Interstellar space is not empty. It is filled with a tenuous mixture of atoms, molecules, dust grains, cosmic rays, and magnetic fields, collectively known as the interstellar medium (ISM). The dominant constituent by mass is neutral atomic hydrogen gas (H I), which can be traced by spectral line radio emission at a wavelength of 21 centimeters. This H I emission is of key importance to understanding the motions, structure, and physical properties of the diffuse ISM -- the arena in which stars form, evolve, and die -- and is also critical to understanding how all galaxies in the universe function as complex systems.

The I-GALFA survey (AO project A2186) is mapping H I in the inner parts of our own Milky Way Galaxy with the Arecibo 305-meter telescope, the largest and most sensitive single radio dish in the world. With the use of the 7-beam Arecibo L-band Feed Array (ALFA) receiver, Arecibo can carry out mapping surveys 7 times faster, allowing a rapid, detailed look at this key region of the Galaxy. Among I-GALFA's targets of scientific study are cold atomic gas in and outside star-forming molecular clouds, faint high-velocity H I emission associated with old, hidden supernovae, shells and chimneys produced by star formation and disk-halo energetics, and stochastic filamentary structure from turbulent energy cascades and magnetic fields.

Like other GALFA H I surveys using the GALSPECT spectrometer, I-GALFA has a 3.35-arcminute FWHM beam and 0.184 km/s channels covering an LSR velocity range of -700 to +700 km/s. Its anticipated full-resolution, empty-field noise is T_rms ~ 0.25 K.

As the following table shows, I-GALFA complements large synthesis array surveys like the Canadian and VLA Galactic plane surveys, which have less sensitivity, velocity resolution, and latitude coverage, but more angular resolution and longitude coverage (see I-GALFA Fields). I-GALFA also complements other single-dish surveys with more sky coverage but lower angular or velocity resolution. The table below compares I-GALFA's empty-field sensitivity to overlapping surveys. Some velocity binning and angular smoothing examples are included, presuming T_rms is proportional to (Beam)^-1 (dV_chan)^-1/2.

Survey	Telescope(s)	Beam	dVchan	Trms
CGPS	DRAO ST + 26m	1'/cos(dec)	0.824 km/s	~ 3 K
VGPS	VLA-D + GBT	1'	0.824 km/s	~ 2 K
I-GALFA (full res)	AO 305m	3.35'	0.184 km/s	0.25 K
I-GALFA (bin1)	AO 305m	3.35'	0.824 km/s (bin1)	0.12 K
I-GALFA (bin2)	AO 305m	3.35'	2.347 km/s (bin2)	0.07 K
I-GALFA (bin1,sm1)	AO 305m	5.65' (sm1)	0.824 km/s (bin1)	0.07 K
GASS	Parkes 64m	15'	0.824 km/s	0.07 K
LAB	Dwingeloo 25m	36'	1.031 km/s	0.07 K

I-GALFA has two commensal astronomy survey partners: the Zone of Avoidance (ZOA) H I search for external galaxies hidden in the plane of the Milky Way (A2144), and the GALFA Continuum Transit Survey (GALFACTS) to map interstellar magnetic field structure with polarized radio continuum emission (A2390). These three projects represent the first triple-survey of its kind undertaken at Arecibo. A smaller H I + continuum double survey (A2174 + A2294) was carried out in 2006 in the Perseus molecular cloud region.