We present an analysis of the dust and gas in Andromeda, using Herschel images sampling the entire far-infrared peak. We fit a modified-blackbody model to ~4000 quasi-independent pixels with spatial resolution of ~140?pc and find that a variable dust-emissivity index (β) is required to fit the data. We find no significant long-wavelength excess above this model, suggesting there is no cold dust component. We show that the gas-to-dust ratio varies radially, increasing from ~20 in the center to ~70 in the star-forming ring at 10?kpc, consistent with the metallicity gradient. In the 10?kpc ring the average β is ~1.9, in good agreement with values determined for the Milky Way (MW). However, in contrast to the MW, we find significant radial variations in β, which increases from 1.9 at 10?kpc to ~2.5 at a radius of 3.1?kpc and then decreases to 1.7 in the center. The dust temperature is fairly constant in the 10?kpc ring (ranging from 17 to 20?K), but increases strongly in the bulge to ~30?K. Within 3.1?kpc we find the dust temperature is highly correlated with the 3.6 μm flux, suggesting the general stellar population in the bulge is the dominant source of dust heating there. At larger radii, there is a weak correlation between the star formation rate and dust temperature. We find no evidence for "dark gas" in M31 in contrast to recent results for the MW. Finally, we obtained an estimate of the CO X-factor by minimizing the dispersion in the gas-to-dust ratio, obtaining a value of (1.9 ± 0.4) × 1020?cm–2 [K km s–1]–1.
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机译：我们使用赫歇尔图像对整个远红外峰进行采样，对仙女座中的尘埃和气体进行了分析。我们将改进的黑体模型拟合到〜4000个近似独立的像素，空间分辨率约为140µpc，发现需要可变的粉尘发射率指数（β）才能拟合数据。我们发现在该模型之上没有明显的长波长超标现象，这表明没有冷尘成分。我们发现，气尘比在径向变化，从中心的〜20到恒星形成环中的〜70在10？kpc时增加，与金属梯度一致。在10？kpc环中，平均β约为1.9，与银河系（MW）的确定值非常吻合。但是，与兆瓦相反，我们发现β的径向变化很大，从10？kpc处的1.9增加到半径为3.1？kpc处的〜2.5，然后在中心减小到1.7。在10？kpc环中，粉尘温度相当恒定（从17到20？K），但在凸起时会急剧增加，达到〜30？K。在3.1kpc以内，我们发现粉尘温度与3.6μm通量高度相关，这表明凸起中的一般恒星种群是那里粉尘加热的主要来源。在较大的半径处，恒星形成速率与粉尘温度之间的相关性较弱。与最近的MW结果相反，我们没有发现M31中“暗气”的证据。最后，我们通过最小化气尘比中的弥散来获得CO X因子的估计值，得到的值为（1.9±0.4）×1020？cm–2 [K km s–1] –1。
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