Ultraviolet Extinction at High Galactic Latitudes II: The Ultraviolet Extinction Function

We present a dust-column--dependent extinction curve parameters for ultraviolet wavelengths at high Galactic latitudes. This extinction function diverges from previous work in that it takes into account the results of Peek & Schiminovich 2013 (Paper I), which demonstrated that there is more reddening in the GALEX bands than would be otherwise expected for E(B-V) < 0.2. We also test the biases in the Planck and SFD extinction maps, and find that the SFD extinction maps are significantly biased at E(B-V) < 0.2. We find that while an extinction function that that takes into account a varying R_FUV with E(B-V) dramatically improves our estimation of FUV-NUV colors, a fit that also includes HI column density dependence is superior. The ultraviolet extinction function we present here follows the model of Fitzpatrick 1999, varying only the amplitude of the FUV rise parameter to be consistent with the data.Comment: 7 pages, 4 figures, submitted to the Ap

A Comparison of Far IR and HI as Reddening Predictors at High Galactic Latitude

Both the Galactic 21-cm line flux from neutral hydrogen (HI) in interstellar medium and the far-infrared (FIR) emission from Galactic dust grains have been used to estimate the strength of Galactic reddening of distant sources. In this work we use a collection of uniform color distant galaxies as color standards to determine whether the HI method or the FIR method is superior. We find that the two methods both produce reasonably accurate maps, but that both show significant errors as compared to the typical color of the background galaxies. We find that a mixture of the FIR and HI maps in roughly a 2-to-1 ratio is clearly superior to either map alone. We recommend that future reddening maps should use both sets of data, and that well-constructed FIR and HI maps should both be vigorously pursued.Comment: Accepted to the ApJ

Do Androids Dream of Magnetic Fields? Using Neural Networks to Interpret the Turbulent Interstellar Medium

The interstellar medium (ISM) of galaxies is composed of a turbulent magnetized plasma. In order to quantitatively measure relevant turbulent parameters of the ISM, a wide variety of statistical techniques and metrics have been developed that are often tested using numerical simulations and analytic formalism. These metrics are typically based on the Fourier power spectrum, which does not capture the Fourier phase information that carries the morphological characteristics of images. In this work we use density slices of magnetohydrodyanmic turbulence simulations to demonstrate that a modern tool, convolutional neural networks, can capture significant information encoded in the Fourier phases. We train the neural network to distinguish between two simulations with different levels of magnetization. We find that, even given a tiny slice of simulation data, a relatively simple network can distinguish sub-Alfv\'enic (strong magnetic field) and super-Alfv\'enic (weak magnetic field) turbulence >98% of the time, even when all spectral amplitude information is stripped from the images. In order to better understand how the neural network is picking out differences betweem the two classes of simulations we apply a neural network analysis method called "saliency maps". The saliency map analysis shows that sharp ridge-like features are a distinguishing morphological characteristic in such simulations. Our analysis provides a way forward for deeper understanding of the relationship between magnetohydrodyanmic turbulence and gas morphology and motivates further applications of neural networks for studies of turbulence. We make publicly available all data and software needed to reproduce our results.Comment: Accepted to ApJ

Magnetically Aligned HI Fibers and the Rolling Hough Transform

We present observations of a new group of structures in the diffuse Galactic ISM: slender, linear HI features we dub "fibers" that extend for many degrees at high Galactic latitude. To characterize and measure the extent and strength of these fibers, we present the Rolling Hough Transform (RHT), a new machine vision method for parameterizing the coherent linearity of structures in the image plane. With this powerful new tool we show the fibers are oriented along the interstellar magnetic field as probed by starlight polarization. We find that these low column density (N(HI) ~ 5 x 10^18 cm^-2) fiber features are most likely a component of the local cavity wall, about 100 pc away. The HI data we use to demonstrate this alignment at high latitude are from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey and the Parkes Galactic All Sky Survey (GASS). We find better alignment in the higher resolution GALFA-HI data, where the fibers are more visually evident. This trend continues in our investigation of magnetically aligned linear features in the Riegel-Crutcher HI cold cloud, detected in the Southern Galactic Plane Survey (SGPS). We propose an application of the RHT for estimating the field strength in such a cloud, based on the Chandrasekhar-Fermi method. We conclude that data-driven, quantitative studies of ISM morphology can be very powerful predictors of underlying physical quantities.Comment: 13 pages, 13 figures. Accepted to the Astrophysical Journa

