HIMALAYA

The HIMALAYA (deep H I observations of MAssive gaLAxies to studY cold gAs) project aims to investigate how isolated massive disk galaxies acquire and regulate their cold gas reservoirs.

Its major scientific goals include but are not limited to the following:

1. to map the distribution and kinematics of diffuse H I around galaxies in order to probe for signatures of cold gas accretion, such as extended disks, anomalous velocity clouds,or inflowing streams;
2. to quantify the diffuse H I fraction and compare it with other baryonic components, thereby constraining the baryon budget in the circumgalactic medium (CGM);
3. to test whether massive galaxies with rotation velocities above 300 km s⁻¹ follow the baryonic Tully–Fisher relation or exhibit deviations at the high-mass end.

To achieve these goals, HIMALAYA has obtained 112 hours of deep VLA D-array H I observations of 14 nearby, massive, and relatively isolated spiral galaxies (v_rot ≥ 300 km s⁻¹, M_* ≥ 10¹¹ M_⊙, d < 150 Mpc). These data reach a H I column density sensitivity of ≲ 10¹⁹ cm⁻², sufficient to detect extended, low-surface-brightness structures beyond the stellar disks. The sample selection excludes galaxies with recent major mergers or strong interactions, ensuring that the detected diffuse gas features are not dominated by tidal effects. In addition, 37 hours of new GBT observations have been allocated in 2025, which will provide the missing short-spacing information and enable us to recover the total H I mass and the diffuse, low-column-density structures down to ∼ 10¹⁷ cm⁻². These complementary single-dish data will be combined with the VLA maps to derive the diffuse H I fraction and better constrain the multiphase baryon budget in the CGM. Together with multi-wavelength data (e.g. ionized gas, X-ray hot gas, and CO gas), HIMALAYA provides a unique dataset to probe the cold gas accretion process and the multiphase CGM in the massive galaxy regime.