Revisiting the Magellanic System with RR Lyrae

Research Summary:

(This paper is available on arXiv and accepted for publication at MNRAS).

The Magellanic clouds are an important piece of the puzzle to help us understand galaxy interactions, as well as the dynamical and star formation history of our own Galaxy. The gaseous features surrounding the LMC and SMC indicate interactions between the two galaxies, and they share a common HI envelope. However, there are still disagreements about the nature of these interactions, such as the Magellanic Bridge connecting the two galaxies. Some studies suggest that interactions between the LMC and SMC stem from multiple pericentric passages of the galaxies as they orbit our Galaxy, with the inter-galactic features resulting from tidal stripping and ram pressure (Ruzicka, Theis and Palous 2009). Other research suggests that the Magellanic clouds have only begun interacting during two recent encounters, estimated to have occurred ∼2 Gyr and ∼250 Myr ago (Kallivayalil et al. 2013; Noel et al. 2015).

We examine the properties of the Magellanic Bridge connecting the Large and Small Magellanic Clouds using ab-type RR Lyrae variables from the extensive dataset of the Optical Gravitational Lensing Experiment (OGLE), Phase IV data release. The metallicities of the RR Lyraes are determined from the characteristics of their light curves, with an average peak of [Fe/H] = -1.790 ±0.011 in the Magellanic Bridge. From the individual reddenings of the OGLE RR Lyrae ab stars, derived via their minimum light curve colors, we determined a peak reddening of E(V–I) = 0.101 ± 0.007 (E(BV) ≈ 0.077). The peak distance modulus of 18.57 ± 0.048 places the Bridge stars at distances between the two systems. The metallicity and distance distributions probe the structure of the Magellanic system as a whole, revealing a smooth distribution between the two satellites. An examination of the HI content does not find a clear correlation between HI emission strength and RR Lyrae spatial distribution, suggesting that the old stellar populations may trace the overlapping halo distributions of the two Magellanic Clouds.

In the figure below, the HI integrated intensity from Putman et al. (2003) is overlaid by the RR Lyrae locations from the OGLE survey indicated in blue. The RR Lyrae that fall into the approximate region of the Bridge are indicated by the red points.

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