Targeted deep surveys of high Galactic latitude HI with the GBT

Over 800 sq. deg. of high Galactic latitude sky have been mapped at 21 cm with the Robert C. Byrd Green Bank Telescope (GBT). An improved knowledge of the telescope's beam characteristics has allowed us to reliably map not only regions of high column density, but also such regions as ELAIS N1, a targeted Spitzer field, which have very low HI column density. The additional fields we have observed cover a cross-section of dynamically and chemically interesting regions as indicated by the presence of intermediate/high velocity gas and/or anomalous far-IR (dust) colour.Comment: 7 pages, 5 figures. To appear in "The Dynamic ISM: A celebration of the Canadian Galactic Plane Survey" ASP Conference Serie

GHIGLS: HI mapping at intermediate Galactic latitude using the Green Bank Telescope

This paper introduces the data cubes from GHIGLS, deep Green Bank Telescope surveys of the 21-cm line emission of HI in 37 targeted fields at intermediate Galactic latitude. The GHIGLS fields together cover over 1000 square degrees at 9.55' spatial resolution. The HI spectra have an effective velocity resolution about 1.0 km/s and cover at least -450 < v < +250 km/s. GHIGLS highlights that even at intermediate Galactic latitude the interstellar medium is very complex. Spatial structure of the HI is quantified through power spectra of maps of the column density, NHI. For our featured representative field, centered on the North Ecliptic Pole, the scaling exponents in power-law representations of the power spectra of NHI maps for low, intermediate, and high velocity gas components (LVC, IVC, and HVC) are -2.86 +/- 0.04, -2.69 +/- 0.04, and -2.59 +/- 0.07, respectively. After Gaussian decomposition of the line profiles, NHI maps were also made corresponding to the narrow-line and broad-line components in the LVC range; for the narrow-line map the exponent is -1.9 +/- 0.1, reflecting more small scale structure in the cold neutral medium (CNM). There is evidence that filamentary structure in the HI CNM is oriented parallel to the Galactic magnetic field. The power spectrum analysis also offers insight into the various contributions to uncertainty in the data. The effect of 21-cm line opacity on the GHIGLS NHI maps is estimated.Comment: Accepted for publication in The Astrophysical Journal, 2015 July 16. 32 pages, 21 figures (Fig. 10 new). Minor revisions from review, particularly Section 8 and Appendix C; results unchanged. Additional surveys added and made available; new Appendix B. Added descriptions of available FITS files and links to four illustrative movies on enhanced GHIGLS archive (www.cita.utoronto.ca/GHIGLS/

Structure formation in a colliding flow: The Herschel view of the Draco nebula

The Draco nebula is a high Galactic latitude interstellar cloud likely to have been formed by the collision of a Galactic halo cloud entering the disk of the Milky Way. Such conditions are ideal to study the formation of cold and dense gas in colliding flows of warm gas. We present Herschel-SPIRE observations that reveal the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity and the turbulence dissipation scale (0.1 pc) that is compatible with that expected if ambipolar diffusion is the main mechanism of energy dissipation in the WNM. The small-scale structures of the nebula are typical of that seen in some molecular clouds. The gas density has a log-normal distribution with an average value of $10^3$ cm$^{-3}$. The size of the structures is 0.1-0.2 pc but this estimate is limited by the resolution of the observations. The mass ranges from 0.2 to 20 M$_{\odot}$ and the distribution of the more massive clumps follows a power law $dN/d\log(M) \sim M^{-1.4}$. We identify a mass-size relation with the same exponent as that found in GMCs ($M\sim L^{2.3}$) but only 15% of the mass of the cloud is in gravitationally bound structures. We conclude that the increase of pressure in the collision is strong enough to trigger the WNM-CNM transition caused by the interplay between turbulence and thermal instability as self-gravity is not dominating the dynamics.Comment: 16 pages, A&A, in pres

