Nir Spectral Synthesis of the Nuclear Regions of Infrared Bright Galaxies

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. The spiral arms could provide the transportation of molecular material onto the ring. Furthermore we see evidence for a bar within a bar but no signs of nuclear activity. Our spectral synthesis of individual knots (Fig.2) in the nuclear starburst ring proofs that they are young stellar clusters, undergoing a decaying starburst. Our analysis shows that the star bursts in the prominent knots of the ring have ages of the order of 1.5x107 years and upper mass cut-offs in the range of 50 M0 or higher. For the central core component we find that most likely a mixture of an old constant and a young decaying starburst is responsible for the observed nuclear properties. This is also supported by our ground-based JHK imaging and the \'-band HST image. The constant component has an age of ;:::1010 yrs with an upper mass cut-offclearly below 50 M8 and the young decaying burst has an age of about 1.3x107yrs, a short decay time of the order of 106 yrs, and an upper mass cut-off 2:50 M8.

29 5 Analysis of Other Starburst Nuclei

IC 342: This source is an almost face-on (i=25°) nearby (1.8 Mpc) spiral galaxy. Boker et al. (1997) collected NIR K- and H-band data using the MPE 3D camera at the Calar Alto· telescope and the WHT. In its center IC 342 show an approximately 50 pc diameter starburst ring that has been imaged in its 12CO(l-O) molecular line emission (Ishizuki et al. 1990), in its radio continuum emission (Turner & Ho 1983) and in the NIR (Boker et al. 1997). Our analysis using both STARS and SPECSYN shows that the nucleus has an age of 15 million years and individual starforming knots within the circum-nuclear ring have an age of about 5x106 years. The simultaneous SPECSYN fitin H- and K-band (Fig.3) is of similar quality as for NGC 7552 and supports the young age of the nuclear region.

NGC 6764: Eckart et al. (1991, 1996) present high resolution images of millimeter-wave 115 GHz 12CO(l-0) as well as near-infrared and optical spectroscopy of the Wolf-Rayet nucleus of the barred LINER galaxy NGC 6764 at a distance of about 32 Mpc. These measurements reveal a dense concentration of molecular gas and a starburst at the nucleus of NGC 6764. A detailed starburst analysis using STARS indicates a recent intense starburst of a few times 107 years . This starburst is most likely responsible for the presence of a fewthousand Wolf-Rayet stars in the nucleus of NGC 6764. A detailed investigation of the Wolf-Rayet-feature at 466 nm indicates that it is spatially extended on a scale of a few arcseconds. The distribution of the 2.12 µm H2 line flux shows extensions approximately perpendicular to the stellar bar which are in agreement with structural features in VLA radio maps and IRA::VI 30m maps of the 12C0(2-1) line emission (Eckart et al. 1991) possibly indicating a nuclear outflow.

6 Summary

Our approach of combining a starburst program based on theoretical tracks of stellar evolution with spectral synthesis using a library of template spectra already available in the literature has proven to be very useful for a detailed understanding of complex, interdependent processes in individual nuclear areas of starburst activity. Especially the incorporation of the spectral data proofs the young age of the knots in the starburst ring. Our observational data, however, shows that these starburst knots still may have a very complex substructure that has to be further resolved in order to avoid dilution of the observed starburst properties by neighbor ing source components. The investigation of the starburst ring with high spatial resolution revealed a complex coupling between ongoing star formation and bar-driven dynamics in the ring. Therefore models including both star formation and dynamics seem to be important to explain starburst rings. From the compiled data and the presented analysis it can be concluded that the understanding of star burst rings is essential for the evolution of galaxies and data with even higher spatial resolution is needed. The spectral synthesis showed that the actual star cluster models that handle continuum luminosities give reasonable answers.

References Bonatto, C., Bica, Allain, D., 1989, Astron. Astrophys., 226, 23. Buta, R., & Combes, F., in Fund.Cosmic Pysics 1996, Vol.17, p.95 Boker, T. et al. 1997, to be submitted E., Charles, P. A., Phillips, M. M., 1985, M.l\.R.A.S., 200, 263. Claussen, M.J., Sahai, R.. 1992, A. J., 103, 1134. Durret, F., Bergeron, J., 1988, Astron. Astrophys. Suppl., 72, 273.

30 Eckart , A., et al. 1996, Ap. J., 472, 588-599. Eckart, A., et al. 1991 ,Ap. J., 372, 67. Feinstein, C., Vega, I., Mendez, M., Forte, J.C., 1990, Astron. Astrophys., 239, 90. Forbes, D.A., Norris, R.R., Williger, G.M., Smith, R.C., 1994a, A. J., 107, 984. Forbes, D.A., Kotilainen, J.K., Moorwood, A.F.M., 1994b, Ap. J. (Letters), 433, Ll3. Genzel, R., et al., 1995, Ap. J., 444, 129. Ishizuki, S., et al. 1990, Nature 344, 224 Kovo & Sternberg 1997, in preparation. Maccacaro T., Perola, G. C., 1981, Ap. J., 246, Lll. :'.v!eynet, G., et al., 1994, Astron. Astrophys. Suppl., 103, 97. Maoz, D., et al., 1996, A. J., 111, 2248. Salpeter, E. E., 1955, Ap. J., 121, 161. Schinnerer, E., 1997, Ap. J., in press Turner, J.L., & Ho, P.T.P, 1983, Ap. J., 178, 623.)

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Fig.1 Contour map of the K-band flux (left) and lOµm flux (right) of �GC 7552 illustrating the positions of the infrared knots and areas (here shown with the solid line l" circular apertures) and the 2" radio knots as defined by Forbes et al. (1994a) shown with the gray lines. Contour levels are in steps of 10% starting with 10%.

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Knot N .!.�c 2e- 15 1 -1 "' � e 5 !! 2.1 2.2 [µm] Fig.2: Measured and synthesised spectra as well as the difference of both as obtained by a 2.3 combination of the starburst program andwavelength spectral synthesis programs for a region in the ring (knot D*; Fig.l) and for the nucleus (knot N; Fig.l). The upper black line shows the measured spectrum and the gray line the synthetic spectrum. The residuals are plotted in the lower part of each panel. For further explanation see Schinnerer et al. (1997) .

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1.6 1.8 1.9 2 2.1 2.2 2.3 2.4 wavelength [µm] 1.7 Fig.3: :\leasured and synthesised H- and K-band spectra of the nuclear 1.25'' in IC 342. See caption of Fig.2.