To investigate the fundamental principles of H 2 formation in a giant molecular cloud, we derive the Hi and H 2 surface density (Σ Hi and Σ H2 ) images of the Perseus molecular cloud on sub-pc scales (∼0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V -band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the Hi data from the Galactic Arecibo L-band Feed Array Hi Survey and an estimate of the local dust-to-gas ratio, we then derive the Σ H2 distribution across Perseus. We find a relatively uniform Σ Hi ∼ 6-8 M pc −2 for both dark and star-forming regions, suggesting a minimum Hi surface density required to shield H 2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Σ H2 /Σ Hi and Σ Hi + Σ H2 . The transition between the Hi-and H 2 -dominated regions occurs at N (Hi) + 2N (H 2 ) ∼ (8-14) × 10 20 cm −2 . Our findings are consistent with predictions for H 2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H 2 formation. However, the importance of warm neutral medium for H 2 shielding, internal radiation field, and the timescale of H 2 formation still remain as open questions. We also compare H 2 and CO distributions and estimate the fraction of "CO-dark" gas, f DG ∼ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V -band extinction.

We present methods and results from "21-cm Spectral Line Observations of Neutral Gas with the EVLA" (21-SPONGE), a large survey for Galactic neutral hydrogen (Hi) absorption with the Karl G. Jansky Very Large Array (VLA). With the upgraded capabilities of the VLA, we reach median root-mean-square (RMS) noise in optical depth of σ τ = 9 × 10 −4 per 0.42 km s −1 channel for the 31 sources presented here. Upon completion, 21-SPONGE will be the largest Hi absorption survey with this high sensitivity. We discuss the observations and data reduction strategies, as well as line fitting techniques. We prove that the VLA bandpass is stable enough to detect broad, shallow lines associated with warm Hi, and show that bandpass observations can be combined in time to reduce spectral noise. In combination with matching Hi emission profiles from the Arecibo Observatory (∼ 3.5 ′ angular resolution), we estimate excitation (or spin) temperatures (T s ) and column densities for Gaussian components fitted to sightlines along which we detect Hi absorption (30/31). We measure temperatures up to T s ∼ 1500 K for individual lines, showing that we can probe the thermally unstable interstellar medium (ISM) directly. However, we detect fewer of these thermally unstable components than expected from previous observational studies. We probe a wide range in column density between ∼ 10 16 and > 10 21 cm −2 for individual Hi clouds. In addition, we reproduce the trend between cold gas fraction and average T s found by synthetic observations of a hydrodynamic ISM simulation by Kim et al. (2014). Finally, we investigate methods for estimating Hi T s and discuss their biases.

We present a catalog of 1964 isolated, compact neutral hydrogen clouds from the Galactic Arecibo L-Band Feed Array Survey Data Release One. The clouds were identified by a custom machine-vision algorithm utilizing the difference of Gaussian kernels to search for clouds smaller than 20 . The clouds have velocities typically between |V LSR | = 20 and 400 km s −1 , line widths of 2.5-35 km s −1 , and column densities ranging from 1 to 35 × 10 18 cm −2 . The distances to the clouds in this catalog may cover several orders of magnitude, so the masses may range from less than a solar mass for clouds within the Galactic disk, to greater than 10 4 M for high-velocity clouds (HVCs) at the tip of the Magellanic Stream. To search for trends, we separate the catalog into five populations based on position, velocity, and line width: HVCs; galaxy candidates; cold low-velocity clouds (LVCs); warm, low positive-velocity clouds in the third Galactic quadrant; and the remaining warm LVCs. The observed HVCs are found to be associated with previously identified HVC complexes. We do not observe a large population of isolated clouds at high velocities as some models predict. We see evidence for distinct histories at low velocities in detecting populations of clouds corotating with the Galactic disk and a set of clouds that is not corotating.

A new 1.4 GHz 19-element, dual-polarization, cryogenic phased array feed (PAF) radio astronomy receiver has been developed for the Robert C. Byrd Green Bank Telescope (GBT) as part of FLAG (Focal L-band Array for the GBT) project. Commissioning observations of calibrator radio sources show that this receiver has the lowest reported beamformed system temperature (T sys ) normalized by aperture efficiency (η) of any phased array receiver to date. The measured T sys /η is 25.4 ± 2.5 K near 1350 MHz for the boresight beam, which is comparable to the performance of the current 1.4 GHz cryogenic single feed receiver on the GBT. The degradation in T sys /η at ∼ 4 (required for Nyquist sampling) and ∼ 8 offsets from the boresight is, respectively, ∼ 1% and ∼ 20% of the boresight value. The survey speed of the PAF with seven formed beams is larger by a factor between 2.1 and 7 compared to a single beam system depending on the observing application. The measured performance, both in frequency and offset from boresight, qualitatively agree with predictions from a rigorous electromagnetic model of the PAF. The astronomical utility of the receiver is demonstrated by observations of the pulsar B0329+54 and an extended H ii region, the Rosette Nebula. The enhanced survey speed with the new PAF receiver will enable the GBT to carry out exciting new science, such as more efficient observations of diffuse, extended neutral hydrogen emission from galactic in-flows and searches for Fast Radio Bursts.

