HD² Science

Scientific Goals and Research Topics

Primary Science Goals

The HD² survey is designed to open a new parameter space in extragalactic HI astronomy: deeper than wide-field surveys and wider than pointed deep surveys, within a field richly covered by DESI, HSC-SSP, and other multiwavelength datasets. Our science goals span from the statistics of neutral gas in the local universe to the HI distribution of the large structure.

1. Census of HI-Bearing Galaxies and the HI Mass Function

A fundamental goal of HD² is to provide a highly complete census of HI-bearing galaxies. By leveraging FAST's exceptional sensitivity over a contiguous 100 deg² area, the survey minimizes selection effects and cosmic variance. This enables a robust measurement of the HI mass function (HIMF), providing a reliable benchmark for cosmological simulations and tracing the distribution of neutral gas in the local universe.

2. HI Scaling Relations and Environmental Effects in Synergy with DESI

The extensive overlap with DESI and the Legacy Survey group catalogs allows for a multi-dimensional study of HI gas fractions and scaling relations. We investigate how the neutral gas content of galaxies correlates with stellar mass, star formation rate, and morphology. Furthermore, by stacking HI spectra across different environments—from isolated field galaxies to groups and clusters—we can quantify how gas reservoirs are affected by their large-scale surroundings and halo properties.

3. Cosmic HI Density ΩHI and Intensity Mapping

HD² constrains the comoving HI density ΩHI through both direct detections and spectral stacking of optically-selected galaxies. The deep FAST data further enable HI intensity mapping to probe the mean HI signal out to z ~ 0.3, tracing the evolution of neutral gas content across cosmic time and providing complementary constraints to measurements from other redshifts.

4. Large-Scale Structure and Clustering

With a high source density of HI detections within the DESI footprint, HD² maps the three-dimensional distribution of HI galaxies with high fidelity. This enables precise measurements of HI galaxy clustering and redshift-space distortions, revealing how HI-selected samples trace the underlying matter distribution and the filamentary structures of the cosmic web.

5. Discovery of Rare and Exotic Populations

The combination of deep FAST data with high-resolution HSC-SSP imaging facilitates the discovery of rare system populations. This includes ultra-diffuse galaxies (UDGs), low surface brightness systems, and potential optically-dark gas clouds. The blind nature of the survey ensures an unbiased search for these exotic objects, maximizing the potential for discovering new classes of HI-rich systems.

Multi-wavelength Resources

The HD2 survey region has been or will be observed by several other facilities, enabling rich multi-wavelength science:

  • Optical spectroscopy and photometry: SDSS, DESI, HSC-SSP, and other surveys for redshift confirmation and stellar population analysis
  • Infrared: WISE data for dust and stellar mass estimates
  • CO observations: Molecular gas measurements to complement HI data
  • UV: GALEX data for star formation rate indicators
  • Radio continuum: NVSS/FIRST, and stars formation tracers