From kleinb@astro.ucla.edu Thu May 23 19:35:21 1996
Date: Thu, 23 May 1996 16:35:32 -0700
From: kleinb@astro.ucla.edu (Beth Klein UCLA Astronomy)
To: gcnews@astro.umd.edu

\documentstyle[11pt,paspconf,epsf]{article}

\begin{document}

\title{2.2 $\mu$m Keck Images of the Galaxy's Central Stellar 
Cluster at 0$.^{\prime\prime}$05 Resolution}

\author{B. L. Klein, A. M. Ghez, M. Morris, E. E. Becklin}
\affil{UCLA Department of Physics and Astronomy, \\8371 M.S. Bldg., 
Los Angeles, CA 90095}


\begin{abstract}
We present preliminary results from a high angular resolution study
of the inner 5$^{\prime\prime}\times$5$^{\prime\prime}$
of the central Galactic stellar cluster.
This program was carried out at the W. M. Keck telescope using the
facility near-infrared camera with a $K$[2.2$\mu$m]-band filter.
A few thousand short exposure frames,
combined using a shift-and-add algorithm,
contribute to the final image which has the
diffraction-limited resolution of 0$.^{\prime\prime}$05.
Even prior to deconvolution,
sources as faint as $\sim15^{th}$ Kmag are detected at the 5$\sigma$ level
and new isolated point sources are identified.
\end{abstract}


% Keywords should be included, but they are not printed in the hardcopy.

\keywords{High-resolution, central stellar cluster, SgrA*(IR)}

\section{Introduction}

Since the discovery of the near-infrared (NIR) emission from the
Galactic Center (Becklin \& Neugebauer 1968), the central stellar
cluster has been the focus of many NIR studies.
Recently, tremendous advances have been made in the angular resolution
of NIR images of this region via methods which compensate for
atmospheric distortions
(Eckart et al. 1995, 1993;  also Close et 
al. 1995, DePoy \& Sharp 1991, Simons \& Becklin 1996).  Nonetheless, 
studies of the Sagittarius A* (IR) complex - the apparent
concentration of infrared sources within $\sim$0$.^{\prime\prime}$5 of SgrA*
- remain confusion limited.  
The W.M. Keck telescope, which has a position-angle-dependent maximum
baseline of 9.0-11.0 meters (Nelson 1989),
offers a unique opportunity to 
probe the detailed structure 
of our Galaxy's central stellar cluster.   

At Mauna Kea, the mean atmospheric seeing conditions 
constrain traditional images to angular resolutions 
of $\theta_{seeing}\sim$0$.^{\prime\prime}$5 at 2.2 $\mu$m.  
Although this image size is excellent by conventional standards, 
it is still a 
factor of 10 worse than the diffraction limit of Keck, which at 
$\lambda=2.2$ $\mu$m is $\theta_{diff}\sim\frac{\lambda}{D}\sim$ 
0$.^{\prime\prime}$05 
(= 420 AU at 8.5 kpc).  
We exploit this high resolving power 
by employing high-resolution imaging techniques to recover
the diffraction limit in post processing.  The viability of diffraction 
limited imaging with the fully-phased Keck telescope has been 
demonstrated by Ghez (1996) and Matthews et al. (1996).

\section{Observations}

Our $K$-band observations incorporated three key elements 
for obtaining data suitable for the reconstruction of diffraction
limited images.
\begin{itemize}
\item {\em Short Exposures:}  120 ms integrations were used in these
observations.  Under good seeing conditions at Keck this has
proven to be sufficiently short to freeze 
the atmospheric distortions and preserve high-resolution information
(see Figure 1).
\item {\em Fine pixel scale:}  A scale of 20 milliarcseconds per pixel is
available with the facility camera, NIRC (256$\times$256 InSb array).  
This is accomplished with the recently installed "image converter" 
(Matthews et al. 1996) that magnifies the field of view from the 
standard 38$^{\prime\prime}$$\times$38$^{\prime\prime}$ to 
5$^{\prime\prime}$$\times$5$^{\prime\prime}$ which is Nyquist sampled at 
2.2 $\mu$m.
\item {\em Telemetry:}  We collected $\sim$10,000 frames centered near the
position of SgrA*.  From this data, 2,741 of the best seeing quality 
frames were selected to be incorporated in the final image presented 
here (Figure 3).
\end{itemize}

