Seyfert and starburst activity in galaxies: Modeling and analysis

Abstract

We carried out an extensive infrared study of galaxies with Seyfert and starburst activity, to constrain the impact of these phenomena on the host galaxy evolution in a local galaxy sample. We developed new tools for stellar population analysis in the infrared, using stellar absorption features. These tools are ideally suited for studies of dusty galaxies and obscured stars in the Milky Way. We used the CO band at 2.3μm to estimate the contribution from red supergiants to the total K-band flux in the central regions of 46 Seyferts from the CfA redshift survey, based on new infrared spectroscopy (R ∼ 700). We removed the non-stellar contribution for 16 Seyferts, and found no evidence for strong starbursts in the majority of them. A comparison to starbursts suggests that the [FeII]¹˙²⁶μm/Paβ¹˙²⁸μm versus H₂²˙¹²¹μm/Brγ²˙¹⁶μm diagram is the best available Seyfert-starburst separator. We created a unique infrared evolutionary population synthesis model for starbursts based on empirical spectral library (R≈2000-3000) of HK spectra of 218 red stars, spanning a range of [Fe/H] from -2.2 to +0.3. We analyzed the behavior of 19 indices, and developed diagnostics for the stellar parameters. Finally, we incorporated the library into Starburst99 (Leitherer et al. 1999), and compared the synthetic spectra with observational data, to constrain the metallicity of starburst and "normal" galaxies. Our new infrared magnesium-based [Fe/H] indicator is in excellent agreement with the strength of the optical iron lines. The correlation between alpha-element and iron-peak element suggests multiple bursts of star formation in the galaxies of our sample.