Stellar Population and Kinematic Profiles in Spiral Bulges & Disks: Population Synthesis of Integrated Spectra

TL;DR

This study analyzes the stellar populations and kinematics of spiral galaxy bulges and disks using deep spectral data, revealing that bulge formation is primarily early and involves complex star formation histories, with merging playing a significant role.

Contribution

Introduces a full population synthesis method for integrated spectra and demonstrates that spiral bulges follow similar age-metallicity relations as ellipticals, with complex star formation histories.

Findings

Bulges are predominantly old and metal-rich in a mass-weighted sense.

Complex star formation histories reduce scatter in age-metallicity relations.

Merging likely plays a key role in bulge formation, with secular processes contributing less.

Abstract

We present a detailed study of the stellar populations (SPs) and kinematics of the bulge and inner disk regions of eight nearby spiral galaxies (Sa-Sd) based on deep Gemini/GMOS data. The long-slit spectra extend to 1-2 disk scale lengths with S/N/Ang>=50. Several different model fitting techniques involving absorption-line indices and full spectrum fitting are explored and found to weigh age, metallicity, and abundance ratios differently. The SPs of spiral galaxies are not well matched by single episodes of star formation; representative SPs must involve average SP values integrated over the star formation history (SFH) of the galaxy. Our "full population synthesis" method is an optimised linear combination of model templates to the full spectrum with masking of regions poorly represented by the models. Our spiral bulges follow the same correlations of increasing light-weighted age and metallicity with central velocity dispersion as those of elliptical galaxies and early-type bulges found in other studies, but when SFHs more complex and realistic than a single burst are invoked, the trend with age is shallower and the scatter much reduced. In a mass-weighted context, all bulges are predominantly composed of old and metal-rich SPs. Bulge formation appears to dominated by early processes that are common to all spheroids, whether they currently reside in disks or not. While monolithic collapse cannot be ruled out in some cases, merging must be invoked to explain the SP gradients in most bulges. Further bulge growth via secular processes or "rejuvenated" star formation generally contributes minimally to the stellar mass budget. (Abridged)