Extragalactic science and cosmology with Stage IV galaxy surveys will rely almost exclusively on redshift measurements derived solely from photometry, which are subject to systematic and statistical uncertainties with numerous analysis choices. Single-survey photometric redshift estimates ought to be improved by combining data from multiple surveys, with common wisdom asserting that optical data benefits from additional infrared (IR) but not ultraviolet (UV) coverage. The degree of improvement for either is not well characterized, and attempts necessitate assumptions of a chosen estimator and its prior information. We apply an information-theoretic metric of potentially recoverable redshift information to assess the impact of multi-survey photometry without assuming an estimator or priors in the context of the Vera C. Rubin Observatory Legacy Survey of Space and Time ( lsst ) in the optical, Roman and Euclid ( roman and euclid ) in the IR, and Cosmological Advanced Survey Telescope for Optical-UV Research ( c astor ) in the UV. Our approach uses mock catalogs to approximate conditional relationships between color and redshift from real samples, but is otherwise independent of estimator and prior information. We conclude that adding UV photometry can benefit redshift determination of certain galaxy populations, but that gain is tempered by their decreased chance of meeting detection criteria at higher wavelengths. We explore the spectral energy distributions of galaxies whose potentially recoverable redshift information is most impacted by additional photometry. The holistic assessment approach we develop here is generic and may be applied to quantify the impact of combining photometric data sets, changing experimental design, optimizing observing strategy, and mitigating systematics.

CASTOR is a proposed wide-field (30 ' x30 '=0.25 deg(2)), high-resolution (FWHM similar to 0.15 ''), 1-m-diameter space telescope that is under development by the Canadian Space Agency and the National Research Council of Canada. Optimized for UV/blue-optical wavelengths, the telescope uses dichroics to enable imaging in three channels (and up to five bands) that cover the 0.15 to 0.55 mu m spectral region, simultaneously. CASTOR will also feature low- and low-medium-resolution spectroscopic capabilities through the use of a deployable grism for low-resolution (R less than or similar to 420) slit-less spectroscopy in its UV and u channels, and low-medium-resolution R similar to 1400 multi-object spectroscopy in a parallel field using a digital micro-mirror device. High-speed, precision photometry will be possible using dedicated CMOS detectors in each of its three channels. We present an overview of the mission, including the optical design, instruments and detectors, payload layout, satellite bus, orbit, and ground segment. We describe the mission's scientific capabilities and expected place within the astronomical landscape in the 2030s. The 5-year lifetime is baselined on a combination of legacy surveys, guest observer programs, and target-of-opportunity science. We summarize scientific plans for the mission in each of eight fields: cosmology, time domain and multi-messenger science, active galactic nuclei, galaxies, near-field cosmology, stellar astrophysics, exoplanets, and solar system studies. We conclude by describing ongoing development efforts, highlighting areas of particular relevance for NASA's Habitable Worlds Observatory.

The Cosmological Advanced Survey Telescope for Optical and ultraviolet Research (CASTOR) is a proposed Canadian-led 1 m class space telescope that will carry out ultraviolet and blue optical wide-field imaging, spectroscopy, and photometry. CASTOR will provide an essential bridge in the post-Hubble era, preventing a protracted UV-optical gap in space astronomy and enabling an enormous range of discovery opportunities from the solar system to the nature of the cosmos, in conjunction with the other great wide-field observatories of the next decade (e.g., Euclid, Roman, Vera Rubin). FORECASTOR (Finding Optics Requirements and Exposure times for CASTOR) will supply a coordinated suite of mission-planning tools that will serve as the one-stop shop for proposal preparation, data reduction, and analysis for the CASTOR mission. We present the first of these tools: a pixel-based, user-friendly, extensible, multi-mission exposure time calculator built in Python, including a modern browser-based graphical user interface that updates in real time. We then provide several illustrative examples of FORECASTOR's use that advance the design of planned legacy surveys for the CASTOR mission: a search for the most massive white dwarfs in the Magellanic Clouds, a study of the frequency of flaring activity in M stars and their distribution and impacts on habitability of exoplanets, mapping the proper motions of faint stars in the Milky Way, wide and deep galaxy surveys, and time-domain studies of active galactic nuclei.

