Galactic Arecibo L-band Feed Array Research Articles

The kinematic structure of magnetically aligned H i filaments

ABSTRACT We characterize the kinematic and magnetic properties of H i filaments located in a high Galactic latitude region (165° < α < 195° and 12° < δ < 24°). We extract three-dimensional filamentary structures using fil3d from the Galactic Arecibo L-Band Feed Array H i (GALFA-H i) survey 21-cm emission data. Our algorithm identifies coherent emission structures in neighbouring velocity channels. Based on the mean velocity, we identify a population of local and intermediate velocity cloud (IVC) filaments. We find the orientations of the local (but not the IVC) H i filaments are aligned with the magnetic field orientations inferred from Planck 353 GHz polarized dust emission. We analyse position–velocity diagrams of the velocity-coherent filaments, and find that only 15 per cent of filaments demonstrate significant major-axis velocity gradients with a median magnitude of 0.5 km s−1 pc−1, assuming a fiducial filament distance of 100 pc. We conclude that the typical diffuse H i filament does not exhibit a simple velocity gradient. The reported filament properties constrain future theoretical models of filament formation.

Shear, writhe, and filaments: Turbulence in the high-latitude molecular cloud MBM 40

Context.Given the structural and thermodynamical complexity of the interstellar medium (ISM), the variety of governing processes, such as stellar feedback, poses challenges to the investigation. High-latitude molecular clouds (HLMCs) with no evidence of internal star formation, as in the case of MBM 40, are excellent sites for studying the chemistry and dynamic evolution of the cold neutral ISM.Aims.We used this high-latitude cloud as an exemplar for the dynamical and chemical processes in the diffuse interstellar medium.Methods.We analyzed new and archival12CO,13CO, CH, HCO+, CS, H2CO, and HCN data from Five College Radio Observatory (FCRAO), Onsala Space Observatory (OSO), Arizona Radio Observatory (ARO), andW. Gordontelescope (Arecibo) combined with the Galactic Arecibo L-band Feed Array H I(GALFA-HI) H I21 cm data set, to study the chemistry, thermal state, and dynamics of MBM 40. A new dynamical analytical approach was adopted by considering each line profile as a line-of-sight probability distribution function (PDF) of the turbulence weighted by gas emissivity.Results.The atomic and molecular gas are smoothly distributed in space and velocity. No steep transition is seen between circumcloud atomic and cloud molecular gas in either radial velocity or structure. We propose a topology of the cloud based on molecular tracers, as a contorted filamentary structure that is shaped by a broad embedding shear flow in the neutral atomic gas. A comparative examination of different molecular tracers shows that13CO, H2CO, and CS only arise from denser molecular cores, whereas12CO, CH, and HCO+trace diffuse gas with a broader range of dynamics.

Discovery of a Filamentary Synchrotron Structure Connected to the Coherent Magnetic Field in the Outer Galaxy

Using data from the Galactic Arecibo L-band Feed Array Continuum Transit Survey, we report the discovery of two previously unidentified, very compressed, thin, and straight polarized filaments approximately centered at Galactic coordinates, (l, b) = (182.°5, − 4.°0), which we call G182.5–4.0. Using data from the Isaac Newton Telescope Galactic Plane Survey, we also find straight, long, and extremely thin Hα filaments coincident with the radio emission. These filaments are positioned in projection at the edge of the Orion-Eridanus superbubble and we find evidence indicating that the filaments align with the coherent magnetic field of the outer Galaxy. We find a lower limit on the total radio flux at 1.4 GHz to be 0.7 ± 0.3 Jy with an average linearly polarized fraction of . We consider various scenarios that could explain the origin of these filaments, including a shell-type supernova remnant (SNR), a bow shock nebula associated with a pulsar, or relic fragments from one or more supernova explosions in the adjacent superbubble, with a hybrid scenario being most likely. This may represent an example of a new class of objects that is neither an SNR nor a bow shock. The highly compressed nature of these filaments and their alignment with the Galactic plane suggests a scenario where this object formed in a magnetic field that was compressed by the expanding Orion-Eridanus superbubble, suggesting that the object is related to this superbubble and implying a distance of ∼400 pc.

