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 GCnewsletter_abstract.tex PhD thesis
Date: Fri, 9 Sep 2005 14:53:43 +0900
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\title{\large \bf Structure and Dynamics at the Central 10 Parsecs of the
Galaxy }

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\author{ Sungho Lee }

\affil{ Korea Astronomy and Space Science Institute, Daejeon 305-348,
Korea }

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Electronic mail: leesh@kasi.re.kr

Thesis work conducted at: School of Earth and Environmental Sciences, Seoul
National University


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The Galactic center influences the current nature as well as the formation,
the evolution
and the future fate of the Milky Way. The Galactic nucleus stands for the
galactic nuclei
of other galaxies and provides an opportunity to
study the environment around a super-massive black hole (SMBH) at high
spatial resolution.
The central 10~pc of the Galaxy, the Sgr A region, contains several
principal components; the SMBH candidate (Sgr A*), the Central cluster, the
circum-nuclear disk
(CND), Sgr A West, a powerful supernova-like remnant (Sgr A East), and
surrounding molecular clouds.
Developing a consistent picture of the interactions between these components
will improve our
understanding of the Galaxy and the nature of galactic nuclei in general.
Previous studies on the spatial and dynamical relationships between the
various objects
are mostly based on indirect and qualitative evidence and leave many
unsolved questions,
which need more robust evidence.
Molecular hydrogen (H$_2$) emission has been used as an excellent tracer and
diagnostic for
interactions between dense molecular clouds and hot, powerful sources.

We observed the ${\rm H_2 ~ 1\!-\!0 ~ S(1)} ~ (\lambda = 2.1218 \micron)$
and
${\rm H_2 ~ 2\!-\!1 ~ S(1)} ~ (\lambda = 2.2477 \micron)$ emission line
spectra from the interaction
regions between Sgr A East, the CND, and the surrounding molecular clouds.
Using the long-slit Cooled Grating Spectrometer 4 (CGS4) with an echelle
grating
at the 3.8~m United Kingdom Infrared Telescope (UKIRT) on Mauna Kea,
we scanned 56 positions in the interaction regions.
We reduced 2-D spectral images using {\sc IRAF} and analyzed a 3-D data cube
using {\sc MIRIAD}.
The data cube has the H$_2$ information both in space (with a resolution of
$\sim 2\arcsec$)
and in velocity (with a resolution of $\sim 18~{\rm km\;s^{-1}}$).
The ${\rm H_2 ~ 1\!-\!0 ~ S(1)}$ data cube was directly compared with the
NH$_3$(3,3) data cube
from McGary, Coil, \& Ho (2001, ApJ, 559, 326) to investigate the gas
kinematics.

Based on the ${\rm H_2 ~ 1\!-\!0 ~ S(1)}$ and ${\rm 2\!-\!1 ~ S(1)}$ line
intensities and gas kinematics,
we concluded that the H$_2$ excitation can be explained by two mechanisms;
a combination of fluorescence and C-shocks in very strong magnetic fields,
or a mixture of slow C-shocks and fast J-shocks.
We estimated shock velocities ($\sim 100~{\rm km\;s^{-1}}$) of Sgr A East by
comparing H$_2$ line profiles
with those of NH$_3$.
From the distribution of the shocked H$_2$ emission, we determined the
interacting boundary
of Sgr A East in projection as an ellipse with the center at $\sim1.5$~pc
offset from Sgr A*
and the dimension of 10.8~pc $\times$ 7.6~pc.
We also determined the positional relationship between Sgr A East and the
molecular clouds
along the line-of-sight and suggested a revised model for the 3-D structure
of the central 10~pc.
From the estimated shock velocities, we deduced the initial explosion energy
(0.2--$4 \times 10^{53}$~ergs)
of Sgr A East. This extremely large energy excludes the hypothesis of a
single, typical, supernova (SN)
for the origin of Sgr A East. We examined other hypotheses (tidal disruption
of a star by the
SMBH, multiple supernovae, and a hypernova) and we concluded that a
hypernova (collapsar or microquasar)
is the most probable origin of Sgr A East.

Based on the energy, we investigated the influences of the Sgr A East-like
explosions
(hypernovae) and normal SNe on the mass inflow to the Galactic nucleus.
We suggest a scenario that the continuous mass inflow into the
Galactic nucleus makes it active by igniting the SMBH or stimulating
a starburst every $\sim 10^8$~yr, but each active phase continues only
${\mathrel{\raise.3ex\hbox{$<$\kern-.75em\lower1ex\hbox{$\sim$}}}} 10^7$~yr
since a large number of SNe resulting from newly born massive stars
cease the mass supply soon. The Galactic nucleus is likely to spend
only about $1/10$ of its life in active.
As for the recent history of the central 10~pc,
the mass inflow restarted several $10^6$~yr ago after a quiescent phase for
$\sim 10^8$~yr.
In its usual schedule, the Galactic nucleus would continue its
activity for a few $10^6$~yr more from now before a huge number of SNe
occur.
However, the active phase was unexpectedly ceased $\sim 10^4$~yr ago, by Sgr
A East.




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