Science Highlights from the GALFA-HI Survey

The GALFA-HI Survey comprises various projects covering a diverse range of scientific questions.  Some of the projects are described below.

1)  The Leo Cloud

This Arecibo-derived image depicts an interstellar gas cloud, full of atomic hydrogen. It lies close to the sun, within 23 light-years, located well inside what is called the "local bubble"--a large nearly empty region inside of which we live. This cloud is about 50 times longer than the moon's diameter, but seems to be beyond razor thin. The internal motions are comparable to the speed of a commercial jet airliner. This cloud is the coldest known, with a temperature of under 20 degrees above absolute zero. We regard these properties as highly unusual for interstellar clouds--yet, as the closest cold cloud to us, the Copernican principle says it should be typical. The cloud, being so near to us, makes an excellent laboratory for the study of the gas that surrounds us in the Galaxy. Ongoing Arecibo observations will reveal further mysteries of this cloud (Heiles & Peek).

An ultra-cold cloud, thought to be one of the closest to the sun.
Intensity represents column density, while color represents the changing velocity
of the cloud, due largely to its bulk motion.

2) A whirlpool of hydrogen in the Milky Way
Our Milky Way is filled with hydrogen atoms, emitting radiation at a wavelength of 21 cm which is invisible to our eyes but easily detectable by sensitive radio telescopes. Galactic hydrogen is highly complex and turbulent, and full of clouds, filaments and knots. This artistic sculpturing of gas is produced by constant stirring from stellar winds and explosions of massive stars (Lee & Stanimirovic).

GALFA-HI peak brightness temperature map of a region in the Milky Way
 showing complex structures in atomic ISM

3) HI clouds in the Galactic halo

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). 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).

HI column density and central velocity along the line of sight for

a subcomplex of HVCs at the tip of the anticenter complex.

4) Gaseous environment of M33


The GALFA-HI Survey probes M33 and its surrounding environment at unprecedented sensitivity.  The data show the full extent and detailed spatial and kinematic structure of M33’s H I.  We find that over 18% of the HI mass of M33 (which is 1.4 × 10 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.  These features are likely to have been pulled out during an interaction with M31 and may now be falling back and fueling the star formation in the disk (Putman et al. 2009).

  HI column density map of M33 (contours), with colors representing the velocity centroids.

5)  Magellanic Stream: Extension and interaction with the hot halo

 Our GALFA-HI observations of the Magellanic Stream (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.

6) 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.

7) HI in the Galactic disk-halo transition region

 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. 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.

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.

8)  The break-up of accreting gas

With the resolution of GALFA-HI and the distance constraint of 10 kpc for Complex C (Thom et al. 2008), we are able to produce a halo cloud mass spectrum for the first time and trace the break-up of the largest accreting structures in the Galactic halo.   Hsu et al. (2011) presents the physical properties of clouds at the tail of Complex C and the Magellanic Stream and determines the density of the surrounding halo gas at the positions of these complexes.   This paper also compares the derived properties to simulations to assess the accuracy of the derivations.  The physical properties of halo clouds have been elusive for 50 years and are crucial to understand their impact and role in galaxy evolution.

The mass spectrum for clouds that are part of the accreting tail of Complex C.

 For more science results check the following websites:
 Highlights from GALFA:

 HIghtlights from I-GALFA: