NIR SPECTRAL SYNTHESIS OF THE NUCLEAR REGIONS OF INFRARED BRIGHT GALAXIES E. Schinnerer1 , A. Eckart1, T. Boker1•2 1 MPI fiir extraterrestrische Physik, Garching, Germany 2 Space Telescope Science Institute, Baltimore, USA Abstract We summarize our first results with a starburst program based on theoretical tracks of stellar evolution combined with a spectral synthesis code using a library of template spectra already available in the literature. The combination of the starburst model with the spectral synthesis demonstrates for the first time the feasibilityof this approach in the near-infrared. This approach validates the estimates of extinction and the contribution from hot dust made from JHK color diagrams and emission line ratios. It also proves in a consistent way that the emission from knots in circum-nuclear rings originate in young stellar clusters. These clusters were probably formed in a decaying starburst about 107 years ago with an upper mass cut off well above 60 solar masses. We carried out a detailed study of the nuclear region of the face-on barred starburst galaxy NGC 7552 applying this method to high spatial resolution imaging spectroscopic data. This approach has proven to be very useful for a detailed understanding of com­ plex, interdependent processes in individual nuclear areas of starburst activity. Especially the incorporation of the spectral data proofs the young age of individual regions of the circum-nuclear starburst ring in NGC 7552. For this galaxy our near-infrared images have subarcsecond resolution and reveal the spatial distribution of the J, H, K continuum - obtained with the MPE SHARP 1 camera - and most of the prominent near-infrared K-band lines like Bry, He!, H2 in emission and stellar absorption features taken with the MPE imaging spectrograph 3D. Our N-band (10.5µm) continuum image (taken with the MPE camera MANIAC) has an angular resolution of about and corresponds very well in structure with the JHK color maps and a published 6cm radio continuum map. The data are combined with radio and far-infrared data available in the literature and interpreted in the framework of a starburst model. As a resultl" we obtained a first insight into the differentstar formation histories of the nucleus and the 1 kpc circum-nuclear ring. We also demonstrate that the star formation rate and efficiency are intimately associated with the dynamics of the stars and gas in the nucleus of NGC 7552. Introduction Starburst rings are a common phenomenon Buta & Combes 1996, Maoz et al. 1996). The 1 ( investigation of circum-nuclear starburst rings is essential to understand star formationin nuclei of galaxies. These rings therefore deserve special attention since they are important tracers of the dvnamics and star formation in the nuclei of their hosts. We therefore choose the barred galaxy NGC which has a prominent circum-nuclear starburst ring to study face-;n 755'.l in detail the properties of the whole ring as well as within the ring. Our results on IC 342 and !'\GC 6764 are summarized at the end. 27 NGC 7552: NGC 7552 is well known for its circum-nuclear starburst ring, and its lack of strong nuclear activity makes it possible to study the intrinsic properties of this structure. NGC 7552 is at a distance of about 20 Mpc (H0=75 kms-1Mpc1); l" corresponds to about 100 pc. The galaxy is nearly face-on with an inclination of �28° (Feinstein et al. 1990). The spectral classification is not quite clear. Bonatto et al. (1989) found giant HII regions concentrated near the corotation radius and identified the object as an HII-galaxy. Durret & Bergeron (1988) detected a weak [OI]>.6300 line classifying the galaxy therefore as a LINER. The galaxy has also been detected in the radio and UV aswell in the X-ray domain (Maccacaro & Perola 1981, Charles & Phillips 1985). Beside the bar and two dominant spiral arms Feinstein et al. (1990) report two weaker rings with radii of 1.9 and 3.4 kpc in the optical. Forbes et al. (1994a) latter discovered another inner ring with a size of only 1 kpc in the radio (3 cm and 6 cm). This ring cannot easily be detected in direct NIR continuum radiation but it is present in the NIR color maps and can also be seen in its Ha and Bq line emission (Forbes et al. 1994a). Both radio and X-ray observations indicate enhanced starburst activity in the nuclear region. Claussen & Sahai (1992) observed the 12CO(l-O) line emission from :.JGC 7552 and report that the object contains a large amount of molecular gas. From the asymmetries in the CO line profile they conclude that the galaxy could be tidally disturbed. 