We present the IRC images of M51, a pair of interacting galaxies. Given the high angular resolution (7.4") and the wide field of view (~ 10') covering almost the entire M51 system, we investigate dust properties and their connection to the spiral arm structure. We have applied image-filtering processes including the wavelet analysis to the N3 image, which traces the total stellar mass best among the IRC bands. From this filtered image, the center, arm, and interarm regions are defined. A color, or flux ratio among the MIR bands, has been measured at each pixel (3.7" in size). We find a wide variety of S7/S11 with a difference between arm and interarm regions. We also find that at some positions S11 seems to be higher than predicted by MW dust models. Estimated contributions from the stellar continuum and gas emission lines to the band are not enough to explain this discrepancy. From these results, we deduce that the PAH ionization condition and its fraction to the total dust mass in M51 are different from those in MW.

ABSTRACT
1. INTRODUCTION
2. DATA REDUCTION
    2.1. Neighbor Dark Frames
    2.2. Earthshine Light Removal
    2.3. PSF Correction
    2.4. Ghost Masking
3. ARM DEFINITION
4. RESULTS
    4.1. Color-Color Diagram
    4.2. Dust Models
5. DISCUSSION
REFERENCES

• FUMI EGUSA(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• I. SAKON(Graduate School of Science, The University of Tokyo)
• T. ONAKA(Graduate School of Science, The University of Tokyo)
• H. MATSUHARA(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• K. ARIMATSU(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Graduate School of Science, The University of Tokyo)
• T. SUZUKI(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• T. WADA(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
• THE IRC TEAM