VIMOS mosaic integral-field spectroscopy of the bulge and disc of the early-type galaxy NGC 4697

Abstract

We present an integral-field study of the internal structure, kinematics and stellar population of the almost edge-on, intermediate-luminosity (L_★) elliptical galaxy NGC 4697. We build extended two-dimensional (2D) maps of the stellar kinematics and line strengths of the galaxy up to ∼0.7 effective radii (R_eff) using a mosaic of eight VIMOS (VIsible Multi-Objects Spectrograph, on the Very Large Telescope) integral-field unit pointings. We find clear evidence for a rotation-supported structure along the major axis from the 2D kinematical maps, confirming the previous classification of this system as a `fast rotator'. We study the correlations between the third and fourth Gauss-Hermite moments of the line-of-sight velocity distribution (LOSVD) h₃ and h₄ with the rotation parameter (V/σ), and compare our findings to hydrodynamical simulations. We find remarkable similarities to predictions from gas-rich mergers. Based on photometry, we perform a bulge/disc decomposition and study the stellar population properties of the two components. The bulge and the disc show different stellar populations, with the stars in the bulge being older (age_bulge=13.5^{+1.4}_{-1.4} Gyr, age_disc=10.5^{+1.6}_{-2.0} Gyr) and more metal poor ({[M/H]_{bulge}} = -0.17^{+0.12}_{-0.1}, {[M/H]_{disc}} = -0.03^{+0.02}_{-0.1}). The evidence of a later-formed, more metal-rich disc embedded in an older, more metal poor bulge, together with the LOSVD structure, supports a mass assembly scenario dominated by gas-rich minor mergers and possibly with a late gas-rich major merger that left a previously rapidly rotating system unchanged. The bulge and the disc do not show signs of different stellar initial mass function (IMF) slopes, and both match well with a Milky Way-like IMF.