Using NIR spectral features in an SDSS sample of early-type galaxies to constrain the low-mass Initial Mass Function

Abstract

The stellar initial mass function (IMF) is a major component in galaxy formation theory, it describes the original distribution of stars as a function of mass at the epoch of formation of the population. To investigate the form and possible variation of the IMF at low stellar-masses, features are measured in spectra from the Sloan Digital Sky Survey DR7 for a sample of ∼2000, low-redshift (0.010 - 0.057) red sequence galaxies and in simple stellar population models that cover a range of elemental abundances and IMFs. Particular attention is paid to the calcium triplet (at ∼8600 Å) and the sodium doublet (at ∼8200 Å) which are characteristic of high-mass ( 8 M ) and low-mass ( 0.5 M ) stars respectively and the NaD (at 5895 Å) and CaI (at 4227 Å) features which show a strong response to their respective elemental abundances.

The combination of these spectral features is a useful technique for separating the effects of elemental abundance and IMF on the size of the measured spectral features, allowing the form and variation of the IMF to be investigated individually. The sodium abundance is constrained from the NaD index, popularly disregarded as an abundance indicator due to the effect of interstellar absorption on this feature. In this work there is no strong evidence to suggest that interstellar absorption has a detrimental effect on the abundance constraints from the NaD index.

Evidence is found for an increasing sodium abundance ([Na/Fe] ∝ σ 1.654±0.050 ) and marginally decreasing calcium abundance ([Ca/Fe] ∝ σ −0.19±0.12 ) with increasing galaxy mass. With these abundances taken into consideration, the IMF variation found is slightly shallower at higher masses but also consistent with zero; ∆x = -0.111±0.071 ∆ log σ (where x is the power-law index describing the slope of the IMF). Taking into consideration the average abundances, the best-fitting IMF is found to be approximately Chabrier-like, with outlying galaxies that correspond to more extreme environments including high sodium abundances and bottom-heavy IMFs.