1. Evidence of a Bottom-heavy Initial Mass Function in Massive Early-type Galaxies from Near-infrared Metal Lines
- Author
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Andrew J. Monson, David J. Lagattuta, S. Eric Persson, Duncan A. Forbes, Jeremy Mould, Nicola Pastorello, Centre de Recherche Astrophysique de Lyon (CRAL), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), and Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)
- Subjects
Initial mass function ,Infrared ,stars: luminosity function ,Metallicity ,FOS: Physical sciences ,Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,01 natural sciences ,Spectral line ,cD ,0103 physical sciences ,galaxies: formation ,galaxies: elliptical and lenticular ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics ,Line (formation) ,Luminosity function (astronomy) ,Physics ,010308 nuclear & particles physics ,Velocity dispersion ,Astronomy and Astrophysics ,Astrophysics - Astrophysics of Galaxies ,Galaxy ,Space and Planetary Science ,mass function ,[SDU]Sciences of the Universe [physics] ,Astrophysics of Galaxies (astro-ph.GA) ,galaxies: stellar content ,galaxies: evolution - Abstract
We present new evidence for a variable stellar initial mass function (IMF) in massive early-type galaxies, using high-resolution, near-infrared spectroscopy from the Folded-port InfraRed Echellette spectrograph (FIRE) on the Magellan Baade Telescope at Las Campanas Observatory. In this pilot study, we observe several gravity-sensitive metal lines between 1.1 $\mu$m and 1.3 $\mu$m in eight highly-luminous ($L \sim 10 L_*$) nearby galaxies. Thanks to the broad wavelength coverage of FIRE, we are also able to observe the Ca II triplet feature, which helps with our analysis. After measuring the equivalent widths (EWs) of these lines, we notice mild to moderate trends between EW and central velocity dispersion ($\sigma$), with some species (K I, Na I, Mn I) showing a positive EW-$\sigma$ correlation and others (Mg I, Ca II, Fe I) a negative one. To minimize the effects of metallicity, we measure the ratio $R$ = [EW(K I) / EW(Mg I)], finding a significant systematic increase in this ratio with respect to $\sigma$. We then probe for variations in the IMF by comparing the measured line ratios to the values expected in several IMF models. Overall, we find that low-mass galaxies ($\sigma \sim 100$ km s$^{-1}$) favor a Chabrier IMF, while high-mass galaxies ($\sigma \sim 350$ km s$^{-1}$) are better described with a steeper (dwarf-rich) IMF slope. While we note that our galaxy sample is small and may suffer from selection effects, these initial results are still promising. A larger sample of galaxies will therefore provide an even clearer picture of IMF trends in this regime., Comment: 17 pages, 10 figures, 6 tables. Accepted for publication in ApJ
- Published
- 2017
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