High-resolution Ultraviolet And Optical Study Of The Nuclear Star Cluster In M31

We performed an observation of image and spectrograph in UltraViolet and Optical band,on detecting the M31 Super Massive Black Hole(SMBH)and the stars(Nuclear Star Cluster)in the central several parsecs.The bulge of M31 has a figure of double surface brightness peaks.We call the optical brighter nucleus as P1,and the fainter nucleus as P2.But in UltraViolet band,P2 is more luminous than P1.Based on the detection of kinematics,we found a SMBH hides inside P2.To study the radiation of the SMBH and the physical property of the nuclear star cluster,we did the following work.By utilizing Near-UltraViolet(NUV)image of HST/COS,we performed two exposures separated by 44 days on M31*.We found the flux difference of M31*in NUV band is(6.9±3.0)×10-18 erg·s-1·cm-2 A-1 in two exposures.We found the mean flux of M31*is 1.7×10-18 erg·s-1·cm2·A-1,by extrapolating the X-ray radiation of M31*to NUV band.Then the flux difference is 4 times of the mean flux in NUV band,so the difference may come from the variation of the SMBH in M31.In addition,we performed a spectral energy distribution(SED)analysis for M31 nuclear star cluster,through HST/WFC3 image,from NUV to NIR.We discovered that,besides the exist of an old stellar pop with mass = 1.54-0.05+0.08×106M?,age = 14.9 gyr,Z = 2.5Z?,P1 also owns a young stellar pop with mass = 8.7-2.0+2.1×103 M?,age=468-14+57 Myr,Z =Z?.P2 also has an old stellar pop the same as the old pop in P1,along with a young component with mass = 5.9-0.6+1.3×103 M?,age = 277-18+62 Myr,Z = Z?.Assuming that the gas accumulated by local stellar wind,we found that the timescale of star forming is longer than age of universe.So the young stellar population might originate from the gas migrating into the central region,or move into the central region after the external formation.We got the Far UltraViolet spectrum of M31 nuclear cluster by HST/COS.This is by far the first FUV spectrum in pc-scale of galaxy center.We got a 117 Myr and a 518 Myr stellar pop in 1320 A-1800 A band,consistent with the result of SED photometry.Furthermore,we found that C IV absorption line is produced by hot intergalactic medium but not by stars.We calculated that the column density of C IV doublet(1548.20 A/1550.77 A)is about 3.5×1014 4cm-2 on the line of sight,with absorption gas temperature-2×105K.