A Survey of Ionized Gas in the Galaxy, made with the Arecibo telescope (SIG-GMA) uses the Arecibo L-band Feed Array (ALFA) to fully sample the Galacticobservable with the telescope in radio recombination lines (RRLs). Processed data sets are being produced in the form of data cubes of 2 • (along l)×4 • (along b)×151 (number of channels), archived and made public. The 151 channels cover a velocity range of 600 km s −1 and the velocity resolution of the survey changes from 4.2 km s −1 to 5.1 km s −1 from the lowest frequency channel to the highest frequency channel, respectively. RRL maps with 3.4 ′ resolution and line flux density sensitivity of ∼ 0.5 mJy will enable us to identify new HII regions, measure their electron temperatures, study the physics of photodissociation regions (PDRs) with carbon RRLs, and investigate the origin of the extended low density medium (ELDM). Twelve Hnα lines fall within the 300 MHz bandpass of ALFA;they are resampled to a common velocity resolution to improve the signal-to-noise ratio (SN) by a factor of 3 or more and preserve the line width. SIGGMA will produce the most sensitive fully sampled RRL survey to date. Here we discuss the observing and data reduction techniques in detail. A test observation toward the HII region complex S255/S257 has detected Hnα and Cnα lines with SN > 10.

The Survey of Ionized Gas of the Galaxy, Made with the Arecibo telescope (SIGGMA) provides a fully-sampled view of the radio recombination line (RRL) emission from the portion of the Galactic plane visible by Arecibo. Observations use the Arecibo L-band Feed Array (ALFA), which has a FWHM beam size of 3 ′ .4. Twelve hydrogen RRLs from H163α to H174α are located within the instantaneous bandpass from 1225 MHz to 1525 MHz. We provide here cubes of average ("stacked") RRL emission for the inner Galaxy region 32 • ≤ ℓ ≤ 70 • , |b| ≤ 1.5 • , with an angular resolution of 6 ′ .The stacked RRL rms at 5.1 km s −1 velocity resolution is ∼ 0.65 mJy beam −1 , making this the most sensitive large-scale fully-sampled RRL survey extant. We use SIGGMA data to catalogue 319 RRL detections in the direction of 244 known H ii regions, and 108 new detections in the direction of 79 H ii Corresponding author: Bin Liu bliu@nao.cas.cn a The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc.2 Liu et al.region candidates. We identify 11 Carbon RRL emission regions, all of which are spatially coincident with known H ii regions. We detect RRL emission in the direction of 14 of the 32 supernova remnants (SNRs) found in the survey area. This RRL emission frequently has the same morphology as the SNRs. The RRL velocities give kinematic distances in agreement with those found in the literature, indicating that RRLs may provide an additional tool to constrain distances to SNRs. Finally, we analyze the two bright star-forming complexes: W49 and W51. We discuss the possible origins of the RRL emission in directions of SNRs W49B and W51C.

The Arecibo L-band Feed Array (ALFA) is being used to conduct a low-Galactic latitude survey, to map the distribution of galaxies and large-scale structures behind the Milky Way through detection of galaxies' neutral hydrogen (H i) 21 cm emission. This Zone of Avoidance (ZOA) survey finds new H i galaxies which lie hidden behind the Milky Way, and also provides redshifts for partially obscured galaxies known at other wavelengths. Before the commencement of the full survey, two low-latitude precursor regions were observed, totaling 138 deg 2 , with 72 H i galaxies detected. Detections through the inner Galaxy generally have no cataloged counterparts in any other waveband, due to the heavy extinction and stellar confusion. Detections through the outer Galaxy are more likely to have Two Micron All Sky Survey counterparts. We present the results of these precursor observations, including a catalog of the detected galaxies, with their H i parameters. The survey sensitivity is well described by a flux-and linewidth-dependent signal-to-noise ratio of 6.5. ALFA ZOA galaxies, which also have H i measurements in the literature, show good agreement between our measurements and previous work. The inner Galaxy precursor region was chosen to overlap the H i Parkes Zone of Avoidance Survey, so ALFA performance could be quickly assessed. The outer Galaxy precursor region lies north of the Parkes sky. Low-latitude large-scale structure in this region is revealed, including an overdensity of galaxies near = 183• and between 5000-6000 km s −1 in the ZOA. The full ALFA ZOA survey will be conducted in two phases: a shallow survey using the observing techniques of the precursor observations, and also a deep phase with much longer integration time, with thousands of galaxies predicted for the final catalog.

The Arecibo L-band Feed Array Zone of Avoidance (ALFA ZOA) Deep Survey is the deepest and most sensitive blind H I survey undertaken in the ZOA. ALFA ZOA Deep will cover about 300 square degrees of sky behind the Galactic Plane in both the inner ( l b 30 7 5; 2°⩽ ⩽°⩽ |°|) and outer ( l b 175 207 ; 2 1°⩽ ⩽°−°⩽ ⩽ +°) Galaxy, using the Arecibo Radio Telescope. First results from the survey have found 61 galaxies within a 15 square degree area centered on l 192 =°and b = −2°. The survey reached its expected sensitivity of rms = 1 mJy at 9 km s −1 channel resolution, and is shown to be complete above integrated flux, F HI = 0.5 Jy km s −1 . The positional accuracy of the survey is 28″ and detections are found out to a recessional velocity of nearly 19,000 km s −1 . The survey confirms the extent of the Orion and Abell 539 clusters behind the plane of the Milky Way and discovers expansive voids, at 10,000 and 18,000 km s −1 . Twenty-six detections (43%) have a counterpart in the literature, but only two of these have known redshifts. Counterparts are 20% less common beyond v hel = 10,000 km s −1 and 33% less common at extinctions higher than A B = 3.5 mag. The ALFA ZOA Deep survey is able to probe large scale structure beyond redshifts that even the most modern wide-angle surveys have been able to detect in the ZOA at any wavelength.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has started the Commensal Radio Astronomy FasT Survey (CRAFTS). In this paper, we use the technical parameters of FAST derived from commissioning observations to simulate the completeness function for extra-galactic HI survey of CRAFTS, HI galaxies from two kinds of mock catalogues are selected. One is generated by Monte-Carlo simulation based on the interpolated Mass-velocity Width Function (MWF) of the ALFALFA $100\%$ (a.k.a. α.100) catalogue. The other is constructed by semi-analytic N-body simulation based on the ΛCDM model. Our results suggest that a 2-pass CRAFTS extra-galactic HI survey will be able to detect nearly 4.8 × 105 galaxies, from which the faint end slope of the HI Mass Function (HIMF) can be recovered to 107 M⊙ and the ”knee mass” of the HIMF can be measured to a redshift of 0.1. Considering the RFI status and sensitivity limitation, CRAFTS will be efficient in detecting HI galaxies at redshifts below 0.1, which implies a tremendous potential in exploring the galaxy interactions in different environments and the spatial distribution of HI galaxies in the local universe.