We introduce the largest to date survey of massive quiescent galaxies at redshift z similar to 1.6. With these data, which cover 2.7.6 deg(2), we can find significant numbers of very rare objects such as ultra-massive quiescent galaxies that populate the extreme massive end of the galaxy mass function, or dense environments that are likely to become present-day massive galaxy clusters. In this paper, the first in a series, we apply our gzKs adaptation of the BzK technique to select our z similar to 1.6 galaxy catalogue and then study the quiescent galaxy stellar mass function with good statistics over M-star similar to 10(10.2)-10(11.7) M-circle dot- a factor of 30 in mass including 60 ultra-massive z similar to 1.6 quiescent galaxies with M-star > 10(11.5) M-circle dot. We rind that the stellar mass function of quiescent galaxies at z similar to 1.6 is well represented by the Schechter function over this large mass range. This suggests that the mass-quenching mechanism observed at lower redshilts must have already been well established by this epoch, and that it is likely due to a single physical mechanism over a wide range or mass. This close adherence to the Schechter shape also suggests that neither merging nor gravitational. lensing significantly affects the observed quenched population. Finally, comparing measurements or M* parameters for quiescent and star-forming populations (ours and from the literature), we find hints of an offset (M-SF* > M-PE*), which could suggest that the efficiency of the quenching process evolves with time.

We perform spatially-resolved, pixel-by-pixel SED fitting on galaxies up to $z\sim2.5$ in the Hubble Extreme Deep Field (XDF). Comparing stellar mass estimates from spatially resolved and spatially unresolved photometry we find that unresolved masses can be systematically underestimated by factors of up to 5. The ratio of the unresolved to resolved mass measurement depends on the galaxy's specific star formation rate (sSFR): at low sSFRs the bias is small, but above sSFR$\ \sim 10^{-9.5}$ yr$^{-1}$ the discrepancy increases rapidly such that galaxies with sSFRs$\ \sim 10^{-8}$ yr$^{-1}$ have unresolved mass estimates of only one half to one fifth of the resolved value. This result indicates that stellar masses estimated from spatially-unresolved datasets need to be systematically corrected, in some cases by large amounts, and we provide an analytic prescription for applying this correction. We show that correcting stellar mass measurements for this bias changes the normalization and slope of the star-forming main sequence and reduces its intrinsic width; most dramatically, correcting for the mass bias increases the stellar mass density of the Universe at high redshift and can resolve the long-standing discrepancy between the directly-measured cosmic star formation rate density at $z\gtrsim1$ and that inferred from stellar mass densities ("the missing mass problem").

We fit model spectral energy distributions to each pixel in 67 nearby ( =0.0057) galaxies using broadband photometry from the Sloan Digital Sky Survey and GALEX. For each galaxy, we compare the stellar mass derived by summing the mass of each pixel to that found from fitting the entire galaxy treated as an unresolved point source. We find that, while the pixel-by-pixel and unresolved masses of galaxies with low specific star formation rates (such as ellipticals and lenticulars) are in rough agreement, the unresolved mass estimate for star-forming galaxies is systematically lower then the measurement from spatially-resolved photometry. The discrepancy is strongly correlated with sSFR, with the highest sSFRs in our sample having masses underestimated by 25% (0.12 dex) when treated as point sources. We found a simple relation to statistically correct mass estimates derived from unresolved broad-band SED fitting to the resolved mass estimates: m_{resolved} = m_{unresolved}/(-0.057log(sSFR) + 0.34) where sSFR is in units of yr^{-1}. We study the effect of varying spatial resolution by degrading the image resolution of the largest images and find a sharp decrease in the pixel-by-pixel mass estimate at a physical scale of approximately 3 kpc, which is comparable to spiral arm widths. The effects we observe are consistent with the "outshining" idea which posits that the youngest stellar populations mask more massive, older -- and thus fainter -- stellar populations. Although the presence of strong dust lanes can also lead to a drastic difference between resolved and unresolved mass estimates (up to 45% or 0.3 dex) for any individual galaxy, we found that resolving dust does not affect mass estimates on average. The strong correlation between mass discrepancy and sSFR is thus most likely due to the outshining systematic bias.

