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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. [ABSTRACT FROM AUTHOR]

High-spatial-resolution H  i observations have led to the realization that the nearby (within few hundreds of parsecs) Galactic atomic filamentary structures are aligned with the ambient magnetic field. Enabled by the high-quality data from the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope for the Galactic ASKAP H  i survey, we investigate the potential magnetic alignment of the |$\gtrsim\!{10}\, {\rm pc}$| -scale H  i filaments in the Small Magellanic Cloud (SMC). Using the Rolling Hough Transform technique that automatically identifies filamentary structures, combined with our newly devised ray-tracing algorithm that compares the H  i and starlight polarization data, we find that the H  i filaments in the north-eastern end of the SMC main body ('Bar' region) and the transition area between the main body and the tidal feature ('Wing' region) appear preferentially aligned with the magnetic field traced by starlight polarization. Meanwhile, the remaining SMC volume lacks starlight polarization data of sufficient quality to draw any conclusions. This suggests for the first time that filamentary H  i structures can be magnetically aligned across a large spatial volume (⁠|$\gtrsim\!{\rm kpc}$|⁠) outside of the Milky Way. In addition, we generate maps of the preferred orientation of H  i filaments throughout the entire SMC, revealing the highly complex gaseous structures of the galaxy likely shaped by a combination of the intrinsic internal gas dynamics, tidal interactions, and star-formation feedback processes. These maps can further be compared with future measurements of the magnetic structures in other regions of the SMC. [ABSTRACT FROM AUTHOR]

For the first time both H α and far-ultraviolet (FUV) observations from an H  i -selected sample are used to determine the dust-corrected star formation rate density (SFRD: $$\dot{\rho }$$) in the local Universe. Applying the two star formation rate indicators on 294 local galaxies, we determine log($$\dot{\rho } _{ {\rm H}\alpha }) = -1.68^{+0.13}_{-0.05}$$  [M⊙ yr−1 Mpc−3] and log($$\dot{\rho }_{ {\rm FUV}}$$) $$= -1.71^{+0.12}_{-0.13}$$ [M⊙ yr−1 Mpc−3]. These values are derived from scaling H α and FUV observations to the H  i mass function. Galaxies were selected to uniformly sample the full H  i mass ($$M_{\rm{H\,\small {I}}}$$) range of the H  i Parkes All-Sky Survey ($$M_{\rm{H\,\small {I}}} \sim 10^{7}$$ to ∼1010.7 M⊙). The approach leads to relatively larger sampling of dwarf galaxies compared to optically selected surveys. The low H  i mass, low luminosity, and low surface brightness galaxy populations have, on average, lower H α /FUV flux ratios than the remaining galaxy populations, consistent with the earlier results of Meurer. The near-identical H α - and FUV-derived SFRD values arise with the low H α /FUV flux ratios of some galaxies being offset by enhanced H α from the brightest and high mass galaxy populations. Our findings confirm the necessity to fully sample the H  i mass range for a complete census of local star formation to include lower stellar mass galaxies which dominate the local Universe. [ABSTRACT FROM AUTHOR]

Understanding gas flows into and out of the most massive dark matter structures in our Universe, galaxy clusters, is fundamental to understanding their evolution. Gas in clusters is well studied in the hot (> 106 K) and cold (< 104 K) regimes, but the warm gas component (104-106 K) is poorly constrained. It is challenging to observe directly, but can be probed through Ly α absorption studies. We produce the first systematic study of the warm gas content of galaxy clusters through synthetic Ly α absorption studies using cosmological simulations of two galaxy clusters produced with ENZO. We explore the spatial and kinematic properties of our cluster absorbers, and show that the majority of the identified absorbers are due to fast moving gas associated with cluster infall from intergalactic medium (IGM) filaments. Towards the centre of the clusters, however, the warm IGM filaments are no longer dominant and the absorbers tend to have higher column densities and metallicities, representing stripped galaxy material. We predict that the absorber velocity distribution should generally be bi-modal and discuss the effects of cluster size, mass, and morphology on the properties of the identified absorbers, and the overall cluster warm gas content. We find tentative evidence for a change in the well-known increasing NHI with decreasing impact parameter for the most massive dark matter haloes. Our results are compared directly to observations of Ly α absorbers in the Virgo cluster, and provide predictions for future Ly α absorption studies. [ABSTRACT FROM AUTHOR]

