Observations of interstellar dust are often used as a proxy for total gas column density N _H. By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS, with accurate (opacity-corrected) H i column densities and newly published OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ ₃₅₃, reddening E(B − V), and the total proton column density N _H in the range (1–30) × 10²⁰ cm⁻², along sightlines with no molecular gas detections in emission. We derive an N _H/E(B − V) ratio of (9.4 ± 1.6) × 10²¹ cm⁻² mag⁻¹ for purely atomic sightlines at ∣ b ∣ > 5 ° , which is 60% higher than the canonical value of Bohlin et al. We report a ∼40% increase in opacity σ ₃₅₃ = τ ₃₅₃/N _H, when moving from the low column density (N _H < 5 × 10²⁰ cm⁻²) to the moderate column density (N _H > 5 × 10²⁰ cm⁻²) regime, and suggest that this rise is due to the evolution of dust grains in the atomic interstellar medium. Failure to account for H i opacity can cause an additional apparent rise in σ ₃₅₃ of the order of a further ∼20%. We estimate molecular hydrogen column densities N H 2 from our derived linear relations, and hence derive the OH/H₂ abundance ratio of X _OH ∼ 1 × 10⁻⁷ for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N H 2 in the range N H 2 ∼ (0.1−10) × 10²¹ cm⁻². This suggests that OH may be used as a reliable proxy for H₂ in this range, which includes sightlines with both CO-dark and CO-bright gas.

We report three new FRBs discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), namely FRB 181017.J0036+11, FRB 181118, and FRB 181130, through the Commensal Radio Astronomy FAST Survey (CRAFTS). Together with FRB 181123, which was reported earlier, all four FAST-discovered FRBs share the same characteristics of low fluence (≤0.2 Jy ms) and high dispersion measure (DM, >1000 pc cm⁻³), consistent with the anticorrelation between DM and fluence of the entire FRB population. FRB 181118 and FRB 181130 exhibit band-limited features. FRB 181130 is prominently scattered (τ _s ≃ 8 ms) at 1.25 GHz. FRB 181017.J0036+11 has full-bandwidth emission with a fluence of 0.042 Jy ms, which is one of the faintest FRB sources detected so far. CRAFTS has started to build a new sample of FRBs that fills the region for more distant and fainter FRBs in the fluence–DM_E diagram, previously out of reach of other surveys. The implied all-sky event rate of FRBs is 1.24 − 0.90 + 1.94 × 10 5 sky⁻¹ day⁻¹ at the 95% confidence interval above 0.0146 Jy ms. We also demonstrate here that the probability density function of CRAFTS FRB detections is sensitive to the assumed intrinsic FRB luminosity function and cosmological evolution, which may be further constrained with more discoveries.

Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas because it forms along with H₂ under similar conditions and forms within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J = 1–0 transitions of ¹²CO, ¹³CO, and C¹⁸O, and the J = 2–1 transition of ¹²CO. The excitation temperatures of OH were found to follow a modified lognormal distribution the peak of which is close to the temperature of the Galactic emission background (CMB+synchrotron). In fact, 90% of the OH has excitation temperatures within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty of mapping this abundant molecule in emission. The opacities of OH were found to be small and to peak around 0.01. For gas at intermediate extinctions (A _V ∼ 0.05–2 mag), the detection rate of OH with a detection limit N(OH) ≃ 10¹² cm⁻² is approximately independent of A _V . We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of “dark molecular gas (DMG).” The measured fraction of DMG depends on the assumed detection threshold of the CO data set. The next generation of highly sensitive low-frequency radio telescopes, such as FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas through decomposing, in velocity, the molecular (e.g., OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.

Stephan’s Quintet (SQ, co-moving radial distance = 85 ± 6 Mpc, taken from the NASA/IPAC Extragalactic Database (NED)¹) is unique among compact groups of galaxies^2–12. Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have probably generated tidal debris in the form of multiple gaseous and stellar filaments^6,8,13, the formation of tidal dwarfs^7,14,15 and intragroup-medium starbursts¹⁶, as well as widespread intergalactic shocked gas^5,10,11,17. The details and timing of the interactions and collisions remain poorly understood because of their multiple nature^18,19. Here we report atomic hydrogen (H i) observations in the vicinity of SQ with a smoothed sensitivity of 1σ = 4.2 × 10¹⁶ cm⁻² per channel (velocity bin-width Δv = 20 km s⁻¹; angular resolution = 4′), which are about two orders of magnitude deeper than previous observations^8,13,20,21. The data show a large H i structure (with linear scale of around 0.6 Mpc) encompassing an extended source of size approximately 0.4 Mpc associated with the debris field and a curved diffuse feature of length around 0.5 Mpc attached to the south edge of the extended source. The diffuse feature was probably produced by tidal interactions in early stages of the formation of SQ (>1 Gyr ago), although it is not clear how the low-density H i gas (N_{H i}≲ 10¹⁸ cm⁻²) can survive the ionization by the intergalactic ultraviolet background on such a long time scale. Our observations require a rethinking of properties of gas in outer parts of galaxy groups and demand complex modelling of different phases of the intragroup medium in simulations of group formation.

Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be underabundant and/or subthermally excited. We performed one of the most sensitive (down to τ rms CO ∼ 0.002 and τ rms HCO + ∼ 0.0008 ) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to A v < 0.32 mag and HCO⁺ was detected down to A v < 0.2 mag , where atomic gas and dark molecular gas start to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures T _ex of 4.1 and 2.7 K, for CO and for HCO⁺, respectively, which are close to the cosmic microwave background and explain their being undercounted in emission surveys. A stronger linear correlation was found between N HCO + and N H 2 (Pearson correlation coefficient P ∼ 0.93) than that of N _CO and N H 2 (P ∼ 0.33), suggesting HCO⁺ is a better tracer of H₂ than CO in diffuse gas. The derived CO-to-H₂ conversion factor (the CO X-factor) of (14 ± 3) × 10²⁰ cm⁻² (K km s − 1 )⁻¹ is approximately six times larger than the average value found in the Milky Way.