All measurements of cosmic star formation must assume the stellar initial mass function (IMF) to extrapolate from the star-formation rate (SFR) measured for massive stars to the total SFR. Traditional determinations of the IMF in galaxies are made at ultraviolet, optical and near-infrared wavelengths, which cannot be probed in dusty gas-rich galaxies. The 13C/18O abundance ratio in the ISM – which were produced mainly by low-mass stars and massive stars, respectively -- has been demonstrated as a sensitive index of the IMF in gas-rich galaxies. Besides the 13C/18O ratio, which is associated with different C and O major isotopes, 17O and 18O are likely even better for tracing the IMF slope because of the same root in the Oxygen family. Here the speaker presents their new ALMA observations and archival data of 17O and 18O in nearby galaxies, covering from low-metallicity dwarf galaxies to gas-rich starburst galaxies. A systematic trend of 17O/18O ratios from starbursts to low-SFR galaxies implies that variable IMF is inevitable. He will also present their recent benchmarking on the chemical evolution against our Milky Way, especially extending to the low-metallicity outer arm region.

BIO
Zhiyu Zhang got his Ph.D. degree in 2014, from the Purple Mountain Observatory, Chinese Academy of Sciences. After that he worked as a postdoc both at the University of Edinburgh and the European Southern Observatory. In May 2019, he joined the faculty of the School of Astronomy and Space Science at the Nanjing University. Zhiyu Zhang is interested in radio/mm/submm astronomy, including physical conditions of multi-phase interstellar medium, radiative transfer between the Cosmic Microwave Background and cold dust and gas in the early Universe, dense molecular gas in galaxies, and astrochemistry. Recently he is more focusing on the stellar initial mass function and elemental abundances in the ISM, connected with galactic chemical evolution.