The GALFA-HI sub-consortium comprises various projects covering a diverse range of scientific questions. Some of the projects are described below. 1) HI in the Galactic disk-halo transition region The circulation of matter between the Galactic disk and halo proceeds through a combination of star-formation disk-halo flows and the accretion of intergalactic matter. Hence, to get a better picture of the mechanisms governing the two-way flow of gas between the disk and halo, the transition region between the Galactic thin disk and the halo, or the gaseous Galactic thick disk (|z|~ 1.5 kpc), is of special interest. The precursor GALFA-HI observations of a region in the Galactic anticenter reveal numerous isolated, small (a few parsecs in size), and cold ( T_kinetic < 400 K) HI clouds at low negative velocities, distinctly separated from the HI disk emission (“low-velocity clouds” (LVCs); Stanimirovic et al. 2006). These clouds are most likely located in the transition region between the Galactic disk and halo (at scale heights of ~ 60–900 pc), yet they have properties of typical cold neutral clouds. LVCs are colder and, most likely, smaller and less massive than Lockman's clouds in the disk/halo interface region of the inner Galaxy. The existence of a large number of LVCs in the outer Galaxy demonstrates that the cloudy and frothy character of the interface region is a general phenomenon, not restricted exclusively to the inner Galaxy. We will systematically search our survey data cubes, catalog LVCs, and examine the LVC properties at various Galactic longitudes.  An example of structure found in the anticenter data cube: three remarkable small HI clouds at the LSR velocity -21 km/s. All pixels with T_B > 12 K have been masked out. Investigation of various types of Galactic HI clouds provides a better understanding of the formation mechanisms for molecular clouds, a census of the abundant cold atomic clouds, and information on the physical state of high-latitude clouds in the Galaxy. High resolution observations of halo clouds (< 5′ and < 1 km/s ) have previously been limited to individual pointings, leaving many questions involving the detailed kinematic and spatial structure of the halo clouds and their interaction with the diffuse halo open to speculation. GALFA-HI is the first survey capable of assessing the detailed properties of a large number of high-velocity clouds (HVCs). Our initial GALFA-HI observations of a subcomplex of HVCs at the tip of the anticenter complex show morphological details suggesting an interaction with the ambient halo medium and differential drag within the cloud subcomplex (Peek et al. 2007).  HI column density and central velocity along the line of sight for a subcomplex of HVCs at
the tip of the anticenter complex. Further, models that reproduce the observed HVCs also predict clouds at lower radial velocities that may easily be confused with Galactic disk (|z| < 1 kpc) gas. We search for these low-velocity halo clouds (LVHCs) using the initial data from GALFA-HI and the IRAS data using a new technique. This technique is based upon the expectation that such clouds should, like HVCs, have very limited infrared thermal dust emission as compared to their HI column density. Using this method we find that there exist low-velocity clouds that have extremely low dust-to-gas ratios, consistent with being Galactic halo LVHCs (Peek et al. 2008). 3) Magellanic Stream: Extension and interaction with the hot halo The Magellanic Stream (MS), a 10° wide tail of HI emanating from the Magellanic Clouds and trailing for almost 100° on the sky (decl.  −60 to +20°),
is the only clear example of a gaseous halo stream in the Milky Way's close proximity. While it is well accepted that the MS is the result of
interactions between the Milky Way (MW) and the Magellanic Clouds, the
relative importance of tidal stripping and various kinds of
gas dynamical interactions is still very much under debate. Our GALFA-HI observations of the MS found four large-scale, coherent HI
streams, extending continuously over a length of 20°, each stream
possessing different morphology and velocity gradients. The newly
discovered streams provide strong support for the tidal model of the MS
formation by Connors et al. (2006),
who suggested a spatial and kinematic bifurcation of the MS. The
observed morphology and kinematics suggest that three of these streams could be interpreted as a three-way splitting of the main MS filament,
while the fourth stream appears much younger and may have originated
from the Magellanic Bridge (Stanimirovic et al. 2008). The velocity field, of the MS tip. Color represents the velocity centroids and brightness represents integrated intensity. 4) Gaseous environment of M33 The evolution of M33 depends on its interaction with its environment. This interaction can be traced through observations of the gaseous halo of M33 to detect possible satellite streams, condensing halo clouds, and/or extensions from M33's disk. The GALFA-HI survey probe the gaseous halo of M33 and its surrounding environment at unprecedented sensitivity. The data shows the full extent and detailed spatial and kinematic structure of M33’s H I. The disruption of the M33 galaxy is evident from its extended gaseous structure. We find that over 18% of the HI mass of M33 ( 1.4 × 10^9 Msolar ) is found beyond the star forming disk. The most distinct features seen in our data 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 extraplanar 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 gaseous disk. The extraplanar 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 (Putman et al. 2009).  HI column density map of M33 (contours). Colours represent the velocity centroids 5) HI in Local Group dwarf galaxies The gas content of dwarf galaxies can help us to understand dwarf galaxy evolution and the hot halo medium of the more massive galaxies with which the dwarfs interact. The GALFA-HI survey can be used to explore HI in the environment of the newly discovered dwarf galaxies and search for additional gas-rich Local Group dwarf galaxies. Previous results using lower sensitivity and resolution data have found all of the new satellites discovered in the SDSS have limits on their HI masses which range from <13 Msolar to <3×104 Msolar, except for Leo T, and that galaxies within 300 kpc of the Milky Way or Andromeda are all undetected in HI to low limits (Grcevich & Putman 2009). The most favored explanation for the lack of HI in dwarf galaxies at small galactocentric distances is ram pressure stripping of the gas in the dwarf galaxy by the larger galaxy's hot halo gas. Local Group dwarf galaxies that have not yet been discovered are likely to be further away and therefore are also more likely to be gas-rich and detected by the GALFA-HI survey. |