The MASSIVE Survey. XVI. The Stellar Initial Mass Function in the Center of MASSIVE Early-type Galaxies

The stellar initial mass function (IMF) is a fundamental property in the measurement of stellar masses and galaxy star formation histories. In this work, we focus on the most massive galaxies in the nearby universe log(M*/M-circle dot) > 11.2. We obtain high-quality Magellan/LDSS-3 long-slit spectroscopy with a wide wavelength coverage of 0.4-1.01 mu m for 41 early-type galaxies (ETGs) in the MASSIVE survey and derive high signal-to-noise spectra within an aperture of R-e/8. Using detailed stellar synthesis models, we constrain the elemental abundances and stellar IMF of each galaxy through full spectral modeling. All the ETGs in our sample have an IMF that is steeper than a Milky Way (Kroupa) IMF. The best-fit IMF mismatch parameter, alpha(IMF) = (M/L)/(M/L)(MW), ranges from 1.1 to 3.1, with an average of <alpha(IMF)> = 1.84, suggesting that on average, the IMF is more bottom heavy than Salpeter. Comparing the estimated stellar masses with the dynamical masses, we find that most galaxies have stellar masses that are smaller than their dynamical masses within the 1 sigma uncertainty. We complement our sample with lower-mass galaxies from the literature and confirm that log(alpha(IMF)) is positively correlated with log(sigma), log(M*), and log(M-dyn). From the combined sample, we show that the IMF in the centers of more massive ETGs is more bottom heavy. In addition, we find that log(alpha(IMF)) is positively correlated with both [Mg/Fe] and the estimated total metallicity [Z/H]. We find suggestive evidence that the effective stellar surface density Sigma(Kroupa) might be responsible for the variation of alpha(IMF). We conclude that sigma, [Mg/Fe], and [Z/H] are the primary drivers of the global stellar IMF variation.