The Andromeda project - I. Deep HST-WFPC2 V, I photometry of 16 fields toward the disk and the halo of the M 31 galaxy. Probing the stellar content and metallicity distribution
The Andromeda project - I. Deep HST-WFPC2 V, I photometry of 16 fields toward the disk and the halo of the M 31 galaxy. Probing the stellar content and metallicity distribution
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DOI：
10.1051/0004-6361:20030455
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发表时间：
2002-12
影响因子：
6.5
通讯作者：
M. Bellazzini;C. Cacciari;L. Federici;F. F. Pecci-F.;R. Bologna;D. PhysicsAstronomy;Ucla
中科院分区：
文献类型：
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作者：
M. Bellazzini;C. Cacciari;L. Federici;F. F. Pecci-F.;R. Bologna;D. PhysicsAstronomy;Ucla
我们在旋涡星系M31附近的16个视场获得了HST-WFPC2 F555W和F814W的测光结果，从~20到~150角分(即~4.5~35kpc)到极限V星等和I星等到~27等不同距离，采集了盘和晕的恒星含量。为每个场获得的颜色-星等图(CMD)显示了复杂恒星群的存在，包括中龄/年轻群和具有广泛金属丰度的老年群。这些叠加在M31圆盘上的场通常显示出蓝色的恒星羽流，我们将其识别为主要的序列成员。根据这种解释，我们发现过去0.5Gyr的恒星形成率随着盘中位置的不同而有很大的不同。所有CMD中最明显的特征是一个突出的红巨星分支(RGB)，其尖端在V波段下降，其金属丰度高于1/10太阳。在所有的田野中都清晰地检测到一团红色的斑块，并经常出现一条微弱的蓝色水平分支。通过比较RGB星和球状星团模板得到的金属丰度分布，都显示出一个很长的，虽然很少填充，但金属贫化的尾巴和一个主要成分的峰值在$\rM[Fe/H]\sim-\，0.6$。丰度分布最值得注意的特征是它们在所有采样领域的总体相似性，覆盖了广泛的环境和银河中心距离。然而，由于金属丰度分布的大体均匀，出现了一些有趣的差异和趋势。例如，中位数[Fe/H]显示出沿短轴(Y)的距离略有下降，直到$Y\simeq 20\arcmin$，但金属丰度梯度完全消失，超过该限制。此外，在一些领域中，明显存在非常富金属的($\Rm[Fe/H]\Ge-0.2$)组分。尽管贫金属星的比例在所有视场中似乎都大致恒定(在几个百分比之内)，但富金属星，特别是非常富金属星的比例随着位置的不同而不同，似乎在叠加在盘面上和/或存在流或亚结构的那些场中更为突出(例如Ibata等人)。2001)。这可能指示并可能跟踪与某个伴星的相互作用效果，例如M32。
We have obtained HST-WFPC2 F555W and F814W photometry for 16 fields in the vicinity of the luminous nearby spiral galaxy M 31, sampling the stellar content of the disk and the halo at different distances from the center, from ~20 to ~150 arcmin (i.e. ~4.5 to 35 kpc), down to limiting V and I magnitudes of ~27. The Color–Magnitude Diagrams (CMD) obtained for each field show the presence of complex stellar populations, including an intermediate age/young population and older populations with a wide range of metallicity. Those fields superposed on the disk of M 31 generally show a blue plume of stars which we identify with main sequence members. According to this interpretation, we find that the star formation rate over the last 0.5 Gyr has varied dramatically with location in the disk. The most evident feature of all the CMDs is a prominent Red Giant Branch (RGB) with a descending tip in the V band, characteristic of metallicity higher than 1/10 Solar. A red clump is clearly detected in all of the fields, and a weak blue horizontal branch is frequently present. The metallicity distributions, obtained by comparison of the RGB stars with globular cluster templates, all show a long, albeit scantly populated, metal-poor tail and a main component peaking at $\rm [Fe/H] \sim - \,0.6$. The most noteworthy characteristic of the abundance distributions is their overall similarity in all the sampled fields, covering a wide range of environments and galactocentric distances. Nevertheless, a few interesting differences and trends emerge from the general uniformity of the metallicity distributions. For example, the median [Fe/H] shows a slight decrease with distance along the minor axis ( Y ) up to $Y\simeq 20 \arcmin$, but the metallicity gradient completely disappears beyond this limit. Also, in some fields a very metal-rich ($\rm [Fe/H] \ge -0.2$) component is clearly present. Whereas the fraction of metal-poor stars seems to be approximately constant (within few percent) in all fields, the fraction of metal-rich and, especially, very-metal-rich stars varies with position and seems to be more prominent in those fields superposed on the disk and/or with the presence of streams or substructures (e.g. Ibata et al. 2001). This might indicate and possibly trace interaction effects with some companion, e.g. M 32.