2 .Data Reduction The near-infrared domain offers the advantage of approximately 10 times lower extinction than the visible thus allowing a deeper insight into nuclear starburst regions that are often heavily obscured. We have therefore chosen near- and mid-infrared imaging and near-infrared spectro­ scopic techniques to extend the existing observational data on the nucleus of the barred spiral '.\/GC 7552. The near-infrared imaging data was taken with IR.AC 2 on the MPE 2.2m tele­ scope, La Silla, Chile, in July 1995 and with higher spatial resolution with the :vIPE SHARP 1 camera at the NTT also in La Silla in July 1992. The new MPE mid-infrared camera .'vfAI\IAC was used in October 1996 to obtain the N-band image. The spectroscopic imaging data was taken with the MPE 3D camera in July 1994 at the 2.2m telescope in La Silla, Chile. The data was sky-subtracted, flat-fielded, dead pixel corrected and the spectroscopic data also corrected for atmospheric emission effects. 3 Starburst Analysis To perform an analysis of the starburst rings we used the population synthesis code STARS (Kovo & Sternberg 1997) which was already applied successfully to several other galaxies and is similar to other codes. The code uses the recent stellar evolutionary tracks of the Genern group (Meynet et al. 1994) to calculate global parameters which depend different on the initial mass function (IMF), the age of the burst as well as the kind of burst. Our IMF varies as proportional to between a lower and upper mass cut-offwith an a of 2.35 (Salpeter 1955). We investigated two types of star formation, a continuous and a decaying starburst with a decay time of about 5 Myrs. As diagnostic tools we used the ratios between the bolometric luminosity rvr-" resembling the total power of the star formation, the K-band luminosity as an indicator of age and evolved late type stars, the Lyman continuum luminosity representing young and massive stars as well as the supernova rate. These parameters can be observed directly via the N-band. the K-band, the Bq line flux in the K-band and the 5 GHz flux. STARS also calculates HR 28 diagrams representing the luminosity distribution in the NIR. These diagrams are used as an input for our spectral synthesis program SPECSYN (see Schinnerer et al. 1997). SPECSYN calculates a synthetic spectrum using the HR diagram and template standard star spectra of the same spectral resolution as the observed spectra which represent different positions in the HR diagram. The reddening of the observed spectra is accounted for via mixed or screen extinction as well as dust emission of a black body with 500 K ::; T ::; 1500 K. The matching of the synthetic with the observed spectrum serves as a test for the population synthesis results as well as the assumed extinction and dust emission. 4 The Analysis of the Starburst Ring in NGC 7552 The Nuclear Region in the Near- and Mid-Infrared: In the high resolution NIR K-band images (Fig.1) of the inner 10" we resolve a nuclear source of about 0.7" FWHM. South-east of the nucleus a bright emission knot is showing up which is not present in the J- or H-band. The NIR continuum of the central few arcseconds is elongated to the north. We compared this data to the available HST V-band images. There we see two prominent dust lanes at the position of the ring around the nucleus and find the nuclear source being unresolved. The appearance of the continuum changes dramatically when we go to the mid-infrared N-band at 10.5 µm (Fig.I). At this wavelength the ring as well as the nucleus are clearly detected. This is similar to the NIR color images which show two dust Janes connecting to the ring which itself is not smooth in its color. The nucleus appears to be slightly reddened. The starburst ring is also observed in the K-band lines of HeI, H2 and the CO-bands. The flux distribution in these lines are not identical indicating different properties within the ring itself. We used the high resolution :,'IRimages to derive the extinction and dust emission of individual regions Bq, within the inner 10" via the JHK color diagram. The K-band luminosity was estimated from this data as well. To derive the Lyman continuum luminosity we used the line fluxes of these regions. The bolometric luminosity we estimated by extrapolation from the IRAS bands using the N-band fluxes as well. To test our population synthesis results we took spectra of the Bq investigated regions and used our spectral synthesis code. Results of the Starburst Analysis: The nucleus of NGC 7552 is powered by a 850 pc parsec diameter circum-nuclear ring. The presence of the ring and the spiral arms starting at its circumference are best seen in the near-infrared color maps and the 10.5µm continuum image.