Discovery of diffuse HI emission in the extended halo of local galaxies

The accretion rate of neutral hydrogen (HI) to the disk of nearby (<25 Mpc) spiral galaxies as observed with radio interferometers is insufficient to explain the star-formation rate (SFR) in these galaxies, indicating that rest of the hydrogen in the circumgalactic medium (CGM) might be diffuse and extended, not probe-able by interferometers due to short-spacing. Because single-dish can probe all spatial scales, we look for 21-cm emission from the diffuse neutral CGM of NGC 891 and NGC 4565 using the Green Bank Telescope. By designing a novel approach complementary to the existing mapping techniques, and by comparing the measurements between single-dish and interferometers, we have detected the diffuse, extended (~100 kpc), massive (~108 Msun), neutral CGM at an unprecedented sensitivity of 1.6 x 1016 cm-2. The scale height and mass distribution of this diffuse HI are consistent with the bathtub model of star formation, attesting to the relation between HI accretion and star formation.