| Abstract |
We present an analysis of ionization and metal enrichment in the
Magellanic Stream (MS), the nearest gaseous tidal stream, using Hubble
Space Telescope/STIS and FUSE ultraviolet spectroscopy of two background
active galactic nuclei. The targets are NGC 7469, lying directly behind
the MS with log N(H I)MS = 18.63 ± 0.03(stat) ±
0.08(syst), and Mrk 335, lying 24fdg7 away with log N(H I)MS
= 16.67 ± 0.05. For NGC 7469, we include optical spectroscopy
from VLT/UVES. In both sight lines, the MS is detected in low-ion (O I,
C II, C III, Si II, Si III, Al II, Ca II) and high-ion (O VI, C IV, Si
IV) absorption. Toward NGC 7469, we measure an MS oxygen abundance
[O/H]MS = [O I/H I] = -1.00 ± 0.05(stat)
± 0.08(syst), supporting the view that the Stream originates in
the Small Magellanic Cloud rather than the Large Magellanic Cloud. We
use CLOUDY to model the low-ion phase of the Stream as a photoionized
plasma using the observed Si III/Si II and C III/C II ratios. Toward Mrk
335, this yields an ionization parameter between log U = -3.45 and
-3.15, a gas density log (n H/cm-3)
between -2.51 and -2.21, and a hydrogen ionization fraction
of 98.9%-99.5%. Toward NGC 7469, we derive sub-solar abundance ratios
for [Si/O], [Fe/O], and [Al/O], indicating the presence of dust in the
MS. The high-ion column densities are too large to be explained by
photoionization, but also cannot be explained by a single-temperature
collisional ionization model (equilibrium or non-equilibrium). This
suggests that the high-ion plasma is multi-phase, with an Si IV region,
a hotter O VI region, and C IV potentially contributing to each. Summing
over the low-ion and high-ion phases, we derive conservative lower
limits on the ratio N(total H II)/N(H I) of gsim19 toward NGC 7469 and
gsim330 toward Mrk 335, showing that along these two directions the vast
majority of the Stream has been ionized. The presence of warm-hot plasma
together with the small-scale structure observed at 21 cm provides
evidence for an evaporative interaction with the hot Galactic corona.
This scenario, predicted by hydrodynamical simulations, suggests that
the fate of the MS will be to replenish the Galactic corona with new
plasma, rather than to bring neutral fuel to the disk.
Based on observations from the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer mission (program P101), operated by Johns Hopkins
University, supported by NASA contract NAS 5-32985; from the NASA/ESA
Hubble Space Telescope (program 9802), obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS 5-26555; and
from the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very
Large Telescope (VLT) Unit 2 (Kueyen) at Paranal, Chile, operated by the
European Southern Observatory, under program ID 081.D-0697(A). |