[Mg/Fe] and variable initial mass function: Revision of [α/Fe] for massive galaxies
arxiv(2024)
Abstract
Observations of nearby massive galaxies have revealed that they are older and
richer in metals and magnesium than their low-mass counterparts. In particular,
the overabundance of magnesium compared to iron, [Mg/Fe], is interpreted to
reflect the short star formation history that the current massive galaxies
underwent early in the Universe. We present a systematic revision of the
[Mg/Fe] - velocity dispersion (σ) relation based on stacked spectra of
early-type galaxies with a high signal-to-noise ratio from the Sloan Digital
Sky Survey (SDSS). Using the penalized pixel-fitting (pPXF) method of
Cappellari Emsellem 2004 and the Vazdekis et al. 2015 MILES single stellar
population (SSP) models, we fit a wide optical wavelength range to measure the
net α-abundance. The combination of pPXF and α-enhanced MILES
models incorrectly leads to an apparently decreasing trend of [α/Fe]
with velocity dispersion. We interpret this result as a consequence of
variations in the individual abundances of the different α-elements.
This warrants caution for a naïve use of full spectral fitting algorithms
paired with stellar population models that do not take individual elemental
abundance variations into account, especially when deriving averaged quantities
such as the mean [α/Fe] of a stellar population. In addition, and based
on line-strength measurements, we quantify the impact of a non-universal
initial mass function on the recovered abundance pattern of galaxies. In
particular, we find that a simultaneous fit of the slope of the initial mass
function and the [Mg/Fe] results in a shallower [Mg/Fe]-σ relation.
Therefore, our results suggest that star formation in massive galaxies lasted
longer than what has been reported previously, although it still occurred
significantly faster than in the solar neighbourhood.
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