We have searched for infrared dust emission from subsets of compact, Galactic neutral hydrogen clouds, with the purpose of looking for dust in high-velocity clouds, identifying low-velocity halo clouds, and investigating the cloud populations defined in the GALFA-H i Compact Cloud Catalog. We do not detect dust emission from high-velocity clouds. The lack of dust emission from a group of low-velocity clouds supports the claim that they are low-velocity halo clouds. We detect dust in the remaining low-velocity clouds, indicating a Galactic origin, with a significantly greater dust-to-gas ratio for clouds with linewidths near 15 km s−1. We propose that this is due to dust associated with ionized gas. [ABSTRACT FROM AUTHOR]

We discuss the support for the cooling of gas directly at the disk-halo interface as a mechanism to continually fuel a galaxy. This may be an important reservoir as there is not enough cold gas observed in galaxy halos and simulations indicate the existing cold clouds will be rapidly destroyed as they move through the surrounding halo medium. We show possible evidence for a net infall of the WIM layer in the Milky Way, simulation results showing the recooling of warm clouds at the disk-halo interface, and GALFA HI data of small, cold HI clouds that could represent this recooling. [ABSTRACT FROM AUTHOR]

ABSTRACT A head-tail high-velocity cloud (HVC) is a neutral hydrogen halo cloud that appears to be interacting with the diffuse halo medium as evident by its compressed head trailed by a relatively diffuse tail. This paper presents a sample of 116 head-tail HVCs across the southern sky ( d < 2°) from the H i Parkes All Sky Survey (HIPASS) HVC catalogue, which has a spatial resolution of 15.5 arcmin (45 pc at 10 kpc) and a sensitivity of cm−2 (5σ). 35 per cent of the HIPASS compact and semi-compact HVCs (CHVCs and:HVCs) can be classified as head-tail clouds from their morphology. The clouds have typical masses of 730 M⊙ at 10 kpc (26 000 M⊙ at 60 kpc) and the majority can be associated with larger HVC complexes given their spatial and kinematic proximity. This proximity, together with their similar properties to CHVCs and:HVCs without head-tail structure, indicates the head-tail clouds have short lifetimes, consistent with simulation predictions. Approximately half of the head-tail clouds can be associated with the Magellanic System, with the majority in the region of the Leading Arm with position angles pointing in the general direction of the movement of the Magellanic System. The abundance in the Leading Arm region is consistent with this feature being closer to the Galactic disc than the Magellanic Stream and moving through a denser halo medium. The head-tail clouds will feed the multi-phase halo medium rather than the Galactic disc directly and provide additional evidence for a diffuse Galactic halo medium extending to at least the distance of the Magellanic Clouds. [ABSTRACT FROM AUTHOR]

We have found the peculiar galaxy NGC 922 to be a new drop-through ring galaxy using multiwavelength (ultraviolet–radio) imaging and spectroscopic observations. Its ‘C’-shaped morphology and tidal plume indicate a recent strong interaction with its companion which was identified with these observations. Using numerical simulations we demonstrate that the main properties of the system can be generated by a high-speed off-axis drop-through collision of a small galaxy with a larger disc system, thus making NGC 922 one of the nearest known collisional ring galaxies. While these systems are rare in the local Universe, recent deep Hubble Space Telescope images suggest they were more common in the early Universe. [ABSTRACT FROM AUTHOR]

Intergalactic HII regions, far from the confines of a galactic disk, represent a mode of star formation in low-density gas outside of galaxies. The figure below (left) shows an R-band continuum image of NGC 1533 from the SINGG Hα survey (Meurer et al. 2006) overlaid with HI contours and the location of three intergalactic HII regions discovered by Ryan-Weber et al. (2004). The HI contours are 1.6, 2.0, 2.4, 2.8, 3.2 and 4.0 ×1020 cm−2 and have a resolution of ~1′. ACS/HRC images of the intergalactic HII regions (right) are composites of UV, V, and I bands. The half-light radii of the clusters associated with regions 1, 2, and 5 are 24.7, 21.7, and 17.0 pc, respectively, at the distance to NGC 1533 (21 Mpc; Tonry et al. 2001). Assuming a Salpeter IMF with Mup = 100, Hα/UV ratios indicate a small number of ionizing O stars relative to the total number of UV-emitting O and B stars. These young (4-6 Myr), intergalactic stellar populations lend valuable insight to our understanding of the methods by which star formation is triggered and may even represent the first episodes of star formation in emerging galaxies. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]