\smallskip

{\centering \leavevmode
\epsfxsize=.40\columnwidth \epsfbox{spklgrm.eps}
\hspace*{.08\columnwidth}
\epsfxsize=.50\columnwidth \epsfbox{psf.eps} \\}


{\centering \leavevmode
\parbox[t]{.40\columnwidth} {Figure~1: A single 120 ms short exposure
(a specklegram) of the inner 3.5 arcseconds of the central stellar
cluster.  High resolution information is clearly contained in this
image.
It is worth noting that the speckle patterns for different sources are
nearly identical across the field. }
\hspace*{.08\columnwidth}
\parbox[t]{.50\columnwidth} {Figure~2: A model fit through measured data
of the shift-and-add point spread
function (PSF) radial profile. The
diffraction limited core is built up from the brightest speckle in each
contributed frame while the 'seeing halo' results from the fainter
surrounding speckles.
The core contains $\sim$10 percent of the flux
and has the expected FWHM of $\theta$ = 0$.^{\prime\prime}$05. } }

\smallskip

\section{Shift-And-Add}

        One method for obtaining diffraction limited images from a
series of short exposures (specklegrams) is "shift-and-add" (e.g., 
Christou 1991).  
In a short exposure such as the one shown in Figure 1, the image 
breaks up into a number of speckles, each of which can be thought 
of as a noisy diffraction limited image of the source.  Shift-and-add 
builds up signal while preserving high resolution information
by registering all the frames on the brightest speckle for each exposure. 
In this scheme the surrounding fainter speckles are treated as noise. 
The resulting image of a point source has a diffraction limited 
core on top of a "seeing halo" (see Figure 2). 

\begin{center}
\epsfxsize=0.9\columnwidth \epsfbox{sgra_cntr.eps}  
\bigskip
\parbox[b]{0.9\columnwidth} {Figure~3: A shift-and-add image of the 
central 5$^{\prime\prime}$$\times$5$^{\prime\prime}$ comprised
of 2,741 seeing selected frames taken in June, 1995.  The contours do
not correspond directly to flux density levels since these images are
not deconvolved.
Nonetheless, comparisons with previously published maps at lower 
resolution indicate that
sources as faint as 15th $K$mag are easily identified.  At the resolution
of 0$.^{\prime\prime}$05 (=420 AU at 8.5 Kpc), new isolated point sources are
detected.} 
\end{center}

\section{Results}
        The results of shift-and-add are shown in
Figure 3.  The (linear) intensity display on top is scaled to show the 
PSF cores of the brighter members of IRS 16.  The contour plot of the 
central 2$.^{\prime\prime}$4$\times$2$.^{\prime\prime}$4 region is scaled 
to display the structure of the SgrA*(IR) complex.  
Sources as faint as $K\sim$15 mag are detected 
at the 5$\sigma$ level in these "raw" (i.e. not deconvolved) images. 
The images shown 
in Figure 3 are in general agreement with the earlier results of 
Eckart et al. (1995 and this volume: resolution 0$.^{\prime\prime}$15).
Much of the extended emission seen in the previous maps
is now resolved into point sources. 

\bigskip 

\acknowledgments
The authors are grateful to the excellent Keck staff whose hard work 
and diligence helped to ensure the success of these observations.  
In particular, many thanks to W. Harrison, A. Conrad, W. Wack, 
J. Aycock, T. Stickel and R. Campbel.
We also thank our Caltech speckle-collaborators K. Matthews, 
G. Neugebauer, A. Weinberger for their support of this project.
This project is funded through the NSF Young Investigator Award (AMG).
BLK is also supported by a UCLA RA/Mentorship fellowship. 


\begin{references}
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\reference DePoy, D. L. \& Sharp, N. A. 1991, \aj, 101, 1324
\reference Eckart, A. et al. 1996, this proceedings
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\reference Eckart, A., Genzel, R., Hofmann, R., Sams, B. J. \& 
    Tacconi-Garman, L. E. 1993, \apjlett, 407, L77
\reference Ghez, A. M. 1996 in NATO Advanced Summer Institute,
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\reference Matthews, K., Ghez, A., Weinberger, A., \& Neugebauer, G.
    1996, \pasp, in press
\reference Nelson, J. 1989, AmSci, 77, 170
\reference Simons, D. \& Becklin, E. E. 1996, \aj, in press

\end{references}


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