PAndAS ' cubs：在仙女座和三角座星系周围发现两个新的矮星系
PAndAS' cubs: discovery of two new dwarf galaxies in the surroundings of the Andromeda and Triangulum galaxies
摘要：
我们提出了两个新的矮星系，仙女座XXI和仙女座二十二，位于仙女座和三角座星系（M31和M33）的周围发现。这些发现源于泛仙女座考古调查（Pan-Andromeda Archaeological Survey，PAndAS）的第一年数据，这是一项利用安装在加拿大-法国-夏威夷望远镜上的Megaprime/MegaCam宽视场相机对M31/M33群进行的测光调查。这两颗卫星都表现为恒星的空间超密度，当绘制在色星等图上时，它们都遵循着距离M31/M33的贫金属（[Fe/H]=-1.8）红巨星分支。仙女座XXI是一个中等亮度的矮星系（M_V=-9.9+/-0.6），尽管表面亮度较低，再次强调许多相对明亮的M31卫星仍有待发现。它也是一颗大型卫星，半光半径接近1千秒差距，是银河系中第四大的本星系群矮球状星系，仅次于最近发现的仙女座XIX、仙女座II和人马座，并支持M31卫星比银河系中的对应卫星更大的趋势。仙女座XXII要暗得多（M_V=-6.5+/-0.8），并且距离M33比距离M31更近（42比224 kpc），这表明它可能是第一颗被发现的三角座卫星。虽然这是一个非常令人兴奋的可能性，在过去的相互作用的M33与M31和其卫星系统的命运的背景下，确认将不得不等待一个良好的距离估计，以确认其物理接近M33。加上之前在M31周围的调查中发现的矮星系，这两颗新卫星使该地区的矮球状星系数量达到20个。
英文摘要：
We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey (PAndAS), a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H]=-1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (M_V=-9.9+/-0.6), albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (M_V=-6.5+/-0.8) and lies a lot closer in projection to M33 than it does to M31 (42 vs. 224 kpc), suggesting that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20.
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