Figure 2.
Microwave (orange),
optical (red, green,
blue), and ultraviolet
(blue) image of the
Phoenix Cluster.
Image courtesy of
the Chandra X-ray
Observatory.
surements are essential to the study of the
expansion history of the universe and hence
to constrain the nature of dark energy.
The supernovae used in the study, discovered with the Palomar Transient Factory,
were confirmed as ordinary Type Ia supernovae by subsequent spectroscopy and by using the Near-Infrared Imager and Spectrometer (NIRI) on the Gemini North telescope to
follow the characteristic fading of the NIR
emission and determine the peak brightness.
One further important selection criterion
was to restrict the study to supernovae at
distances large enough so the overall expansion of the universe (the Hubble flow) determines the motion of their host galaxies,
independent of local peculiar motions; i.e.,
redshifts 0.03 < z < 0.09 (Figure 1). While earlier work had already indicated the greater
uniformity of supernova emission in the NIR,
this is the first large study to obtain highquality measurements of more distant supernovae. Although these observations are
more difficult because the distant supernovae appear fainter, this avoids the complication of local motions and results in the most
precise known standard candle for cosmological measurements.
Beginning to Solve the Cooling
Flow Problem
Clusters of galaxies are full of hot gas that
emits copious X-ray radiation. This emission should lead to a “cooling flow,” whereby
cooling material sinks to the dense center of
the cluster. In turn, we expect this inflowing
reservoir of relatively cool gas to stimulate
star formation in the galaxy located at the
cluster’s core, rather than result in runaway
cooling of the cluster gas. The problem, until now, is that observations of such central
galaxies have revealed them to be quiescent, showing little evidence for ongoing
star formation.
26
GeminiFocus
Michael McDonald (Massachusetts Institute
of Technology) and colleagues have now
detected the first evidence for significant
cooling-flow-induced star formation in a
central cluster galaxy (McDonald et al., Nature, 488: 349, 2012). The cluster itself, designated SPT-CLJ2344-4243, was detected
with the South Pole Telescope. Follow-up
spectra obtained using the Gemini MultiObject Spectrograph (GMOS) at Gemini
South provided some of the first hints that
the central galaxy was unlike the red, wellformed elliptical galaxies typical of cluster
cores. The researchers also used additional
measurements of other cluster members to
determine the baseline redshift (z = 0.6) for
comparison of other observations.
The more complete analysis of the so-called
“Phoenix Cluster” and its central galaxy (Figure 2) emerges from observations spanning
X-ray to far-infrared energies. The central
galaxy possesses an active nucleus in addition to star formation at a rate of 740 MSun/
year. The star formation rate is still too low
to prevent runaway cooling, given the measured cooling flow rate of 3800 MSun/year,
suggesting that the feedback mechanism
is not fully established in this example.
Nonetheless, the high star formation rate
points to this mode of star formation from
intracluster gas as an important element
December2012