On the O II ground configuration energy levels

The most accurate way to measure the energy levels for the O II 2p^3 ground configuration has been from the forbidden lines in planetary nebulae. We present an analysis of modern planetary nebula data that nicely constrain the splitting within the ^2D term and the separation of this term from the ground ^4S_{3/2} level. We extend this method to H II regions using high-resolution spectroscopy of the Orion nebula, covering all six visible transitions within the ground configuration. These data confirm the splitting of the ^2D term while additionally constraining the splitting of the ^2P term. The energies of the ^2P and ^2D terms relative to the ground (^4S) term are constrained by requiring that all six lines give the same radial velocity, consistent with independent limits placed on the motion of the O+ gas and the planetary nebula data.Comment: 20 pages, 3 figures. To be published in Ap

Temperature Variations from \u3cem\u3eHubble Space Telescope\u3c/em\u3e Spectroscopy of the Orion Nebula

We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the fractional mean-square Te variation (t2A)across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in order to estimate Te and t2A in the N+region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [N II] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5 arcsec square (matching the slit width). The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t2A varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 to 10 232 K; t2A varies from 0.00584 to 0.0175. The measurements of Te reported here are an average along each line of sight. Therefore, despite finding remarkably low t2A, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II]Te exceeds the average [O III]Te confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by θ1 Ori C; and indirectly, evidence of high electron density

A photoionized Herbig-Haro object in the Orion nebula

The spectra of Herbig Haro objects are usually characteristic of ionization and excitation in shock-heated gas, whether an internal shock in an unsteady outflow or a bow shock interface with the interstellar medium. We examine the eastern-most shock -- the leading optically visible shock -- of a Herbig Haro outflow (HH 529) seen projected on the face of the Orion Nebula, using deep optical echelle spectroscopy, showing that the spectrum of this gas is consistent with photoionization by $\theta^1$ Ori C. By modeling the emission lines, we determine a gas-phase abundance of Fe which is consistent with the depleted (relative to solar) abundance found in the Orion nebula -- evidence for the presence of dust in the nebula and therefore in the Herbig Haro outflow. The spectrum also allows for the calculation of temperature fluctuations, $t^2$, in the nebula and the shock. These fluctuations have been used to explain discrepancies between abundances obtained from recombination lines versus those obtained from collisionally-excited lines, although to date there has not been a robust theory for how such large fluctuations ($t^2 > 0.02$) can exist.Comment: 50 pages, 8 figures, To be published in Ap

Deviations from He I Case B Recombination Theory and Extinction Corrections in the Orion Nebula

We are engaged in a comprehensive program to find reliable elemental abundances in and to probe the physical structure of the Orion Nebula, the brightest and best-resolved H II region. In the course of developing a robust extinction correction covering our optical and ultraviolet FOS and STIS observations, we examined the decrement within various series of He I lines. The decrements of the 2^3S-n^3P, 2^3P-n^3S and 3^3S-n^3P series are not in accord with caseB recombination theory. None of these anomalous He I decrements can be explained by extinction, indicating the presence of additional radiative transfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDY photoionization equilibrium models including radiative transfer are developed to predict the observed He I decrements and the quantitative agreement is quite remarkable. Following from these results, select He I lines are combined with H I and [O II] lines and stellar extinction data to validate a new normalizable analytic expression for the wavelength dependence of the extinction. In so doing, the He+/H+ abundance is also derived.Comment: 42 pages, 10 figures. To be published in Ap

Accurate Galactic 21-cm H I measurements with the NRAO Green Bank Telescope

Aims: We devise a data reduction and calibration system for producing highly-accurate 21-cm H I spectra from the Green Bank Telescope (GBT) of the NRAO. Methods: A theoretical analysis of the all-sky response of the GBT at 21 cm is made, augmented by extensive maps of the far sidelobes. Observations of radio sources and the Moon are made to check the resulting aperture and main beam efficiencies. Results: The all-sky model made for the response of the GBT at 21 cm is used to correct for "stray" 21-cm radiation reaching the receiver through the sidelobes rather than the main beam. This reduces systematic errors in 21-cm measurements by about an order of magnitude, allowing accurate 21-cm H I spectra to be made at about 9' angular resolution with the GBT. At this resolution the procedures discussed here allow for measurement of total integrated Galactic H I line emission, W, with errors of 3 K km s^-1, equivalent to errors in optically thin N_HI of 5 x 10^18 cm^-2.Comment: 49 pages, 25 figures; A&A, in pres