Vertical Population Gradients in NGC 891. I. ∇pak Instrumentation and Spectral Data

We have measured vertical and radial stellar population gradients in NGC 891. We compare these gradients to those known for the Milky Way from studies of resolved stars. Optical spectroscopic measurements extend spatially from the disk midplane up to 2.6 kpc in height and out to a radius of 12 kpc on both sides of the galaxy. Data were acquired with ∇Pak, a variable-pitch fiber integral field unit (IFU) on the WIYN telescope. We describe the laboratory and on-sky performance of ∇Pak, as well as modifications to the standard observational and analysis procedures necessary to calibrate data taken with this unique IFU. ∇Pak has a mean throughput of 80% at 5500 Å. To achieve an estimated precision of 10% in light-weighted mean age and metallicity, we define a set of spatial apertures in radius and height in which spectra are binned to achieve a signal-to-noise ratio of ∼ 20 Å-1. We use spectral indices to measure age, metallicity, and abundance, indicating that NGC 891' s stellar populations have 0.2 < Z Z < 1 and +0.2 dex α-enhancement on average. We find a clear transition from young (<3 - 5 Gyr) to old (>7 Gyr) stellar populations at 0.4 kpc, roughly the scale height of the thin disk. We also find a slight trend toward younger populations at larger radii, consistent with flaring in an inside-out disk formation scenario. The vertical age gradient in NGC 891 is in remarkable qualitative agreement with a model for disk heating tuned to studies of the Milk Ways solar cylinder. © 2018. The American Astronomical Society. All rights reserved.