A Comparison of Multiphase Magnetic Field Tracers in a High Galactic Latitude Region of the Filamentary Interstellar Medium

Abstract Understanding how the Galactic magnetic field threads the multiphase interstellar medium (ISM) remains a considerable challenge, as different magnetic field tracers probe dissimilar phases and field components. We search for evidence of a common magnetic field shared between the ionized and neutral ISM by comparing 1.4 GHz radio continuum polarization and H i line emission from the Galactic Arecibo L-Band Feed Array Continuum Transit Survey (GALFACTS) and Galactic Arecibo L-Band Feed Array H i (GALFA-H i) survey, respectively. We compute the polarization gradient of the continuum emission and search for associations with diffuse/translucent H i structures. The polarization gradient is sensitive to changes in the integrated product of the thermal electron density and line-of-sight field strength (B ∥) in warm ionized gas, while narrow H i structures highlight the plane-of-sky field orientation in cold neutral gas. We identified one region in the high Galactic latitude Arecibo sky, G216+26 centered on (ℓ, b) ∼ (216°, +26°), containing filaments in the polarization gradient that are aligned with narrow H i structures roughly parallel to the Galactic plane. We present a comparison of multiphase observations and magnetic field tracers of this region, demonstrating that the warm ionized and cold neutral media are connected likely via a common magnetic field. We quantify the physical properties of a polarization gradient filament associated with Hα emission, measuring a line-of-sight field strength B ∥ = 6 ± 4 μG and a plasma beta β = 2.1 − 2.1 + 3.1 . We discuss the lack of widespread multiphase magnetic field alignments and consider whether this region is associated with a short-timescale or physically rare phenomenon. This work highlights the utility of multitracer analyses for understanding the magnetized ISM.

λ21-cm Interstellar HI Profiles, Critical Ionization Velocities, and Derived Electron Densities

Analysis of neutral hydrogen λ21-cm profiles at high northern and southern Galactic latitudes reveals unexpected properties, connects unanticipated associations, and confirms the fundamental parameters of the interstellar medium. Gaussian decomposition of profiles from the second data release of the Galactic Arecibo L-Band Feed Array (GALFA) confirms earlier results of a pervasive component with a linewidth of order 34 km s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , consistent with the critical ionization velocity (CIV) of helium. The ratio of the column density of this component to the entire profile is proportional to the interstellar helium abundance. The results are comparable to the accepted value of the cosmic abundance, 0.085, in the southern Galactic hemisphere, but different for directions in the northern Galactic hemisphere. Normalizing to the cosmic abundance allows us to estimate how much HI is “missing” from the λ21-cm observations in the northern sky, confirming an asymmetry that has been known for decades. These CIV results also predict the volume density of interstellar electrons along different lines of site, which match values derived from pulsar dispersion measures and Hα observations. These results provide strong support for the hypothesis that interstellar λ21-cm HI profile linewidths are modulated by the CIV plasma phenomenon.

The H i Structure of the Local Volume Dwarf Galaxy Pisces A

Abstract Dedicated H i surveys have recently led to a growing category of low-mass galaxies found in the Local Volume. We present synthesis imaging of one such galaxy, Pisces A, a low-mass dwarf originally confirmed via optical imaging and spectroscopy of neutral hydrogen (H i) sources in the Galactic Arecibo L-band Feed Array H i survey. Using H i observations taken with the Karl G. Jansky Very Large Array, we characterize the kinematic structure of the gas and connect it to the galaxy’s environment and evolutionary history. While the galaxy shows overall ordered rotation, a number of kinematic features indicate a disturbed gas morphology. These features are suggestive of a tumultuous recent history and represent % of the total baryonic mass. We find a total baryon fraction if we include these features. We also quantify the cosmic environment of Pisces A, finding an apparent alignment of the disturbed gas with nearby, large-scale filamentary structure at the edge of the Local Void. We consider several scenarios for the origin of the disturbed gas, including gas stripping via ram pressure or galaxy–galaxy interactions, as well as accretion and ram pressure compression. Though we cannot rule out a past interaction with a companion, our observations best support the suggestion that the neutral gas morphology and recent star formation in Pisces A are a direct result of its interactions with the intergalactic medium.

Producing synthetic maps of dust polarization using a velocity channel gradient technique

ABSTRACT In modern cosmology, many efforts have been made to detect the primordial B-mode of cosmic microwave background polarization from gravitational waves generated during inflation. Considering the foreground dust contamination of microwave polarization maps, it is essential to obtain a precise prediction for polarization in dust emission. In this work, we show a new method to produce synthetic maps of dust polarization in the magnetized turbulent interstellar medium from more abundant high-resolution H i data. By using the velocity channel gradient technique, we are able to predict both the direction and degree of dust polarization by investigating spectroscopic H i information in position–position–velocity space. We apply our approach to the Galactic Arecibo L-band Feed Array H i data, and find a good correspondence between synthesized maps and Planck’s polarization measurements at 353 GHz.

