Browsing by Author "Zhao, Gang"

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    Central velocity dispersion catalogue of LAMOST-DR7 galaxies
    (2020) Napolitano, Nicola R.; D'Ago, Giuseppe; Tortora, Crescenzo; Zhao, Gang; Luo, A-Li; Tang, Baitian; Zhang, Wei; Zhang, Yong; Li, Rui
    The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is a major facility to carry out spectroscopic surveys for cosmology and galaxy evolution studies. The seventh data release of the LAMOST ExtraGAlactic Survey (LEGAS) is currently available and including redshifts of 193 361 galaxies. These sources are spread over similar to 11 500 deg(2) of the sky, largely overlapping with other imaging (SDSS and HSC) and spectroscopic (BOSS) surveys. The estimated depth of the galaxy sample, r similar to 17.8, the high signal-to-noise ratio, and the spectral resolution R = 1800, make the LAMOST spectra suitable for galaxy velocity dispersion (VD) measurements, which are invaluable to study the structure and formation of galaxies and to determine their central dark matter content. We present the first estimates of central VD of similar to 86 000 galaxies in LAMOST footprint. We have used a wrap-up procedure to perform the spectral fitting using PPXF, and derive VD measurements. Statistical errors are also assessed by comparing LAMOST VD estimates with the ones of SDSS and BOSS over a common sample of similar to 51 000 galaxies. The two data sets show a good agreement, within the statistical errors, in particular when VD values are corrected to 1 effective radius aperture. We also present a preliminary mass-sigma relation and find consistency with previous analyses based on local galaxy samples. These first results suggest that LAMOST spectra are suitable for galaxy VD measurements to complement the available catalogues of galaxy internal kinematics in the Northern hemisphere. We plan to expand this analysis to next LAMOST data releases.
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    Portable adaptive optics for exoplanet imaging
    (2021) Zhu, Yong-Tian; Dou, Jiang-Pei; Zhang, Xi; Zhao, Gang; Guo, Jing; Infante, Leopoldo
    The portable adaptive optics (PAO) device is a low-cost and compact system, designed for 4-meter class telescopes that have no adaptive optics (AO) system, because of the physical space limitation at the Nasmyth or Cassegrain focus and the historically high cost of conventional AO. The initial scientific observations of the PAO are focused on the direct imaging of exoplanets and sub-stellar companions. This paper discusses the concept of PAO and the associated high-contrast imaging performance in our recent observational runs. PAO deliver a Strehl ratio better than 60% in H band under median seeing conditions of 1 ''. Combined with our dedicated image rotation and subtraction (IRS) technique and the optimized IRS (O-IRS) algorithm, the averaged contrast ratio for a 5 <= V-mag <= 9 primary star is 1.3 x 10(-5) and 3.3 x 10(-6) at angular distance of 0.36 '' with exposure time of 7 minutes and 2 hours, respectively. PAO has successfully revealed the known exoplanet of kappa And b in our recent observation with the 3.5-meter ARC telescope at Apache Point Observatory. We have performed the associated astrometry and photometry analysis of the recovered kappa And b planet, which gives a projected separation of 0.984 '' +/- 0.05 '', a position angle of 51.1 degrees +/- 0.5 degrees and a mass of 10.15(-1.255)(+2.19) M-Jup. These results demonstrate that PAO can be used for direct imaging of exoplanets with medium-sized telescopes.