A comparison of 10 galaxies with estimated TRGB distances with those having primary distances estimated using Cepheids or RR Lyrae stars was presented by Lee, Freedman & Madore (1993). These galaxies covered a wide range of morphological types (dwarf ellipticals: NGC 147, NGC 185 and NGC 205, an early-type spiral: M31, a late-type spiral: M33, and several irregulars: NGC 3109, NGC 6822, IC 1613 and WLM), covering a range of luminosity, metallicity, and distances up to (m - M) = 25.5 mag. The results of this comparison with Cepheid and RR Lyrae distances were extremely encouraging: the difference in the relative distances amounts to less than ± 10% (rms), or 5% in distance.
Figure 22. A montage of modern I - (V -
diagrams for the two Local Group galaxies IC 1613 (lower right) and
NGC 6822 (upper left). While red and blue
supergiants are marked along
with the AGB population, it is clear that the dominant contributor is
the RGB with its broad and distinctive upper luminosity plateau
marking the TRGB. The NGC 6822 data are adapted from the thesis of
Motivated by this result, we undertook a number of follow-up studies including simulations (Madore & Freedman 1995) to investigate the sources of error and factors limiting the application of this method. Then, using the combined facilities of the Palomar 5m and Las Campanas 2.5m we have begun a complete survey of TRGB distances to all galaxies in the Local Group and as far beyond it as aperture and seeing conditions will permit. To date, the results for two galaxies slightly peripheral to the Local Group, Sextans A and Sextans B have been submitted for publication. These objects were chosen to allow a further comparison with the Cepheid distance scale, given that Cepheids have been observed in both of these galaxies and the derived distances indicated that the TRGB would be readily detected from the ground. And indeed it was, with both galaxies showing the tip at I ~ 21.7 mag. Other galaxies currently being under study are NGC 3109, WLM, M33, and the various dwarf elliptical companions to M31.
Figure 23. The I - (V - I)
color-magnitude diagram for stars in M32 as derived from WFPC-2 HST
observations by Grillmar et al. (private communication). Note
that while the reddest (metal-rich) bright giant stars are found
progressively at fainter magnitudes the bluest (metal-poor) giants still
define the brightest tip. The horizontal lines mark the expected
magnitude for the TRGB
based on three apparent Cepheid moduli derived for various fields in
M31, which is thought to be the parent galaxy of
M32, and therefore at the same distance.
Figure 24. Sextans A: The left panel
shows the I - (V - I) color-magnitude diagram for the main body of the dwarf
irregular galaxy Setxans A. The horizontal lines at I ~ 21.7 mag
mark the derived apparent magnitude level of the TRGB. The upper right
panel shows the apparent I-band luminosity function for the stars in
Sextans A; the lower right panel shows the Sobel
identifying the position of the discontinuity in the luminosity
(Sakai, Madore & Freedman
A number of researchers have already begun to use HST to apply the TRGB method to determining distances beyond the seeing/resolution limit of ground-based telescopes. For instance, the halo of the peculiar elliptical radio galaxy NGC 5128 (= Cen A) has been resolved by Soria et al. (1996) and their I-(V-I) color-magnitude diagram shows the TRGB at I = 24.1 mag, thereby providing a distance of 3.6 ± 0.2 Mpc. Similarly, Elson (private communication) reports the detection of the TRGB in the southern hemisphere lenticular galaxy NGC 3115, and quotes a preliminary distance of ~ 10 Mpc. And most recently, Sakai et al. (1997) report the detection of the TRGB in the giant elliptical galaxy NGC 3379 (= M105) at a distance of 11.5 ± 1.6 Mpc. This latter application is of special interest given that NGC 3379 is a member of a group (the Leo I Group) which has two spiral galaxy members which have had independent Cepheid-based distances derived. Tanvir et al. (1995) find a distance of 11.9 ± 0.9 Mpc for NGC 3368 (= M96), while Graham et al. (1997) derive a slightly lower modulus for their target spiral NGC 3351, corresponding to a distance of 10.0 ± 0.3 Mpc. While the differences in these Population I distances may indicate back-to-front geometry, they agree, individually and in the mean, to within ~ 10% of the Population II TRGB distance.
At the time of writing the authors were aware of two HST proposals to pursue applications of the TRGB method in the Virgo cluster. One proposal by Harris and collaborators was to image two low-metallicity dwarf galaxies directly; those exposures are still to be taken. On the other hand, early reports indicate that Ferguson et al. apparently have detected the red giant population between galaxies in the Virgo cluster core. However, at these large distances, very long integration times are essential to achieve the required signal-to-noise needed to detect stars at I > 27 mag, and consequently no color information is yet in hand.
Figure 25. NGC 3379 and the Comparison
of Cepheid Distances with the TRGB. The right panel shows both the I-band
luminosity function (upper) and the tip response function (lower) for
the HST observation of stars in the giant elliptical galaxy NGC 3379
(Sakai et al. (1997).
The left panel shows a comparison of the
NGC 3379 TRGB distance (marked by the Leo I Group)
corresponding Cepheid-based distance to the group. Other Cepheid-TRGB
comparisons are also shown for closer galaxies, Sextans A, NGC 3109,