Left: (Ks, F606W-Ks) color–magnitude diagram of
NGC 1851; the detail of the double SGB is shown in the inset. Right:
same as the left panel with average photometric (random) uncertainties
indicated. Overlaid is the fiducial line with the approximate locations
of the main sequence turnoff and main sequence knee highlighted by red
dots.
Credit: NASA/JPL-Caltech/SSC/Ricardo Schiavo (UVA)
Expecting to resolve stars deep into the crowded field of a globular
cluster is a tall order for ground-based telescopes. However, Paolo
Turri (University of Victoria, Canada) and colleagues have used the
Gemini Multi-conjugate adaptive optics System (GeMS) with the Gemini
South Adaptive Optics Imager (GSAOI) to do just that. Their data present
the most accurate and deepest near-infrared photometry from the ground
of a crowded field. It also illustrates the remarkable potential of
MCAO-equipped Extremely Large Telescopes of the future.
Their Ks measurements of the Galactic globular cluster NGC
1851 are combined with HST photometry and the resulting color-magnitude
diagram demonstrates that the ground-based data is of an unprecedented
depth and precision for crowded field observations. The delivered image
quality approaches Gemini’s diffraction limit, with an average measured
full-width at half-maximum (FHWM) of 0.09 arcsecond. The work is published in The Astrophysical Journal Letters.
Abstract:
The Extremely Large Telescopes currently under construction have a
collecting area that is an order of magnitude larger than the present
largest optical telescopes. For seeing-limited observations the
performance will scale as the collecting area, but with the successful
use of adaptive optics (AO), for many applications it will scale as D4
(where D is the diameter of the primary mirror). Central to the success
of the ELTs, therefore, is the successful use of multi-conjugate
adaptive optics (MCAO) which applies a high degree of correction over a
field of view larger than the few arcseconds that limits classical AO
systems. In this Letter, we report on the analysis of crowded field
images taken on the central region of the galactic globular cluster NGC
1851 in the Ks band using the Gemini Multi-conjugate Adaptive
Optics System (GeMS) at the Gemini South Telescope, the only
science-grade MCAO system in operation. We use this cluster as a
benchmark to verify the ability to achieve precise near-infrared
photometry by presenting the deepest Ks photometry in crowded
fields ever obtained from the ground. We construct a color–magnitude
diagram in combination with the F606W band from the Hubble Space
Telescope/Advanced Camera for Surveys. As well as detecting the “knee”
in the lower main sequence at Ks '20.5, we also detect the
double subgiant branch of NGC 1851, which demonstrates the high
photometric accuracy of GeMS in crowded fields.
Source: Gemini Observatory
