NGC 1808

   
1 Introduction

As hosts of vigorous episodes of star formation activity, starburst galaxies in the local Universe offer a unique opportunity to study the star formation process in extreme environments reminiscent of the conditions presumably prevailing in primordial galaxies. Studies in recent years have underscored the prominent role of starbursts in galaxy formation and evolution as well as their significant contribution to the extragalactic background, and to the chemical enrichment and heating of the intergalactic medium up to the highest redshifts (e.g. Steidel et al. 1996; Puget et al. 1996; Smail et al. 1997; Franx et al. 1997; Pettini et al. 1998; Elbaz et al. 2002). Understanding the starburst phenomenon is thus a key issue of local and cosmological relevance.

Nuclear starburst galaxies generally emit the bulk of their luminosity in the infrared, primarily as reprocessed radiation by interstellar dust grains heated by important populations of hot massive stars. In addition, a substantial fraction - if not most - of the star formation in starburst systems is heavily obscured at optical and ultraviolet wavelengths by large amounts of dust. This has been particularly dramatically illustrated by mid-infrared imaging of the colliding pair NGC 4038/4039 (Mirabel et al. 1998). Studies at infrared wavelengths are thus crucial in defining the properties of starburst galaxies and the interplay between the primary energy sources and the reprocessing material. The growing evidence for the presence and significant role of dust in distant galaxies further emphasizes the need for infrared investigations of nearby dust-rich templates (e.g. Rowan-Robinson 2001; Blain et al. 2002; and references therein). However, except for a limited sample, the detailed spatial and spectral energy distributions of starburst galaxies in this regime are still poorly known. Progress has been hindered by instrumental limitations, and by the difficulties inherent to ground-based and air-borne observations at these wavelengths.

The Infrared Space Observatory (ISO; Kessler et al. 1996) has revolutionized the field of infrared astronomy (see the reviews by Cesarsky & Sauvage 1999; Genzel & Cesarsky 2000), with unprecedented capabilities compared to its predecessor the Infrared Astronomical Satellite (IRAS). In the mid-infrared (MIR, $\lambda = 5.0$- $16.5~{\rm\mu m}$), the camera ISOCAM (Cesarsky et al. 1996c) provided an improvement in sensitivity and spatial resolution by factors of $\sim$1000 and $\sim$60, respectively. In addition to broad- and narrow-band filters, ISOCAM featured continuously variable filters (CVFs) allowing arcsecond-scale spectrophotometric imaging with full wavelength coverage at a resolution of $R \equiv \lambda/\Delta\lambda \sim 40$. The MIR regime contains a variety of key features tracing different components of the interstellar medium (ISM) and is subject to relatively little extinction by dust ( $A_{\rm 5 - 17~\mu m} < 0.1~A_{V}$). Spectrophotometric observations permit studies of the most prominent features to characterize the spatial distribution and infer the properties of the ISM and exciting sources, deep into obscured star-forming regions.

In this paper, we present results of ISOCAM CVF observations of M 82, NGC 253, and NGC 1808. M 82 and NGC 253 are two of the nearest and brightest starburst galaxies (at distances of D = 3.3and 2.5 Mpc, respectively, $1^{\prime\prime} = 16$ and 12 pc; Freedman & Madore 1988; Davidge & Pritchet 1990). Both have been extensively studied in all accessible spectral regions and have often been considered as prototypical objects for the starburst phenomenon (see Telesco 1988; Rieke et al. 1993; and Engelbracht et al. 1998 for reviews). NGC 1808 is a more distant system ( $D = 10.9~{\rm Mpc}$for $H_{0} = 75~{\rm km~s^{-1}~Mpc^{-1}}$, $1^{\prime\prime} = 53~{\rm pc}$; Sandage & Tammann 1987) which closely resembles M 82 and NGC 253 in its global properties (e.g. Dahlem et al. 1990; Junkes et al. 1995). In all three galaxies, starburst activity takes place within the central $\sim$0.1- $1~{\rm kpc}$ regions which are severely obscured at optical and ultraviolet wavelengths. The large extinction levels likely result in part from high disk inclination angles of $80^{\circ}$, $78^{\circ}$, and $57^{\circ}$ for M 82, NGC 253, and NGC 1808 (Götz et al. 1990; Pence 1981; Reif et al. 1982).

At comparable infrared luminosities $L_{\rm IR} \equiv L_{\rm 8-1000~\mu m}$ of $3.7 \times 10^{10}$, $1.4 \times 10^{10}$, and $3.4 \times 10^{10}~{L_{\odot}}$ for M 82, NGC 253, and NGC 1808, these galaxies also share key morphological features including a kiloparsec-scale stellar bar, large amounts of molecular gas in the central $\sim$ $1~{\rm kpc}$ predominantly concentrated in circumnuclear ring-, spiral-, or bar-like structures, populations of compact and luminous "super star clusters'' as well as of compact non-thermal radio sources associated with supernova remnants, and a large-scale outflowing starburst wind (Förster Schreiber et al. 2001; Engelbracht et al. 1998; Tacconi-Garman et al. 1996; and references therein). The triggering of starburst activity in M 82 is generally attributed to gravitational interaction as evidenced by the extended filamentary tidal features in H I gas distribution threading M 82 and its neighbours M 81 and NGC 3077 or, alternatively, to the stellar bar which may itself have been induced by the interaction (e.g. Yun et al. 1993,1994). In NGC 253, the bar itself appears to be the primary mechanism responsible for the onset of starburst activity as there are no obvious signatures of interaction with a nearby companion, although a past merger or accretion event involving a small galaxy has been suggested based on kinematical evidence (e.g. Anantharamaiah & Goss 1996; Böker et al. 1998). Tidal interaction with NGC 1792 has been proposed for NGC 1808 based on circumstantial evidence (e.g. Dahlem et al. 1990; Koribalski et al. 1993). The recent high sensitivity H I observations by Dahlem et al. (2001) do not indicate the presence of any tidal feature, dwarf galaxy or other debris that could support a close passage in the past, leaving the stellar bar as more probable trigger or a possible merger/accretion event.

In view of their similar nature, we focus on the comparison of the spatial and spectral properties of M 82, NGC 253, and NGC 1808 at MIR wavelengths. Though small, this sample also allows us to probe the transition regime between normal spiral and irregular galaxies and the more extreme luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs for which $L_{\rm IR} > 10^{11}$ and $> 10^{12}~{L_{\odot}}$, respectively). Section 2 describes the observations and data reduction procedure, and Sect. 3 presents the results. Section 4 discusses the origin and spatial distibution of the continuum and emission features. Section 5 addresses the issues of extinction effects, variations in spectral properties, and indicators of star formation activity. Section 6 summarizes the paper.

NGC 1808