Systematic variations of the Initial Mass Functionof
early-type galaxies • Francesco La Barbera (INAF/OAC, IT)• Ignacio Ferreras (UCL/MSSL, UK)• Alejandro Vazdekis (IAC, ES)• Ignacio Martín Navarro (IAC, ES)• Reinaldo R. de Carvalho (INPE, BR)• Carsten Weidner (IAC, ES) • Jesús Falcón Barroso (IAC, ES)• Ignacio G. de la Rosa (IAC, ES)• Marina Trevisan (INPE, BR)• Beatriz Barbuy (IAG, BR)• Elena Ricciardelli (Valencia, ES)
Sample and Models
SPIDER sample of 39,993 bright (Mr200/Å, no sky contamination issues
unimodal (single power-law) IMF
bimodal (low-mass tapered; Vazdekis+’96) IMF
MILES extended (MIUSCAT) SSP models (Vazdekis+’12)
0.06
Spectral indices vs. σ and [α/Fe](Ferreras+’13; La Barbera+’13)
Mg4780, TiO1, TiO2, NaI8190, CaT
IMF (4700
Fitting results – observed vs. model indicesIM
F sl
o pe
Metallicity
IMF
slo p
e
Metallicity
All (optical+NIR) indices point to an increase of age, [Z/H], and IMF slope with σ0(see Conroy & van Dokkum ‘12; Spiniello+’14a, b)
(Ferreras+’13; La Barbera+’13)
Unimodal and bimodal models fit equally well (i.e. cannot be singled out from indices)
IMF slope and M*/L vs. σ
Trend from a Kroupa-like IMF (σ≤150km/s), to a bottom-heavy IMF at high σ.F 0
.5=
Indices constrain the mass fraction in low-mass stars at birth in the IMF, F0.5.
(Ferreras+’13; La Barbera+’13)
dynamical Mdyn/LMdyn∝5Reσ2
stellar M*/L
Bimodal 2x smaller than unimodal M*/L. one can have strong IMF-sensitive features, but “low” M*/L (see Smith&Lucey 2013)(see Läsker+’13)
(Vazdekis+’96,’97; Weidner+’13)The time-dependent IMF of ETGs
F0.5=0.6
F0.5=0.3
Kroupa-like IMF matches age-metallicity of ETGs, but not F0.5
bottom-heavy at all times never reaches metallicity [Z/H]>-1 !!
F0.5=0.6F0.5=0.5
D
B
time-dependent IMF is needed to match simultaneously: - age and [Z/H]- F0.5- fraction of stellar remnants (i.e. LMXB rates; see Peacock+’14)
A
Chemical enrichment toy models (Ferreras&Silk2000a,b)
Martín-Navarro et al. (arXiv:1404.6533)
… suggest that the bottom-heavy IMF is confined to the galaxy central regions.
A bottom-heavy IMF in the cores of ETGs ?
…. of optical+NIR features ….
Long-slit observations at 10.4m GTC….
(see POSTER by I. Martín-Navarro)
See also arXiv:1407.4455: “The stellar initial mass function at z>1” by Martìn-Navarro et al.
SummarySignificant trend of IMF slope to increase with σ for the whole population of ETGs
σ, rather than [Mg/Fe], seems to drive the IMF trend.
We cannot distinguish different IMF models (unimodal vs. bimodal) with line strengths, implying a large uncertainty on M*/L.
In contrast, the fraction in low-mass (