The Cosmological Advanced Survey Telescope for Optical and uv Research (CASTOR) is a proposed CSA-led mission that would carry out panoramic imaging in the ultraviolet and blue-optical region (≈ 150–550 nm). Operating close to the diffraction limit, the 1m CASTOR telescope would have a spatial resolution comparable to the Hubble Space Telescope (HST), but with an instantaneous field of view about two hundred times larger. The scientific impact from such a facility would be immense, covering topics ranging from small bodies in the outer solar system to the equation of state of the universe. CASTOR has the potential to be a significant, unique and highly strategic Canadian contribution to the international portfolio of astronomical facilities in the 2020s, complementing highprofile optical/IR space missions (Euclid, WFIRST) and ground-based telescopes (LSST) currently under development in Europe and the United States. By placing Canada at the forefront of astronomical research in the coming decade, CASTOR would showcase the technological capabilities of Canadian industries to an international audience, and inspire the next generation of young Canadians to pursue careers in science, engineering and technology. To achieve maximum scientific impact, and to enable effective international partnerships (including collaborations with the Euclid, LSST and WFIRST development teams), CASTOR should launch no later than the middle of the next decade. Since the time of the 2010 Long Range Plan for Canadian Astronomy, CSA has taken significant steps to advance CASTOR through the mission concept stage, including ongoing technology development studies. However, it is now imperative that CASTOR moves promptly to a Phase 0 study in order to refine estimates for cost and schedule, optimize the mission design and survey strategies, and lay the groundwork for international partnerships. Subject headings: telescopes – satellites – instrumentation – techniques – surveys – cosmology – dark energy – dark matter – galaxies – stellar astrophysics – planetary systems – solar system – ultraviolet, optical and infrared astronomy – time domain astronomy 1. BACKGROUND AND CONTEXT In the next decade, two landmark space missions will transform astronomy by carrying out deep, high1 NRC Herzberg Astronomy & Astrophysics, 5071W. Saanich Road, Victoria, BC 2 COM DEV Ltd, 303 Terry Fox Drive, Kanata, ON 3 Department of Astronomy, University of Toronto, 50 St. George Street, Toronto, ON 4 Magellan Aerospace, 3160 Derry Road East, Mississauga, ON 5 Department of Physics and Astronomy, University of Waterloo, Waterloo, ON 6 Northeast Space Company Inc., Box 355 – 900 Greenbank Road, Ottawa, ON 7 Department of Mechanical & Aeronautical Engineering, Clarkson University, Potsdam, New York 8 B-Con Engineering Inc., 14 Capella Court, Nepean, ON 9 Canadian Space Agency, 6767 Boulevard de l’Aroport, SaintHubert, QC 10 Département de Physique, de Génie Physique et d’Optique, Université Laval, QC 11 Xiphos Technologies, 3981 St.Laurent Boulevard, Suite 500, Montreal, QC 12 ABB Analytical, 585 Boulevard Charest Est, Suite 300, Québec, QC 13 Department of Astronomy and Physics and Institute for Computational Astrophysics, Saint Mary’s University, 923 Robie Street, Halifax, NS 14 Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC 15 BMV Optical Technologies, 26 Concourse Gate, Ottawa, ON resolution, wide-field imaging in the red-optical and infrared (IR) spectral region (0.55 ≤ λ . 2 μm). The first of these, Euclid, is an ESA-led mission that is scheduled for launch in 2020 (Laureijs et al. 2011). Euclid will image an area of at least 15 000 deg in the IR region (Y JH), as well as in a single broad filter (VIS) at red-optical wavelengths. Around 2023, Euclid will be joined by NASA’s WFIRST mission (Spergel et al. 2013), which will also carry out red-optical/IR imaging (Y JH and F184), but to a depth ∼ 3 mag deeper than Euclid over a smaller (' 2200 deg) field. Both missions are primarily motivated by a desire to understand dark energy — a mysterious component of the universe that causes an acceleration in the cosmic expansion rate — but their legacy value is so immense that a vast amount of ancillary science will be enabled. On the ground, the Large Synoptic Survey Telescope (LSST) is expected to begin its decade-long survey operations in 2022. LSST (Ivezić et al. 2008, Abell et al. 2009) will revolutionize time-domain astronomy by repeatedly imaging an area of ∼ 20 000 deg every few nights. The combination of optical imaging from LSST and IR imaging from Euclid/WFIRST is expected to be a powerful resource that astronomers will exploit for decades to come. Indeed, LSST and WFIRST emerged as the topranked projects in groundand space-based astronomy

The Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) is a proposed CSA mission that would make a unique, powerful, and lasting contribution to astrophysics by providing panoramic, high-resolution imaging in the UV/optical (0.15 - 0.55 μm) spectral region. This versatile `smallSAT'-class mission would far surpass any ground-based optical telescope in terms of angular resolution, and would provide ultra-deep imaging in three broad lters to supplement longer-wavelength data from planned international dark energy missions (Euclid, WFIRST) as well as from the ground-based Large Synoptic Survey Telescope (LSST). Combining the largest focal plane ever own in space, with an innovative optical design that delivers HST-quality images over a eld two orders of magnitude larger than Hubble Space Telescope (HST), CASTOR would image about 1/8th of the sky to a (u-band) depth ~1 magnitude fainter than will be possible with LSST even after a decade of operations. No planned or proposed astronomical facility would exceed CASTOR in its potential for discovery at these wavelengths.

We study how the addition of onboard optical photometric bands to future space-based weak-lensing instruments could affect the photometric redshift estimation of galaxies and hence improve estimations of the dark energy parameters through weak lensing. Basing our study on the current proposed Euclid configuration and using a mock catalog of galaxy observations, various onboard options are tested and compared with the use of ground-based observations from the Large Synoptic Survey Telescope (LSST) and Pan-STARRS. Comparisons are made through the use of the dark energy figure of merit, which provides a quantifiable measure of the change in the quality of the scientific results that can be obtained in each scenario. Effects of systematic offsets between LSST and Euclid photometric calibration are also studied. We find that adding two optical bandpasses (U and G) or even solely the U bandpass to the space-based infrared instrument greatly improves its photometric redshift performance, bringing it close to the level that would be achieved by combining observations from both space-based and ground-based surveys while freeing the space mission from reliance on external data sets.

We study how the addition of on-board U and/or G photometric bands to the Euclid satellite could affect the photometric redshift estimation of galaxies, and hence improve estimations of the dark energy parameters through weak lensing. Using a mock catalog of galaxy observations, various on-board options are tested and compared with the use of ground-based observations from the Large Synoptic Survey Telescope (LSST) and Pan-STARRS. Comparisons are made through the use of the dark energy Figure of Merit, which provides a quantifiable measure of the change in the quality of the scientific results that can be obtained in each scenario. Effects of systematic offsets between LSST and Euclid photometric calibration are also studied. We find that adding two (U and G) or even one (U) on-board optical band-passes to the Euclid satellite greatly improves its photometric redshift performance. It brings Euclid's performance close to the level that would be achieved by combining observations from Euclid and LSST while freeing the mission from reliance on external datasets.

We explore the feasibility and limitations of using the 1.6 mu m bump as a photometric redshift indicator and selection technique, and use it to study the rest-frame H-band galaxy luminosity and stellar mass functions (SMFs) at redshift z similar to 2. We use publicly available Spitzer/IRAC images in the GOODS fields and find that color selection in the IRAC bandpasses alone is comparable in completeness and contamination to BzK selection. We find that the shape of the 1.6 mu m bump is robust, and photometric redshifts are not greatly affected by choice of model parameters. Comparison with spectroscopic redshifts shows photometric redshifts to be reliable. We create a rest-frame NIR-selected catalog of galaxies at z similar to 2 and construct a galaxy SMF. Comparisons with other SMFs at approximately the same redshift but determined using shorter wavelengths show good agreement. This agreement suggests that selection at bluer wavelengths does not miss a significant amount of stellar mass in passive galaxies. Comparison with SMFs at other redshifts shows evidence for the downsizing scenario of galaxy evolution. We conclude by pointing out the potential for using the 1.6 mu m bump technique to select high-redshift galaxies with the JWST, whose. > 0.6 mu m coverage will not be well suited to selecting galaxies using techniques that require imaging at shorter wavelengths.