Kinetic Tomography. II. A second method for mapping the velocity field of the Milky Way Interstellar Medium and a comparison with spiral structure models

In this work, we derive a spatially resolved map of the line-of-sight velocity of the interstellar medium and use it, along with a second map of line-of-sight velocity from Paper I of this series, to determine the nature of gaseous spiral structure in the Milky Way. This map is derived from measurements of the 1.527 $\mu$m diffuse interstellar band (DIB) in stellar spectra from the APOGEE survey and covers the nearest 4-5 kpc of the Northern Galactic plane. We cross-check this new DIB-based line-of-sight velocity map with the map derived in Paper I and find that they agree. We then compare these maps with line-of-sight velocity maps derived from simulations of quasi-stationary density wave spiral structure and dynamic, or material, spiral structure in a Milky Way-like galaxy. While none of the maps derived from these simulations is an exact match to the measured velocity field of the Milky Way, the measurements are more consistent with simulations of dynamic spiral structure. In the dynamic spiral structure simulation that best matches the measurements, the Perseus spiral arm is being disrupted.Comment: Submitted to AAS journal

Revealing the Milky Way's Hidden Circumgalactic Medium with the Cosmic Origins Spectrograph Quasar Database for Galactic Absorption Lines

Every quasar (QSO) spectrum contains absorption-line signatures from the interstellar medium, disk-halo interface, and circumgalactic medium (CGM) of the Milky Way (MW). We analyze Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectra of 132 QSOs to study the significance and origin of SiIV absorption at $|v_{\rm LSR}|\leq100$ km/s in the Galactic halo. The gas in the north predominantly falls in at $-50\lesssim v_{\rm LSR}\lesssim 0$ km/s, whereas in the south, no such pattern is observed. The SiIV column density has an average and a standard deviation of $\langle N_{\rm SiIV}\rangle=(3.8\pm1.4)\times10^{13}$ cm$^{-2}$. At $|b|\gtrsim 30$ degree, $N_{\rm SiIV}$ does not significantly correlate with $b$, which cannot be explained by a commonly adopted flat-slab geometry. We propose a two-component model to reconstruct the $N_{\rm SiIV}$-$b$ distribution: a plane-parallel component $N_{\rm DH}^{\perp}$ to account for the MW's disk-halo interface and a global component $N_{\rm G}$ to reproduce the weak dependence on $b$. We find $N_{\rm DH}^{\perp}=1.3^{+4.7}_{-0.7}\times10^{12}$ cm$^{-2}$ and $N_{\rm G}=(3.4\pm0.3)\times10^{13}$ cm$^{-2}$ on the basis of Bayesian analyses and block bootstrapping. The global component is most likely to have a Galactic origin, although its exact location is uncertain. If it were associated with the MW's CGM, we would find $M_{\rm gas, all}\gtrsim4.7\times10^9\ M_{\odot} (\frac{C_f}{1})(\frac{R}{75\ {\rm kpc}})^2 (\frac{f_{\rm SiIV}}{0.3})^{-1}(\frac{Z}{0.3\ Z_{\odot}})^{-1}$ for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at $|v_{\rm LSR}|\leq100$ km s$^{-1}$ hidden in the MW's CGM. Along with this work, we make our QSO dataset publicly available as the COS Quasar Database for Galactic Absorption Lines (COS-GAL).Comment: Published on ApJ. https://iopscience.iop.org/article/10.3847/1538-4357/aaf6eb/met

