THE VIRUS-P EXPLORATION OF NEARBY GALAXIES (VENGA): THE XCO GRADIENT IN NGC 628
THE VIRUS-P EXPLORATION OF NEARBY GALAXIES (VENGA): THE XCO GRADIENT IN NGC 628
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DOI：
10.1088/0004-637x/764/2/117
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发表时间：
2012-12
期刊：
影响因子：
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通讯作者：
G. Blanc;A. Schruba;N. Evans;S. Jogee;A. Bolatto;A. Leroy;Mimi Song;R. V. D. van den Bosch;N. Drory;M. Fabricius;D. Fisher;K. Gebhardt;A. Heiderman;I. Marinova;S. Vogel;T. Weinzirl
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文献类型：
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作者：
G. Blanc;A. Schruba;N. Evans;S. Jogee;A. Bolatto;A. Leroy;Mimi Song;R. V. D. van den Bosch;N. Drory;M. Fabricius;D. Fisher;K. Gebhardt;A. Heiderman;I. Marinova;S. Vogel;T. Weinzirl
我们测量了NGC 628中12CO（1-0）到H2转换因子（XCO）的径向分布。利用VENGA积分场光谱的Hα发射来绘制恒星形成速率（SFR）表面密度（ΣSFR）。我们通过反转分子恒星形成定律（SFL）从ΣSFR估计出分子气体表面密度（ΣH2），并将其与CO强度进行比较以测量XCO。我们使用不同的SFR示踪剂（Hα与远紫外+ 24 μm）和不同望远镜（单碟望远镜和干涉仪）的CO图，通过改变SFL的斜率来研究系统不确定性的影响。观测到的XCO剖面对这些系统是稳健的，从R ~ 7 kpc到星系中心下降了2倍，并且通过梯度Δlog(XCO) = 0.06±0.02指数kpc−1很好地拟合。我们研究了XCO随金属丰度、气体密度和电离参数变化的变化。理论模型表明，XCO中的梯度可以用金属丰度减小和ΣH2随半径减小的组合来解释。来自局部紫外辐射场的光电加热似乎有助于高密度区域XCO的降低。我们的研究结果表明，星系环境在恒星形成区域的物理条件中起着重要的作用，特别是分子络合物中碳的化学性质，以及CO排放的辐射转移。当气体表面密度或金属丰度发生较大变化时，我们告诫不要采用单一的XCO值。
We measure the radial profile of the 12CO(1–0) to H2 conversion factor (XCO) in NGC 628. The Hα emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (ΣSFR). We estimate the molecular gas surface density (ΣH2) from ΣSFR by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure XCO. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (Hα versus far-UV plus 24 μm), and CO maps from different telescopes (single-dish and interferometers). The observed XCO profile is robust against these systematics, drops by a factor of two from R ∼ 7 kpc to the center of the galaxy, and is well fit by a gradient Δlog(XCO) = 0.06 ± 0.02 dex kpc−1. We study how changes in XCO follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in XCO can be explained by a combination of decreasing metallicity, and decreasing ΣH2 with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of XCO in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star-forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single XCO value when large changes in gas surface density or metallicity are present.