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The VIRUS-P Exploration of Nearby Galaxies (VENGA): The X CO Gradient in NGC 628

机译:邻近星系(VENGA)的VIRUS-P探索:NGC 628中的X CO梯度

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摘要

We measure the radial profile of the ^(12)CO(1-0) to H_2 conversion factor (X_(CO)) in NGC 628. The Hα emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (Σ_(SFR)). We estimate the molecular gas surface density (Σ_(H2)) from Σ_(SFR) by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure X_(CO). 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 X_(CO) 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(X_(CO)) = 0.06 ± 0.02 dex kpc^(–1). We study how changes in X_(CO) follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in X_(CO) can be explained by a combination of decreasing metallicity, and decreasing Σ_(H2) with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of X_(CO) 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 X_(CO) value when large changes in gas surface density or metallicity are present.
机译:我们在NGC 628中测量^(12)CO(1-0)到H_2转换因子(X_(CO))的径向轮廓。VENGA积分场光谱学中的Hα发射用于映射恒星形成率(SFR )表面密度(Σ_(SFR))。我们通过逆转分子星形成定律(SFL),从Σ_(SFR)估算分子气体表面密度(Σ_(H2)),并将其与CO强度进行比较以测量X_(CO)。我们使用不同的SFR示踪剂(Hα与远紫外线加24μm)和来自不同望远镜(单皿和干涉仪)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X_(CO)轮廓对这些系统很稳健,从R〜7 kpc到星系中心下降了两倍,并且与梯度Δlog(X_(CO))= 0.06±0.02 dex很好地拟合kpc ^(– 1)。我们研究X_(CO)的变化如何跟随金属性,气体密度和电离参数的变化。理论模型表明,X_(CO)中的梯度可以通过降低金属性和随半径减小Σ_(H2)来解释。来自局部紫外线辐射场的光电加热似乎有助于降低高密度区域的X_(CO)。我们的结果表明,银河环境在设定恒星形成区域的物理条件(尤其是分子复合物中碳的化学性质以及CO的辐射转移)方面起着重要作用。我们警告不要在气体表面密度或金属性发生较大变化时采用单个X_(CO)值。

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