There is no doubt that the future of SBF lies in a combination of space-based and Adaptive Optics (AO) observations. The SBF fluctuation signal in the H-band is a factor of 30 larger than that in the I-band. Equally important, for HST/NICMOS observations the sky background is more than a factor of 100 lower compared to IR observations with ground-based telescopes. The HST WFC3/IR channel offers a unique tool with high spatial resolution (FWHM = 0.13″), wide field of view (2.05′ × 2.26′), good set of filters for SBF measurements, and good detector characteristics (e.g., sensitivity, noise, cosmetics, etc). WFC3/IR observations in F110W (1.25 µm, ≈ J) and F160W (1.6 µm, ≈ H) as well as ACS z′-band (0.85 µm) measurements will be useful complements to existing (ground-based) studies.
The most promising approach is a complete space-based SBF IR survey to map out the nearby universe. Such a project would allow scientists to constrain the stellar population and mass content of ‘dynamically hot’ galaxies and act as a powerful complement to ongoing surveys studying the properties of nearby galaxies in detail, such as SAURON (de Zeeuw et al. 2002) or ATLAS3D (Cappellari et al. 2011) 8. A crucial complement to space observations is offered by AO observations with NIR instruments [e.g., ALTtitude conjugate Adaptive optics for the InfraRed (ALTAIR), OH-Suppressing InfraRed Integral-field Spectrograph (OSIRIS), Nasmyth Adaptive Optics System Near-Infrared Imager and Spectrograph (NACO), InfraRed Camera and Spectrograph (IRCS)] with excellent seeing conditions between ≲ 0.′′03 − 0.′′05 FWHM in the IR wavelength regime, feasible with current state-of-the-art 8-m class telescopes such as Gemini, Keck, VLT, or Subaru. Future 30-m class telescopes such as the E-ELT 9 or TMT 10 will be operated consecutively using AO instruments.
Multi-band SBF studies across a broad wavelength regime will be the ultimate
key to understand the stellar populations of stellar systems and their
properties. If it is possible to conceive completely the dependence of
across the full
wavelength spectrum, the SBF method will then be independent from the
calibration of the primary distance indicators of Cepheids.
The future JWST will open a new window for SBFs in the MIR, which is currently prohibitively expensive and challenging from the perspective of ground-based observations (telescope exposure time and uncertainties in the bright sky background). Promising wavelength ranges are the L (3.5 µm) and M (4.8 µm) bands, as both are expected to be highly sensitive to the age content of old, evolved stellar populations. In particular, the MIR SBFs will allow astronomers to constrain the mass-loss rates of AGB stars and pin down whether there is a relation between the mass-loss and metallicity of a stellar population as expected from stellar synthesis modelling (González-Lópezlira et al. 2010).
Moreover, the Gaia astrometry satellite, 11 expected to launch in May 2013, will have a dramatic impact through a re-definition of the cosmic distance scale. Proper motions for stars and new trigonometric parallaxes of ten-thousand (partly long-period) Cepheids will be measured within 5 kpc; hence geometric distances of the most powerful ‘primary’ distance indicators (e.g., Cepheids, RR Lyrae stars, statistical/trigonometric parallaxes) will be improved with unprecedented accuracy (between 10 to 100 µarcsec, ∼ 25 µarcsec at V = 15 mag). Distances with precision at sub-percentage-level will pave the way for a direct refinement of the calibration of ‘secondary’ distance indicators (e.g., SBF, SNe Ia/II, Novae, Dn − σ, Tully-Fisher relation) and a global re-assessment of the entire cosmic distance ladder (see also Section 1.2). Moreover, due to the frequent repeated observations of the same areas on the sky, Gaia will also detect and locate transient events, such as gamma-ray-bursts (GRBs), binaries and SNe with a completeness down to G = 20 mag, to a high accuracy.
The combination of both high-resolution trigonometric parallaxes and NIR-AO observations will lift the SBF technique to the next level, establish an ultimate reference point for the cosmic distance scale, and settle the controversy regarding the local Hubble flow (Lynden-Bell et al. 1988; Tonry et al. 2000). The future of the SBF method still shines extremely brightly.
Acknowledgments
The author is grateful to Professor Joseph B. Jensen for stimulating discussions and many valuable comments and suggestions on an earlier version of the draft. The anonymous referee is thanked for a constructive review, which improved the clarity of the manuscript. The author acknowledges support from a VIPERS Fellowship through a PRIN-INAF 2008 grant (VIPERS) and partial support from grant HST-GO-10826.01 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
8 ATLAS3D: http://www-astro.physics.ox.ac.uk/atlas3d Back.
9 E-ELT: http://www.eso.org/public/teles-instr/e-elt Back.
10 TMT: http://www.tmt.org Back.
11 Gaia satellite: http://gaia.esa.int/ Back.