HI 21cm Absorption at z∼2.347 towards PKS B0438-436
N. Kanekar, R. Subrahmanyan, S. L. Ellison, W. M. Lane, J. N. Chengalur
Abstract
We report the detection of redshifted HI~21cm absorption in the
z∼2.347
damped Lyman-
α
absorber (DLA) towards PKS B0438-436, with the Green Bank Telescope. This is the second-highest redshift at which 21cm absorption has been detected in a DLA. The absorption extends over
∼60
km/s and has two distinct components, at
z=2.347477(12)
and
z=2.347869(20)
. A similar velocity structure is seen in optical metal lines, although the peak absorption here is offset by
∼11
km/s from the peak in the 21cm line. We obtain a high spin temperature
T
s
∼(886±248)×(f/0.58)
K, using a covering factor estimated from 2.3 GHz VLBI data. However, the current data cannot rule out a low spin temperature. The non-detection of CO 3-2 absorption places the upper limit
N
CO
<3.8×
10
15
×(
T
x
/10)
cm
−2
on the CO column density.