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arXiv 1002.3334astro-ph.GA
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星暴演化:明亮IRAS星系射电光谱中的自由-自由吸收

Starburst evolution: free-free absorption in the radio spectra of luminous IRAS galaxies

M. S. Clemens, A. Scaife, O. Vega, A. Bressan

摘要:

我们描述了在244和610 MHz的20个明亮和超明亮的IRAS星系的样本的射电观测。这些是31个物体样本的一个子集,这些物体具有至少23 GHz的良好测量的无线电频谱。这些天体在1.4 GHz以下的射电谱表现出多种多样的形式,很少是在较高频率下同步辐射谱的简单幂律外推。大多数这类天体的射电频谱都有频谱翻转或弯曲。我们解释这些光谱的自由-自由吸收在星爆环境。有几个天体的射电光谱有两个分量,在不同的频率上有自由-自由转换(包括Arp 220和Arp 299),这表明同步辐射来自不同的辐射测量区域。在这些来源中,使用一个简单的模型的超新星速率,我们估计的时间,同步辐射受到电离气体的强自由-自由吸收,并比较这与预期的HII区域的寿命。我们发现,电离气体的寿命是一个数量级大于合理的寿命为个别HII区域。我们讨论了这一结果的影响,并认为,这些来源有一个显着的无线电组件具有较强的自由-自由吸收是那些恒星形成率仍在增加随着时间的推移。我们注意到,如果电离损失修改固有的同步加速器光谱,使其陡峭的更高的频率,经常观察到的赤字通量高于~10 GHz将大大减少。

英文摘要:

We describe radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set of a sample of 31 objects that have well-measured radio spectra up to at least 23 GHz. The radio spectra of these objects below 1.4 GHz show a great variety of forms and are rarely a simple power-law extrapolation of the synchrotron spectra at higher frequencies. Most objects of this class have spectral turn-overs or bends in their radio spectra. We interpret these spectra in terms of free-free absorption in the starburst environment. Several objects show radio spectra with two components having free-free turn-overs at different frequencies (including Arp 220 and Arp 299), indicating that synchrotron emission originates from regions with very different emission measures. In these sources, using a simple model for the supernova rate, we estimate the time for which synchrotron emission is subject to strong free-free absorption by ionized gas, and compare this to expected HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII regions. We discuss the implications of this result and argue that those sources which have a significant radio component with strong free-free absorption are those in which the star formation rate is still increasing with time. We note that if ionization losses modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.

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