arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1008.0519astro-ph.GA
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两个红色团块和X形Milky Way凸起

Two Red Clumps and the X-Shaped Milky Way Bulge

Andrew McWilliam, Manuela Zoccali

摘要:

从2 MASS红外测光中,我们发现两个红团(RC)种群共存于纬度|b| >5.5 °的银河系隆起的同一场中,范围在经度13度和纬度20度以上。只有当这些RC峰仅仅反映了相隔约2.3 kpc的两个恒星星族时,我们才能理解这些数据;在(l,b)=(+1,-8)时,两个RC分别位于6.5和8.8+/-0.2 kpc。双峰RC是不一致的倾斜酒吧形态。我们的大多数场显示两个RC与经度的距离大致恒定,这也与倾斜的酒吧不一致,尽管可能存在底层酒吧。两个RC中的恒星密度随经度变化很大:在正经度侧,前景RC占主导地位,而背景RC占主导地位的负经度。连接前景和背景群体的最大值的线相对于视线倾斜约20 +/-4 °,类似于银河系的倾斜。两个RC之间的距离向银道面方向逐渐减小;从边缘看,凸起是X形的,类似于一些河外凸起和N体模拟的结果。这个X的中心与银河系中心的距离一致,尽管如果凸起比47 Tuc年轻2-3 Gyr,则会出现更好的一致性。如果两个RC星群几乎相切地从银河棒的两端发散出来,每一边都像漏斗或喇叭,我们的观测就可以理解了。或者,X,或双漏斗形状,可能会继续到银河系中心。从太阳方向看,这看起来像花生/盒子形状,但当切向观察时,它是X形的。

英文摘要:

From 2MASS infra-red photometry we find two red clump (RC) populations co-existing in the same fields toward the Galactic bulge at latitudes |b|>5.5 deg., ranging over ~13 degrees in longitude and 20 degrees in latitude. We can only understand the data if these RC peaks simply reflect two stellar populations separated by ~2.3 kpc; at (l,b)=(+1,-8) the two RCs are located at 6.5 and 8.8+/-0.2 kpc. The double-peaked RC is inconsistent with a tilted bar morphology. Most of our fields show the two RCs at roughly constant distance with longitude, which is also inconsistent with a tilted bar, although an underlying bar may be present. The stellar densities in the two RCs changes dramatically with longitude: on the positive longitude side the foreground RC is dominant, while the background RC dominates negative longitudes. A line connecting the maxima of the foreground and background populations is tilted to the line of sight by ~20 +/-4 deg., similar to claims for the tilt of a Galactic bar. The distance between the two RCs decreases towards the Galactic plane; seen edge-on the bulge is X-shaped, resembling some extra-galactic bulges and the results of N-body simulations. The center of this X is consistent with the distance to the Galactic center, although better agreement would occur if the bulge is 2-3 Gyr younger than 47 Tuc. Our observations may be understood if the two RC populations emanate, nearly tangentially, from the ends of a Galactic bar, each side shaped like a funnel or horn. Alternatively, the X, or double funnel shape, may continue to the Galactic center. This would appear peanut/box shaped from the Solar direction, but X-shaped when viewed tangentially.

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