AKARI view of star formation in NGC 1313

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

Received: 28 August 2012
Accepted: 18 February 2013

Abstract

Context. In the southwest region of NGC 1313, patchy star-forming regions are found. In the neighborhood, is a giant supershell with a diameter of 3.2 kpc. However, the direct association between star-forming regions and the giant supershell is still unclear.

Aims. We investigate the relation between the surface densities of the gas (Σ_gas) and star formation rate (Σ_SFR) within the disk of NGC 1313 to obtain the spatial distribution of the star formation efficiency (SFE).

Methods. NGC 1313 was observed with the Infrared Camera (IRC) and Far-Infrared Surveyor (FIS) onboard AKARI in ten bands at 3.2, 4.1, 7, 11, 15, 24, 65, 90, 140, and 160 μm. With these AKARI ten-band images, we spectrally decomposed the stellar and polycyclic aromatic hydrocarbon (PAHs) components in addition to the cold (~20 K) and warm (~60 K) dust emission components. The cold and warm dust components were converted into the gas mass and the SFR.

Results. The SFE shows high values in the regions around the giant supershell, with the highest SFE in the galaxy in the southern spiral arm region. A similar trend can be seen in the spatial distribution of the PAH abundance relative to big grains. We find a power-law dependence of the star formation rate per unit area on the column density Σ_SFR proportional to Σ_gas^N. From a region-by-region analysis, the power-law index N for the northern spiral arm is found to be N ≃ 2.0, which is observed on typical spiral arms, whereas those for the southern spiral arm and giant supershell regions have an index of N ~ 1.6.

Conclusions. The PAH abundance and the SFE show anomalously high values in regions around the giant supershell. The enhanced PAH abundance can be caused by shattering of big grains through slow shocks (40 km s^-1). The power-law index N = 1.62 ± 0.06 obtained in the giant supershell region can be accounted for by the star formation scenario with collect-and-collapse in the expanding giant supershell. These pieces of evidence strongly indicate that the giant supershell has triggered the star formation and enhanced the SFE.