The Nuclear Starburst in NGC 253

Abstract

We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands, broadband images in the J, H, and bands, narrowband images centered at the wavelengths of Brc and 0) S(1), and K s H 2 (1, imaging spectroscopy centered on [Ne II] (12.8 km). We have subtracted a composite stellar spectrum from the galaxy spectrum to measure faint emission lines that otherwise would be buried in the complicated continuum structure. We use these data and data from the literature in a comprehensive reassessment of the starburst in this galaxy. We conÐrm that the [Fe II] emission is predominantly excited by supernova explosions and show that the rate of these events can be derived from the strength of the infrared [Fe II] lines. Although the emission superÐcially resembles a thermally excited spectrum, H 2 most of the infrared luminosity is excited by Ñuorescence in low-density gas. We conÐrm the presence H 2 of a bar and also show that this galaxy has a circumnuclear ring. The relation of these features to the gaseous bar seen in CO is in agreement with the general theoretical picture of how gas can be concentrated into galaxy centers by bars. We derive a strong upper limit of D37,000 K for the stars exciting the emission lines. We use velocity-resolved infrared spectra to determine the mass in the starburst region. Most of this mass appears to be locked up in the old, preexisting stellar population. Using these constraints and others to build an evolutionary synthesis model, we Ðnd that the initial mass function (IMF) originally derived to Ðt the starburst in M82 also accounts for the properties of NGC 253 ; this IMF is similar to a modiÐed Salpeter IMF. The models indicate that rapid massive star formation has been ongoing for 20È30 million yr in NGC 253 ; that is, it is in a late phase of its starburst. Its optical spectrum has characteristics of a transitional H II/weak-[O I] LINER. We model the emission-line spectrum expected from a late phase starburst and demonstrate that it reproduces these characteristics.

Excited OH+, H2O+, and H3O+in NGC 4418 and Arp 220

“…Since the γ−rays are thought to be the result of cosmic rays accelerated by supernova remnant shocks that interact with high density target material, a high rate of cosmicray ionization indicates a high rate of supernovae in the central region of starburst galaxies (ν SN ∼ 0.03 yr −1 in NGC 253, Engelbracht et al 1998), which would be expected to be accompanied by observable galactic outflows. These are indeed seen in both NGC 253 and M82 (e.g.…”

The AMIGA sample of isolated galaxies

“…Most methods used to determine the stellar populations are based on templates of stellar absorption-line spectra that are used as the basis to fit the stellar component (e.g. Ho et al 1997;Engelbracht et al 1998;Kauffmann et al 2003b;Cid Fernandes et al 2005;Hao et al 2005). The library of templates should be large to allow the fitting of stellar templates that reflect a wide dispersion in galaxy properties such as metallicities, ages and velocity dispersions.…”

Active Galactic Nuclei in the Sloan Digital Sky Survey. I. Sample Selection

“…The library is typically composed of star spectra generated from a population synthesis code ( Bica 1988;Saraiva et al 2001;Kauffmann et al 2003b) or direct observational spectra of absorption-line galaxies ( Ho et al 1997a) or stars ( Engelbracht et al 1998). In this paper the library is constructed by applying the principal component analysis ( PCA) technique (e.g., Connolly et al 1995;Lahav et al 1996;Bromley et al 1998;Eisenstein et al 2003;Yip et al 2004) to a sample of pure absorption-line galaxies.…”