Physical state of 21-cm absorber at Zabs ∼ 0.3 towards a radio-loud abs quasar J1443+0214

Abstract

Understanding the physical state of the interstellar medium of protogalaxy at high red- shift plays a key role in our understanding of galaxy formation. Although it is not possible to directly observe the highly redshifted galaxies, but their signature as absorp- tion line in the background quasar spectrum allow us to study them in both optical and radio regions. In this regard we study the quasar-galaxy pair (QGP) J1443+0214 with zqso =1.82, zabs = 0.3714. The foreground galaxy in this QGP is identified by the detec- tion of emission lines in the SDSS spectrum of QSO. The foreground galaxy is invisible otherwise. The QGP J1443+0214 was observed on 2012 June 2 (5.7h on source) with Giant Me- terwave Radio Telescope (GMRT) using a baseband bandwidth of 4.17 MHz split into 512 spectral channels centered on the redshifted 21-cm frequency of foreground galaxy. We reduce the data using Astronomical Image Processing System (AIPS) to obtain HI 21-cm absorption line. This reduction process consists of steps like loading the data into AIPS, editing and flagging the bad data, calibrating it through various steps of gain calibration, bandpass calibration and finally the self calibration. Further, we fit Gaussian components to the continuum and absorption line profile separately; then divide the continuum from multi-channel source data using Interactive Data Language (IDL). We estimate parameters like optical depth and spin temperature from this spectrum obtained.