TY - JOUR
T1 - Evidence of a Bottom-heavy Initial Mass Function in Massive Early-type Galaxies from Near-infrared Metal Lines
AU - Lagattuta, David J.
AU - Mould, Jeremy R.
AU - Forbes, Duncan A.
AU - Monson, Andrew J.
AU - Pastorello, Nicola
AU - Persson, S. Eric
N1 - Funding Information:
The authors wish to thank the referee for thoroughly reading the manuscript and providing several useful comments that greatly improved the quality of the paper. Thanks to Rob Simcoe for assistance with the FIRE pipeline. France Allard provided invaluable advice on synthetic stellar spectra. We also thank Blesson Mathew and Nidia Morrell for their contributions. We appreciate the use of Charlie Conroy’s FSPS and Michele Cappellari’s pPXF codes. D.J.L. acknowledges support from the ERC starting grant 336736-CALENDS. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. D.A.F. thanks the ARC for financial support via DP130100388.
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.
PY - 2017/9/10
Y1 - 2017/9/10
N2 - 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 and 1.3 μm in eight highly luminous () 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 (σ), with some species (K i, Na i, Mn i) showing a positive EW-σ 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 σ. 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 ( km s-1) favor a Chabrier IMF, while high-mass galaxies ( 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.
AB - 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 and 1.3 μm in eight highly luminous () 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 (σ), with some species (K i, Na i, Mn i) showing a positive EW-σ 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 σ. 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 ( km s-1) favor a Chabrier IMF, while high-mass galaxies ( 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.
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U2 - 10.3847/1538-4357/aa8563
DO - 10.3847/1538-4357/aa8563
M3 - Article
AN - SCOPUS:85029523844
VL - 846
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 2
M1 - 166
ER -