Seyfert and Starburst Activity in Galaxies: Modeling and Analysis

Abstract

During recent years, there has been a growing interest in active galactic nuclei and starburst galaxies. These two spectacular phenomena can dominate completely the total spectrum of their host, and they are present in a significant fraction of the local galaxy population. There is growing evidence that they might play an important part in the galaxy evolution at high redshift (e.g., A. Dey et al. 1997, ApJ, 490, 698; A. Cimatti et al. 1999, A&A, 352, L45), prompting us to carry out a detailed study of nearby galaxies with active nuclei and starbursts in order to take advantage of the more detailed analysis that can be carried out in the local universe. Seyfert galaxies.—We used new infrared (IR) spectroscopy of the CO band at 2.3 mm of 46 Seyfert galaxies from the CfA sample (J. Huchra & R. Burg, 1992, ApJ, 393, 90) to constrain the populations of young stars in their central regions. This is the first set of IR spectra of a complete sample of Seyfert galaxies. The CO band appears diluted by a nonstellar featureless component, most likely warm dust near the central engine. We devised a technique based on JHKL colors to separate the stellar and nonstellar components at 2.3 mm and corrected 16 Seyfert galaxies for dilution. A comparison with the CO indices of ellipticals and pure starburst galaxies indicates that no red supergiant phase starbursts dominate the Seyfert galaxies currently. The best Seyfert-starburst separator in the near-IR appears to be the [Fe ii] (1.26 mm)/Pab versus H2 (2.12 mm)/Brg diagram. The narrow emission line properties of our sample are significantly different from those of ultraluminous IR galaxies from the sample of T. W. Murphy, Jr., et al. (2001, AJ, 121, 97), implying that most of the latter (∼ ) cannot contain 23 hidden active nuclei. The other ∼ can still harbor completely 13 obscured AGNs, invisible even in the K band. Spectral library.—We have compiled a moderately high resolution ( ) H(1.6 mm) and K-band (2.2 mm) R ≈ 2000–3000 spectral atlas containing 218 stars. The atlas spans most of the ranges of , and [Fe/H] relevant for modeling starburst T log g, eff galaxies. We report indices for 19 spectral features and develop new diagnostic techniques for estimating parameters of individual stars. Magnesium indices are the best near-IR metallicity indicators because they are less sensitive to the stellar than Teff the others. The IR Mg i indices are just as good indicators for the abundance in composite systems as their optical counterparts. We have calibrated the strength of Mg i at 1.50 mm as function of the stellar [Fe/H]. We also calibrated a number of line ratios as indicators and luminosity class separators. Teff Despite the lack of suitable iron lines, the light-to-heavy element abundance ratio can be estimated in the IR by using the Ca i at 2.26 mm, because it has been shown from optical observations (G. Worthey 1998, PASP, 110, 888) that the calcium abundance follows the iron abundance. Evolutionary population synthesis.—The spectral library was incorporated into an evolutionary population synthesis model for young stellar populations (≤30 Myr) aimed specifically at starbursts. The model is based on Starburst99, developed by C. Leitherer et al. (1999, ApJ, 123, 3). It predicts the time evolution of near-IR features for single bursts of star formation (SF). They all are very weak during the first 5–7 Myr, while most of the flux comes from hot blue supergiants. The indices become stronger as the red supergiants begin to develop at 7–10 Myr. The indices weaken as the supergiants explode as supernovae at 12–15 Myr and stabilize at to 13 of the peak values after that. Our models, when combined 12 with age estimates from hydrogen recombination emission lines, can be used to probe directly the abundance of the starbursts, because the index strengths are scaled by the metallicity. Metallicities of starbursts.—We applied the Mg i 1.50 mm indicator to determine the metallicity of the starbursts from C. Engelbracht (1997, Ph.D. thesis, Univ. Arizona) and derived near-solar abundances for the majority of them. The technique yielded [Fe/H] p 0.3–0.4 for giant ellipticals. Our estimates correlate very well with the optical iron indices. We demonstrated by means of a simple chemical enrichment model that the fraction of iron peak elements can be influenced significantly by the preceding SF history. This poses a problem for [Fe/H] estimates based on a-elements (e.g., Mg) but also opens up a possibility to explore the previous SF history.