New constraints on the nature and origin of the Leading Arm of the Magellanic Stream

P. Richter, A. J. Fox, B. P. Wakker, J. C. Howk, N. Lehner, K. A. Barger, E. D'Onghia, F. J. Lockman

Published 2018-08-28Version 1

We present a new precision measurement of gas-phase abundances of S, O, N, Si, Fe, P, Al, Ca as well as molecular hydrogen (H_2) in the Leading Arm (region II, LAII) of the Magellanic Stream (MS) towards the Seyfert galaxy NGC 3783. The results are based on high-quality archival ultraviolet/optical/radio data from various different instruments (HST/STIS, FUSE, AAT, GBT, GB140ft, ATCA). Our study updates previous results from lower-resolution data and provides for the first time a self-consistent component model of the complex multi-phase absorber, delivering important constraints on the nature and origin of LAII. We derive a uniform, moderate alpha abundance in the two main absorber groups at +245 and +190 km s^-1 of alpha/H = 0.30 pm 0.05 solar, a low nitrogen abundance of N/H = 0.05 pm 0.01 solar, and a high dust content with substantial dust depletion values for Si, Fe, Al, and Ca. These alpha, N, and dust abundances in LAII are similar to those observed in the Small Magellanic Cloud (SMC). From the analysis of the H_2 absorption, we determine a high thermal pressure of P/k = 1680 K cm^-3 in LAII, in line with the idea that LAII is located in the inner Milky Way halo at a z-height of <20 kpc where it hydrodynamically interacts with the ambient hot coronal gas. Our study supports a scenario, in which LAII stems from the break-up of a metal- and dust-enriched progenitor cloud that was recently (200-500 Myr ago) stripped from the SMC.