We examine the prospects for measuring the dark energy equation-of-state parameter w within the context of any uncertain redshift evolution of galaxy cluster structure (building on our earlier work) and show that including the redshift-averaged cluster power spectrum (P cl ) and direct mass measurements of 100 clusters helps tremendously in reducing cosmological parameter uncertainties. Specifically, we show that when combining the redshift distribution and the power spectrum information for a particular X-ray survey (DUET ) and two SunyaevZel'dovich effect surveys (South Pole Telescope and Planck), the constraints on the dark energy equation-of-state w can be improved by roughly a factor of 4. Because surveys designed to study the redshift distribution of clusters will have all the information necessary to constructP cl , the benefit of addingP cl in reducing uncertainties comes at no additional observational cost. Combining detailed mass studies of 100 clusters with the redshift distribution improves the parameter uncertainties by a factor of 3-5. The data required for these detailed mass measurementsassumed to have 1 uncertainties of 30%-are accumulating in the XMM-Newton and Chandra archives. The best constraints are obtained when one combines both the power spectrum constraints and the mass measurements with the cluster redshift distribution; when using the survey to extract the parameters and evolution of the massobservable relations, we estimate uncertainties on w of $4%-6%. These parameter constraints are possible with cluster surveys that are carried out over large, contiguous regions of the sky, so that the cluster power spectrum can be reasonably measured. In combination with cosmic microwave background or distance measurements that have different parameter degeneracies, cluster studies of dark energy will provide enhanced constraints and allow for cross-checks of systematics.

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a deep z ′ -band imaging survey covering the Spitzer SWIRE Legacy fields designed to create the first large homogeneouslyselected sample of massive clusters at z > 1 using an infrared adaptation of the cluster red-sequence method. We present an overview of the northern component of the survey which has been observed with CFHT/MegaCam and covers 28.3 deg 2 . The southern component of the survey was observed with CTIO/MOSAICII, covers 13.6 deg 2 , and is summarized in a companion paper by Wilson et al. (2008). We also present spectroscopic confirmation of two rich cluster candidates at z ∼ 1.2. Based on Nodand-Shuffle spectroscopy from GMOS-N on Gemini there are 17 and 28 confirmed cluster members in SpARCS J163435+402151 and SpARCS J163852+403843 which have spectroscopic redshifts of 1.1798 and 1.1963, respectively. The clusters have velocity dispersions of 490 ± 140 km/s and 650 ± 160 km/s, respectively which imply masses (M 200 ) of (1.0 ± 0.9) x 10 14 M ⊙ and (2.4 ± 1.8) x 10 14 M ⊙ . Confirmation of these candidates as bonaf ide massive clusters demonstrates that two-filter imaging is an effective, yet observationally efficient, method for selecting clusters at z > 1.

We examine the prospects for measuring the dark energy equation of state parameter w within the context of the still uncertain redshift evolution of galaxy cluster structure. We show that for a particular X-ray survey (SZE survey) the constraints on w degrade by roughly a factor of 3 (factor of 2) when one accounts for the possibility of non-standard cluster evolution. With followup measurements of a cosmology independent, mass-like quantity it is possible to measure cluster evolution, improving constraints on cosmological parameters (like w & Ω M ). We examine scenarios where 1%, 10% and 100% of detected clusters are followed up, showing that even a modest followup program can enhance the final cosmological constraints. For the case of followup measurements on 1% of the cluster sample with an uncertainty of 30% on individual cluster mass-like quantities, constraints on w are improved by a factor of 2 to 3. For the best case scenario of a zero curvature universe, these particular X-ray and SZE surveys can deliver uncertainties on w of ∼4% to 6%.