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Figure 2.

galaxies using the Gemini Multi-object Spectrograph (GMOS) on the Gemini North 8-meter telescope. Figure 1 shows an example of our observational setup and extracted radially resolved spectra for the grand design spiral galaxy NGC 628 (also known as M74).

Uncovering the Stellar Populations The high-quality GMOS spectra were used to develop a “full population synthesis” technique to determine the stellar content of each spectrum. The method consists of an optimized linear combination of Simple Stellar Population (SSP) model templates to the full spectrum while masking regions poorly represented by the models. Each model SSP represents the spectral energy distribution for a single burst of star formation at a given age and metallicity (Z). Establishing the relative

Comparison of our “full population synthesis” model fits (red) to the observed central GMOS spectra (black) of spiral galaxies NGC 628 (top) and the emission-line dominated NGC 7495 (bottom). The average lightweighted age and Z, effective dust extinction, τV, and goodness-of-fit measure, χ2, of the fits are indicated in each figure. The bottom panels show the percent data– model residuals.

contribution of each SSP to the integrated galaxy spectrum thus provides a stochastically-sampled SFH, yielding the true average stellar population parameters for each spectrum. This contrasts with many previous A detailed breakdown of the age, metallicity (Z), and kinematic properties of the stellar population (SP) content comprising bulges of all types along the Hubble sequence of galaxies is a very a useful probe in discerning between formation scenarios. Information about both light- and mass-weighted quantities is needed to form a comprehensive picture of the star formation history (SFH) of a given system. For nearby galaxies, whose SPs can be resolved into individual stars in deep photometric observations, SFHs can be derived from a detailed analysis of the distribution of their stars in the color-magnitude plane. Beyond our Local Group, however, observations are limited to

studies, which provide SSP-equivalent values that are heavily biased to the last episode of star formation, which dominates the optical light even when its contribution to the stellar mass budget is minimal. Two examples of our full population synthesis fits are shown in Figure 2. The gray shading indicates regions that are not represented in the models, i.e., any nonstellar contributions, and are thus masked in the fit. These can include the CCD gap regions (green vertical dash-dotted lines), variable sky lines that are difficult to model and subtract accurately (the locations of which are indicated by the dashed and dotted vertical lines), and emission lines from the surrounding gas prevalent

the integrated light along a given line-of-sight, which

Figure 3.

must then be deconvolved into the relative fractions

Average age (top) and Z (bottom) from the full population synthesis fits as a function of central velocity dispersion, σ0. Black solid squares: lightweighted values. Red open squares: mass-weighted values. The dotted lines are linear regressions to the data.

of stars of a given population that contribute to the total luminosity. This challenge is especially acute for spiral galaxies that are known to harbor a mixture of young and old stars, and may also suffer from the reddening and extinction effects of interstellar dust. However, with the tremendous recent progress in stellar population modeling, combined with highquality data from large-aperture telescopes with fast and sensitive detectors, many of these obstacles can now be met head-on. To tackle these issues, our group collected deep longslit spectroscopy for a sample of eight nearby spiral



Issue 38 - June 2009  


Issue 38 - June 2009