Research On The Very Low-luminance Galaxies Of The Nearest Universe

Using a small telescope's wide field and long exposure deep imaging technology can be used to detect more very low surface brightness galaxies.The imaging system of the small telescope is simple and can reduce the internal reflected light and scattered light,it is conductive to improve the surface brightness limit.We use the small telescope's wide field and long exposure time deep imaging to search for the Ultra-Diffuse Galaxies(UDGs)in the nearby universe and introduce the characteristics of UDGs finally in this thesis.We present a detection of 105 candidates of UDGs in a 3°×3°field centered on HCG 95 using deep g and r-band images taken with the Purple Mountain Observatory 1 meter schmidt wide field survey telescope at the Xuyi station,which include 68 plausible ultra-diffuse galaxies and 37 possible UDG candidates.This field contains one rich galaxy cluster(Abell 2588 at z = 0.199)and two poor clusters(Pegasus ?galaxy cluster at z = 0.013 and Pegasus ? galaxy cluster at z =0.040).Pegasus ?cluster separated from more distant Pisces-Perseus Supercluster and its velocity dispersion may be lowest so far,our HCG 95 field is located in sparser environment than Coma.Besides,we find that most of UDGs are surrounded by the two poor clusters,and it indicates that UDGs may be corrected with the two poor clusters.The UDG candidates are mainly identified with half-light radius re>1.5 kpc and g-band central surface brightness ?(g,0)>24.0 mag aresec-2.Deep z'-band images are available for 99 of the 105 UDG candidates from the Dark Energy Camera Legacy Survey(DECaLS),confirming that these galaxies have extremely low central surface brightness.Interestingly,we find that one UDG(H95F)that belongs to HCG 95 has rich HI gas(H I mass is about 1.08 ×109M?)detected by the Very Large Array(VLA).This gas-rich UDG(H95F)has ?(g,0)= 24.33 mag arcsec-2,half-light radius re=3.28 kpc,stellar mass M*?1.82 ×108 M?.UDGs in the HCG 95 field span over a wide color range,the majority of UDGs belongs to the red-sequence galaxies,still,a number of UDGs exhibit color as blue as typical blue cloud galaxies and H I-selected galaxies,this indicates these blue UDGs are still forming stars.The diversity of color indicates UDGs may be formed through different star forming histories,which may be corrected with the environments.In this thesis,we discuss the possible formation mechanism of UDGs,especially for gas-rich UDGs.In addition,the average color of UDGs in the HCG 95 field seems to be bluer than that of UDGs in Coma,it may be related with their environments.However,a high abundance of UDGs in the HCG 95 field is favored by the environment of poor galaxy clusters residing in HI-rich large-scale structure.