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 masses and galaxy star formation histories. In this work we focus on most massive galaxies nearby universe $\log(M_{\star}/M_{\odot})>11.2$. We obtain high quality Magellan/LDSS-3 long slit spectroscopy with wide wavelength coverage $0.4\mu{\rm m}-1.01\mu{\rm m}$ for 41 early-type (ETGs) MASSIVE survey, derive S/N spectra within an aperture $R_{\rm e}/8$. Using detailed synthesis models, constrain elemental abundances IMF each through full spectral modeling. All ETGs our sample have that steeper than Milky Way (Kroupa) IMF. best-fit mismatch parameter, $\alpha_{\rm IMF}=(M/L)/(M/L)_{\rm MW}$, ranges from 1.12 to 3.05, average $\langle \alpha_{\rm IMF} \rangle=1.84$, suggesting average, more bottom-heavy Salpeter. Comparing estimated dynamical mass, find smaller their $1\sigma$ uncertainty. complement lower-mass literature, confirm $\log(\alpha_{\rm IMF})$ positively correlated $\log(\sigma)$, $\log(M_{\star})$, $\log(M_{\rm dyn})$. centers bottom-heavy. addition, both [Mg/Fe] total metallicity [Z/H]. suggestive evidence effective surface density $\Sigma_{\rm Kroupa}$ might be responsible variation IMF}$. conclude $\sigma$, [Z/H] are primary drivers global variation.