Kiloparsec-scale star formation law in M 81 and M 101 based on AKARI far-infrared observations

¹ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252–5210, Japan e-mail: suzuki@ir.isas.jaxa.jp
² Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464–8602, Japan
³ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
⁴ Graduate School of Science, Osaka University, Toyonaka, Osaka 560–0043, Japan

Received: 29 March 2010
Accepted: 10 June 2010

Abstract

Aims. We assess the relationships between the surface densities of the gas and star formation rate (SFR) within spiral arms of the nearby late-type spiral galaxies M 81 and M 101. By analyzing these relationships locally, we empirically derive a kiloparsec scale Kennicutt-Schmidt Law (Σ_SFR Σ_gas^N).

Methods. Both M 81 and M 101 were observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared bands at 65, 90, 140, and 160 μm.

Results. The spectral energy distributions of the whole galaxies show the presence of the cold dust component (T_C ~ 20 K) in addition to the warm dust component (T_W ~ 60 K). We deconvolved the cold and warm dust emission components spatially by making the best use of the multi-band photometric capability of the FIS. The cold and warm dust components show power-law correlations in various regions, which can be converted into the gas mass and the SFR, respectively. We find a power-law correlation between the gas and SFR surface densities with significant differences in the power-law index N between giant H ii regions (N = 1.0 ± 0.5) and spiral arms (N = 2.2 ± 0.2) in M 101. The power-law index for spiral arms in M 81 is similar (N = 1.9 ± 0.4) to that of spiral arms in M 101.

Conclusions. The power-law index is not always constant within a galaxy. The difference can be attributed to the difference in the star formation processes on a kiloparsec scale. N 2 seen in the spiral arms in M 81 and M 101 supports the scenario of star formation triggered by cloud-cloud collisions enhanced by a spiral density wave, while N 1 derived in giant H ii regions in M 101 suggests the star formation induced by the Parker instability triggered by high-velocity H i gas infall. The present method can be applied to a large galaxy sample for which the AKARI All Sky Survey provides the same 4 far-infrared band data.