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Sub-Arcsecond Resolution Mid-IR Imaging of The Starburst NGC 520
´ Bojan Nikolic, Paul Alexander, Garret Cotter, Malcolm Longair AP Group, Cavendish Laboratory, Cambridge, U.K. Marcel Clemens Universita degli Studi di Padova, Italy [3], which is N0 = 2.6 â 1053 s-1, by revising their K-band extinction estimate to AK = 3. · Using IRAS data, and the Radio-FIR correlation, we estimate the total energy release in the nuclear starburst in NGC 520 to be 6.7 â 1010L . Mid-IR data indicate around 7 â 108L , or 1% of the total luminosity, is emitted in the 7.7 µ m UIB. · The luminosity and N0 estimates indicates an age of 5.2 Myr and starburst mass of 1.0 â 108M (assuming an instantaneous burst of star-formation). This can be compared to the mass of cold molecular material = 4.3 â 109M .

Abstract
We present sub-arcsecond resolution mid-infrared images of the nearby starburst NGC 520 in four narrow filters (7.9, 9.7 , 11.6 and 12.5 µ m). These data, together with matched-resolution VLA data at four frequencies and Br data, have enabled us to calculate, in a selfconsistent way, the properties of the starburst. This model agrees with our estimate of extinction from the 9.7 µ m silicate absorption feature, which we have also used to produce a map of the column density of the obscuring material.

The intensity of UIB is an accurate tracer of UV radiation field, while their ratios can tell us a lot about the environment of PAH molecules. For example, the 3.3 µ m UIB is the highest in energy, and is therefore emitted by the smallest PAHs and/or the PAHs exposed to the hardest UV field. Comparison to lower energy UIBs could therefore constrain PAH size distribution or the hardness of the UV field.

3 Observations
Observations were carried out with the MICHELLE instrument on UKIRT on the nights of 26thJuly (filters at 7.9, 9.7 and 11.6 µ m) and 27thJuly (12.5 µ m filter) in photometric conditions. Background subtraction was performed with the standard technique of chopping of the secondary and nodding of the primary mirrors, both with a throw of 19.76 arcsecs. Our field of view was 70 x 50 .

1 Introduction
We have commenced a programme of using mid-infrared (Mid-IR) observations to study the interstellar medium, in particular the dust component, in a sample of nearby infra-red bright starburst/AGN galaxies. In this poster we present our data for NGC 520 (Arp 157), a merging system[1] at a distance of 30 Mpc (1 = 145 pc). As there is no evidence for an AGN in this system, we can attribute its entire IR luminosity, LI R = 8.9 â 1010L , to embedded star formation. The high extinction present in this system is clear from comparison of R-band (c = 0.6 µ m, figure 1) and K-band (c = 2.2 µ m, figure 2) images.

3.1

Supporting observations

We have produced VLA radio maps of this source at matched resolution at four frequencies (figure 4). We also used an archival ISOCAM Circular Variable Filter (CVF) spectrum (figure 5).

· We find that the relative variations in our Mid-IR data -- that is, the colour maps -- are consistent with a constant source spectrum which suffers absorption by a varying column density of intervening material. The inferred map of the line-of-sight gas column density is shown in figure 8. The average column density over the starburst disk is 6 â 1022 Hcm-2 which corresponds to AK = 4, a value close to that indicated by Br data.

· Using detailed stellar population models[5] we used the above parameters to estimate the supernova rate to be SN 0.1 yr-1. This is close to the value inferred from synchrotron radio emission component, using the method of Condon 1992[6], SN 0.15 yr-1.

Figure 1. NGC 520: R-band (colour, from [2]) and ISOCAM 15 µ m (contoured at 0.2 mJy arcsecs-2 â 0.5n)

Figure 2. NGC 520: K-band (colour, from [3]) and ISOCAM 15 µ m (contoured at 0.2 mJy arcsecs-2 â 0.5n)

Figure 7. Map of Br emission, from [3] (overlaid with own contours at peakâ0.5n)

Figure 8. Gas column density â1023 Hcm-2, estimated from the 9.7 µ m absorption feature (different scale to previous figures)

5
Figure 4. VLA observations of NGC 520 (contours) overlaid on 12.5 µ m image (colour) , top row: 1.4 and 4.9 GHz; bottom row: 8.5 and 15 GHz. Contours are at 21, 3.2, 4.4 and 7.4 mJy beam-1 â 0.5n respectively.

