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7. CONCLUSIONS

Surface Brightness Fluctuations are most likely the most efficient technique to measure the distance to unresolved nearby stellar systems: In terms of accuracy, depth, and completeness, the SBF technique represents a unique way of measuring extragalactic distances to early-type galaxies and spiral bulges. Moreover, as SBFs are an intrinsic property of an integrated stellar population, they give insight into and allow scientists to set constraints on the stellar populations and chemistry of globular clusters and early-type galaxies. Precise extragalactic distances are still extremely important and fundamental for various applications in astrophysics. High-accuracy distance measurements are essential because errors in distance usually translate into large uncertainties in the derived physical properties of individual galaxies, such as absolute luminosities, linear sizes, star formation rates and timescales, black hole masses, and (total) masses of galaxies, as well as dark matter halo mass estimates. Moreover, for the nearby universe, accurate distances serve as a reliable and trustworthy substitute for redshift as a distance indicator and provide the possibility of mapping the three-dimensional structure and velocity field of the local volume.

Multi-wavelength SBF studies involving optical and NIR observations offer a unique way of analysing the properties and content of composite stellar populations, as different photometric bandpasses are sensitive to different phases of the stellar evolution. In particular, stellar population gradients based on SBF measurements allow one to probe the merger and assembly history of nearby galaxies and unresolved stellar systems. An interesting byproduct of detailed SBF data are possible constraints on the stellar populations and properties of globular cluster systems, such as understanding the nature and origin of the bi/multi-modal globular cluster colour distributions in giant elliptical galaxies, which remains an unsolved puzzle (Conroy & Spergel 2011).

Apart from ground-based observations, a complete optical SBF survey from space will allow us to construct a complete map of the individual velocity patterns of galaxies and galaxy clusters down to an accuracy in distances where the bulk motions become just a few percent (≲ 3%) of the Hubble flow (see Section 3.1 for details). Based on WFC3/IR observations of 600 Cepheids to calibrate the magnitude-redshift relation of 253 SNe Ia, the most recent measurement of the Hubble constant was derived down to an accuracy of 3%: H0 = 73.8 ± 2.4(stat) ± 3.4(sys) km s−1 Mpc−1 (Riess et al. 2011). Future observations will be able to measure the Hubble constant to a precision of 2% using the most accurate distance indicators. The extension of SBF studies with WFC3/IR will be a highly valuable asset from this perspective, both for the stellar content and for precise mass estimates.

The combination of a precise direct measurement of the Hubble constant H0 with constraints from the CMB provides a powerful test of different models of dark energy (Hu 2005). In particular, the two independent approaches together allow the investigation of the evolution of cosmic energy density w to 10% accuracy across a broad cosmic epoch of 0 < z < 1100 into the early universe of the ‘dark ages’ (∼ 13.5 Gyr). However, w is highly susceptible to uncertainties in the value of H0, with the errors in w being twice the fractional errors in H0, thereby keeping w and the flatness constant (Weinberg et al. 2012). The real challenge for future studies is therefore an accurate measurement of the local H0 value to 1% precision, encompassing both statistical (random) errors and systematic uncertainties.

The drawback of an individual H0 value is the restriction to a single number at a fixed redshift, so that only one particular model of dark energy can be tested but not possible deviations from the theoretical predictions. A high-precision measurement of H0, however, significantly increases the power of other dark energy constraints by up to 40% (Weinberg et al. 2012). Moreover, a direct H0 determination has the potential to uncover possible departures from the smooth evolution of dark energy at low redshift and could settle the controversy if the universe is still in a phase of acceleration.

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