A MERLIN survey of 4.7-GHz excited OH masers in star-forming regions

Phase-referenced observations of 13 star-forming regions in the  ²Π_1/2, J = 1/2  transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of  ∼10⁷ K  at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north–south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H  ii region and has a brightness temperature of  ∼4 × 10⁵ K  . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc.     

Probing subparsec structure in the Lyman α forest with gravitational microlensing

We present the results of microlens ray-tracing simulations showing the effect of absorbing material between a source quasar and a lensing galaxy in a gravitational lens system. We find that, in addition to brightness fluctuations due to microlensing, the strength of the absorption line relative to the continuum varies with time, with the properties of the variations depending on the structure of the absorbing material. We conclude that such variations will be measurable via ultraviolet spectroscopy of image A of the gravitationally lensed quasar Q2237+0305 if the Lyman α clouds between the quasar and the lensing galaxy possess structure on scales smaller than ∼0.1 pc. The time-scale for the variations is on the order of years to decades, although very short-term variability can occur. While the Lyman α lines may not be accessible at all wavelengths, this approach is applicable to any absorption system, including metal lines.     

A search for the 53-MHz OH line near G48.4−1.4 using the National MST Radar Facility

We present the results of a search for the ground-state hyperfine transition of the OH radical near 53 MHz using the National Mesosphere–Stratosphere–Troposphere (MST) Radar Facility at Gadanki, India. The observed position was G48.4−1.4 near the Galactic plane. The OH line is not detected. We place a 3σ upper limit for the line flux density at 39 Jy from our observations. We also did not detect recombination lines (RLs) of carbon, which were within the frequency range of our observations. The 3σ upper limit of 20 Jy obtained for the flux density of carbon RLs, along with observations at 34.5 and 327 MHz, are used to constrain the physical properties of the line-forming region. Our upper limit is consistent with the line emission expected from a partially ionized region with electron temperature, density and path lengths in the range 20–300 K, 0.03–0.3 cm⁻³ and 0.1–170 pc, respectively.     

The 21-cm forest

We examine the prospects for studying the pre-reionization intergalactic medium (IGM) through the so-called 21-cm forest in spectra of bright high-redshift radio sources. We first compute the evolution of the mean optical depth τ for models that include X-ray heating of the IGM gas, Wouthuysen–Field coupling, and reionization. Under most circumstances, the spin temperature T _S grows large well before reionization begins in earnest; this occurs so long as the X-ray luminosity of high-redshift starbursts (per unit star formation rate) is comparable to that in nearby galaxies. As a result,  τ≲ 10⁻³  throughout most of reionization, and background sources must sit well beyond the reionization surface in order to experience absorption that is measurable by square-kilometre class telescopes. H  ii regions produce relatively large 'transmission gaps' and may therefore still be observable during the early stages of reionization. Absorption from sheets and filaments in the cosmic web fades once T _S becomes large and should be rare during reionization. Minihaloes can produce strong (albeit narrow) absorption features. Measuring their abundance would yield useful limits on the strength of feedback processes in the IGM as well as their effect on reionization.     

kSZ from patchy reionization: The view from the simulations

We present the first calculation of the kinetic Sunyaev–Zel’dovich (kSZ) effect due to the inhomogeneus reionization of the universe based on detailed large-scale radiative transfer simulations of reionization. The resulting sky power spectra peak at ℓ = 2000–8000 with maximum values of [ℓ(ℓ + 1)C_ℓ/(2π)]_max  4–7 × 10 ⁻¹³. The scale roughly corresponds to the typical ionized bubble sizes observed in our simulations, of 5–20 Mpc. The kSZ anisotropy signal from reionization dominates the primary CMB signal above ℓ = 3000. At large-scales the patchy kSZ signal depends only on the source efficiencies. It is higher when sources are more efficient at producing ionizing photons, since such sources produce larger ionized regions, on average, than less efficient sources. The introduction of sub-grid gas clumping in the radiative transfer simulations produce significantly more power at small-scales, but has little effect at large-scales. The patchy reionization kSZ signal is dominated by the post-reionization signal from fully-ionized gas, but the two contributions are of similar order at scales ℓ  3000 − 10⁴, indicating that the kSZ anisotropies from reionization are an important component of the total kSZ signal at these scales.