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arXiv 1408.6296astro-ph.GA
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Galerkin的积分射电连续谱

Integrated Radio Continuum Spectra of Galaxies

Joshua Marvil, Frazer Owen, Jean Eilek

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13 pages, 13 figures, 5 tables; accepted for publication in the Astronomical Journal
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摘要：

我们调查的光谱形状的总连续辐射，在74 MHz和5 GHz（400至6厘米的波长）之间，为一个大样本的明亮星系。我们利用VLA低频巡天、Westerbork北方巡天、NRAO VLA巡天和Green Bank 6 cm巡天的重叠调查覆盖范围，实现了比以前更好的分辨率、灵敏度和样本量。对于我们的250个明亮星系的样本，我们测量的平均光谱指数，$α$，为-0.69之间的1.4和4.85 GHz，-0.55之间的325 MHz和1.4 GHz，和-0.45之间的74和325 MHz，这相当于在平均光谱的曲率检测。当用具有恒定曲率的广义函数拟合时，该曲率的大小约为每对数频率十年${Δα}$ = -0.2。在我们的数据中，低频光谱平坦化与星系倾角的关系没有明显的趋势，这表明自由-自由吸收并不是对观测到的曲率的令人满意的解释。热辐射与非热辐射的比值是通过两种独立的方法估算的，（1）使用IRAS远红外通量，（2）使用总光谱指数值。方法（1）得到的1.4 GHz热分数分布为9% ${\pm}$ 3%，这与以前的研究一致，而方法（2）得到的平均1.4 GHz热分数为51%，色散为26%。方法（2）产生的非常不合理的值表明，典型的幂律热和非热分量的总和对于325和1.4 GHz之间的总频谱指数不是一个可行的模型。

英文摘要：

We investigate the spectral shape of the total continuum radiation, between 74 MHz and 5 GHz (400 to 6 cm in wavelength), for a large sample of bright galaxies. We take advantage of the overlapping survey coverage of the VLA Low-Frequency Sky Survey, the Westerbork Northern Sky Survey, the NRAO VLA Sky Survey, and the Green Bank 6 cm survey to achieve significantly better resolution, sensitivity, and sample size compared to prior efforts of this nature. For our sample of 250 bright galaxies we measure a mean spectral index, $α$, of -0.69 between 1.4 and 4.85 GHz, -0.55 between 325 MHz and 1.4 GHz, and -0.45 between 74 and 325 MHz, which amounts to a detection of curvature in the mean spectrum. The magnitude of this curvature is approximately ${Δα}$ = -0.2 per logarithmic frequency decade when fit with a generalized function having constant curvature. No trend in low frequency spectral flattening versus galaxy inclination is evident in our data, suggesting that free-free absorption is not a satisfying explanation for the observed curvature. The ratio of thermal to non-thermal emission is estimated by two independent methods, (1) using the IRAS far-IR fluxes, and (2) with the value of the total spectral index. Method (1) results in a distribution of 1.4 GHz thermal fractions of 9% ${\pm}$ 3%, which is consistent with previous studies, while method (2) produces a mean 1.4 GHz thermal fraction of 51% with dispersion 26%. The highly implausible values produced by method (2) indicate that the sum of typical power-law thermal and non-thermal components is not a viable model for the total spectral index between 325 and 1.4 GHz.

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