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Ground-Based Mid Infra-Red Observations of Nearby Starburst and AGN Galaxies
Bo jan Nikolic , Paul Alexander, Garret Cotter and Malcolm Longair
Astrophysics Group, Cavendish Lab, Cambridge, U.K.

Marcel Clemens
Universita degli Studi di Padova, Italy Abstract. We present preliminary results of recent spatially resolved mid infrared (Mid-IR) spectroscopy with the MICHELLE instrument on UKIRT of NGC 7469, observed as part of a larger programme to study a sample of starburst and AGN in the Mid-IR. Keywords: galaxies: active -- galaxies: individual (NGC 7469) -- galaxies: nuclei -- galaxies: starburst -- infrared: galaxies

1. Intro duction Mid-Infrared (Mid-IR) observations from high altitude ground-based observatories offer similar resolution to optical observations but have the advantage that extinction at the source is at least an order of magnitude smaller, making these an imp ortant to ol for probing the inner regions of heavily obscured AGN and starburst galaxies. We present preliminary results of recent observations with the United Kingdom Infra-Red Telescop e (UKIRT) using MICHELLE, a new generation Mid-IR imager and sp ectrograph. The diffraction limited b eam width of UKIRT at 10 µm is 0.7 arcseconds. These observations are a part of our multi-wavelength programme to map, at matched resolution, a sample of 20 nearby starburst and AGN in the Mid Infrared and at four radio frequencies. Our aim is to investigate the spatial extent of the "Unidentified Infrared Bands (UIB)" in these galaxies and relate these to tracers of the underlying heating sources such as the radio continuum and optical emission-lines. UIB are very commonly seen in galactic and extragalactic ob jects, with the ma jor bands centred at rest wavelengths of 6.2 µm, 7.7 µm 8.6 µm, 11.3 µm and 12.7 µm. The sp ecific questions we aim to answer are: - Are the UIB and heating sources co-spatial?
email:bn204@mrao.cam.ac.uk; BN would like to acknowledge support of the U.K. Particle Physics and Astronomy Research Council (PPARC)

c 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Figure 1. HST NICMOS image of NGC7469 at 1.1 µm (left) and CFHT V-band image (right)

- How are the UIB features mo dified in extreme environments (sho cks, intense UV flux, etc.) and what can we learn ab out the life cycles of small grains and PAHs? - Can we use the UIB features to determine the heating source in dusty ob jects. We selected a sample of 20 targets according to following criteria: 1) a sp ecific 12 µm luminosity > 1023 W Hz-1 as selected from the extended 12 µm galaxy sample (Rush et al., 1993); 2) go o d quality ISO data to constrain the level of continuum in our sp ectra; and 3) angular size < 90 arcsecs at 10 µm (where available, otherwise K band) to match the fo cal plane area of the MICHELLE detector. In our first run we detected five sources, and here present preliminary results for one of these ob jects, NGC 7469.

2. NGC 7469 The spiral galaxy NGC 7469 (z = 0.016) has a Seyfert 1 nucleus and is classified as an Ultra Luminous Infra Red Galaxy (ULIRG) with L8-1000 µm = 1011.56 L . A V band CFHT image of NGC7469 is shown in figure 1, together with a 1.1 µm HST image. ISO sp ectroscopic observations of this galaxy show a pronounced 11.3 µm UIB (figure 2, top trace), which is surprising considering the evidence for an AGN with X-ray flux of 3.4 â 10 -11 ergs-1 cm-2 (Fabbiano et al., 1992). Ground based Mid-IR imaging by Miles, Houck and

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Hayward (Miles et al., 1994), show evidence of an IR bright circumnuclear ring which is probably p owered by a starburst. Comprehensive Near IR imaging and sp ectroscopy (fig. 1, and Genzel et al., 1995) also supp ort this hyp othesis. We observed NGC 7469 with MICHELLE on the 10 June 2002, in go o d conditions. For atmospheric subtraction we employed the standard observing strategy of chopping of the secondary (20 arcsecs throw) and no dding the primary (40 arcsecs throw) b oth along the slit. We observed a nearby standard for calibration purp oses and after slewing onto the target we employed a p eak-up sequence to ensure the 0.7 arcsecs wide slit was p ositioned on the p eak of Mid-IR continuum emission. Basic data reduction was done with the ORAC-DR pip eline and then analysed with our own software. The spatially integrated sp ectrum we obtained is shown in figure 2. It is clear from these preliminary data that the equivalent width of the 11.3 µm UIB is far smaller then that observed by ISO with its large b eam width. We conclude that in the case of NGC 7469, the source of the 11.3 µm UIB is not asso ciated with the source of brightest 10 µm continuum emission. By analysing sp ectra at p ositions along the slit, we were able to measure the equivalent width of the 11.3 µm UIB as a function of distance from the continuum p eak. Preliminary results are shown in figure 3 indicating that the equivalent width p eaks at around 2 arcsecs from the p eak. This is consistent with the hyp othesis that the source of the 11.3 µm feature is the circumnuclear starburst ring and that the 11.3 µm feature is suppressed in the vicinity of the AGN.

Acknowledgements This work is based on data taken with the UKIRT op erated by the JAC on b ehalf PPARC. Also based on observations with ISO, an ESA pro ject with instruments funded by ESA Memb er States and with the participation of ISAS and NASA; data from the Canada-France-Hawaii Telescop e (CFHT) archive; and on data from the NASA/ESA Hubble Space Telescop e, obtained from the data archive at the STScI.

References
Fabbiano, G., D.-W. Kim, and G. Trinchieri: 1992, `An X-ray catalog and atlas of galaxies'. Astrophysical Journal Supplement Series 80, 531­644.

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Figure 2. The Mid Infrared spectrum of NGC7469: the ISOPHOT mode 40 spectrum (top line, circular markers, data available only to 11.6 microns); and spatially integrated MICHELLE spectrum (bottom line, diamond markers). The spectra are not absolutely calibrated.

Figure 3. Equivalent width of the 11.3 µm UIB as function of distance from the peak of continuum emission. Genzel, R., L. Weitzel, L. E. Tacconi-Garman, M. Blietz, M. Cameron, A. Krabbe, D. Lutz, and A. Sternberg: 1995, `Infrared imaging and spectroscopy of NGC 7469'. Astrophysical Journal 444, 129­145. Miles, J. W., J. R. Houck, and T. L. Hayward: 1994, `High-resolution mid-infrared observations of NGC 7469'. Astrophysical Journal Letters 425, L37­L40. Rush, B., M. A. Malkan, and L. Spinoglio: 1993, `The extended 12 micron galaxy sample'. Astrophysical Journal Supplement Series 89, 1­33.

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