Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
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DOI：
10.1086/504035
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发表时间：
2006-01
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影响因子：
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通讯作者：
T. Thompson;E. Quataert;E. Waxman;N. Murray;C. Martin
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文献类型：
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作者：
T. Thompson;E. Quataert;E. Waxman;N. Murray;C. Martin
我们估计了一个星系样本的最小能量磁场(Bmin)，这些星系的气体表面密度是从正常螺旋到发光星暴的四个数量级以上。结果表明，随着表面密度的增加，ISM中的最小能量磁压与总压之比显著减小。对于超高亮度红外星系Arp 220，这一比例约为10-4。因此，如果最小能量估计是适用的，那么与正常的恒星形成螺旋星系相比，恒星爆发中的磁场在动力学上比重力弱。然而，我们认为，恒星爆发中相对论电子的快速冷却使最低能量估计失效。我们评估了一些独立的限制星暴星系磁场强度的因素。特别是，我们认为FIR-射电关联的存在意味着宇宙线电子的同步冷却时间标度比它们从银盘逃逸的时间短得多；这反过来又意味着星暴中的真实磁场明显大于Bmin。反对如此大的场的最有力的论据是，人们可能会认为星暴具有陡峭的射电光谱，表明强烈的同步加速器冷却，而这是没有观测到的。然而，我们发现，即使在快速冷却的情况下，电离和韧致辐射损失也可以使星暴星系的非热谱变平，从而与观测光谱更好地吻合。我们进一步证明，电离和韧致辐射损耗在塑造大多数GHz频率的星暴的射电频谱中可能是重要的，从而保持了FIR-无线电关联的线性。因此，我们得出结论，恒星爆发中的磁场比Bmin大得多。我们重点介绍了几个可以检验这一结论的观察结果。
We estimate minimum energy magnetic fields (Bmin) for a sample of galaxies with measured gas surface densities, spanning more than four orders of magnitude in surface density, from normal spirals to luminous starbursts. We show that the ratio of the minimum energy magnetic pressure to the total pressure in the ISM decreases substantially with increasing surface density. For the ultraluminous infrared galaxy Arp 220, this ratio is ~10-4. Therefore, if the minimum energy estimate is applicable, magnetic fields in starbursts are dynamically weak compared to gravity, in contrast to normal star-forming spiral galaxies. We argue, however, that rapid cooling of relativistic electrons in starbursts invalidates the minimum energy estimate. We assess a number of independent constraints on the magnetic field strength in starburst galaxies. In particular, we argue that the existence of the FIR-radio correlation implies that the synchrotron cooling timescale for cosmic-ray electrons is much shorter than their escape time from the galactic disk; this in turn implies that the true magnetic field in starbursts is significantly larger than Bmin. The strongest argument against such large fields is that one might expect starbursts to have steep radio spectra indicative of strong synchrotron cooling, which is not observed. However, we show that ionization and bremsstrahlung losses can flatten the nonthermal spectra of starburst galaxies even in the presence of rapid cooling, providing much better agreement with observed spectra. We further demonstrate that ionization and bremsstrahlung losses are likely to be important in shaping the radio spectra of most starbursts at GHz frequencies, thereby preserving the linearity of the FIR-radio correlation. We thus conclude that magnetic fields in starbursts are significantly larger than Bmin. We highlight several observations that can test this conclusion.