Column Density Distribution Function Research Articles

HI Column Density Distribution Function at z = 0: Connection to Damped Lyα Statistics

AbstractWe present a measurement of the HI column density distribution function f(Nhi) at the present epoch for column densities > 1020 cm−2. These high column densities compare to those measured in damped Lyα lines seen in absorption against background quasars. Although observationally rare, it appears that the bulk of the neutral gas in the Universe is associated with these damped Lyα systems. In order to obtain a good anchor point at z = 0 we determine f(Nhi) in the local Universe by using 21 cm synthesis observations of a complete sample of spiral galaxies. We show that f(Nhi) for damped Lyα systems has changed significantly from high z to the present and that change is greatest for the highest column densities. The measurements indicate that low surface brightness galaxies make a minor contribution to the cross section for HI, especially for NHI > 1021 cm−2.

APM z ≳ 4 survey: distribution and evolution of high column density H i absorbers

Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2.8<z(abs)<4.4 (8 with z(abs)>3.5). High resolution echelle spectra (0.8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4.383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0.008 < z < 4.7 to study the redshift evolution and the column density distribution function for absorbers with log N(HI)>=17.2. In the HI column density distribution f(N)=kN^-beta we find evidence for breaks in the power law, flattening for 17.2< log N(HI)<21 and steepening for log N(HI)>21.2. The column density distribution function for the data with log N(HI)>=20.3 is better fit with the form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N(z) for z>3.5.

Uncertainties in the interpretation of the Lyman alpha forest lines

We study the formation of hydrogen HI Lyα forest lines in turbulent media accounting for the effects of a finite correlation length, l, in the velocity field. This is in contrast to the usual procedure, in which line formation is treated in the microturbulent limit (l≡0). Our results show clearly that a finite velocity correlation length (l > 0) affects strongly (i) the profile of the Lyα line, (ii) its equivalent width, and (iii) the intensity ratios of Lyα and higher order Lyman lines. We argue that the column density distribution function f(N) has to be reexamined. A deficit of absorption systems revealed in the Lyα forest in the range 1019 cm−2≤N ≤ 1016 cm−2 may simply be an artifact.

Evidence for structure in the H I column density distribution of QSO absorbers

The H I column density distribution function of QSO absorption line systems is investigated using recent data with high spectral resolution, and extensive surveys of the Lyman limit systems and damped Ly-alpha systems. The hypothesis that the differential distribution function is fitted by a single power law is rejected at the 99 percent confidence level. A double power law, with a break at N(H I) = 10 exp 16/sq cm, also provides a poor fit over the range in which the sample is complete. While there are no discontinuities in the observed distribution, there is a clear flattening at N(H I) of about 10 exp 16/sq cm, compared to lower column densities. These observed features can be understood using models of photoionized clouds which are confined by an external pressure with density profiles governed by gravity. In particular, the flattening at N(H I) of about 10 exp 16/sq cm can be explained in terms of a transition between metal-poor and metal-rich systems.

High-redshift QSO absorbing clouds and the background ionizing source

Echelle spectra of the high-redshift QSO PKS 2000-330 have been analyzed to provide redshifts, H I column densities, and velocity dispersions of Lyman line absorbing systems in the redshift range of 3.02-3.75. The H I column density distribution function may not be well approximated by a power law, as had been suggested in earlier work, but instead shows a possible flattening towards lower values of the H I column density. For simple uniform cloud models, the redshift evolution of this distribution could in principle be used to infer the redshift dependence of the density of the gas within the clouds. It is found that the source of ionization of these clouds cannot be the integrated light from background quasars, if the quasar density distribution cuts off between z of about 2 and z of about 4 as is commonly thought. 25 references.