The Discovery and Origin of A Very-High Velocity Cloud Toward M33

We report the detection of a largely ionized very-high velocity cloud (VHVC; $v_{\rm LSR}\sim-350$ km/s) toward M33 with the Hubble Space Telescope/Cosmic Origin Spectrograph. The VHVC is detected in OI, CII, SiII, and SiIII absorption along five sightlines separated by ~0.06-0.4 degree. On sub-degree scales, the velocities and ionic column densities of the VHVC remain relatively smooth with standard deviations of +/-14 km/s and +/-0.15 dex between the sightlines, respectively. The VHVC has a metallicity of [OI/HI]=-0.56+/-0.17 dex (Z=0.28+/-0.11 Z$_{\odot}$). Despite the position-velocity proximity of the VHVC to the ionized Magellanic Stream, the VHVC's higher metallicity makes it unlikely to be associated with the Stream, highlighting the complex velocity structure of this region of sky. We investigate the VHVC's possible origin by revisiting its surrounding HI environment. We find that the VHVC may be: (1) a MW CGM cloud, (2) related to a nearby HI VHVC -- Wright's Cloud, or (3) connected to M33's northern warp. Furthermore, the VHVC could be a bridge connecting Wright's Cloud and M33's northern warp, which would make it a Magellanic-like structure in the halo of M33.Comment: Accepted for publication in Ap

Neutral hydrogen structures trace dust polarization angle: Implications for cosmic microwave background foregrounds

Using high-resolution data from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey, we show that linear structure in Galactic neutral hydrogen (HI) correlates with the magnetic field orientation implied by Planck 353 GHz polarized dust emission. The structure of the neutral interstellar medium is more tightly coupled to the magnetic field than previously known. At high Galactic latitudes, where the Planck data are noise-dominated, the HI data provide an independent constraint on the Galactic magnetic field orientation, and hence the local dust polarization angle. We detect strong cross-correlations between template maps constructed from estimates of dust intensity combined with either HI-derived angles, starlight polarization angles, or Planck 353 GHz angles. The HI data thus provide a new tool in the search for inflationary gravitational wave B-mode polarization in the cosmic microwave background, which is currently limited by dust foreground contamination.Comment: Accepted to PRL. 7 pages, 5 figures, incl Supplemental Materia

Finding gas-rich dwarf galaxies betrayed by their ultraviolet emission

We present ultraviolet (UV) follow-up of a sample of potential dwarf galaxy candidates selected for their neutral hydrogen (HI) properties, taking advantage of the low UV background seen by the GALEX satellite and its large and publicly available imaging footprint. The HI clouds, which are drawn from published GALFA-HI and ALFALFA HI survey compact cloud catalogs, are selected to be galaxy candidates based on their spatial compactness and non-association with known high-velocity cloud complexes or Galactic HI emission. Based on a comparison of their UV characteristics to those of known dwarf galaxies, half (48%) of the compact HI clouds have at least one potential stellar counterpart with UV properties similar to those of nearby dwarf galaxies. If galaxies, the star formation rates, HI masses, and star formation efficiencies of these systems follow the trends seen for much larger galaxies. The presence of UV emission is an efficient method to identify the best targets for spectroscopic follow-up, which is necessary to prove that the stars are associated with the compact HI. Further, searches of this nature help to refine the salient HI properties of likely dwarfs (even beyond the Local Group). In particular, HI compact clouds considered to be velocity outliers relative to their neighbor HI clouds have the most significant detection rate of single, appropriate UV counterparts. Correcting for the sky coverage of the two all-Arecibo sky surveys yielding the compact HI clouds, these results may imply the presence of potentially hundreds of new tiny galaxies across the entire sky.Comment: 16 pages, 12 figures, accepted to Ap

The GALFA-HI Survey: Techniques

We explain the entire process by which we conduct the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey. The survey is a high resolution (3.4'), large area (13000 deg^2), large Galactic velocity range (-750 to +750 km/s), high spectral resolution (0.18 km/s) survey of the Galaxy in the 21 cm line hyperfine transition of hydrogen conducted at Arecibo Observatory. We touch on some of the new Galactic science being conducted using the GALFA-HI survey, ranging from High-Velocity Clouds to HI narrow-line self-absorption. We explain the many technical challenges that confront such a survey, including baseline ripple, gain variation and asymmetrical beam shapes. To correct for these systematic effects we use various newly developed methods, which we describe in detail. We also explain the data reduction process step by step, starting with the raw time-ordered data and ending with fully calibrated maps. The effects of each step of the data reduction on the final data product is shown sequentially. We conclude with future directions for the ongoing survey.Comment: 25 pages, 10 figure