摘要
活动星系核是一类存在着剧烈物理过程的天体,从射电到高能辐射的全电磁波段都有均匀辐射,这使得它成为各个波段探测的重要对象.活动星系核的高光度使得我们能够对高红移天体的光谱进行细致研究.因此,研究活动星系核对于理解星系的形成、结构和演化有重要意义.目前,活动星系核的研究已经成为现代天体物理研究的热点之一.活动星系核中大约有10%是射电噪的,伴随着相对论性喷流.射电星系根据形态的不同可以分为FR I和FR II两种,其中FR I是边缘暗的射电结构,而FR II是在边缘增亮型射电结构,在射电瓣的边缘处可以看到明亮的热斑.FR II的射电功率通常高于FR I,在P178 MHz=1025 W/Hz处分界.目前,导致它们在观测上表现出不同的原因并不清楚,部分原因是喷流与周围介质的相互作用导致的,也可能是因为FR I和FR II的中央引擎吸积模式不同导致的.其实,射电星系除了可以根据射电形态进行分类之外,还可以根据其光学光谱分成高激发射电星系和低激发射电星系两种:其中,高激发星系的[O III]/Hα>0.2以及[O III]发射线的等值宽度EW>3 A,而低激发星系的[O III]发射线相对较弱.射电星系的两种分类不是完全一致的,虽然高激发星系的核光度普遍高于低激发星系,但是仍有小部分低激发星系是明亮的射电源,并且具有典型的FR II射电形态.
Active galactic nucleus is a kind of astronomical object with dramatic physical processes.Their radiation covers from radio to high-energy bands,which makes it become an important object for research at all electromagnetic bands.The high luminosities of active galactic nucleus enable us to proceed a detailed spectroscopic research of the objects at high redshift.The central structures and activities of active galactic nucleus are important for understanding the formation,structure,and evolution of galaxies.At present,the research of active galactic nuclei has become one of hot topics in the modern astrophysics.About 10%of the active galactic nuclei are radio-loud,showing a highly collimated,relativistic radio jet.Radio galaxies are classified as FR I or FR II sources according to their radio morphology.FR I radio galaxies have edge-darkened radio structure,while FR II galaxies are defined by edge-brightened radio jets terminating in compact hot spots.The jet power of FR II radio galaxies is systematically higher than FR Is.The transition luminosity for FR I/II is at P178MHz=1025 W/Hz.So far the reason of such difference between FR I and FR II galaxies is still unclear.There are different interpretations for the FR dichotomy,such as,the interaction of the jet with the ambient medium or the intrinsic nuclei properties of accretion.Besides the FR I/II dichotomy,radio galaxies can also be classified by their spectroscopic nuclear properties into high excitation galaxies(HEG)and low excitation galaxies(LEG):high excitation galaxies have[O III]/Hα>0.2 and equivalent width(EW)of[O III]>3?A,while LEGs have relatively weaker[O III].The two classification methods are not consistent.Although HEGs have systematically higher nuclear luminosities than LEGs,a few LEGs are bright radio sources and have typical FR II morphology distinguishable from HEGs.
作者
胡京馥
HU Jing-fu(Shanghai Astronomical Observatory,Chinese Academy of Sciences,Shanghai 200030)
出处
《天文学报》
CSCD
北大核心
2018年第4期124-126,共3页
Acta Astronomica Sinica
作者简介
胡京馥,2016-07-01获得博士学位,导师:上海天文台曹新伍研究员;xwcao@shao.ac.cn。
