Implications of 21-cm observations for damped Lyalpha systems

Abstract

We present Giant Metrewave Radio Telescope HI 21cm absorption observations, of candidate and confirmed damped Lyman-α systems. The derived spin temperatures T spin are high, in all cases, ∼ 1000 K or higher. We have also collated from the published literature a list of damped absorbers for which 21cm observations exist, and discuss the implications of the observations for the nature of these systems.A cross-comparison of the HI 21cm profiles (which trace the cold gas) with the low ionization metal profiles (which trace all the neutral HI, both cold and warm) shows that in all cases for which both spectra are available, the 21cm absorption coincides in velocity with the deepest metal line feature. This is consistent with models in which the deep metal line features arise from discrete clouds but not with models where the deepest features are the result of velocity crowding.We also find that the typical derived spin temperatures of damped Lyman-α systems are considerably higher than those in the Milky Way or nearby spiral galaxies. The only exceptions are systems which are known to be associated with the disks of spirals; these do, in fact, show low spin temperature. In a multi-phase medium the derived spin temperature is a weighted average of the temperatures of the individual phases. High apparent T spin values are hence to be expected from small, low metallicity objects since these objects should (as per existing theoretical models of the formation of a multi-phase ISM in the Milky Way and high redshift proto-galaxies) have a lower fraction of the cold phase in their ISM as compared to large galaxies. The high T spin is hence consistent with the observed low metallicities of damped Lyman-α absorbers as well as with recent findings that damped absorption is associated with a variety of galaxy types (as opposed to being confined to the disks of large spirals).Finally, although the number of systems for which observations are available is small, we suggest that the following trend may be identified : at low redshift, damped Lyman-α absorption arises from a range of systems, including spiral galaxy disks, while, at high redshift, absorption occurs predominantly in smaller systems.

The accretion history of the universe with the SKA

“…Using equation 1 from Chengalur & Kanekar (2000) with a spin temperature of T S = 100 K (see e.g. Beswick et al, 2004) and unity covering factor we find a target velocity-integrated optical depth τ dV ≈ 0.06 km s −1 .…”

Cii* Absorption in Damped Lyα Systems. II. A New Window on the Star Formation History of the Universe

“…This is supported by the excessive number of low-ion clouds spread across a velocity interval of $ 270 km s À1 (Ellison et al 2001); by comparison, the median velocity interval of DLAs is %100 km s À1 (PW97). Note that Chengalur & Kanekar (2000) would not have detected this configuration, since it would have an apparent optical depth ð21Þ ¼ ð10 pc=40 pcÞ 2 5 CNM ð21Þ < 0:01, which is less than the 2 upper limit of 0.022. Ellison et al (2001) carried out an imaging study of the field surrounding Q0201+11 that provides a test of the CNM hypothesis.…”

“…Whereas spin temperatures in nearby spirals are less than 300 K (Dickey & Lockman 1990), the spin temperatures in most DLAs exceed 500 K (Chengalur & Kanekar 2000). The discrepancy is greatest at z > 3, where several DLAs exhibit T s > 2000 K. Because the kinetic temperature, T, equals the spin temperature in most scenarios, the high values of T s have been interpreted as indicators of gas in the WNM rather than CNM phase (Carilli et al 1996;Kanekar & Chengalur 2001;Chengalur & Kanekar 2000). This poses an interesting dilemma, as C ii* absorption, which must arise in the CNM in most DLAs, is detected in two DLAs with high inferred spin temperatures.…”

Cii* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two‐Phase Medium

“…More specifically, we suppose that all low-ion transitions in DLAs, such as Ly 1215, Si ii 1527, Fe ii 1608, etc., arise in low-density gas with T % 8000 K. According to the Norman & Spaans (1997) hypothesis, this occurs because P < P min ; i.e., CNM gas does not exist in high-z DLAs. However, the detection of 21 cm absorption with spin temperatures T s < 600 K in Q0458À02 (Wolfe et al 1985) and T s < 1200 K in Q1331+17 (Wolfe & Davis 1979;Chengalur & Kanekar 2000) rules out a pure WNM and is consistent with the presence of CNM in some cases. As a result, we retain the two-phase hypothesis but assume that the CNM covering factor is so low that many sight lines miss the CNM phase and encounter only the WNM phase.…”