We present a detailed study of an estimator of the HI column density, based on a combination of HI 21cm absorption and HI 21cm emission spectroscopy. This isothermal estimate is given by $N_{rm HI,ISO} = 1.823 times 10^{18} int left[ tau_{rm tot} tim
es {rm T_B} right] / left[ 1 - e^{-tau_{rm tot}} right] {rm dV}$, where $tau_{rm tot}$ is the total HI 21cm optical depth along the sightline and ${rm T_B}$ is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived $N_{rm HI,ISO}$ with the true HI column density $N_{rm HI}$. The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H{sc i} column density to the isothermal estimate, $N_{rm HI}/{N_{rm HI, ISO}}$, is within a factor of 2 of unity while the 68.2% confidence intervals are within a factor of $approx 3$ of unity, out to high HI column densities, $le 5 times 10^{23}$,cm$^{-2}$ per 1 km s$^{-1}$ channel, and high total optical depths, $le 1000$. The isothermal estimator thus provides a significantly better measure of the HI column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high HI column density gas in the Milky Way.
We report Westerbork Synthesis Radio Telescope and Arecibo Telescope observations of the redshifted satellite OH-18cm lines at $z sim 0.247$ towards PKS1413+135. The conjugate nature of these lines, with one line in emission and the other in absorpti
on, but with the same shape, implies that the lines arise in the same gas. The satellite OH-18cm line frequencies also have different dependences on the fine structure constant $alpha$, the proton-electron mass ratio $mu = m_p/m_e$, and the proton gyromagnetic ratio $g_p$. Comparisons between the satellite line redshifts in conjugate systems can hence be used to probe changes in $alpha$, $mu$, and $g_p$, with few systematic effects. The technique yields the expected null result when applied to Cen.A, a nearby conjugate satellite system. For the $z sim 0.247$ system towards PKS1413+135, we find, on combining results from the two telescopes, that $[Delta G/G] = (-1.18 pm 0.46) times 10^{-5}$ (weighted mean), where $G = g_p [mu alpha^2]^{1.85}$; this is tentative evidence (with $2.6 sigma$ significance, or at 99.1% confidence) for a smaller value of $alpha$, $mu$, and/or $g_p$ at z~0.247, i.e. at a lookback time of ~2.9 Gyrs. If we assume that the dominant change is in $alpha$, this implies $[Delta alpha /alpha ] = (-3.1 pm 1.2) times 10^{-6}$. We find no evidence that the observed offset might be produced by systematic effects, either due to observational or analysis procedures, or local conditions in the molecular cloud.
The OH molecule, found abundantly in the Milky Way, has four transitions at the ground state rotational level(J = 3/2) at cm wavelengths. These are E1 transitions between the F+ and F- hyperfine levels of the Lambda doublet of the J=3/2 state. There
are also forbidden M1 transitions between the hyperfine levels within each of the doublet states occuring at frequencies 53.171 MHz and 55.128 MHz. These are extremely weak and hence difficult to detect. However there is a possibility that the level populations giving rise to these lines are inverted under special conditions, in which case it may be possible to detect them through their maser emission. We describe the observational diagnostics for determining when the hyperfine levels are inverted, and identify a region near W44 where these conditions are satisfied. A high-velocity-resolution search for these hyperfine OH lines using the low frequency feeds on four antennas of the GMRT and the new GMRT Software Backend(GSB) was performed on this target near W44. We place a 3-sigma upper limit of ~17.3 Jy (at 1 km/s velocity resolution) for the 55 MHz line from this region. This corresponds to an upper limit of 3 X 10^8 for the amplification of the Galactic synchrotron emission providing the background.
We report the detection of OH 1667 MHz and wide HI 21cm absorption at $z sim 0.67$ towards the red quasar 1504+377, with the Green Bank Telescope and the Giant Metrewave Radio Telescope. The HI 21cm absorption extends over a velocity range of $sim 60
0$ km/s blueward of the quasar redshift ($z=0.674$), with the new OH 1667 MHz absorption component at $sim -430$ kms, nearly coincident with earlier detections of mm-wave absorption at $z sim 0.6715$. The atomic and molecular absorption appear to arise from a fast gas outflow from the quasar, with a mass outflow rate ${dot M} sim 12 M_odot$ yr$^{-1}$ and a molecular hydrogen fraction $f_{rm H_2} equiv (N_{rm H_2}/N_{rm HI}) sim 0.2$. The radio structure of 1504+377 is consistent with the outflow arising due to a jet-cloud interaction, followed by rapid cooling of the cloud material. The observed ratio of HCO$^+$ to OH column densities is $sim 20$ times higher than typical values in Galactic and high-$z$ absorbers. This could arise due to small-scale structure in the outflowing gas on sub-parsec scales, which would also explain the observed variability in the HI 21cm line.
