PhD Thesis (PDF) - Department of Astronomy - University of Virginia

is the Advanced CCD Imaging Spectrometer (ACIS). As a non-dispersive imaging
spectrometer, the location, time, and energy of every detected photon in the 0.08–
10 keV range is recorded. Since an early-type galaxy will typically fit on a single 1024×
1024 chip with 0.492 ′′ ×0.492 ′′ pixels, the more sensitive, backside illuminated S3 chip
is typically used. At ∼ 1.5 keV, the effective area including the quantum efficiency
(QE) of the S3 chip is ∼ 650 cm 2 , and the 1σ point spread function (PSF) is ∼ 0. ′′ 5
near the optical axis of the telescope (Weisskopf et al. 2002). (As a point source moves
off-axis, its PSF becomes larger and more complex.) The combination of sensitivity
and resolution is sufficient to detect bright, active LMXBs (LX 10 37 ergs s −1 ) at the
distance of the Virgo cluster in reasonable exposure times.
1.6 The Hubble Space Telescope
Like Chandra, the Hubble Space Telescope (HST) is part of NASA’s Great Ob-
servatories program. Since HST orbits above the atmosphere of the Earth, it is a
diffraction-limited instrument. While ground-based instruments typically have ∼ 1 ′′
resolution, HST can achieve ∼ 0.1 ′′ resolution or better. The Wide Field Channel of
the Advanced Camera for Surveys (ACS) has a 202 ′′ × 202 ′′ field-of-view (FOV), can
observe light from 3700–11,000 ˚A, and has a peak efficiency of 48% (Pavlovsky et al.
2004), including the effects of quantum efficiency and the optics (Pavlovsky et al.
2004). The half-light diameter of a typical GC (∼ 6 pc, Harris 1996) at the distance
of the Virgo cluster corresponds approximately to ∼ 1.6 pixel, where 1 pixel ∼ 0.049 ′′ .
The PSF of the HST-ACS is well enough calibrated that one can detect such a spatial
extent and measure the half-light radius of the GC. With HST-ACS, we can not only
detect the majority of GCs at the distance of the Virgo cluster, but can also measure
their sizes.
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