Conclusions
· We have presented new Mid-IR and Radio data which further constrain the properties of the starburst in NGC 520 and indicate it is more deeply embedded then previously thought (AK 3.5). · We show that Mid-IR emission accurately and efficiently traces intense star formation. · We demonstrate that Mid-IR imaging in several filters around the 9.7 µ m absorption feature can be used to make meaningful maps of the column density of the obscuring material. We believe that with a larger number of number of narrow-band filters (around six) it should be possible to accurately dis-entangle the mid-infrared UIB and continuum and the effects of extinction. These data should be sufficient on their own to constrain UV radiation field, heating source spectrum and extinction along the line of sight in deeply embedded starbursts, all at the high angular resolution possible from the ground.

Figure 3. Michelle observations of NGC 520. Top row, left: 7.9 µ m; right: 9.7 µ m; Bottom row, left: 11.6 µ m; right: 12.5 µ m (contours are at 90, 10, 22 and 55 mJy arcsecs-2 â 0.75n respectively). Figure 5. ISOCAM CVF spectrum of NGC 520

Acknowledgments
We would like to thank C. Xu and J.K. Kotilainen for kindly letting us have their data in electronic format. B.N. would like to acknowledge a PPARC studentship. Based on data from UKIRT; VLA; and ISO.

2 The Case for Mid-Infrared Observations
The Mid-IR is increasingly being seen as the key to understanding the phenomena of Luminous IR galaxies in the local universe as well as intense star-formation at medium and high redshift. The reasons for this are: · Much smaller extinction compared to optical wavelengths (A0.5 µ m/A7.7 µ m 50).

References
[1] A. Toomre and J. Toomre. Galactic Bridges and Tails. Astrophysical Journal, 178:623­666, December 1972. [2] C. Xu, Y. Gao, J. Mazzarella, N. Lu, J. W. Domingue. Mapping Infrared Enhancements ing Spiral-Spiral Pairs. I. ISO CAM and ISO Astrophysical Journal, 541:644­659, October Sulentic, and D. L. in Closely InteractSWS Observations. 2000.

· Possibility of diffraction-limited seeing from the ground on 8-meter class telescope, giving resolution of 0. 4.

· A number of emission/absorption features which could be used to determine the intensity & spectrum of the heating source as well as the extinction along the line of sight. The Mid-IR spectrum of NGC 520 in the 6-15 µ m range is shown in figure 5. It displays the three components often seen in the midinfrared: · The Unidentified Infrared Bands (UIB) centered at 3.3, 7.7, 11.3 and 12.7 µ m. These are attributed to transiently heated Polycyclic Aromatic Hydrocarbons (PAH) molecules with 100 atoms. is attributed to Very Small Grains (VSG), again tranby single UV photons. These must be sufficiently high temperatures when heated by single photons, not to have sharp emission features.

[3] J. K. Kotilainen, J. Reunanen, S. Laine, and S. D. Ryder. Nearinfrared line imaging of the starburst galaxies NGC 520, NGC 1614 and NGC 7714. Astronomy and Astrophysics, 366:439­450, February 2001.
Figure 6. Interstellar extinction curve from [4] overlaid with the pass bands of our filters

[4] A. Li and B. T. Draine. Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium. Astrophysical Journal, 554:778­ 802, June 2001. [5] C. Leitherer, D. Schaerer, J. D. Goldader, R. M. G. Delgado, C. Robert, D. F. Kune, D. F. de Mello, D. Devost, and T. M. Heckman. Starburst99: Synthesis Models for Galaxies with Active Star Formation. Astrophysical Journal Supplement Series, 123:3­40, July 1999. [6] J. J. Condon. Radio emission from normal galaxies. Annual Review of Astronomy and Astrophysics, 30:575­611, 1992.

· The continuum siently heated small to reach yet big enough

4 Discussion
Good quality data at a large number of wavelengths has allowed to draw detailed conclusions about the starburst in NGC 520: · Radio free-free emission indicates UV photon density of N0 = 8.1 â 1053 s-1. This can be reconciled with estimate by Kotilainen et al

· The deep silicate absorption at 9.7 µ m (see figure 6).