H [CSC]i[/CSC] 21 Centimeter Absorption in Two Low-Redshift Damped L[CLC]y[/CLC]α Systems

Abstract

We report the discovery of two low redshift HI 21cm absorbers, one at z = 0.2212 towards the z em = 0.630 quasar OI 363 (B0738+313), and the other at z = 0.3127 towards PKS B1127-145 (z em = 1.187). Both were found during a survey of MgII selected systems at redshifts 0.2 < z < 1 using the new UHF-high system at the Westerbork Synthesis Radio Telescope (WSRT). New HST/FOS observations also identify both systems as damped Lyα (DLa) absorbers. By comparing the column density from the DLa line with that from the HI 21cm line, we calculate the spin temperature, T s of the two systems. We find T s ≈ 1000 K for both of these low redshift absorbers.For the z = 0.3127 system towards PKS B1127-145, two galaxies have been previously identified with emission lines at the absorber redshift (Bergeron & Boissé, 1991), with the galaxy at a closer projected distance to the quasar assumed to be responsible for the absorption system. An ESO-NTT/EFOSC2 spectrum of a 3rd, fainter companion at 3.9 arcsec or 11 h −1 100 kpc from the line of sight of PKS 1127-145 reveals [OIII]4958 and 5007 at z = 0.3121 ± 0.0003. We consider this object the most likely to be responsible for the 21cm absorption, as it is much closer to the QSO sightline than the two galaxies identified by Bergeron & Boissé.

Implications of 21-cm observations for damped Lyalpha systems

“…This small size implies that the covering factor of the absorbing system is likely to be close to unity. Assuming a covering factor of unity results in a spin temperature Ts of ∼ 910 ± 160 K, consistent within the error bars with the value of 1000 ± 200 K obtained by Lane et al (1998).…”

“…In the case of 1451-375, the milli-arcsecond structure is not known. It does, however, have an unresolved sub-arcsecond sized core, with an inverted spectrum and a total flux of 1.1 Jy at 1.4 GHz, If one assumes that the absorber completely covers this central core, the 3σ lower limit to the spin temperature is ∼ 1400 K. PKS 1127-145 : The 21cm absorption line system towards PKS 1127-145 was discovered by Lane et al (1998), using the WSRT. The observations, however, did not resolve out the line and were also corrupted by low-level interference close to the line location.…”

“…The channel spacing is ∼ 2 km/s and the RMS noise after a single Hanning smoothing is ∼ 3.5 mJy. The broad feature detected by Lane et al (1998) breaks up into a number of narrow components. There is also an entirely new component, ∼ -50 km s −1 from z = 0.3127, not detected in the WSRT data.…”

“…Interestingly, 1127 − 145 is the only system which has both a large 21cm velocity spread and a high temperature; all other systems with large velocity widths are identified with low Ts spirals. There are, however, at least three galaxies at the redshift of the damped Lyman-α absorber (Lane et al 1998). It is possible that they are interacting and that some of the HI in this system is associated with tidal features and has a high spin temperature.…”

“…Wolfe & Davis 1979, Wolfe, Briggs & Jauncey 1981, Wolfe et al 1985, Carilli et al 1996, Lane et al 1998, Chengalur & Kanekar 1999. Observing the 21cm line yields a number of interesting physical parameters.…”