Pedro Salas

The ionized gas in the Milky Way is a major component of the interstellar medium. Observations of extinction-free tracers, such as hydrogen radio recombination lines (HRRLs), have revealed the presence of a dense (electron density = 1–100 cm ⁻³ ) warm ionized medium. Motivated by advances in radio instrumentation, the existence of fully sampled HRRL maps, and a better knowledge about the population of discrete H ii regions in our Galaxy, we have acquired new low-frequency ( ν ≲ 1 GHz) observations of HRRLs to characterize the properties of this gas. We target three positions in the Galactic plane, with few or no known H ii regions, using the 342 and 800 MHz feeds of the Green Bank Telescope (GBT). We detect HRRL emission from all three positions. We combine these with the fully sampled HRRL 5.8 GHz cubes from the GBT Diffuse Ionized Gas Survey to determine the gas properties using a forward modeling approach. From our analysis we find electron densities between 6 and 15 cm ⁻³ , and that to determine the gas temperature and emission measure, we require informative priors or higher signal-to-noise ratio observations.

We present the first detection of helium radio recombination lines (HeRRLs) at 800 MHz from the diffuse ionized gas (DIG) in the Cygnus X region, with an ionized helium-to-hydrogen ratio ( n H e + / n H + ) of 0.06 ± 0.02, consistent with values observed in Galactic H ii regions ionized by O6 stars or earlier. The data were obtained from a multifrequency RRL survey (342, 800, and 5800 MHz) using the Robert C. Byrd Green Bank Telescope, toward a region centered at l = 80 . ° 0 , b = 1 . ° 4 , with an extent of ± 2 . ° 85 . Several techniques were applied to mask out RRL emission from H ii regions, and the remaining data were averaged to achieve the sensitivity required to detect HeRRLs from the DIG. Analysis of the pixel-to-pixel hydrogen line central velocities suggests that the 342 and 800 MHz line-forming regions are associated. Modeling of the averaged data indicates that the emission at these frequencies is dominated by an ionized component with an electron density of 2–15 cm ⁻³ and an EM of 750–1600 pc cm ⁻⁶ for an electron temperature of 8000 K. The derived properties of the DIG in Cygnus X are consistent with those reported for diffuse gas in the inner Galaxy. Previous attempts to detect HeRRLs in the inner Galaxy only yielded upper limits of 0.013, much lower than the value found in Cygnus X. Our detection has significant implications, including the potential for an additional hydrogen ionization source and changes in dust properties in the inner Galaxy.

Understanding the transition from atomic gas to molecular gas is critical to explain the formation and evolution of molecular clouds. However, the gas phases involved, cold H i and CO-dark molecular gas, are challenging to directly observe and physically characterize. We observed the Cygnus X star-forming complex in carbon radio recombination lines (CRRLs) at 274–399 MHz with the Green Bank Telescope at 48 ′ (21 pc) resolution. Of the 30 deg² surveyed, we detect line-synthesized C273α emission from 24 deg² and produce the first large-area maps of low-frequency CRRLs, which likely originate in CO-dark molecular gas. The morphology of the C273α emission reveals arcs, ridges, and extended possibly sheet-like gas, which are often found on the outskirts of CO emission. We find a correlation between velocity-integrated C273α and the 8 μm intensity with a power-law slope of 1.3 ± 0.2. We interpret the relation as the dependence of cool dark gas emission on the far-ultraviolet radiation field, G ₀ ≈ 40−160. We determine the typical angular separation between C273α and ¹³CO emission to be 12 pc. Velocity differences between C273α and ¹³CO are apparent throughout the region and have a typical value of 2.9 km s⁻¹. We estimate gas densities of n ≈ 20−900 cm⁻³ with a nominal n ≈ 400 cm⁻³ in the C⁺/H₂ layer. The evolution of the C273α gas seems to be dominated by turbulent pressure, with a characteristic timescale to form H₂ of about 2.6 Myr. These observations underline the richness of low-frequency CRRLs to provide revelatory insights into the characteristics of (CO-)dark gas and the evolution of molecular gas.

