The star formation history of the Magellanic Clouds derived from long-period variable star counts

Abstract

We present the first reconstruction of the star formation history (SFH) of the Large and Small Magellanic Clouds (LMC and SMC) using Long Period Variable stars. These cool evolved stars reach their peak luminosity in the near-infrared; thus, their K-band magnitudes can be used to derive their birth mass and age, and hence the SFH can be obtained. In the LMC, we found a 10-Gyr old single star formation epoch at a rate of ∼ 1.5 M ⊙ yr −1 , followed by a relatively continuous SFR of ∼ 0.2 M ⊙ yr −1 , globally. In the core of the LMC (LMC bar), a secondary, distinct episode is seen, starting 3 Gyr ago and lasting until ∼ 0.5 Gyr ago. In the SMC, two formation epochs are seen, one ∼ 6 Gyr ago at a rate of ∼ 0.28 M ⊙ yr −1 and another only ∼ 0.7 Gyr ago at a rate of ∼ 0.3 M ⊙ yr −1 . The latter is also discernible in the LMC and may thus be linked to the interaction between the Magellanic Clouds and/or Milky Way, while the formation of the LMC bar may have been an unrelated event. Star formation activity is concentrated in the central parts of the Magellanic Clouds now, and possibly has always been if stellar migration due to dynamical relaxation has been effective. The different initial formation epochs suggest that the LMC and SMC did not form as a pair, but at least the SMC formed in isolation.

Planck2015 results

“…The Commander free-free amplitude at 10 GHz is 141 Jy, which gives an SFR of 0.10 M yr −1 ; the LSF model gives 0.08 M yr −1 , and the LSF-CMB model gives 0.09 M yr −1 . These are lower than the estimate of 0.2 M yr −1 for the average SFR from analyses of stellar populations by Harris & Zaritsky (2009) and Rezaeikh et al (2014), but agree well with the recent star formation rate of 0.06 M yr −1 calculated by Whitney et al (2008) based on young stellar objects; these authors also give a range of SFR estimates of 0.05-0.25 M yr −1 from infrared and Hα data. However, all of these estimates correspond to an SFR over different timescales (e.g., Murphy et al 2012), as well as being subject to systematic effects; a more detailed study would be necessary to disentangle these effects.…”

Long-period variable stars in NGC 147 and NGC 185 – I. Their star formation histories

“…Our method, which is explained in section 3, Javadi et al (2011Javadi et al ( , 2016 and Rezaeikh et al (2014), is based on the luminosity peak of LPVs in the K band. Figures 2 and 3 show the location of the LPVs in the near-infrared CMDs (middle panels), as well as the carbon stars for comparison (right panels).…”

“…We have developed a novel method to use Long-Period Variable stars (LPVs) to reconstruct the SFH, which we have successfully applied the Local Group galaxies M 33 (Javadi, van Loon & Mirtorabi 2011;Javadi et al 2016) and the Magellanic Clouds (Rezaeikh et al 2014). The most evolved stars with low to intermediate mass, at the tip of the Asymptotic Giant Branch (AGB) show brightness variations on timescales of ≈ 100 to > 1000 days due to radial pulsation.…”

Modeling Dust in the Magellanic Clouds

“…This is in agreement with the current star formation rates in the MCs ( In general, observations of stellar populations in dwarf galaxies suggest that the star formation bursts are sporadic, separated by millions to billions of years even in isolated systems (e.g., Weisz et al 2008). According to their star-formation histories (Harris & Zaritsky 2004, 2009Rezaei et al 2014), the MCs appear to be typical members of this class of galaxies. MC-type ISECs may be then prototypical of dwarf galaxies of low metallicity, with high gas content (see e.g., Cecchi-Pestellini et al 2014b).…”