The Interstellar Medium

(Jayaram N. Chengalur, Swarna K. Ghosh, Nissim Kanekar, Nimisha Kantharia, Subhashis Roy, Narendra Nath Patra)

The life cycle of a galaxy consists of an interplay between its two main constituents, the stars and the interstellar medium (ISM), with the dark matter halo providing the background stage. Stars initially form by the collapse of molecular clouds in the ISM, and then pollute the ISM with metals, both during their life (via stellar winds) and especially towards the end of their life, via supernovae. These metals cause further cooling of the ISM gas, which gives rise to further star formation. Understanding physical conditions in the ISM of galaxies, and the evolution of the ISM, is hence a major area of research in astronomy and at NCRA-TIFR.
The ISM consists of various gas phases, including neutral atomic, ionized, and molecular gas, at different temperatures, pressures, and densities. The neutral atomic ISM is best probed with the hyperfine spectral line of neutral atomic hydrogen (HI) at a rest frequency of 1420.40575 MHz (21cm). NCRA-TIFR astronomers have long used this HI 21cm spectral transition to study physical conditions in the neutral gas in the Milky Way. For example, HI 21cm absorption studies with the GMRT have found evidence for a phase transition in neutral hydrogen in the Milky Way, above a critical HI surface density. At surface densities higher than this threshold, the hydrogen is present in both the cold and the warm phases (at temperatures ~100 K, and >1000 K, respectively); however, at surface densities below this threshold, the hydrogen appears to exist predominantly in the warm phase. Such GMRT HI 21cm absorption studies have also been used to find evidence that the standard two-phase models of neutral hydrogen may not be applicable in the Milky Way, as a significant fraction of the neutral gas appears to lie in the thermally unstable phase. NCRA-TIFR astronomers continue to use the HI 21cm transition as a critical tool to probe physical conditions in the Milky Way.
Besides HI 21cm studies, NCRA-TIFR astronomers have also used radio recombination lines to probe the temperature and density of ionized gas in the Milky Way. Studies of complex molecules like acetaldehyde have been used to determine the spatial extent of organic molecules, and to show that such organic molecules are widespread in the star-forming regions and not confined to tiny hot cores. Radio continuum studies of ionized regions around massive stars (HII regions) and supernova remnants have also been used to glean information about physical conditions in the ionized gas.

Recent Results
Detection of the Galactic warm neutral medium in HI 21cm absorption
Patra et al. report a deep GMRT search for Galactic HI 21-cm absorption towards the quasar B0438-436, yielding the detection of wide, weak HI 21-cm absorption, with a velocity-integrated HI 21-cm optical depth of 0.0188 +/- 0.0036 km/s. Comparing this with the HI column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 +/- 365 K, one of the highest measured in the Galaxy. This is consistent with most of the HI along the sightline arising in the stable warm neutral medium. The low-peak HI 21-cm optical depth towards B0438-436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the HI 21-cm absorption and emission spectra. This yields a gas kinetic temperature T_k <= (4910 +/- 1900) K, and a spin temperature T_s = (1000 +/- 345) K for the gas that gives rise to the HI 21-cm absorption. The GMRT data are consistent with the HI 21-cm absorption arising from either the stable WNM, with T_s << T_k, T_k~5000 K, and little penetration of the background Lyman-alpha radiation field into the neutral hydrogen, or the unstable neutral medium, with T_sT_k~1000 K. The figure shows results of the multi-Gaussian joint decomposition of the (A) HI 21-cm emission and (B) HI 21-cm absorption spectra. The top panels show the best-fit model (solid curve) overlaid on the two spectra, while the bottom two panels show the residuals from the fit.
The temperature of the warm neutral medium in the Milky Way
Roy et al. used deep, high velocity resolution HI 21cm absorption spectra towards 32 sources, obtained with the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT) to probe physical conditions in the Galactic neutral hydrogen (HI). The HI 21cm absorption spectra are sufficiently sensitive to detect HI 21cm absorption by the warm neutral medium (WNM). Comparing these spectra with HI 21 cm emission spectra from the Leiden-Argentine-Bonn (LAB) survey, Roy et al. show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the HI 21cm absorption spectra yielded very few components with linewidths in the temperature range of stable WNM, with no such WNM components detected for 16 of the 30 sightlines. Some of the detected HI 21cm absorption along 13 of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, the authors use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total HI column density, at least 28 per cent of the neutral hydrogen must have temperatures in the thermally unstable range (200-5000 K). The GMRT and WSRT data hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models. The figure shows the maximum kinetic temperature of the different components on each of the 30 sightlines, from the multi-Gaussian fits. The stable WNM temperature range (5000–8000 K) is indicated by the horizontal dashed lines. Components with temperatures consistent with stable WNM are indicated by open circles, while those definitely outside the above range (at >= 3 sigma significance) are shown as filled circles.
The temperature of the diffuse HI in the Milky Way - I. High resolution HI 21 cm absorption studies
Roy et al. used the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT) spectra to obtain deep, high velocity resolution HI 21cm absorption spectra of 32 compact extra-galactic sources. These are amongst the deepest HI 21cm absorption spectra ever obtained, with optical depth root-mean-square noise < 0.001 per 1 km/s velocity channel, sufficiently sensitive to detect HI 21cm absorption from the warm neutral medium along all sightlines. HI 21cm absorption was detected against all background sources but one, B0438-436. Roy et al. used the detected HI 21cm spectra to infer the spin temperature as a function of velocity along each sightline. On every sightline, the maximum spin temperature detected (at >= 3 sigma significance) is >= 1000 K, indicating that the warm neutral medium is being detected along most sightlines. This is by far the largest sample of Galactic HI 21 cm absorption spectra of this quality, providing a sensitive probe of physical conditions in the neutral atomic interstellar medium. The figure shows the HI 21cm emission spectrum (top), the HI 21cm absorption spectrum (middle), and the spin temperature spectrum (bottom) for six of the 32 targets. HI 21cm absorption is clearly detected against all sources.

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