Abstract: NGC5128 is the nearest, easily observable giant elliptical galaxy, and the only one in which red giant branch (RGB) stars can be resolved from the ground. The only way to derive the surface brightness profile in these outer regions is to resolve and count individual halo stars. The authors derive photometry result of two field image data in the outer stellar halo of NGC5128. CCD imaging data is obtained using the Magellan/IMACS f/4 camera in the V and I bands. The RGB stars candidates are selected by color and magnitude, and 4719 and 980 RGB star candidates are obtained in the inner and outer field respectively. The outer field is used as a control field to measure the level of contamination from unresolved background galaxies. By applying a Sersic law to the surface brightness profile in the inner field, major axis profile is derived with the Sersic parameters: r_e = 6.21 arcmin, ?μ_e = 22.05 mag/arcsec², n = 1.90, with fitting error 0.37 mag for V-band; r_e = 6.57 arcmin, μ_e =20.79 mag/arcsec², n = 1.90, with fitting error 0.41 mag for I-band. The result shows consistent results between the V and I bands within the limits of the data, and no evidence of a break in a profile that would signify a transition to a halo population.

Key words: elliptical galaxy, NGC5128, red giant branch, outer stellar halo

摘要： NGC5128是距离地球最近的、可通过地面望远镜直接分辨其中红巨星分支恒星的巨椭球星系。通过分辨并计数该星系外晕中的红巨星分支恒星, 研究其星系晕的表面亮度轮廓, 并对位于该星系外恒星晕主轴方向的内外两个视场的图像进行测光研究, 这些CCD图像来自Magellan/IMACS f/4望远镜的V和I两个波段。通过星等和颜色对红巨星分支恒星进行筛选, 最终在两个视场的图像中分别得到包含4719和980个红巨星分支恒星候选的样本, 并将外场样本用于对内场污染的估计和矫正。采用 Sersic定律对内场表面亮度轮廓进行拟合, 得到的最佳拟合参数为: V波段, r_e = 6.21角分, μ_e = 22.05 mag/arcsec², n = 1.90, 拟合误差为0.37星等; I波段, r_e = 6.57角分, μ_e =20.79 mag/arcsec², n = 1.90, 拟合误差为0.41星等。结果显示V和I波段的拟合结果一致, 并且在数据的精度内, 没有发现轮廓向晕族突变的迹象。

关键词: 巨椭球星系, NGC5128, 红巨星分支, 星系外恒星晕