We present an analysis of new extremely deep images of the resolved stellar population of the Bootes I ultra faint dwarf spheroidal galaxy. These new data were taken with the Hubble Space Telescope, using the Advanced Camera for Surveys (Wide Field Camera) and Wide Field Camera 3 (UVIS), with filters F606W and F814W (essentially V and I), as part of a program to derive the low-mass stellar initial mass function in this galaxy. We compare and contrast two approaches to obtaining the stellar photometry, namely ePSF and DAOPHOT. We identify likely members of Bootes I based on the location of each star on the color-magnitude diagram, obtained with the DAOPHOT photometry from the ACS/WFC data. The probable members lie close to stellar isochrones that were chosen to encompass the known metallicity distribution derived from spectroscopic data of brighter radial-velocity member stars and are consistent with the main-sequence turnoff. The resulting luminosity function of the Bootes I galaxy has a 50% completeness limit of 27.4 in F814W and 28.2 in F606W (Vega magnitude system), which corresponds to a limiting stellar mass of $\le 0.3 M_\odot$.

中文翻译：

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微弱星系中的微弱星星。I. Boötes I 超微暗矮星系的超深光度测量

我们对 Bootes I 超微弱矮椭球星系的已解析恒星群的新极深图像进行了分析。这些新数据是使用哈勃太空望远镜拍摄的，使用高级巡天相机（广角相机）和广角相机 3（UVIS），带有过滤器 F606W 和 F814W（基本上是 V 和 I），作为推导程序的一部分这个星系中的低质量恒星初始质量函数。我们比较和对比了两种获得恒星光度测量的方法，即 ePSF 和 DAOPHOT。我们根据颜色-星等图上每颗恒星的位置确定了 Bootes I 的可能成员，该位置是通过 ACS/WFC 数据的 DAOPHOT 光度法获得的。可能的成员靠近恒星等时线，这些等时线被选择来包含已知的金属丰度分布，这些分布是从较亮的径向速度成员星的光谱数据得出的，并且与主序关闭一致。Bootes I 星系的最终光度函数在 F814W 中的 50% 完整性极限为 27.4，在 F606W（织女星等）中为 28.2，这对应于 $\le 0.3 M_\odot$ 的极限恒星质量。