We use the Karl G. Jansky Very Large Array (VLA) to conduct a high-sensitivity survey of neutral hydrogen (Hi) absorption in the Milky Way. In combination with corresponding Hi emission spectra obtained mostly with the Arecibo Observatory, we detect a widespread warm neutral medium (WNM) component with excitation temperature T s = 7200 +1800 −1200 K (68% confidence). This temperature lies above theoretical predictions based on collisional excitation alone, implying that Ly-α scattering, the most probable additional source of excitation, is more important in the interstellar medium (ISM) than previously assumed. Our results demonstrate that Hi absorption can be used to constrain the Ly-α radiation field, a critical quantity for studying the energy balance in the ISM and intergalactic medium yet notoriously difficult to model because of its complicated radiative transfer, in and around galaxies nearby and at high redshift.

We derive two-dimensional spatial power spectra of four distinct interstellar medium tracers, H i, 12CO(J = 1–0), 13CO(J = 1–0), and dust, in the Perseus molecular cloud, covering linear scales ranging from ∼0.1 pc to ∼90 pc. Among the four tracers, we find the steepest slopes of −3.23 ± 0.05 and −3.22 ± 0.05 for the uncorrected and opacity-corrected H i column density images. This result suggests that the H i in and around Perseus traces a non-gravitating, transonic medium on average, with a negligible effect from opacity. On the other hand, we measure the shallowest slope of −2.72 ± 0.12 for the 2MASS dust extinction data and interpret this as the signature of a self-gravitating, supersonic medium. Possible variations in the dust-to-gas ratio likely do not alter our conclusion. Finally, we derive slopes of −3.08 ± 0.08 and −2.88 ± 0.07 for the 12CO(1–0) and 13CO(1–0) integrated intensity images. Based on theoretical predictions for an optically thick medium, we interpret these slopes of roughly −3 as implying that both CO lines are susceptible to the opacity effect. While simple tests for the impact of CO formation and depletion indicate that the measured slopes of 12CO(1–0) and 13CO(1–0) are not likely affected by these chemical effects, our results generally suggest that chemically more complex and/or fully optically thick media may not be a reliable observational tracer for characterizing turbulence.

We present initial results from a deep neutral hydrogen (Hi) survey of the HALOGAS galaxy sample, which includes the spiral galaxies NGC891, NGC925, NGC4414, and NGC4565, performed with the Robert C. Byrd Green Bank Telescope (GBT). The resulting observations cover at least four deg 2 around these galaxies with an average 5σ detection limit of 1.2×10 18 cm −2 over a velocity range of 20 km s −1 and angular scale of 9.1 . In addition to detecting the same total flux as the GBT data, the spatial distribution of the GBT and original Westerbork Synthesis Radio Telescope (WSRT) data match well at equal spatial resolutions. The Hi mass fraction below Hi column densities of 10 19 cm −2 is, on average, 2%. We discuss the possible origins of low column density Hi of nearby spiral galaxies. The absence of a considerable amount of newly detected Hi by the GBT indicates these galaxies do not have significant extended diffuse Hi structures, and suggests future surveys planned with the SKA and its precursors must go at least as deep as 10 17 cm −2 in column density to significantly increase the probability of detecting Hi associated with the cosmic web and/or cold mode accretion.

We investigate turbulent properties of the non-star-forming, translucent molecular cloud, MBM16 by applying the statistical technique of a two-dimensional spatial power spectrum (SPS) on the neutral hydrogen (HI) observations obtained by the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey. The SPS, calculated over the range of spatial scales from 0.1 to 17 pc, is well represented with a single power-law function, with a slope ranging from -3.3 to -3.7 and being consistent over the velocity range of MBM16 for a fixed velocity channel thickness. However, the slope varies significantly with the velocity slice thickness, suggesting that both velocity and density contribute to HI intensity fluctuations. By using this variation we estimate the slope of 3D density fluctuations in MBM16 to be −3.7 ± 0.2. This is significantly steeper than what has been found for HI in the Milky Way plane, the Small Magellanic Cloud, or the Magellanic Bridge, suggesting that interstellar turbulence in MBM16 is driven on scales > 17 pc and that the lack of stellar feedback could be responsible for the steep power spectrum.