Reddening map and recent star formation in the Magellanic Clouds based on OGLE IV Cepheids^⋆

Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263002, India
e-mail: yogesh@aries.res.in

Received: 5 November 2018
Accepted: 11 June 2019

Abstract

Context. The reddening maps of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are constructed using the Cepheid period–luminosity (P–L) relations.

Aims. We examine reddening distribution across the LMC and SMC through large data sets on classical Cepheids provided by the OGLE Phase IV survey. We also investigate the age and spatio-temporal distributions of Cepheids to understand the recent star formation history in the LMC and SMC.

Methods. The V and I band photometric data of 2476 fundamental mode (FU) and 1775 first overtone mode (FO) Cepheids in the LMC, and 2753 FU and 1793 FO Cepheids in the SMC were analysed for their P–L relations. We converted the period of FO Cepheids to the corresponding period of FU Cepheids before combining the two modes of Cepheids. Both galaxies were divided into small segments and combined FU and FO P–L diagrams were drawn in two bands for each segment. The reddening analysis was performed on 133 segments covering a total area of about 154.6 deg² in the LMC and 136 segments covering a total area of about 31.3 deg² in the SMC. By comparison with well-calibrated P–L relations of these two galaxies, we determined reddening E(V − I) in each segment and equivalent reddening E(B − V) assuming the normal extinction law. The period–age relations were used to derive the age of the Cepheids.

Results. Reddening maps were constructed using reddening values in different segments across the LMC and SMC. We find clumpy structures in the reddening distributions of the LMC and SMC. From the reddening map of the LMC, highest reddening of E(V − I) = 0.466 mag is traced in the region centred at α ∼ 85_.^°13, δ ∼ −69_.^°34 which is in close vicinity of the star forming HII region 30 Doradus. In the SMC, maximum reddening of E(V − I) = 0.189 mag is detected in the region centred at α ∼ 12_.^°10, δ ∼ −73_.^°07. The mean reddening values in the LMC and SMC are estimated as E(V − I)_LMC = 0.113 ± 0.060 mag, E(B − V)_LMC = 0.091 ± 0.050 mag, E(V − I)_SMC = 0.049 ± 0.070 mag, and E(B − V)_SMC = 0.038 ± 0.053 mag.

Conclusions. The LMC reddening map displays heterogeneous distribution having small reddening in the central region and higher reddening towards the eastern side of the LMC bar. The SMC has relatively small reddening in its peripheral regions but larger reddening towards the south-west region. In these galaxies, we see evidence of a common enhanced Cepheid population at around 200 Myr ago which appears to have occurred due to a close encounter between the two clouds.