Browsing by Author "Nagai, Daisuke"

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    Missing thermal energy of the intracluster medium
    (2007) Afshordi, Niayesh; Lin, Yen-Ting; Nagai, Daisuke; Sanderson, Alastair J. R.
    The Sunyaev-Zel'dovich (SZ) effect is a direct probe of thermal energy content of the Universe, induced in the cosmic microwave background (CMB) sky through scattering of CMB photons off hot electrons in the intracluster medium (ICM). We report a 9 sigma detection of the SZ signal in the CMB maps of Wilkinson Microwave Anisotropy Probe (WMAP) three-year data, through study of a sample of 193 massive galaxy clusters with observed X-ray temperatures greater than 3 keV. For the first time, we make a model-independent measurement of the pressure profile in the outskirts of the ICM, and show that it closely follows the profiles obtained by X-ray observations and numerical simulations. We find that our measurements of the SZ effect would account for only half of the thermal energy of the cluster, if all the cluster baryons were in the hot ICM phase. Our measurements indicate that a significant fraction, 35 +/- 8 per cent, of baryonic mass is missing from the hot ICM, and thus must have cooled to form galaxies, intracluster stars, or an unknown cold phase of the ICM. There does not seem to be enough mass in the form of stars or cold gas in the cluster galaxies or intracluster space, signalling the need for a yet-unknown baryonic component (at 3 sigma level), or otherwise new astrophysical processes in the ICM.
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    Morphological Parameters and Associated Uncertainties for 8 Million Galaxies in the Hyper Suprime-Cam Wide Survey
    (Wiley, 2023) Ghosh, Aritra; Urry, C. Megan; Mishra, Aayush; Perreault-Levasseur, Laurence; Natarajan, Priyamvada; Sanders, David B.; Nagai, Daisuke; Tian, Chuan; Cappelluti, Nico; Kartaltepe, Jeyhan S.; Powell, Meredith C.; Rau, Amrit; Treister, Ezequiel
    We use the Galaxy Morphology Posterior Estimation Network (GaMPEN) to estimate morphological parameters and associated uncertainties for & SIM;8 million galaxies in the Hyper Suprime-Cam Wide survey with z & LE; 0.75 and m & LE; 23. GaMPEN is a machine-learning framework that estimates Bayesian posteriors for a galaxy's bulge-to-total light ratio (L ( B )/L ( T )), effective radius (R ( e )), and flux (F). By first training on simulations of galaxies and then applying transfer learning using real data, we trained GaMPEN with <1% of our data set. This two-step process will be critical for applying machine-learning algorithms to future large imaging surveys, such as the Rubin-Legacy Survey of Space and Time, the Nancy Grace Roman Space Telescope, and Euclid. By comparing our results to those obtained using light profile fitting, we demonstrate that GaMPEN's predicted posterior distributions are well calibrated (& LSIM;5% deviation) and accurate. This represents a significant improvement over light profile fitting algorithms, which underestimate uncertainties by as much as & SIM;60%. For an overlapping subsample, we also compare the derived morphological parameters with values in two external catalogs and find that the results agree within the limits of uncertainties predicted by GaMPEN. This step also permits us to define an empirical relationship between the Sersic index and L ( B )/L ( T ) that can be used to convert between these two parameters. The catalog presented here represents a significant improvement in size (& SIM;10x), depth (& SIM;4 mag), and uncertainty quantification over previous state-of-the-art bulge+disk decomposition catalogs. With this work, we also release GaMPEN's source code and trained models, which can be adapted to other data sets.