||Observational Constraints to the Initial Mass Function in Normal Late-Type Galaxies
||440, UP2010: Have Observations Revealed a Variable Upper End of the Initial Mass Function?
||Boselli, A.; Boissier, S.; Cortese, L.; Buat, V.; Hughes, T.; Gavazzi, G.
||We use GALEX far-ultraviolet (FUV, 1539 Å) and Hα
data for a large sample of nearby objects to study the
shape of the initial mass function (IMF) of normal late-type
galaxies in the high mass (m ≥ 2 M☉) stellar range.
The data are corrected for dust attenuation using the most
accurate techniques at present available, namely the Balmer
decrement for Hα data and the total far-infrared to
FUV flux ratio for GALEX data. The Hα to FUV flux ratio
of the observed galaxies increases with their total stellar mass
by a factor of ∼ 1.4 dex in the stellar mass range
107 ≤ Mstar ≤ 1011 M☉ but reduces
to ∼ 0.2 dex when AGN, highly inclined or starburst galaxies
are excluded, becoming a weak trend whose statistical significance
needs still to be confirmed. We show however that this weak
trend might be totally removed using different extinction
correction recipes and is thus not robust evidence of an
intrinsically varying Hα over FUV flux ratio in
late-type galaxies. If real, this trend can be explained
by variations of the slope of the IMF from γ = 2.35
for massive galaxies to γ = 2.60 in low luminosity
systems. We show however that this observed trend can also
be due to the different micro history of star formation
in massive galaxies with respect to dwarf systems.