HI4PI: A full-sky HI survey based on EBHIS and GASS

Abstract: Measurement of the Galactic neutral atomic hydrogen (HI) column density, NHI, and brightness temperatures, Tb, is of high scientific value for a broad range of astrophysical disciplines. In the past two decades, one of the most-used legacy HI datasets has been the Leiden/Argentine/Bonn Survey (LAB). We release the HI 4$\pi$ survey (HI4PI), an all-sky database of Galactic HI, which supersedes the LAB survey. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn HI Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). EBHIS and GASS share similar angular resolution and match well in sensitivity. Combined, they are ideally suited to be a successor to LAB. The new HI4PI survey outperforms the LAB in angular resolution (16.2', FWHM) and sensitivity (RMS: 43 mK). Moreover, it has full spatial sampling and thus overcomes a major drawback of LAB, which severely undersamples the sky. We publish all-sky column density maps of the neutral atomic hydrogen in the Milky Way, along with full spectroscopic data, in several map projections including HEALPix.

• The paper presents an integrated full-sky HI survey combining EBHIS and GASS that achieves an angular resolution of 16.2 arcminutes and an RMS noise level of 43 mK.
• It employs advanced baseline modeling and RFI mitigation techniques to ensure precise calibration and consistency across overlapping regions.
• The survey offers comprehensive Galactic HI maps that facilitate detailed studies of Galactic structures, extra-planar gas, and super-shell dynamics.

An Overview of the HI4PI Full-Sky Neutral Hydrogen Survey

The paper "HI4PI: A full-sky \ion{H}{i} survey based on EBHIS and GASS" presents a comprehensive survey of the Galactic neutral atomic hydrogen (\ion{H}{i}), leveraging data from the Effelsberg--Bonn \ion{H}{i} Survey (EBHIS) and the Galactic All-Sky Survey (GASS). This work aims to supersede the legacy Leiden/Argentine/Bonn Survey (LAB), offering enhancements in angular resolution and sensitivity, while providing an extensive dataset that spans the entire sky.

Data Integration and Methodological Advancements

The HI4PI survey integrates EBHIS and GASS data, which possess compatible angular resolutions and sensitivities. The consolidation of these data sources is significant: EBHIS surveys the northern hemisphere, while GASS covers the southern hemisphere, creating a full-sky \ion{H}{i} dataset. Both surveys implement advanced techniques in baseline modeling and RFI mitigation to enhance data quality, employing distinct yet complementary strategies in data acquisition and processing.

Key survey parameters include an angular resolution of 16.2 arcminutes and an RMS noise level of 43 mK, positioning HI4PI as a high-fidelity successor to LAB. HI4PI also utilizes the HEALPix format for uniform data representation, facilitating compatibility with other astronomical datasets. The survey addresses previous limitations in spatial sampling observed in LAB and provides data in multiple map projections to cater to a range of scientific applications.

Numerical Results and Data Quality

The survey demonstrates robust calibration and consistency between EBHIS and GASS intensities, indicating no significant biases when compared with LAB. The paper details comparisons across overlapping regions between EBHIS and GASS, identifying minor discrepancies largely attributable to calibration peculiarities or intrinsic sensitivities within the respective instruments. Such rigorous data validation underscores the survey's reliability.

Discrepancies in radial velocity alignments between EBHIS and GASS are acknowledged, with a noted shift of approximately -0.35 km/s, suggesting potential refinements in future data cross-referencing procedures. Nevertheless, the study confirms the intensity calibration precision across various column densities, ensuring accuracy for applications in the analysis of Galactic structures and phenomena.

Implications and Prospects

The HI4PI survey has substantial implications for astrophysical research. Its enhanced data quality and full-sky coverage enable detailed investigations into the structure and dynamics of \ion{H}{i} in the Milky Way, exploration of extra-planar gas, and thorough analysis of Galactic super-shells. Furthermore, by providing comprehensive column density maps, the HI4PI survey facilitates cross-comparison studies with datasets from other spectral observations, supporting multi-wavelength research initiatives.

The paper discusses the contamination by extragalactic objects, noting the necessity for cautious interpretation when using data near high-velocity clouds and intergalactic regions. This reflects the need for continuous updates and potential caveats when separating Galactic \ion{H}{i} from overlapping extra-galactic emissions.

Looking forward, the HI4PI collaboration sets a foundation for future surveys and refinements, particularly with planned upgrades in data acquisition and processing capabilities. The prospect of a second sky coverage for EBHIS will potentially reconcile any residual discrepancies, enhancing the dataset's value.

In conclusion, the HI4PI survey represents a significant advancement in \ion{H}{i} mapping. Its comprehensive data and methodological rigor offer a pivotal resource for ongoing and future astrophysical research, spanning Galactic to cosmological scales.