EVIDENCE FOR A MILD STEEPENING AND BOTTOM-HEAVY INITIAL MASS FUNCTION IN MASSIVE GALAXIES FROM SODIUM AND TITANIUM-OXIDE INDICATORS
- Kapteyn Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen (Netherlands)
We measure equivalent widths (EWs)-focusing on two unique features (NaI and TiO{sub 2}) of low-mass stars ({approx}< 0.3 M{sub Sun })-for luminous red galaxy spectra from the Sloan Digital Sky Survey (SDSS) and X-Shooter Lens Survey (XLENS) in order to study the low-mass end of the initial mass function (IMF). We compare these EWs to those derived from simple stellar population models computed with different IMFs, ages, [{alpha}/Fe], and elemental abundances. We find that models are able to simultaneously reproduce the observed NaD {lambda}5895 and Na I {lambda}8190 features for lower-mass ({approx}{sigma}{sub *}) early-type galaxies (ETGs) but deviate increasingly for more massive ETGs, due to strongly mismatching NaD EWs. The TiO{sub 2} {lambda}6230 and the Na I {lambda}8190 features together appear to be a powerful IMF diagnostic, with age and metallicity effects orthogonal to the effect of IMF. We find that both features correlate strongly with galaxy velocity dispersion. The XLENS ETG (SDSS J0912+0029) and an SDSS ETG (SDSS J0041-0914) appear to require both an extreme dwarf-rich IMF and a high sodium enhancement ([Na/Fe] = +0.4). In addition, lensing constraints on the total mass of the XLENS system within its Einstein radius limit a bottom-heavy IMF with a power-law slope to x {<=} 3.0 at the 90% CL. We conclude that NaI and TiO features, in comparison with state-of-the-art simple stellar population models, suggest a mildly steepening IMF from Salpeter (dn/dm {proportional_to} m {sup -x} with x = 2.35) to x Almost-Equal-To 3.0 for ETGs in the range {sigma} = 200-335 km s{sup -1}.
- OSTI ID:
- 22047690
- Journal Information:
- Astrophysical Journal Letters, Vol. 753, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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