The recent star-formation history of the Large and Small Magellanic Clouds

Abstract

Aims. Recent interactions between the Large and the Small Magellanic Clouds (LMC and SMC) and the Milky Way can be understood by studying their recent star formation history. This study aims to detect any directional or propagating star formation in the last 500 Myr.

Methods. We traced the age of the last star-formation event (LSFE) in the inner Large and Small Magellanic Cloud (L&SMC) using the photometric data in V and I passbands from the Optical Gravitational Lensing Experiment (OGLE-III) and the Magellanic Cloud Photometric Survey (MCPS). The LSFE is estimated from the main sequence turn-off point in the color-magnitude diagram (CMD) of a subregion. After correcting for extinction, the turn-off magnitude is converted to age, which represents the LSFE in a region.

Results. The spatial distribution of the age of the LSFE shows that the star-formation has shrunk to within the central regions in the last 100 Myr in both the galaxies. The location as well as age of LSFE is found to correlate well with those of the star cluster in both the Clouds. The SMC map shows two separate concentrations of young star-formation, one near the center and the other near the wing. We detect peaks of star-formation at 0–10 Myr and 90–100 Myr in the LMC, and 0–10 Myr and 50–60 Myr in the SMC. The quenching of star-formation in the LMC is found to be asymmetric with respect to the optical center such that most of the young star forming regions are located to the north and east. On deprojecting the data onto the LMC plane, the recent star-formation appears to be stretched in the northeast direction and the HI gas is found to be distributed preferentially in the north. We found that the centroid is shifted to the north during the time interval 200–40 Myr, whereas it is found to have shifted to the northeast in the last 40 Myr. In the SMC, we detect a shift in the centroid of the population younger than 500 Myr and as young as 40 Myr in the direction of the LMC.

Conclusions. We propose that the HI gas in the LMC has been pulled to the north of the LMC in the last 200 Myr because of the gravitational attraction of our Galaxy at the time of perigalactic passage. The shifted HI gas was preferentially compressed in the north during the time interval 200–40 Myr and in the northeast in the last 40 Myr, owing to the motion of the LMC in the Galactic halo. The recent star-formation in the SMC is due to the combined gravitational effect of the LMC and the perigalactic passage.