Salpeter normalization of the Stellar Initial Mass Function for massive galaxies at z~1

Abstract

The stellar initial mass function (IMF) is a key parameter to study galaxy evolution. Here we measure the IMF mass normalization for a sample of 68 field galaxies in the redshift range 0.7 to 0.9 within the Extended Groth Strip. To do this we derive total (stellar + dark matter) mass-to-light [$(M/L{)}_{\mathrm{dyn}}$] using axisymmetric dynamical models. Within the region where we have kinematics (about one half-light radius), the models assume: (i) that mass-follows-light, implying negligible differences between the stellar and total density profiles; (ii) constant velocity anisotropy (${\beta }_{\mathrm{z}}\equiv 1-{\sigma }_z^2/{\sigma }_R^2=0.2$); (iii) that galaxies are seen at the average inclination for random orientations (i.e. $i=60^{\circ }$, where $i=90^{\circ }$ represents edge-on). The dynamical models are based on anisotropic Jeans equations, constrained by HST/ACS imaging and the central velocity dispersion of the galaxies, extracted from good-quality spectra taken by the DEEP2 survey. The population $(M/L{)}_{\mathrm{pop}}$ are derived from full-spectrum fitting of the same spectra with a grid of simple stellar population models. Recent dynamical modelling results by the $ATLA{S}^{3D}$ project and numerical simulations of galaxy evolution indicate that the dark matter fraction within the central regions of our galaxies should be small. This suggest that our derived total $(M/L{)}_{\mathrm{dyn}}$ should closely approximate the stellar $M/L$. Our comparison of $(M/L{)}_{\mathrm{dyn}}$ and $(M/L{)}_{\mathrm{pop}}$ then imply that for galaxies with stellar mass $M_{\odot }\ge 10^{11}$ $M_{\odot }$, the $average$ normalization of the IMF is consistent with a Salpeter slope, with a substantial scatter. This is similar to what is found within a similar mass range for nearby galaxies.

7 Pages, 4 Figures, 1 Table, Accepted for publication in ApJ Letters

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• Evolution of central dark matter of early-type galaxies up to z ~ 0.8
• Redshift evolution of the dynamical properties of massive galaxies from SDSS-III/BOSS
• The kinematics of massive quiescent galaxies at $1.4<z<2.1$: dark matter fractions, IMF variation, and the relation to local early-type galaxies
• Observed trend in the star formation history and the dark matter fraction of galaxies at redshift $z\approx 0.8$
• The last 6 Gyr of dark matter assembly in massive galaxies from the Kilo Degree Survey
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• What Regulates Galaxy Evolution? Open Questions in Our Understanding of Galaxy Formation and Evolution
• Variations of the Initial Mass Function in Semi-Analytical models
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• Implications of galaxy buildup for putative IMF variations in massive galaxies
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• The Fornax3D project: Intrinsic Correlations between Orbital Properties and the Stellar Initial Mass Function
• Stellar velocity dispersion and initial mass function gradients in dissipationless galaxy mergers
• Optical+NIR analysis of a Newly Confirmed Einstein ring at z$\sim$1 from the Kilo-Degree Survey: Dark matter fraction, total and dark matter density slope and IMF
• Fornax 3D project: assessing the diversity of IMF and stellar population maps within the Fornax Cluster
• Initial Mass Function for Massive Galaxies at z$\sim$1