[论文解读] Investigating hot gas in the halos of two massive spirals: Observations and cosmological simulations

J. Rasmussen, Jesper Sommer‐Larsen+5 more|arXiv (Cornell University)|Oct 30, 2006

Galaxies: Formation, Evolution, Phenomena参考文献 4被引用 4

一句话总结

本研究首次提出了对大质量螺旋星系NGC 5746周围热X射线发射晕的初步探测，其光度约为~7×10³⁹ erg s⁻¹，延伸至约20 kpc，与外部吸积的热气体通过辐射冷却形成的结果一致，而非盘面外流。宇宙学模拟证实了这种吸积情景，支持热晕作为大质量螺旋星系中重子燃料储存库的作用。

ABSTRACT

Models of disk galaxy formation commonly predict the existence of an extended reservoir of hot gas surrounding massive spirals at low redshift. As a test of these models, we have obtained X-ray and optical data of the two massive edge-on spirals NGC 5746 and NGC 5170, in order to investigate the amount and origin of hot gas in their disks and halos. Chandra observations of NGC 5746 reveal evidence for diffuse X-ray emission with a total luminosity of ~7 x 10^39 erg/s surrounding this galaxy out to at least ~20 kpc from the disk, whereas an identical study of the less massive NGC 5170 fails to detect any extraplanar X-ray emission. Unlike the case for other disk galaxies with detected X-ray halos, the halo emission around NGC 5746 is not accompanied by extraplanar H-alpha or radio emission, and there is no evidence for significant nuclear or starburst activity in the disk. In contrast to these other cases, the emission around NGC 5746 therefore appears to arise from the cooling of externally accreted material rather than from disk outflows. To verify this idea, we present results of cosmological simulations of galaxy formation and evolution, showing our observations to be in good agreement with expectations for cosmological accretion, while also confirming that the X-ray halos of other spirals do not fit well into an accretion scenario. We find that the estimated cooling rate of hot halo gas around NGC 5746 would provide sufficient material for star formation in the disk to proceed at its present rate. This lends support to the idea that a supply of hot ambient gas is potentially available as fuel for star formation in massive, nearby spirals, and suggests that accretion of hot gas could be important for maintaining the stellar disks of such galaxies.

研究动机与目标

• 检验大质量螺旋星系是否如宇宙学星系形成模型所预测的那样拥有延伸的热气体晕。
• 确定两个大质量侧向螺旋星系NGC 5746和NGC 5170晕中弥漫X射线发射的起源。
• 区分X射线发射的来源是热气体吸积还是盘面驱动的外流。
• 评估热晕气体对星系中缺失重子质量的贡献。

提出的方法

• 对NGC 5746和NGC 5170进行钱德拉X射线观测，以探测弥漫的星系平面上方X射线发射。
• 利用光学Hα成像搜索与恒星形成或外流相关的电离气体。
• 使用热等离子体模型对X射线发射进行光谱分析，以估算温度和金属丰度。
• 利用宇宙学星系形成模拟，将观测到的X射线特性与热晕吸积的预测进行比较。
• 估算热晕气体的冷却速率，并与NGC 5746盘面中观测到的恒星形成速率进行比较。
• 对NGC 5170应用统计极限，以评估非探测结果的显著性，并与模拟结果的一致性。

实验结果

研究问题

• RQ1 NGC 5746晕中的弥漫X射线发射是否与外部吸积的热气体冷却一致，而非盘面外流？
• RQ2 在未检测到相关Hα或射电发射的情况下，NGC 5746的X射线晕发射起源为何？
• RQ3 宇宙学模拟在多大程度上能再现NGC 5746中观测到的X射线光度和温度？
• RQ4 热晕气体的冷却速率是否足以解释NGC 5746盘面当前的恒星形成速率？
• RQ5 热晕气体在多大程度上贡献于大质量螺旋星系中缺失的重子质量？

主要发现

• NGC 5746中弥漫的X射线发射在0.3–2 keV能量段的总光度约为~7×10³⁹ erg s⁻¹，延伸至至少20 kpc的盘面之外。
• X射线谱最符合一个温度约为~0.5 keV、金属丰度Z ≤ 0.1 Z⊙的热等离子体模型，但需更深度的数据以确认。
• 未检测到相关的Hα或射电发射，排除了显著的恒星形成或外流作为X射线发射来源的可能性。
• NGC 5170中未探测到X射线发射，其3σ上限约为~3×10³⁹ erg s⁻¹，与模拟预测一致。
• 宇宙学模拟与NGC 5746观测到的X射线特性高度一致，支持晕气源于吸积的结论。
• 估算的热晕气体冷却速率足以维持NGC 5746盘面当前的恒星形成速率，表明其提供了可靠的重子燃料供应。