揭示汉尼的Voorwerp：IC 2497的射电观测
Revealing Hanny's Voorwerp: radio observations of IC 2497
摘要：
我们目前的多波长无线电观测的螺旋星系IC 2497和邻近的发射星云被称为“汉尼的Voorwerp”的方向。我们的WSRT连续观测在1.4 GHz和4.9 GHz，揭示了扩展的发射在星云的位置的存在下，虽然大部分的发射在星系的中心仍然没有解决。e-VLBI 1.65GHz观测表明，IC 2497中存在一个毫角秒量级的暗中心致密源，其亮温度超过4 E5 K。利用WSRT，我们在IC 2497附近探测到大量中性氢。IC 2497以南有一个总质量为5.6E9 Msol的云团，包括汉尼的沃尔沃普。另一个云复合体位于IC 2497以西约100 kpc的一个小星系群，质量为2.9E9 Msol。我们的数据暗示了两个复合体之间的物理联系。我们还检测HI的吸收对中央连续源IC 2497。我们的观测结果强烈支持这一假设，即汉尼的Voorwerp正在被IC 2497中心的活动星系核电离。在这种情况下，与活动星系核相关的等离子体喷流，清除了一条通过ISM/IGM的路径，朝着星云的方向。大尺度射电连续辐射可能来源于该喷流与Hanny's Voorwerp所嵌入的大云复合体之间的相互作用。HI的运动学不符合有规律的自转，因此IC 2497周围的云团可能是潮汐成因的。从HI对中心源的吸收，我们推导出HI柱密度的下限为2.8E21 +-0.4E21atoms/sqcm。然而，假设检测到的气体是非标准条件，我们不能排除IC 2497中心的活动星系核是康普顿厚度的可能性。
英文摘要：
We present multi-wavelength radio observations in the direction of the spiral galaxy IC 2497 and the neighbouring emission nebula known as "Hanny's Voorwerp". Our WSRT continuum observations at 1.4 GHz and 4.9 GHz, reveal the presence of extended emission at the position of the nebulosity, although the bulk of the emission remains unresolved at the centre of the galaxy. e-VLBI 1.65 GHz observations show that on the milliarcsecond-scale a faint central compact source is present in IC 2497 with a brightness temperature in excess of 4E5 K. With the WSRT, we detect a large reservoir of neutral hydrogen in the proximity of IC 2497. One cloud complex with a total mass of 5.6E9 Msol to the South of IC 2497, encompasses Hanny's Voorwerp. Another cloud complex is located at the position of a small galaxy group ~100 kpc to the West of IC 2497 with a mass of 2.9E9 Msol. Our data hint at a physical connection between both complexes. We also detect HI in absorption against the central continuum source of IC 2497. Our observations strongly support the hypothesis that Hanny's Voorwerp is being ionised by an AGN in the centre of IC 2497. In this scenario, a plasma jet associated with the AGN, clears a path through the ISM/IGM in the direction towards the nebulosity. The large-scale radio continuum emission possibly originates from the interaction between this jet and the large cloud complex that Hanny's Voorwerp is embedded in. The HI kinematics do not fit regular rotation, thus the cloud complex around IC 2497 is probably of tidal origin. From the HI absorption against the central source, we derive a lower limit of 2.8E21 +- 0.4E21 atoms/sqcm to the HI column density. However, assuming non-standard conditions for the detected gas, we cannot exclude the possibility that the AGN in the centre of IC 2497 is Compton-thick.
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