Context. Carbon radio recombination lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Observations of these lines allow for physical parameters of this phase to be measured. Aims. We observed CRRLs with the recently commissioned New Extension in Nançay Upgrading LOFAR (NenuFAR) telescope towards two of the brightest sources at low-frequency (10–85 MHz): Cassiopeia A and Cygnus A (hereafter, Cas A and Cyg A, respectively). We then measured the density, n_e, and temperature, T_e, of the electrons in line-of-sight clouds. Methods. We used NenuFAR’s beam-forming mode and integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a reduction pipeline primarily aimed at removing the radio frequency interference (RFI) contamination and correcting the baselines. We then performed a first fitting of the spectral lines observed in absorption associated with the line-of-sight clouds. Results. Cas A is the brightest source in the sky at low frequencies and represents an appropriate test bench for this new telescope. On this source, we detected 398 Cα lines between the principal quantum numbers n = 426 and n = 826. Cyg A is also a bright source, however, the Cα lines were observed to be fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. For both sources, we reached a significantly higher signal-to-noise ratio (S/N) and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of the spectral line widths with the electronic quantum number provides constraints on the physical properties of the clouds: T_e, n_e, and the temperature, T₀, of the radiation field, the mean turbulent velocity, ν_t, and the typical size of the cloud. Conclusions. Our final constraints differ from those inferred from LOFAR results, with ∼50% lower T_e, ∼35% lower n_e, and from 10 to 80% higher ν_t, on average. The NenuFAR observations sample a larger space volume than LOFAR’s towards the same sources due to the differences in instrumental beam sizes. These discrepancies highlight the sensitivity of low-frequency CRRLs as probes of the diffuse ISM, paving the way towards large area surveys of CRRLs in our Galaxy.

We present a study of hydrogen, helium, and carbon millimeter-wave radio-recombination lines (RRLs) toward 10 representative positions throughout the Orion Nebula complex, using the Yebes 40 m telescope in the Q band (31.3 GHz to 50.6 GHz) at an angular resolution of about 45″ (~0.09 pc). The observed positions include the Orion Nebula (M42) with the Orion Molecular Core 1, M43, and the Orion Molecular Core 3 bordering on NGC 1973, 1975, and 1977. While hydrogen and helium RRLs arise in the ionized gas surrounding the massive stars in the Orion Nebula complex, carbon RRLs stem from the neutral gas of the adjacent photo-dissociation regions (PDRs). The high velocity resolution (0.3 km s⁻¹) enables us to discern the detailed dynamics of the RRL emitting neutral and ionized gas. We compare the carbon RRLs with SOFIA/upGREAT observations of the [C II] 158 µm line and IRAM 30 m observations of the ¹³CO (J = 2−1) line (the complete map is presented here for the first time). We observe small differences in peak velocities between the different tracers, which cannot always be attributed to geometry but potentially to shear motions. Using the far-infrared [C II] and [¹³C II] intensities with the carbon RRL intensities, we can infer physical conditions (electron temperature T_e and electron density n_e, converted to hydrogen nuclei density n_H by dividing by the carbon gas-phase abundance 𝒜_c ≃ 1.4 × 10⁻⁴) in the PDR gas using nonlocal thermal equilibrium excitation models. For positions in OMC1, we infer n_e ≃ 20–40 cm⁻³ and T_e ≃ 210–240 K. On the border between OMC1 and M43, we observe two gas components with n_e ≃ 2 cm⁻³ and n_e ≃ 8 cm⁻³, and T_e ≃ 100 K and T_e ≃ 150 K. In M43, we infer n_e ≃ 2–3 cm⁻³ and T_e ≃ 140 K. The Extended Orion Nebula southeast of OMC1 is characterized by n_e ≃ 2 cm⁻³ and T_e ≃ 180 K, while OMC3 has n_e ≃ 1 cm⁻³ and T_e ≃ 130 K. Our observations are sensitive enough to detect faint lines toward two positions in OMC1, in the BN/KL PDR and the PDR close to the Trapezium stars, that may be attributed to RRLs of C⁺ or O⁺. In general, the RRL line widths of both the ionized and neutral gas, as well as the [C II] and ¹³CO line widths, are broader than thermal, indicating significant turbulence in the interstellar medium, which transitions from super-Alfvénic and subsonic in the ionized gas to sub-Alfvénic and supersonic in the molecular gas. At the scales probed by our observations, the turbulent pressure dominates the pressure balance in the neutral and molecular gas, while in the ionized gas the turbulent pressure is much smaller than the thermal pressure.

We use Green Bank Telescope radio recombination line (RRL) observations to constrain the morphology of warm ionized gas in the Fermi Bubble. Our limits on hydrogen RRL emission allow us to constrain the emission measure of the ionized gas and to estimate the beam filling factor of previous Wisconsin H-Alpha Mapper observations. Our results indicate that the warm ionized gas on the Fermi Bubble boundary spans at least 1000 pc² with a depth of less than a pc, suggesting either a sheet-like structure or a structure consisting of many small clumps. Our data will enable us to properly calibrate future Hα observations and aid in calculating the Lyman continuum flux needed to maintain the ionized gas.

The MeerKAT Absorption Line Survey (MALS) has observed 391 telescope pointings at the L band (900–1670 MHz) at δ ≲ +20°. We present radio continuum images and a catalog of 495,325 (240,321) radio sources detected at a signal-to-noise ratio (S/N) > 5 over an area of 2289 deg² (1132 deg²) at 1006 MHz (1381 MHz). Every MALS pointing contains a central bright radio source (S _{1 GHz} ≳ 0.2 Jy). The median spatial resolution is 12″ (8″). The median rms noise away from the pointing center is 25 μJy beam⁻¹ (22 μJy beam⁻¹) and is within ∼15% of the achievable theoretical sensitivity. The flux density scale ratio and astrometric accuracy deduced from multiply observed sources in MALS are 8), confirm the flattening of spectral indices with decreasing flux density, and identify 140 ultra-steep-spectrum (α < −1.3) sources as prospective high-z radio galaxies (z > 2). We have identified 1308 variable and 122 transient radio sources comprising primarily active galactic nuclei that demonstrate long-term (26 yr) variability in their observed flux densities. The MALS catalogs and images are publicly available at https://mals.iucaa.in.

