Gemini's Near-Infrared Imager Images Over 120,000 Stars in the Nearby Galaxy M33
February 26, 2003
Gemini Near-infrared Imager (NIRI) has been working reliably and
delivering excellent images at wavelengths between 1 and 5 microns. In
one NIRI program, very sharp images (0.35 arcsecs FWHM without adaptive
optics correction) of 11 different sub-regions of the nearby spiral
galaxy M33 were collected over a period of two nights in September
The infrared NIRI images probe through the
intervening dust in M33 and allow us a clear view of the many evolved
giant stars in this galaxy. In the inset infrared (IR) image, 22,000
individual giant stars were detected and measured. The image on the
left shows what this same field looks like at visible wavelengths with
a seeing of 0.75 arcsecs (FWHM). The dust seen in the optical image all
but disappears in the NIRI IR image (a composite of 1.6 and 2.1 micron
Taking a census of the stars in other galaxies
is a very important part of understanding the history and composition
of this and other galaxies. From the numbers, colors, and brightness of
the stars, astronomers can infer how galaxies form, how their
compositions have evolved, and when their stars were born. NIRI's field
of view and high IR sensitivity make it possible to extend the stellar
census of M33 and other galaxies to many more and much fainter stars.
When the 11 fields like the one above are considered together, a total
of 120,000 stars were measured.
Among the 22,000 stars in this IR image, there
are a handful of Cepheid-variable stars. Cepheids pulsate with a rate
that is directly related to their intrinsic mass and luminosity.
Cepheid variables are the gold nuggets in images like this one because
they can be used to accurately determine the distance between us and
the Cepheids. Distance measurements to far-away galaxies are very
difficult to make, and finding large numbers of Cepheid variables in
galaxies like M33 makes the job much easier. Over 100 Cepheids can be
found among the 120,000 stars measured as part of this program, making
it one of the largest compilations of Cepheids in any galaxy.
Accurately measuring the Cepheid variables in M33 will allow
astronomers to improve the accuracy of distances measured to galaxiesas far away as 60 million light years.
NIRI was built by the University of Hawai`i Institute for Astronomy, Honolulu, HI.