25. The scorecard
As a concluding summary we return to the list proffered at the
beginning of the article as criteria that a successful extragalactic
distance indicator might aspire to, and try to assess the TRGB in that
- Indeed the luminosity generated by the
helium core flash is enormous, being comparable to the instantaneous
output of the rest of galaxy. Sadly, the duration of this burst is
only a few seconds, and even more tragically, virtually all of that
energy is absorbed internally within the star as it restructures
itself en route to quiescent helium core burning on the horizontal
branch. Nevertheless, the luminosity at which the upward evolution of
the red giant star is terminated by core ignition is still quite
luminous by stellar standards. In the I band the TRGB magnitude is as
bright as a 5-day Cepheid variable.
- By extragalactic standards, it is fair to say that any distance
indicator that can claim a precision of ±0.20 mag (or 10% in
distance) qualifies as being an excellent distance indicator. The TRGB
method boasts an externally defined dispersion of less than ± 0.1
mag, comparable to Cepheid distance moduli.
- For the metallicity range -2.2 < [Fe/H] < -0.7 dex, the I-band
color-magnitude diagrams for Galactic globular clusters calibrate the
zero point of the TRGB method to the same level that the RR Lyrae
distance scale is now known. As the RR Lyrae zero point is improved, the
TRGB zero point will also get better with time.
- To first order, nuclear physics determines the bolometric
luminosity at which all low-mass stars terminate.
- Theory predicts (and observations confirm) that the bolometric
(I-band) magnitude is insensitive to
chemical composition variations in the range -2.2 < [Fe/H] <
-0.7 dex, and for ages in the calculated range 7-17 Gyr.
- The halos of all galaxies are synonymous
with Population II stars. If there is a measurable surface brightness
defining these halos then the red giant branch stars must be present
- Every galaxy (elliptical, spiral or
irregular) must have had a first generation of stars. The low-mass and
low-metallicity members of that generation of stars must now be
populating the red giant branch, and defining the tip.
- At the color and approximate
magnitude level expected for TRGB stars other populations of stars do
little more than add noise to the distinctive discontinuity in the
red-giant-branch luminosity function. However, care should be taken
that the TRGB is truly triggering the edge detector by having a fair
sample of luminosity function well sampled below the tip before
incompleteness in the photometry sets in. All too often that may prove
to be a luxury in limiting applications, but the warning still stands.