Items filtered by date: June 2020

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Items filtered by date: June 2020

Identifying galaxy groups and clusters in the high-z universe

Galaxies are not distributed at random. Rather, they are found to be clustered, with half or more of them residing in groups (with intermediate-to-low abundance of member galaxies) or clusters (with high abundance). Galaxy groups and clusters are commonly used to link galaxies with their host dark matter halos, as current galaxy formation models predict that galaxies form and evolve in dark matter halos. Therefore, identification of galaxy groups from observational samples is a crucial step toward a complete picture of the galaxy-halo connection. In the past decades, much effort has been dedicated to identifying galaxy groups in various galaxy surveys, both photometric and spectroscopic. Most of these studies have been limited to low-redshift surveys such as the Sloan Digital Sky Survey (SDSS). Next-generation redshift surveys will extend these to high redshifts, such as the Subaru Prime Focus Spectrograph (PFS) project which will observe about 0.3 million galaxies at 0.7 < z < 1.7 over ~15 square degrees in the sky -- an ambitious survey that is a factor of 10 larger than existing surveys at similar redshifts such as zCOSMOS.

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Bars enhance the central star formation and gas inflow in nearby galaxies

Two thirds of spiral galaxies in the local Universe are observed to have a bar-shaped structure in their central region. Theoretically, galactic bars are expected to transfer angular moment from the center outwards through driving cold gas inward. As a result, the gas density at the galactic center will increase and thus trigger star formation, leading to the formation and growth of the central bulge. Therefore, galactic bars play important roles in the secular evolution of galaxies, through driving the inflow of cold gas, enhancing the central star formation, the consumption of cold gas, the growth of the (pseudo-)bulge, and probably that of the central super massive black hole as well. These processes have been seen in numerical simulations of disc galaxy evolution. Observationally, however, it is not easy to directly see the whole process due to the lack of suitable data. In recent years surveys of integral field spectroscopy such as CALIFA and MaNGA have obtained spatially resolved spectroscopy for large samples of galaxies at low redshifts, allowing the star formation history and stellar populations to be mapped across the whole galaxy. Meanwhile, mapping of cold gas content has also become available for a considerable number of galaxies at the same redshifts. These new data have enabled astronomers to study the role of bars in star formation and cold gas inflow in great detail.

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