A Correlated Search for Local Dwarf Galaxies in GALFA-H i and Pan-STARRS

Abstract In recent years, ultrafaint dwarf (UFD) galaxies have been found through systematic searches of large optical surveys. However, the existence of Leo T, a nearby gas-rich dwarf, suggests that there could be other nearby UFDs that are optically obscured but have gas detectable at nonoptical wavelengths. With this in mind, we perform a search of the full Galactic Arecibo L-band Feed Array H i (GALFA-H i) survey, a radio survey that covers one-third of the sky at velocities −650 &lt; V LSR &lt; +650 km s−1, for neutral hydrogen sources. We are able to probe regions of the sky at lower Galactic latitudes and smaller compared to previous explorations. We use the Source Finding Application on GALFA-H i and select all sources with similar properties to Leo T and other local dwarf galaxies. We find 690 dwarf galaxy candidates, one of which is particularly promising and likely a new galaxy near the Galactic plane (b = −8°) that is comparable in velocity width and H i-flux to other recently discovered local volume galaxies. We find we are sensitive to Leo T-like objects out to 1 Mpc at velocities clear from background H I emission. We check each candidate’s corresponding optical fields from Pan-STARRS and fit stars drawn from isochrones, but find no evidence of stellar populations. We thus find no other Leo T-like dwarfs within 500 kpc of the Milky Way in the one-third of the sky covered by the GALFA-H i footprint, and discuss our nondetection in a cosmological context.

Where Are All of the Gas-bearing Local Dwarf Galaxies? Quantifying Possible Impacts of Reionization

Abstract We present an approach for comparing the detections and non-detections of Local Group (LG) dwarf galaxies in large H i surveys to the predictions of a suite of n-body simulations of the LG. This approach depends primarily on a set of empirical scaling relations to connect the simulations to the observations, rather than making strong theoretical assumptions. We then apply this methodology to the Galactic Arecibo L-band Feed Array Hi (GALFA-HI) Compact Cloud Catalog (CCC), and compare it to the suite Exploring the Local Volume In Simulations (ELVIS) of simulations. This approach reveals a strong tension between the naïve results of the model and the observations: while there are no LG dwarfs in the GALFA-HI CCC, the simulations predict ∼10. Applying a simple model of reionization can resolve this tension by preventing low-mass halos from forming gas. However, and if this effect operates as expected, the observations provide a constraint on the mass scale of the dwarf galaxy that reionization impacts. Combined with the observed properties of Leo T, the halo virial mass scale at which reionization impacts dwarf galaxy gas content is constrained to be , independent of any assumptions about star formation.

The GALFA-H i Survey Data Release 2

Abstract We present the Galactic Arecibo L-Band Feed Array H i (GALFA-H i) survey data release 2 (DR2). The survey covers the 21 cm hyperfine transition of Galactic H i from −650 to 650 , with 0.184 channel spacing, 4′ angular resolution, and 150 mK rms noise per 1 velocity channel. DR2 covers the entirety of the sky available from the William E. Gordon 305 m antenna at Arecibo, from decl. −1°17′ to decl. + 37°57′ across all R.A.: 4 steradians or 32% of the sky. DR2 differs in a number of ways from data release 1, which was released in 2011. DR2 is built from a largely separate set of observations from DR1, which were taken in a much more consistent mode. This consistency, coupled with more careful attention to systematics and more advanced data reduction algorithms, leads to a much higher-quality DR2 data product. We present three data sets for public use: H i data cubes, far-sidelobe stray-radiation-corrected column density maps, and results of the Rolling Hough Transform linear feature detection algorithm.

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 353GHz 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 353GHz 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.

MAGNETICALLY ALIGNED H I 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.

CHARACTERIZING THE TURBULENT PROPERTIES OF THE STARLESS MOLECULAR CLOUD MBM 16

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\pm0.2. 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.

THE GALFA-H i COMPACT CLOUD CATALOG

ABSTRACT 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 × 1018 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 104 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 HIGH-RESOLUTION STUDY OF THE H I-H2TRANSITION ACROSS THE PERSEUS MOLECULAR CLOUD

To investigate the fundamental principles of H2 formation in a giant molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and Sigma_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 Sigma_H2 distribution across Perseus. We find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and star-forming regions, suggesting a minimum HI surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2. The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2) ~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 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.

THE GALFA-HI SURVEY: DATA RELEASE 1

We present the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey, and its first full data release (DR1). GALFA-HI is a high resolution (~ 4'), large area (13000 deg^2), high spectral resolution (0.18 km/s), wide band (-700 < v_LSR < +700 km/s) survey of the Galactic interstellar medium in the 21-cm line hyperfine transition of neutral hydrogen conducted at Arecibo Observatory. Typical noise levels are 80 mK RMS in an integrated 1 km/s channel. GALFA-HI is a dramatic step forward in high-resolution, large-area Galactic HI surveys, and we compare GALFA-HI to past, present, and future Galactic HI surveys. We describe in detail new techniques we have developed to reduce these data in the presence of fixed pattern noise, gain variation, and inconsistent beam shapes, and we show how we have largely mitigated these effects. We present our first full data release, covering 7520 square degrees of sky and representing 3046 hours of integration time, and discuss the details of these data.

