VLT observations of metal-rich extra galactic HII regions. I. Massive star populations and the upper end of the IMF

TL;DR

This study uses VLT spectra of metal-rich HII regions to identify Wolf-Rayet stars, analyze their features, and set lower limits on the upper mass cutoff of the IMF, finding no evidence for variation with metallicity.

Contribution

It provides the first large set of metal-rich WR regions and robustly estimates the upper mass cutoff of the IMF using direct star population data.

Findings

Detection of WR stars in 27 metal-rich HII regions.

Derived lower limit for IMF upper mass cutoff of 60-90 Msun.

No evidence for systematic variation of the IMF upper end with metallicity.

Abstract

We have obtained high quality FORS1/VLT optical spectra of 85 disk HII regions several nearby spiral galaxies. Our sample of metal-rich HII regions with metallicities close to solar and higher reveal the presence of Wolf-Rayet (WR) stars in 27 objects from the blue WR bump and 15 additional candidate WR regions. This provides for the first time a large set of metal-rich WR regions. Approximately half of the WR regions also show broad CIV emission attributed to WR stars of the WC subtype (...). The WR regions show quite clear trends between their observed WR features and the Hbeta emission line. Detailed synthesis models are presented to understand/interpret these observations. (...) The availability of a fairly large sample of metal-rich WR regions allows us to improve existing estimates of the upper mass cut-off of the IMF in a robust way and independently of detailed modeling: from the observed maximum Hbeta equivalent width of the WR regions we derive a LOWER LIMIT for M_up of 60-90 Msun in the case of a Salpeter slope and larger values for steeper IMF slopes. From our direct probe of the massive star content we conclude that there is at present no evidence for systematic variations of the upper mass cut-off of the IMF in metal-rich environments, in contrast to some claims based on indirect nebular diagnostics. (abridged abstract)