[论文解读] Gas and dust cooling along the major axis of M33 (HerM33es) -- Herschel/PACS [CII] and [OI] observations

C. Krämer, Thomas Nikola+18 more|Kölner Universitäts PublikationsServer (Universität zu Köln)|May 7, 2020

Galaxies: Formation, Evolution, Phenomena被引用 4

一句话总结

本研究利用赫歇尔/PACS对M33星系主轴方向上[ C II ] 158 μm和[ O I ] 63 μm谱线的观测，研究气体与尘埃的冷却机制，发现[ C II ]/TIR比值随远红外 luminosity 增加而降低，这是由于尘埃柱密度上升所致。数据最符合中等PDR模型（n ~ 2×10² cm⁻³，G₀ ~ 60），表明 beam 填充因子较低，且与[ C II ]发射受尘埃遮蔽限制而非FUV辐射场变化一致。

ABSTRACT

M33 is a gas rich spiral galaxy of the Local Group. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. We mapped the emission of gas and dust in M33 using the far-infrared lines of [CII] and [OI](63um) and the TIR. The line maps were observed with Herschel/PACS. These maps have 50pc resolution and form a ~370pc wide stripe along its major axis covering the sites of bright HII regions, but also more quiescent arm and inter-arm regions from the southern arm at 2kpc galacto-centric distance to the south out to 5.7kpc distance to the north. Full-galaxy maps of the continuum emission at 24um from Spitzer/MIPS, and at 70um, 100um, and 160um from PACS were combined to obtain a map of the TIR. TIR and [CII] intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G0,obs~2 to 200 in units of the average Galactic radiation field. The binned [CII]/TIR ratio drops with rising TIR, with large, but decreasing scatter. Fits of modified black bodies (MBBs) to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the [OI] data before comparing it with models of photon dominated regions (PDRs). Most of the ratios of [CII]/[OI] and ([CII]+[OI])/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n~2x10^2 cm-3, G0~60, and and a second low-FUV solution at n~10^4 cm-3, G0~1.5. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ~1. A fraction of the gas may, however, be represented by the second solution.

研究动机与目标

• 沿M33主轴测量[ C II ] 158 μm和[ O I ] 63 μm发射的空间分布，以研究星际介质中的冷却机制。
• 量化[ C II ]/TIR与[ O I ]/TIR比值和尘埃发射（TIR）之间的关系，以理解光致分解区（PDRs）中的物理条件。
• 利用VLA H I数据和辐射转移修正，评估冷中性介质（CNM）和前景吸收对[ C II ]和[ O I ]谱线发射的贡献。
• 将观测到的谱线比值与标准PDR模型对比，以约束M33盘面中气体密度、FUV辐射场和beam填充因子。
• 评估金属度梯度或尘埃柱密度变化是否主导了在恒星形成区中观测到的[ C II ]/TIR下降。

提出的方法

• 在10–15角秒分辨率下，获取了赫歇尔/PACS对M33主轴方向上[ C II ] 158 μm和[ O I ] 63 μm发射的深度映射。
• 结合斯皮兹勒/IRS 70–160 μm连续谱数据，拟合改进的黑体（MBB）模型，推导出尘埃温度和面密度。
• 利用VLA H I数据估算冷中性介质（CNM）对[ C II ]发射的贡献，并应用K2013校正方法。
• 使用双气体层模型（冷前景层 + 温暖背景层）对[ O I ]谱线强度进行前景吸收校正，估计校正因子在高H I柱密度下可达3.3。
• 对[ O I ]吸收进行校正后，将观测到的[ C II ]/[ O I ]和([ C II ]+[ O I ])/TIR比值与PDR模型（Kaufman et al. 2006）对比，以推导物理条件。
• 将数据按TIR的0.5 dex区间分箱，分析[ C II ]/TIR和[ O I ]/TIR随远红外 luminosity 增加的趋势。

实验结果

研究问题

• RQ1 在M33的主轴方向上，[ C II ]/TIR比值如何变化？其在高TIR区域的下降由哪些物理过程驱动？
• RQ2 在M33中，观测到的[ C II ]/TIR下降在多大程度上是由于尘埃柱密度增加，而非随星系半径变化的金属度降低？
• RQ3 冷中性介质（CNM）对观测到的[ C II ]发射贡献有多大？其如何影响PDR诊断的解释？
• RQ4 前景和背景气体层如何影响[ O I ] 63 μm谱线强度？进行准确PDR建模需要哪些校正？
• RQ5 哪些PDR模型参数（密度、FUV场、beam填充因子）最能重现M33中观测到的[ C II ]/[ O I ]和([ C II ]+[ O I ])/TIR比值？

主要发现

• [ C II ]/TIR比值从低TIR区域的1.1±0.4%下降至高TIR、旋臂明亮区域的0.5±0.1%，表明其与尘埃柱密度存在强烈负相关。
• 该[ C II ]/TIR下降最合理的解释是：在尘埃温度和发射指数恒定的情况下，尘埃质量面密度上升，MBB拟合结果证实了这一点。
• 对于H II区BCLMP 691，[ C II ]/TIR的下降与[ C II ]表面亮度恒定、TIR因尘埃柱密度上升而增加一致，而非FUV场变化所致。
• [ O I ]/[ C II ]比值在~0.2至20之间变化，平均值为4.5±2.6，仅在TIR明亮区域表现出强相关性，表明稀薄气体与致密气体的激发条件不同。
• ([ C II ] + [ O I ])/TIR比值平均为0.75±0.3%，范围在0.3%至3%之间，表明这些谱线主导了M33盘面的冷却预算。
• PDR建模支持中等解，即n ~ 2×10² cm⁻³，G₀ ~ 60，暗示beam填充因子较低（~1）；而低FUV解（n ~ 10⁴ cm⁻³，G₀ ~ 1.5）因填充因子过高而不合理。