Synchrotron spectral index and interstellar medium densities of star-forming galaxies
Synchrotron spectral index and interstellar medium densities of star-forming galaxies
复制标题
恒星形成星系的同步加速器光谱指数和星际介质密度
DOI：
10.1093/mnras/stv510
复制
发表时间：
2015
影响因子：
4.8
通讯作者：
Subhashis Roy
中科院分区：
文献类型：
--
作者：
A. Basu;R. Beck;P. Schmidt;Subhashis Roy
同步辐射的光谱指数是理解宇宙射线电子（CREs）和星际介质（ISM）性质的重要参数。我们在亚kpc线性尺度上测定了四个附近恒星形成星系NGC 4736、NGC 5055、NGC 5236和NGC 6946的同步加速器光谱指数（$\alpha_{\rm nt}$）。所有星系的$\alpha_{\rm nt}$都在0.33到1.4 GHz之间。我们发现光谱指数在完全中性（原子+分子）气体表面密度（$\Sigma_{\rm gas} \gtrsim \rm 50~M_\odot pc^{-2}$）的区域更平坦（$\gtrsim -0.7$），特别是在星系的臂部和内部。在含有$\Sigma_{\rm gas} \lesssim \rm 50~M_\odot pc^{-2}$的区域，特别是在星系的臂间和外部区域，光谱指数急剧变陡至$<-1.0$。$\alpha_{\rm nt}$的变平不太可能是由于0.33 GHz的无热吸收引起的。我们的结果与在$\sim0.3$ GHz以下频率发射的CREs主要受轫致辐射和/或电离损失的情况一致。对于密度较大的介质（$\Sigma_{\rm gas} \gtrsim \rm 200~M_\odot pc^{-2}$），具有强磁场（$\sim 30~\mu$ G）， $\alpha_{\rm nt}$看起来比$-0.5$平坦，这可能是由于电离损失造成的。我们发现，由于ISM的块状性质，这样密集的区域只覆盖了星系的一小部分（$\lesssim5$ ％）。因此，星系集成频谱可能不会显示出这种损耗机制的迹象，并且在广泛的无线电频率范围内（$\sim 0.1$至10 GHz之间）仍然是幂律。
The spectral index of synchrotron emission is an important parameter in understanding the properties of cosmic ray electrons (CREs) and the interstellar medium (ISM). We determine the synchrotron spectral index ($\alpha_{\rm nt}$) of four nearby star-forming galaxies, namely NGC 4736, NGC 5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The $\alpha_{\rm nt}$ was determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral index to be flatter ($\gtrsim -0.7$) in regions with total neutral (atomic + molecular) gas surface density, $\Sigma_{\rm gas} \gtrsim \rm 50~M_\odot pc^{-2}$, typically in the arms and inner parts of the galaxies. In regions with $\Sigma_{\rm gas} \lesssim \rm 50~M_\odot pc^{-2}$, especially in the interarm and outer regions of the galaxies, the spectral index steepens sharply to $<-1.0$. The flattening of $\alpha_{\rm nt}$ is unlikely to be caused due to thermal free--free absorption at 0.33 GHz. Our result is consistent with the scenario where the CREs emitting at frequencies below $\sim0.3$ GHz are dominated by bremsstrahlung and/or ionization losses. For denser medium ($\Sigma_{\rm gas} \gtrsim \rm 200~M_\odot pc^{-2}$), having strong magnetic fields ($\sim 30~\mu$G), $\alpha_{\rm nt}$ is seen to be flatter than $-0.5$, perhaps caused due to ionization losses. We find that, due to the clumpy nature of the ISM, such dense regions cover only a small fraction of the galaxy ($\lesssim5$ percent). Thus, the galaxy-integrated spectrum may not show indication of such loss mechanisms and remain a power-law over a wide range of radio frequencies (between $\sim 0.1$ to 10 GHz).