Dissertation Presentation
| Date & Time | Thursday November 19, 2009
9:00-10:00am |
| Location | Astronomy Building, Room 201 |
| Title | The Magellanic Stream |
| Speaker | David Nidever, University of Virginia | | Abstract | We have performed a detailed HI study of the Magellanic Stream (MS).
Using a Gaussian decomposition of the HI velocity profiles in the
Leiden-Argentine-Bonn (LAB) all-sky HI survey, we expose the MS to be
composed of two filaments distinct both spatially (as first pointed out by
Putman et al. 2003) and in velocity. Using the velocity coherence of the
filaments, one can be traced back to its origin in what we identify as the
SouthEast HI Overdensity (SEHO) of the Large Magellanic Cloud (LMC), which
includes 30 Doradus. Parts of the Leading Arm (LA) can also be traced back
to the SEHO in velocity and position. Therefore, at least one-half of the
trailing Stream and most of the LA originates in the LMC, contrary to previous
assertions that both the MS and the LA originate in the Small Magellanic
Cloud (SMC) and/or in the Magellanic Bridge. The two MS filaments show
strong periodic, undulating spatial and velocity patterns that we speculate
are an imprint of the LMC rotation curve. If true, then the drift rate of the
Stream gas away from the Magellanic Clouds is ~49 km/s and the age of the
MS is ~1.74 Gyr. The Staveley-Smith et al. (2003) high-resolution HI data of
the LMC show gas outflows from supergiant shells in the SEHO that seem to
be creating the LA and LMC filament of the MS. Blowout of LMC gas is an
effect not previously accounted for but one that probably plays an important
role in creating the MS and LA. We lay out a new model, using SGS blowout,
for the formation of the MS that fits all of the available data and solves some
longstanding problems.
We also conducted a ~200 deg^2 21-cm survey with the Robert C. Byrd Green
Bank Telescope at the tip of the MS to bridge the ~10 degree gap between the
"classical" MS and the MS-like emission reported by Braun & Thilker (2004). Our
survey, in combination with the Arecibo survey by Stanimirovic et al. (2008),
shows that the MS gas is continuous across this gap and that the MS is at least
~140 degrees long. The MS-tip is composed of a multitude of forks and
filaments. We identify a new filament on the eastern side of the MS that
significantly deviates from the nominal MS equator for ~45 degrees. Additionally,
we find a previously unknown velocity inflection in the MS-tip near MS longitude
L_MS=-120 degree at which the velocity reaches a minimum and then starts to
increase. The mass of the newly-found ~40 degree extension of the MS-tip is
~5x10^7 M_sun which increases the total mass of the MS by ~10%. We find that
five compact high velocity clouds from de Heij et al. (2002) as well as Wright's
Cloud might be associated with the MS because they match the MS in position
and velocity. From our combined map of the entire MS, we find that the total
column density (integrated transverse to the MS) drops markedly along the MS and
follows an exponential decline with L_MS of N(HI)=5.9x10^21 exp(L_MS/19.3 degree)
1/cm^2. Under the assumption that the observed sinusoidal velocity pattern of
the LMC filament of the MS is due to the origin of the MS from a rotating LMC,
we estimate that the age of the ~140 degree-long MS is ~2.5 Gyr. This coincides
with bursts of star formation in the Magellanic Clouds and a possible close
encounter of these two galaxies with each other that could have triggered the
formation of the MS. These new observational characteristics of the MS offer
additional new constraints on MS simulations. |
| Additional Information | Public Talk To Be Followed By Closed Oral Session
Note: Copy of Dissertation in Astronomy Office (Blue Binder)
A brief public question and answer session will follow the talk and precede the closed oral exam with the dissertation committee. |
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