A 2.18 To 2.28 Micron Study Of The Stellar Population In The Nucleus Of M82

To examine the properties of the stars found in the central 50 pc of the starburst galaxy M82, we have acquired a high signal-to-noise (approximately 100), low-resolution (R = 640) spectrum of the nucleus between 2.18 and 2.28-mu-m. By comparing the relative strengths of the strong atomic absorption features seen in M82 to those of solar neighborhood giants and supergiants, we look for peculiarities in the stellar population of the nucleus. We find that the strengths of stellar absorption features in the spectrum of M82 at 2.227 and 2.238-mu-m appear enhanced relative to the other strong features at 2.207 and 2.263-mu-m. We investigate potential explanations for these peculiarities using both observed and synthesized stellar spectra. Spectra of solar neighborhood late class stars demonstrate that the ratios of these strong absorption features remain constant over a large range in effective temperature and gravity. We conclude that we cannot reproduce the peculiar feature ratios in M82 with any combination of solar neighborhood stars.By turning to spectral synthesis, we can better identify the atomic species responsible for these features and explore what effect changes in other parameters of the stellar atmospheres have on the feature ratios. At our resolution, none of the features in the 2.18-2.28-mu-m spectrum of late class stars can be attributed to any single atomic species. Nevertheless, the behavior of these blends can be investigated. Examination of the synthesis spectra produced from a grid of model atmospheres [T(eff) = 3750-5000 K, log (g) = 0.75-2.25, and [Z/H] = -0.5 to 0.5] strengthens the conclusion drawn from examination of the field stars; the ratios of these features are relatively uninfluenced by changes in temperature and gravity. This modeling also points out that changes in turbulent velocity, overall metal abundance, and continuous opacity have only a small effect on the ratios of these features. The only significant changes in these ratios result from altering individual elemental abundances. We find that we can reproduce the relative strengths of many of the features in our spectrum of M82 by increasing Fe and Ti abundances relative to lighter elements, such as Ca, Sc, and Na, by almost-equal-to 0.5 dex.Many studies of the gas-phase chemistry in the central region of M82 have also indicated the existence of enhanced abundances as a result of extensive stellar processing. If our interpretation of the data is correct, this calls into question the results of simple stellar population studies which employ photometric indices for their analysis, as they assume that all the stellar populations have the same intrinsic characteristics. For example, enhanced metallicity in a stellar population causes the giant branch to become redder, and furthermore causes stars to remain there longer. These changes in the evolutionary tracks and isochrones will cause the population to appear redder. Further, the metal enhancement will also change the molecular equilibrium in these cool stars, which will result in the strengthening of the molecular bands, such as TiO and CO. Ignoring this effect may lead to the classification of a population of metal-rich giants as a population of supergiants. In order to properly analyze the populations of the nuclear regions of galaxies, especially regions with recent histories of extensive star formation and nucleosynthetic evolution, the effects of peculiar metal abundances must be taken into account. This analysis must include both a direct measurement of the elemental abundances in the region, and a good understanding of how these metal abundances affect both the structure and isochrones of the stars in the population. Methods to satisfy these requirements must be further developed, especially for regions of active star formation into which the near-infrared is our only window.