Spatially Resolved Stellar Populations and Kinematics with KCWI: Probing the Assembly History of the Massive Early-type Galaxy NGC 1407

Abstract

Abstract Using the newly commissioned Keck Cosmic Web Imager (KCWI) instrument on the Keck II telescope, we analyze the stellar kinematics and stellar populations of the well-studied massive early-type galaxy (ETG) NGC 1407. We obtained high signal-to-noise integral field spectra for a central and an outer (around one effective radius toward the southeast direction) pointing with integration times of just 600 s and 2400 s, respectively. We confirm the presence of a kinematically distinct core also revealed by VLT/MUSE data of the central regions. While NGC 1407 was previously found to have stellar populations characteristic of massive ETGs (with radially constant old ages and high alpha-enhancements), it was claimed to show peculiar supersolar metallicity peaks at a large radius that deviated from an otherwise strong negative metallicity gradient, which is hard to reconcile within a “two-phase” formation scenario. Our outer pointing confirms the near-uniform old ages and the presence of a steep metallicity gradient, but with no evidence for anomalously high metallicity values at large galactocentric radii. We find a rising outer velocity dispersion profile and high values of the fourth-order kinematic moment—an indicator of possible anisotropy. This coincides with the reported transition from a bottom-heavy to a Salpeter initial mass function, which may indicate that we are probing the transition region from the “in situ” to the accreted phase. With short exposures, we have been able to derive robust stellar kinematics and stellar populations in NGC 1407 to ∼1 effective radius. This experiment shows that future work with KCWI will enable 2D kinematics and stellar populations to be probed within the low surface brightness regions of galaxy halos in an effective way.