Lyman cloud

In gas clouds containing one or more hot stars (7 cn > 30 000 K), hydrogen atoms are ionized by the stellar UV radiation in the Lyman continuum and recombine to excited levels their decay gives rise to observable emission lines such as the Balmer series (see, for example, Fig. 3.22). Examples are planetary nebulae (PN), which are envelopes of evolved intermediate-mass stars in process of ejection and... 

Fig. 12.15. Growth of metallicity with time in regions of differing overdensity, corresponding approximately to clusters of galaxies (5 = 103 thick curve), DLA and Lyman-limit systems (5 = 102 thinner curve), moderate-column-density Ly-a clouds (5 = 10 dotted curve) and low-column-density Ly-a clouds (8 = 0 dashed curve), The dot-dashed curve shows the global average. After Cen and Ostriker (1999). Courtesy Renyue Cen.

Analysis is carried out on certain types of clouds, called damped Lyman alpha systems (or DLA systems). These clouds are essentially neutral, with high column densities, of the order of 10 ° atoms per square centimetre. 

In fact, the D/H has been recently measured in several high-z hydrogen clouds. These hydrogen clouds are seen by their distinctive Lyman-alpha features in the spectra of several QSOs. Here, we consider the new results reported by Kirkman et al. (2003) they announce the detection of D in an absorption system at z=2.526 in the spectrum of Q1243+3047 they show how they have improved the exploration of some factors which determine the measurement errors. We adopt their best estimate of the primordial D/H values towards five QSOs ... 

Atomic carbon plays an important role in both heating and cooling of interstellar clouds. Because its ionization potential (11.3 eV) is less than the Lyman... 

The ionization limit of C is considerably lower than that of hydrogen (see Table 1) on the other hand in interstellar space the radiation density beyond of the Lyman continuum is high enough to ionize most carbon. The transition region between the CII region and the Cl region is relatively sharp. Cl regions occur only in the inner part of dense clouds (see Section II. D). 

A couple more "warm" candidates probably go about here, including mirror or shadow neutrinos or majorons (ApOl, Sect. 12.5). These must have masses in excess of 0.25 - 0.4 keV or something bad will come down the chimney (probably reionization that smears out the Lyman alpha forest clouds). A 40 eV neutrino popped up in Ap97 (Sect. 12.2), and it was not clear how to avoid having so many of these that the universe would fold up into Pauli s pocket. (See Ap02 for generic objections to warm DM of any sort.)... 

The question of loss of metals from galaxies is profound because of the existence of metals in low column density Lyman-a forest systems (Ellison et al. 1999), which are probably gas clouds residing outside of galaxies. Where the heavy elements came from in these systems is still a mystery it is possible that they were seeded with elements... 

The SFR Distribution is obtained from various tracers (Lyman-a continuum, pulsars, SN remnants, molecular clouds) and shows that the SFR increases with decreasing galactocentric distance reaching a peak at 4-6 kpc in correspondence of the gas peak. 

A level of metal enrichment of 10-3 to 10 2 of solar in regions of the IGM with N(H I) > 1014 cm-2 may still be understood in terms of supernova driven winds from galaxies. The work of Aguirre et al. (2001) shows that such outflows which, as we shall shortly see ( 4.5) are observed directly in Lyman break galaxies at z = 3, may propagate out to radii of several hundred kpc before they stall. However, if O VI is also present in Lya forest clouds of lower column density, as claimed by Schaye et al. (2000), an origin in pregalactic stars at much earlier epochs is probably required (Madau, Ferrara, Rees 2001). 

ABSTRACT. Recent work on radiative processes and collisional excitation in molecular Hydrogen and its deuterated isotopic substitute and in molecular Carbon is reviewed. Particular attention is drawn to non-adiabatic coupling effects on the intensities of Lyman and Werner band systems of the vacuum ultraviolet spectrum of Hj and to the role of nuclear spin on ortho-para transitions in Hj due to collisions. The inter-relation between those processes and state to state chemistry is stressed out. We discuss the implications of these new data in a recent comprehensive model of diffuse interstellar clouds (Viala et al., 1987). 

As discussed in Sec. 1.1, cosmic-ray ionisation of H2 produces secondary electrons which excite H2 and cause it to fluoresce in the Lyman and Werner bands. Since cosmic rays penetrate cold cores easily, these UV photons are generated internally throughout the cloud and are not subject to the levels of extinction suffered by externally-generated photons. In principle these photons can both dissociate gas-phase molecules and photodesorb mantle material the latter process is known as cosmic ray-induced photodesorption. 

---