PHYSICAL PROPERTIES OF COMPLEX C HALO CLOUDS

Observations from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey of the tail of Complex C are presented and the halo clouds associated with this complex cataloged. The properties of the Complex C clouds are compared to clouds cataloged at the tail of the Magellanic Stream to provide insight into the origin and destruction mechanism of Complex C. Magellanic Stream and Complex C clouds show similarities in their mass distributions (slope = -0.7 and -0.6, respectively) and have a common linewidth of 20 - 30 km/s (indicative of a warm component), which may indicate a common origin and/or physical process breaking down the clouds. The clouds cataloged at the tail of Complex C extend over a mass range of 10^1.1 to 10^4.8 solar masses, sizes of 10^1.2 to 10^2.6 pc, and have a median volume density of 0.065 cm^(-3) and median pressure of (P/k) = 580 K cm^{-3}. We do not see a prominent two-phase structure in Complex C, possibly due to its low metallicity and inefficient cooling compared to other halo clouds. From assuming the Complex C clouds are in pressure equilibrium with a hot halo medium, we find a median halo density of 5.8 x 10^(-4) cm^(-3), which given a constant distance of 10 kpc, is at a z-height of ~3 kpc. Using the same argument for the Stream results in a median halo density of 8.4 x 10^(-5) x (60kpc/d) cm^(-3). These densities are consistent with previous observational constraints and cosmological simulations. We also assess the derived cloud and halo properties with three dimensional grid simulations of halo HI clouds and find the temperature is generally consistent within a factor of 1.5 and the volume densities, pressures and halo densities are consistent within a factor of 3.

COMPACT H I CLOUDS FROM THE GALFA-H I SURVEY

The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of numerous isolated, very compact HI clouds at low Galactic velocities, which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ surveyed so far, we have detected 96 of such compact clouds. The detected clouds are cold with a median T$_{k,max}$ (the kinetic temperature in the case in which there is no non-thermal broadening) of 300 K. Moreover, these clouds are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5 \times 10^{18}$ cm$^{-2}$ in HI column density. Most of the clouds deviate from Galactic rotation at the 20-30 km/s level, and a significant fraction show evidence for a multiphase medium and velocity gradients. No counterparts for these clouds were found in other wavebands. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of the compact clouds are related to the Galactic disk, and their distances are likely to be in the range of 0.1 to a few kpc. We discuss various possible scenarios for the formation and maintenance of this cloud population and its significance for Galactic ISM studies.

THE DISRUPTION AND FUELING OF M33

The disruption of the M33 galaxy is evident from its extended gaseous structure. We present new data from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey that show the full extent and detailed spatial and kinematic structure of M33's neutral hydrogen. Over 18% of the HI mass of M33 (M_HI(tot) =1.4 x 10^9 Msun) is found beyond the star forming disk as traced in the far-ultraviolet (FUV). The most distinct features are extended warps, an arc from the northern warp to the disk, diffuse gas surrounding the galaxy, and a southern cloud with a filament back to the galaxy. The features extend out to 22 kpc from the galaxy center (18 kpc from the edge of the FUV disk) and the gas is directly connected to M33's disk. Only five discrete clouds (i.e., gas not directly connected to M33 in position-velocity space) are catalogued in the vicinity of M33, and these clouds show similar properties to Galactic and M31 halo clouds. M33's gaseous features most likely originate from the tidal disruption of M33 by M31 1-3 Gyr ago as shown from an orbit analysis which results in a tidal radius < 15 kpc in the majority of M33's possible orbits. M33 is now beyond the disruptive gravitational influence of M31 and the gas appears to be returning to M33's disk and redistributing its star formation fuel. M33's high mean velocity dispersion in the disk (~18.5 km/s) may also be consistent with the previous interaction and high rate of star formation. M33 will either exhaust its star formation fuel in the next few Gyrs or eventually become star formation fuel for M31. The latter represents the accretion of a large gaseous satellite by a spiral galaxy, similar to the Magellanic Clouds' relationship to the Galaxy.

A peculiar H i cloud near the distant globular cluster Pal 4

We present 21-cm observations of four Galactic globular clusters, as part of the on-going Galactic Arecibo L-band Feed Array (GALFA) H i survey at Arecibo. We discovered a peculiar H i cloud in the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties and likelihood of association with the cluster. We conclude that an association of the H i cloud and Pal 4 is possible, but that a chance coincidence between Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether. New, more stringent upper limits were derived for the other three clusters: M 3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas and the interaction of compact clouds with the Galactic halo gas.