1,290 research outputs found
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 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 km/s in the Galactic halo. The gas in
the north predominantly falls in at km/s,
whereas in the south, no such pattern is observed. The SiIV column density has
an average and a standard deviation of cm. At degree,
does not significantly correlate with , which cannot be
explained by a commonly adopted flat-slab geometry. We propose a two-component
model to reconstruct the - distribution: a plane-parallel
component to account for the MW's disk-halo interface and
a global component to reproduce the weak dependence on . We find
cm and cm 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 for the cool gas at all
velocities in the Galactic halo. Our analyses show that there is likely a
considerable amount of gas at km s 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;
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). 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
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