Chemical Abundances in the Leading Arm of the Magellanic Stream^∗

Abstract

The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph and Green Bank Telescope spectra of four AGN sightlines passing through the LA and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0 +2.0 −2.0 % solar to 12.6 +6.0 −4.1 % solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H=35±7% solar (Lu et al. 1998), but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, two of the three nearby sightlines show high-velocity clouds with H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 • further to the Galactic northwest.

New Constraints on the Nature and Origin of the Leading Arm of the Magellanic Stream

“…Our study confirms earlier results on the global metal abudance of LA II (Lu et al 1998;Fox et al 2018) and provides new information on the dust properties and physical conditions in the gas, further constraining of the origin of the LA and its evolution.…”

“…The analysis of the COS G130M data of NGC 3783 presented in Fox et al (2018) yields a slightly lower S ii column density than our analysis of the STIS E140M data (COS: log N (S ii) = 14.41 ± 0.12; STIS: log N (S ii) = 14.57 ± 0.03), although both studies use the same S ii line at 1253.8Å. In Fig.…”

“…Yet, it is still only half of that measured in the LMC filament of the MS (R13). As discussed in Fox et al (2018), the observed metallicity spread in the LA most likely indicates an episodic removal of gas from the SMC, with LA III being the oldest part stripped from the SMC together with the main body of the MS 1 − 2 Gyr ago, followed by the later removal of LA I and LA II due to more recent encounters of SMC and LMC. Our abundance measurements therefore set important new constraints for the numerical modeling of the the Magellanic System (see Pardy et al 2018).…”

“…The local value for U can estimated from the measured Si iii/Si ii column density ratios, an approach that has been used in many of our previous studies of the MS and the LA (see Fox et al 2013Fox et al , 2018R13). One drawback of this method is that it does not account for the possible multi-phase nature of the absorbers that may be composed of denser cores and more diffuse gas layers where the local Si iii/Si ii ratios may vary considerably.…”

Spectroscopy of the Young Stellar Association Price-Whelan 1: Origin in the Magellanic Leading Arm and Constraints on the Milky Way Hot Halo

“…A number of HST studies have found that the MS has a mean metallicity of [Fe/H] ≈ −1 dex across its length (Fox et al 2010(Fox et al , 2013b, although there is evidence for a more metal-rich component (Richter et al 2013;Fox et al 2013b). The Magellanic Bridge, the gas between the MCs, also has a metallicity of ≈ −1 (Lehner et al 2008) as does the LA (Lu et al 1998;Fox et al 2018;Richter et al 2018). The consistency in the metallicities for these distinct gaseous features that lead, trail, and connect the MCs suggests that they share a common originating system: the MCs, themselves.…”