Hector Galaxy Survey: Linking the low- and high-mass ends of the initial mass function in star-forming galaxies

Abstract

The stellar initial mass function (IMF) is a fundamental ingredient in galaxy evolution, linking observed integrated light to stellar masses, star-formation rates, and chemical enrichment histories. Constraining the full IMF shape beyond the Milky Way remains challenging, as most studies focus either on the low-mass end of quiescent galaxies or the high-mass end of star-forming galaxies. Here we present the first simultaneous analysis of both ends of the stellar initial mass function (IMF) in 214 star-forming galaxies from the Hector survey (z ∼ 0.01–0.07). We estimate the low-mass end slope ( α _low ) using a stellar population approach that fits IMF-sensitive absorption features with extended star formation histories, while the high-mass end slope ( α _high ) is derived via the Kennicutt diagnostic, which compares the observed H α equivalent width and g – r colour with stellar population synthesis models. We find substantial diversity in IMF shapes, with galaxies spanning combinations of bottom-heavy/light and top-heavy/light slopes. A weak but statistically robust correlation between the low- and high-mass IMF slopes is observed, but partial correlation analysis indicates that this apparent link is largely driven by their mutual dependence on stellar mass and metallicity. Both IMF slopes show significant correlations with stellar mass, star formation activity (traced by H α luminosity and surface density), and stellar metallicity ([M/H]). In general, higher stellar mass, stronger star formation activity, and higher metallicity are associated with both bottom-heavy and top-heavy IMFs. We find that the full IMF shape seems to be modulated by total stellar mass. Partial correlation analysis reveals that α _low is primarily driven by [M/H], whereas α _high is mainly linked to stellar mass and recent star formation. Because α _low traces the IMF over long-term averages and α _high captures only recent (≲ 10 Myr) star formation, the processes shaping each end likely occur over different and possibly decoupled timescales. Our findings challenge the universality of the IMF and emphasise the need for galaxy evolution and stellar population models to incorporate a flexible IMF prescription. Accounting for these variations is essential to build an IMF-consistent picture of galaxy evolution across cosmic time.