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The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a new 400-800 MHz radio interferometer under development for deployment in South Africa. HIRAX will comprise 1024 six meter parabolic dishes on a compact grid and will map most of the southern sky over the course of four years. HIRAX has two primary science goals: to constrain Dark Energy and measure structure at high redshift, and to study radio transients and pulsars. HIRAX will observe unresolved sources of neutral hydrogen via their redshifted 21-cm emission line ('hydrogen intensity mapping'). The resulting maps of large-scale structure at redshifts 0.8-2.5 will be used to measure Baryon Acoustic Oscillations (BAO). BAO are a preferential length scale in the matter distribution that can be used to characterize the expansion history of the Universe and thus understand the properties of Dark Energy. HIRAX will improve upon current BAO measurements from galaxy surveys by observing a larger cosmological volume (larger in both survey area and redshift range) and by measuring BAO at higher redshift when the expansion of the universe transitioned to Dark Energy domination. HIRAX will complement CHIME, a hydrogen intensity mapping experiment in the Northern Hemisphere, by completing the sky coverage in the same redshift range. HIRAX's location in the Southern Hemisphere also allows a variety of cross-correlation measurements with large-scale structure surveys at many wavelengths. Daily maps of a few thousand square degrees of the Southern Hemisphere, encompassing much of the Milky Way galaxy, will also open new opportunities for discovering and monitoring radio transients. The HIRAX correlator will have the ability to rapidly and efficiently detect transient events. This new data will shed light on the poorly understood nature of fast radio bursts (FRBs), enable pulsar monitoring to enhance long-wavelength gravitational wave searches, and provide a rich data set for new radio transient phenomena searches. This paper discusses the HIRAX instrument, science goals, and current status.

The WISE Catalog of Galactic H II Regions contains ∼2000 H II region candidates lacking ionized gas spectroscopic observations. All candidates have the characteristic H II region mid-infrared morphology of WISE12 m m emission surrounding 22 m m emission, and additionally have detected radio continuum emission. We here report Green Bank Telescope hydrogen radio recombination line and radio continuum detections in the X-band (9 GHz; 3 cm) of 302 WISE H II region candidates (out of 324 targets observed) in the zone ℓ 225 20   -, b 6. | |   Here we extend the sky coverage of our H II region Discovery Survey, which now contains nearly 800 H II regions distributed across the entire northern sky. We provide LSR velocities for the 302 detections and kinematic distances for 131 of these. Of the 302 new detections, 5 have ℓ b v , , () coordinates consistent with the Outer Scutum-Centaurus Arm (OSC), the most distant molecular spiral arm of the Milky Way. Due to the Galactic warp, these nebulae are found at Galactic latitudes >1°in the first Galactic quadrant, and therefore were missed in previous surveys of the Galactic plane. One additional region has a longitude and velocity consistent with the OSC but lies at a negative Galactic latitude (G039.183−01.422; −54.9 km s 1-). With Heliocentric distances >22 kpc and Galactocentric distances >16 kpc, the OSC H II regions are the most distant known in the Galaxy. We detect an additional three H II regions near ℓ 150  whose LSR velocities place them at Galactocentric radii >19 kpc. If their distances are correct, these nebulae may represent the limit to Galactic massive star formation.

We derive infrared and radio flux densities of all ∼ 1000 known Galactic H II regions in the Galactic longitude range 17• . 5 < < 65• . Our sample comes from the Wide-Field Infrared Survey Explorer (WISE) catalog of Galactic H II regions (Anderson et al. 2014). We compute flux densities at six wavelengths in the infrared (Spitzer GLIMPSE 8 µm, WISE 12 µm and 22 µm, Spitzer MIPSGAL 24 µm, and Herschel Hi-GAL 70 µm and 160 µm) and two in the radio (MAGPIS 20 cm and VGPS 21 cm). All H II region infrared flux densities are strongly correlated with their ∼ 20 cm flux densities. All H II regions used here, regardless of physical size or Galactocentric radius, have similar infrared to radio flux density ratios and similar infrared colors, although the smallest regions (r < 1 pc), have slightly elevated IR to radio ratios. The colors log 10 (F 24 µm /F 12 µm ) ≥ 0 and log 10 (F 70 µm /F 12 µm ) ≥ 1.2, and log 10 (F 24 µm /F 12 µm ) ≥ 0 and log 10 (F 160 µm /F 70 µm ) ≤ 0.67 reliably select H II regions, independent of size. The infrared colors of ∼ 22% of H II regions, spanning a large range of physical sizes, satisfy the IRAS color criteria of Wood & Churchwell (1989a) for H II regions, after adjusting the criteria to the wavelengths used here. Since these color criteria are commonly thought to select only ultra-compact H II regions, this result indicates that the true ultra-compact H II region population is uncertain. Comparing with a sample of IR color indices from star-forming galaxies, H II regions show higher log 10 (F 70 µm /F 12 µm ) ratios. We find a weak trend of decreasing infrared to ∼ 20 cm flux density ratios with increasing R gal , in agreement with previous extragalactic results, possibly indicating a decreased dust abundance in the outer Galaxy.

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