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We study the properties of HI gas in the outer regions (~2r_25) of a spiralgalaxy, UGC 00439 (z = 0.01769), using HI 21-cm absorption towards differentcomponents of an extended background radio source, J0041$-$0043 (z = 1.679).The radio source exhibits a compact core coincident with the optical quasar andtwo lobes separated by ~7 kpc, all at an impact parameter ~25 kpc. The HI 21-cmabsorption detected towards the southern lobe is found to extend over ~2 kpc^2.The absorbing gas shows sub-kpc-scale structures with the line-of-sightvelocities dominated by turbulent motions. Much larger optical depth variationsover 4-7 kpc-scale are revealed by the non-detection of HI 21-cm absorptiontowards the radio core and the northern lobe, and the detection of NaI and CaIIabsorption towards the quasar. This could reflect a patchy distribution of coldgas in the extended HI disc. We also detect HI 21-cm emission from UGC 00439and two other galaxies within ~150 kpc to it, that probably form an interactinggroup. However, no HI 21-cm emission from the absorbing gas is detected.Assuming a linear extent of ~4 kpc, as required to cover both the core and thesouthern lobe, we constrain the spin temperature <~300 K for the absorbing gas.The kinematics of the gas and the lack of signatures of any ongoing in situstar formation are consistent with the absorbing gas being at the kinematicalminor axis and corotating with the galaxy. Deeper HI 21-cm observations wouldhelp to map in greater detail both the large- and small-scale structures in theHI gas associated with UGC 00439.

Mapping kiloparsec-scale structures in the extended HI disc of the galaxy UGC 00439 by HI 21-cm absorption