Stellar population and kinematic profiles in spiral bulges and discs: population synthesis of integrated spectra

Abstract

We present a detailed study of the stellar populations (SPs) and kinematics of the bulge and inner disc regions of eight nearby spiral galaxies (Sa–Sd) based on deep Gemini/GMOS data. The long-slit spectra extend to 1–2 disc scalelengths with S/N/Å≥ 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. We find that the SPs of spiral galaxies are not well matched by single episodes of star formation; more representative SPs must involve average SP values integrated over the star formation history (SFH) of the galaxy. Our 'full population synthesis' method is an optimized linear combination of model templates to the full spectrum with masking of regions poorly represented by the models. Realistic determinations of the SP parameters and kinematics (rotation and velocity dispersion) also rely on careful attention to data/model matching (resolution and flux calibration). The population fits reveal a wide range of age and metallicity gradients (from negative to positive) in the bulge, allowing for diverse formation mechanisms. The observed positive age gradients within the effective radius of some late-type bulges helps reconcile the long-standing conundrum of the coexistence of secular-like kinematics, light profile shape and stellar bar with the 'classical'-like old and α-enhanced SPs in the Milky Way bulge. The discs, on the other hand, almost always show mildly decreasing to flat profiles in both age and metallicity, consistent with inside-out formation. 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, however, all bulges are predominantly composed of old and metal-rich SPs. While secular contributions to the evolution of many of our bulges are clearly evident, with young (0.001–1 Gyr) SPs contributing as much as 90 per cent of the optical (V-band) light, the bulge mass fraction from young stars is small (≾25 per cent). The implies a bulge formation dominated by early processes that are common to all spheroids, whether they currently reside in discs 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, with the relative secular weight increasing with decreasing central velocity dispersion.

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