Deep Narrowband Imaging of the Galactic Center
Science:
Conduct a deep narrowband imaging survey of the Galactic Center region, -0.5 £ l £ 1.5, b = 0.0 ± 0.25 degrees, in Br g, 2.12 micron H2 , and 1.64 micron [Fe II] emission lines; and in broadband J H KS . These emission lines are diagnostic of physical conditions in shock-excited gas, and frequently identify morphologically interesting objects. The broadband filters will be used for continuum subtraction; KS in combination with longer wavelength data characterizes continuum sources (stars) in this heavily extincted region. The science has two threads, investigations of energy exchange in the interstellar medium and of the stellar content of this unique and still poorly understood part of the Galaxy. While stars can be used as tracers of large scale structures, the emission line observations go directly into the physics of energetic nonstellar components on a wide range of length scales. Specific science goals include:
NEWFIRM on the 4-m telescope offers
the opportunity to efficiently and deeply survey star formation and energy
exchange in the unique conditions found in the Galactic center. The NEWFIRM
fields are being observed in a variety of surveys, including 1.1 mm continuum
(Bolocam), 21 and 6 cm (IGPS and VLA), and mid-IR (Spitzer). Known individual
objects are the target of a variety of pointed-observation studies with 8-10 m
telescopes; I expect many more targets for followup will be identified with the
NEWFIRM data in combination with other surveys. There is also a rich heritage
of data at all wavelengths in the literature and in electronic form. So there
is both significant immediate science return and substantial archival value for
the dataset. This minisurvey is scoped to be “in bounds” for the Science
Verification program. It can be expanded in area subsequently during regular
science operations.
System performance issues:
The broadband imaging is similar to
what has been done before. The project stresses use of the narrowband filters.
Individual exposures will very likely be background limited, while the
composite images will likely be confusion limited (and certainly will be, for
the broadband data). Data rates will be moderate for the broadband imaging, ~1
frame/minute, and low for the narrowband imaging, ~1 frame every 5-10 minutes.
Performance issues of interest include
·
Electronic and thermal stability of
the arrays with exposure times of many minutes—for example, signal offset due
to readout-induced self-heating
·
Does this impact mixing broadband
and narrowband exposures during observing?
·
Use of the reference pixels
incorporated in the array structure to correct such effects in pipeline
processing
·
Guider performance—is microstepping
possible?—and PSF stability over hours, for intercomparison of images in
different filters at finest possible spatial scales
·
Data-taking protocols for best sky
background and flatfield definition in a very crowded field at high airmass—this
may vary between broad and narrow bandpass filters
·
Definition of best quick-look
reductions and realtime diagnostics for narrowband imaging
·
How best to use broadband images for
continuum subtraction, a photometrically iterative process
·
Uncovering systematics of whatever
origin—instrument, telescope, sky—that may limit ultimate sensitivity for deep
narrowband imaging
Observing strategy:
We will obtain very deep narrowband
images, with commensurate shorter exposures in J, H, KS
to reach the same stellar flux limits as in the narrowband filters. Integration
times per filter are estimated from our previous experience.
Area coverage: Four NEWFIRM
pointings along –0.5 £
l £
1.5, b = 0.0. In equatorial
coordinates the Galactic center is at (a, d)
17:46, -28:56 (J2000). Dithered background limited exposures will be used to
accumulate the total on-source integration times indicated. Apart from the high
airmass, there is nothing novel about the basic observing procedures. However,
definition of the sky background and sky flats in this very crowded region may
be challenging even with narrowband filters. We will attempt to derive mean
background sky frames and flatfields from the on-source data, while initially
obtaining some off-source sky frames for comparison. The time estimate assumes
use of the on-source data for this purpose. If it proves necessary to spend
equal time off-source for less crowded sky fields, either the total integration
time will double or, more likely, the area coverage will be decreased to fit
available time. Off-source flatfields can be acquired, at modest additional
cost in observing time, before or after observing the target fields but at the
same declination.
Telescope time, including allowance
for overheads:
3
NB filters, x 3600 sec / filter = 4 hrs/ field, x 4 fields = 16 hours
3
BB filters, x 600 sec / filter = 1 hr/
field, x 4
fields = 4 hours
==================================================
Total
time
20 hours
This region is accessible from
The three SV programs use an identical
approach to obtain ultra-deep narrow-band images along with conmeasurate
shorter exposures in J, H, K (to reach the same stellar flux limits as in the
NB filters). In each field, the science goals are:
1 Obtain a flux-limited measure of molecular
and atomic and/or low-ionization shocks using H2 and FeII. Measure energy/momentum injection rate by shocks. Constrain
sources of turbulent momentum in a cloud.
2 Use broad-band colors to estimate and map
extinction along LOS. Use to estimate
extinction correction for shocks. Combine with Spitzer surveys to
identify young stars. Determine K-luminosity functions, color-color
diagrams, and measure large-scale distributions of YSOs as
functions of reddening/IR-excess.
3 In Galactic Plane and
Estimate stellar population K-band
luminosity fcn. Use colors to identify stellar types. Identify star
clusters and massive stars.
Notes: These three fields are being observed
by a variety of surveys, including 1.1 mm continuum (Bolocam), 21 and 6 cm
(IGPS and VLA), IR (Spitzer), and in some cases visual (Mosaic).
* Deep H2, Br gamma, [Fe II] imaging of the
Ultra Deep imaging of 1 square
degree extending from l=-0.5 to l=1.5,
+/-0.25 degrees.
Coverage: 4 fields with NEWFIRM
3 NB filters, x 3600 sec / filter
= 4 hrs/ field
3 BB filters, x 600 sec / filter =
1 hr/ field
==============================================
Total = 20 hrs