C-band Mueller matrices for the Green Bank Telescope are presented here which enable on-sky Stokes parameters for point sources at the beam center to be determined. Standard calibrators, 3C 138 and 3C 286, were observed using the Spider program to steer the telescope across a broad range of R.A. on both sides of the zenith transit. For this analysis, only the observations at the peak of the Spider pattern were used rather than the full sweep of the runs. Therefore, the results presented here only apply to point sources at the beam center. The Mueller matrices are shown to vary with frequency and with use of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain between the X and Y components of the feed. However, the relative calibration gain can be determined from observations of a source with known polarization. Correcting the data for the relative calibration gain prior to data analysis allows for use of a frequency-independent Mueller matrix. This generic Mueller matrix is shown to provide reliable C-band polarization measurements.

The BL Lac object PKS 1413+135 was observed by the Large Survey Project MeerKAT Absorption Line Survey (MALS) in the L-band, at 1139 MHz and 1293–1379 MHz, targeting the H I and OH lines in absorption at z = 0.24671. The radio continuum might come from the nucleus of the absorbing galaxy or from a background object at redshift lower than 0.5, as suggested by the absence of gravitational images. The H I absorption line is detected at a high signal-to-noise ratio, with a narrow central component, and with a red wing, confirming previous results. The OH 1720 MHz line is clearly detected in (maser) emission, peaking at a velocity shifted by −10 to −15 km s⁻¹ with respect to the H I peak. The 1612 MHz line is lost due to radio frequency interference. The OH 1667 MHz main line is tentatively detected in absorption, but not the 1665 MHz line. Over 30 years a high variability is observed in optical depths, due to the rapid changes of the line of sight caused by the superluminal motions of the radio knots. The H I line has varied by 20% in depth, while the OH-1720 MHz depth has varied by a factor of ∼3. The position of the central velocity and the widths also varied. The absorbing galaxy is an early-type spiral (maybe S0) seen edge-on, with a prominent dust lane, covering the whole disk. Given the measured mass concentration and the radio continuum size at centimeter wavelengths (100 mas corresponding to 400 pc at z = 0.25), the width of the absorption lines from the nuclear regions are expected up to 250 km s⁻¹. The narrowness of the observed lines (< 15 km s⁻¹) suggests that the absorption comes from an outer gas ring, as frequently observed in S0 galaxies. The millimetric lines are even narrower (< 1 km s⁻¹), which corresponds to the continuum size restricted to the core. The radio core is covered by individual 1 pc molecular clouds, whose column density is a few 10²² cm⁻², which is compatible with the gas screen detected in X-rays.

We report the detection of stimulated hydrogen radio recombination line (RRL) emission from ionized gas in a z = 0.89 galaxy using 580–1670 MHz observations from the MeerKAT Absorption Line Survey. The RRL emission originates in a galaxy that intercepts and strongly lenses the radio blazar PKS 1830−211 (z = 2.5). This is the second detection of RRLs outside of the local Universe and the first clearly associated with hydrogen. We detect effective H144α (and H163α) transitions at observed frequencies of 1156 (798) MHz by stacking 17 (27) RRLs with 21σ (14σ) significance. The RRL emission contains two main velocity components and is coincident in velocity with H i 21 cm and OH 18 cm absorption. We use the RRL spectral line energy distribution and a Bayesian analysis to constrain the density (n _e ) and the volume-averaged path length (ℓ) of the ionized gas. We determine log ( n e ) = 2.0 − 0.7 + 1.0 cm⁻³ and log ( ℓ ) = − 0.7 − 1.1 + 1.1 pc toward the northeast (NE) lensed image, likely tracing the diffuse thermal phase of the ionized ISM in a thin disk. Toward the southwest (SW) lensed image, we determine log ( n e ) = 3.2 − 1.0 + 0.4 cm⁻³ and log ( ℓ ) = − 2.7 − 0.2 + 1.8 pc, tracing gas that is more reminiscent of H scii regions. We estimate a star formation (surface density) rate of Σ_SFR ∼ 0.6 M _⊙ yr⁻¹ kpc⁻² or SFR ∼ 50 M _⊙ yr⁻¹, consistent with a star-forming main-sequence galaxy of M _⋆ ∼ 10¹¹ M _⊙. The discovery presented here opens up the possibility of studying ionized gas at high redshifts using RRL observations from current and future (e.g., SKA and ngVLA) radio facilities.