. Observational capabilities in this spectral region will expand greatly in the next decade as new observatories are developed both in space and on the ground. These facilities include the Space Infrared Telescope Facility (SIRTF), the far-infrared and submillimeter telescope (FIRST), and the millimeter array (MMA). In the longer term, the requirements of high angular resolution (comparable to that of HST), full wavelength coverage, and high sensitivity (approaching the fundamental limit imposed by photon counting statistics) will motivate the development of far-IR and submillimeter space interferometry using cold telescopes and incoherent detector arrays.Comment: 10 pages, LaTeX, including 5 postscript figures, and requiring aipproc.sty and epsfig.sty. To appear in the proceedings of the 9th Annual October Astrophysics Conference in Maryland, ``After the Dark Ages: When Galaxies were Young", edited by S. S. Holt and E. P. Smit

Collisional excitation of far-infrared line emissions from warm interstellar carbon monoxide (CO)

Motivated by recent observations with Herschel/PACS, and the availability of new rate coefficients for the collisional excitation of CO (Yang et al. 2010), the excitation of warm astrophysical CO is revisited with the use of numerical and analytic methods. For the case of an isothermal medium, results have been obtained for a wide range of gas temperatures (100 to 5000 K) and H2 densities (1E+3 to 1E+9 cm-3), and presented in the form of rotational diagrams, in which the logarithm of the column density per magnetic substate, log (N[J]/g[J]), is plotted for each state, as a function of its energy, E[J]. For rotational transitions in the wavelength range accessible to Herschel/PACS, such diagrams are nearly linear when n(H2) > 1E+8 cm-3. When log10(n[H2]) = 6.8 to 8, they exhibit significant negative curvature, whereas when log10(n[H2]) < 4.8 the curvature is uniformly positive throughout the PACS-accessible range. Thus, the observation of a positively-curved CO rotational diagram does not NECESSARILY require the presence of multiple temperature components. Indeed, for some sources observed with Herschel/PACS, the CO rotational diagrams show a modest positive curvature that can be explained by a single isothermal component. Typically, the required physical parameters are H2 densities in the 1E+4 to 1E+5 cm-3 range and temperatures, T, close to the maximum at which CO can survive. Other sources exhibit rotational diagrams with more curvature than can be accounted for by a single temperature component. For the case of a medium with a power-law distribution of gas temperatures, with dN/dT proportional to T to the power -b, results have been obtained for H2 densities 1E+3 to 1E+9 cm-3 and power-law indices, b, in the range 1 to 5; such a medium can account for a CO rotational diagram that is more positively curved than any resulting from an isothermal medium.Comment: Accepted for publication in the Astrophysical Journa

Lyman alpha radiation in external galaxies

The Ly alpha line of atomic hydrogen is often a luminous component of the radiation emitted by distant galaxies. Except for those galaxies which have a substantial central source of non-stellar ionizing radiation, most of the Ly alpha radiation emitted by galaxies is generated within regions of the interstellar medium which are photoionized by starlight. Conversely, much of the energy radiated by photoionized regions is carried by the Ly alpha line. Only hot, massive stars are capable of ionizing hydrogen in the interstellar medium which surrounds them, and because such stars are necessarily short-lived, Ly alpha emission traces regions of active star formation. Researchers argue that the strength of the Ly alpha emission observed from external galaxies may be used to estimate quantitatively the dust content of the emitting region, while the Ly alpha line profile is sensitive to the presence of shock waves. Interstellar dust particles and shock waves are intimately associated with the process of star formation in two senses. First, both dust particles and shock waves owe their existence to stellar activity; second, they may both serve as agents which facilitate the formation of stars, shocks by triggering gravitational instabilities in the interstellar gas that they compress, and dust by shielding star-forming molecular clouds from the ionizing and dissociative effects of external UV radiation. By using Ly alpha observations as a probe of the dust content in diffuse gas at high redshift, we might hope to learn about the earliest epochs of star formation