[论文解读] Confirmation Of Two New Galactic Bulge Globular Clusters: FSR 19 and FSR 25

C. Obasi, M. Gómez+2 more|arXiv (Cornell University)|Jun 16, 2021

Stellar, planetary, and galactic studies参考文献 54被引用 21

一句话总结

本研究利用来自VVVX和2MASS巡天的深近红外测光数据，结合Gaia EDR3的天体测量学和自行数据，确认了FSR 19和FSR 25为银河系银心区域真实的、低光度的球状星团。分析结果表明，这两个星团的距离约为7 kpc，年龄约为11 Gyr，[Fe/H] = -0.5 dex，其总绝对Ks波段星等分别为MKs ≈ -7.7和-7.3 mag，属于目前已知最暗淡的银心球状星团之一。

ABSTRACT

Globular clusters (GCs) in the Milky Way (MW) bulge are very difficult to study because: i) they suffer from the severe crowding and galactic extinction; which are characteristic of these inner Galactic regions ii) they are more prone to be affected by dynamical processes. Therefore, they are relatively faint and difficult to map. However, deep near-infrared photometry like that provided by the VISTA Variables in the Via L\'actea Extended Survey (VVVX) is allowing us to map GCs in this crucial yet relatively uncharted region. Our results confirm with high confidence that both FSR 19 and FSR 25 are genuine MW bulge GCs. Each of the performed tests and resulting parameters provides clear evidence for the GC nature of these targets. We derive distances of 7.2$\pm$0.7 kpc and D=7.0$\pm$0.6 (corresponding to distance moduli of 14.29$\pm$0.08 and 14.23$\pm$0.07) for FSR 19 and FSR 25, respectively. Their ages and metallicities are 11 Gyr and [Fe/H]= -0.5 dex for both clusters, which were determined from Dartmouth and PARSEC isochrone fitting. The integrated luminosities are M$_{Ks}$(FSR 19) = -7.72 mag and M$_{Ks}$(FSR 25) = -7.31 mag which places them in the faint tail of the GC Luminosity Function. By adopting a King profile for their number distribution, we determine their core and tidal radii ($r_c$, $r_t$). For FSR 19, r$_{c}$= 2.76$\pm$0.36 pc and r$_{t}$=5.31$\pm$0.49 pc, while FSR 25 appears more extended with r$_{c}$= 1.92$\pm$0.59 pc and r$_{t}$=6.85$\pm$1.78 pc. Finally their mean GC PMs (from Gaia EDR3) are $\mu_{\alpha^\ast}$= -2.50 $\pm$0.76 mas $yr^{-1}$, $\mu_{\delta}$= -5.02 $\pm$0.47 mas $yr^{-1}$ and $\mu_{\alpha^\ast}$= -2.61 $\pm$ 1.27 mas $yr^{-1}$ , $\mu_{\delta}$= -5.23 $\pm$0.74 mas $yr^{-1}$ for FSR 19 and FSR 25, respectively. }

研究动机与目标

• 确认FSR 19和FSR 25为银河系银心区域中因暗淡且严重消光而难以观测的球状星团。
• 测量这两个候选体的关键物理参数——距离、年龄、金属量、光度、结构半径及自行。
• 通过深度近红外测光和精确天体测量，克服银心区域高消光和恒星密集带来的挑战。
• 完善银河系内区低光度球状星团的星族统计，该区域历史上难以测绘。

提出的方法

• 利用VVVX和2MASS巡天的近红外测光数据，减轻银河系银心区域的消光影响。
• 结合VVVX、2MASS和Gaia EDR3数据，获得恒星群体的精确测光、天体测量和自行数据。
• 采用Dartmouth和PARSEC模型进行颜色-星等图（CMD）分析与等年龄线拟合，推导年龄和金属量。
• 应用统计检验，包括对光度函数的强Kolmogorov-Smirnov检验，确认星团的星场恒星过量分布。
• 对径向星数分布拟合King模型，测量核心半径和潮汐半径（rc和rt）。
• 采用稳健的筛选标准，基于自行和测光一致性，排除前景恒星，确保样本纯净。

实验结果

研究问题

• RQ1 FSR 19和FSR 25是否为真实的球状星团，还是场星场的恒星过量或其他恒星系统？
• RQ2 FSR 19和FSR 25的距离、年龄、金属量和光度是多少？与已知的银心球状星团相比如何？
• RQ3 FSR 19和FSR 25的结构参数（核心半径和潮汐半径）以及自行特征是否支持其作为真实球状星团的分类？
• RQ4 FSR 19和FSR 25的参数与仅基于2MASS或H波段距离方法的先前估计相比如何？
• RQ5 它们的低光度和低金属量在银河系形成与演化背景下的意义是什么？

主要发现

• 基于多项独立检验，FSR 19和FSR 25被高置信度确认为银河系银心区域的真实球状星团。
• FSR 19的距离为7.2 ± 0.7 kpc（距离模数14.29 ± 0.08 mag），FSR 25的距离为7.0 ± 0.6 kpc（距离模数14.23 ± 0.07 mag）。
• 两个星团的年龄一致为11 Gyr，金属量[Fe/H] = -0.5 dex，由近红外CMD的等年龄线拟合得出。
• FSR 19的Ks波段总绝对星等为MKs = -7.72 mag，FSR 25为MKs = -7.31 mag，位于球状星团光度函数的暗淡尾部。
• FSR 19的核心半径为2.76 ± 0.36 pc，潮汐半径为5.31 ± 0.49 pc；FSR 25更为弥散，其核心半径为1.92 ± 0.59 pc，潮汐半径为6.85 ± 1.78 pc。
• 其Gaia EDR3自行数据一致：FSR 19的µα∗ ≈ -2.5 mas yr⁻¹，µδ ≈ -5.0 mas yr⁻¹；FSR 25的µα∗ ≈ -2.6 mas yr⁻¹，µδ ≈ -5.2 mas yr⁻¹。