Tip of the Red Giant Branch as a Distance Indicator

The tip of the red giant branch (TRGB) method is a powerful, Pop II distance indicator. It uses the I-band luminosity of the brightest RGB stars. It turns out that in this wavelength, the magnitude of the TRGB stars is very insensitive to metallicity, and also to age.

Here, you can see the insensitivity of the TRGB magnitudes to the age and metallicity. Plotted on the top figures are four Galactic globular clusters spanning various metallicities (left), and four theoretical isochrone (Padova models) of ages from 5 up to 12 Gyrs. In the middle, I plotted M82 I-(V-I) color magnitude diagram on the left. M82 is a starburst galaxy in the M81 group of galaxies, at 3.3 Mpc. The I-band luminosity function and its corresponding filter output are also shown (middle and right figures). The edge-detection filter was applied to the luminosity function to obtain the filter output. Another example of the TRGB application, IC 1613, which is a dwarf Irregular galaxy in the Local Group, is shown on the bottom.

Because the TRGB method is a Population II distance indicator unlike the Cepheid period-luminosity relation, it can be applied to any morphological types of galaxies, from ellipticals to irregulars to spirals. Taking advantage of this, we have been obtaining the TRGB distances to all the galaxies observable from ground, and also using the HST. The motivation behind in observing all these galaxies is to study the distribution and dynamical history of galaxies in the Local Supercluster. Surprisingly, the details of the local Universe dynamics is very uncertain, mostly due to lack of good distance data. This is now possible using the TRGB method with largest telescopes, and especially mosaic cameras which allows us to observe the entire galaxy at once. I was granted an LTSA award for this project.

A figure showing the comparison of Cepheid and TRGB distances is shown here. The straight line is not a fit, but represents a line of slope 1.

Testing the metallicity dependence of the Cepheid period-luminosity relation: It has become increasingly important to understand the systematic uncertainties in the Cepheid distance scale. One of the outstanding sources of systematic errors is the metallicity dependence of the Cepheid variable stars. We have compiled a database of galaxies whose distances have been measured by both the Cepheid PL relation and the TRGB method. Because the TRGB magnitude is remarkably insensitive to the metallicity, the difference between the TRGB and Cepheid distances can be plotted as a function of the Cepheid metallicity to test for the existence of any trend. The figure below shows that test: it offers strong evidence for the weak metallicity dependence of the Cepheid PL relations.

In collaboration with Laura Ferrarese, Robert Kennicutt and Abi Saha, we have gotten 26 orbits of HST time during Cycle 9 to pursue this further. We will be obtaining the TRGB distances to six more galaxies in order to test the metallicity dependence of the Cepheid variables.

Extending the TRGB Method to IR: future telescopes/instruments will be optimized for IR observations. Some simulations have shown that the NGST will allow us to observe the TRGB stars as far out as in Coma cluster. I have been obtaining the JHK data of Galactic globular clusters, which will be used to

References

Observations of the "Garland" Region in the Halo of NGC 3077 Using the Hubble Space Telescope, Sakai and Madore, in preparation

The Tip of the Red Giant Branch Distance to the Large Magellanic Cloud Sakai, Zaritsky and Kennicutt, AJ, 119, 1197

Detection of the Red Giant Branch Stars in M82 Using the Hubble Space Telescope Sakai and Madore, ApJ, 526, 599

Tip of the Red Giant Branch as a Distance Indicator Sakai, IAU Symposium #183, Kyoto Japan, August 1997

Cepheid and Tip of the Red Giant Branch Distances to the Dwarf Irregular Galaxy IC 10 Sakai, Madore and Freedman, ApJ, 511, 671-679

Tip of the Red Giant Branch Distances to Galaxies. IV. Sextans B Sakai, Madore and Freedman, ApJ, 480, 589

Detection of the Tip of the Red Giant Branch in NGC 3379 (M105) in the Leo I Group Using the Hubble Space Telescope Sakai, Madore, Freedman, Lauer, Ajhar and Baum, ApJ, 478, 49

Tip of the Red Giant Branch Distances to Galaxies. III. The Dwarf Galaxy Sextans A Sakai, Madore and Freedman, ApJ, 461, 713


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