射電星系的中央引擎和統(tǒng)一模型研究

胡京馥

(中國科學(xué)院上海天文臺上海200030)

活動星系核是一類存在著劇烈物理過程的天體,從射電到高能輻射的全電磁波段都有均勻輻射,這使得它成為各個波段探測的重要對象.活動星系核的高光度使得我們能夠?qū)Ω呒t移天體的光譜進(jìn)行細(xì)致研究.因此,研究活動星系核對于理解星系的形成、結(jié)構(gòu)和演化有重要意義.目前,活動星系核的研究已經(jīng)成為現(xiàn)代天體物理研究的熱點之一.

活動星系核中大約有10%是射電噪的,伴隨著相對論性噴流.射電星系根據(jù)形態(tài)的不同可以分為FR I和FR II兩種,其中FR I是邊緣暗的射電結(jié)構(gòu),而FR II是在邊緣增亮型射電結(jié)構(gòu),在射電瓣的邊緣處可以看到明亮的熱斑.FR II的射電功率通常高于FR I,在P178MHz=1025W/Hz處分界.目前,導(dǎo)致它們在觀測上表現(xiàn)出不同的原因并不清楚,部分原因是噴流與周圍介質(zhì)的相互作用導(dǎo)致的,也可能是因為FR I和FR II的中央引擎吸積模式不同導(dǎo)致的.其實,射電星系除了可以根據(jù)射電形態(tài)進(jìn)行分類之外,還可以根據(jù)其光學(xué)光譜分成高激發(fā)射電星系和低激發(fā)射電星系兩種:其中,高激發(fā)星系的[O III]/Hα>0.2以及[O III]發(fā)射線的等值寬度EW>3?A,而低激發(fā)星系的[O III]發(fā)射線相對較弱.射電星系的兩種分類不是完全一致的,雖然高激發(fā)星系的核光度普遍高于低激發(fā)星系,但是仍有小部分低激發(fā)星系是明亮的射電源,并且具有典型的FR II射電形態(tài).

在第2章中,我們選取了111個紅移小于0.3的射電星系并研究他們的活動核中心性質(zhì).樣本中黑洞的質(zhì)量是利用寄主星系的速度彌散以及寬線區(qū)發(fā)射線的寬度估計的.我們統(tǒng)計發(fā)現(xiàn)表征高低激發(fā)線相對強(qiáng)度的激發(fā)指數(shù)和愛丁頓比之間呈現(xiàn)明顯的正相關(guān).考慮到窄發(fā)射線區(qū)的尺度與光度之間的經(jīng)驗關(guān)系我們利用CLOUDY程序作了光致電離模型計算發(fā)現(xiàn),理論結(jié)果與觀測結(jié)果相符合.模型計算中我們討論了帶有明顯大藍(lán)包和沒有大藍(lán)包兩種不同的SED模板,但是模型計算結(jié)果顯示其對SED譜形并不敏感.這意味著低激發(fā)星系和高激發(fā)星系之間的不同性質(zhì)并不是吸積模型的不同導(dǎo)致的.我們認(rèn)為,也許這兩類星系之間的不同是由于窄線區(qū)尺度的不同造成的.活動星系核的統(tǒng)一模型認(rèn)為所有的活動星系核都有相同的基本結(jié)構(gòu):中央超大質(zhì)量黑洞、吸積盤以及在某些觀測方向能遮擋中心核輻射的塵埃環(huán).據(jù)此,可以將活動星系核分成兩類:可以直接觀測到吸積盤輻射的I型活動星系核(光學(xué)光譜中具有明顯寬發(fā)射線)和寬線區(qū)被塵埃環(huán)遮擋的II型活動星系核,其光學(xué)光譜中只能觀測到窄發(fā)射線.

曹新伍等(見Cao X W,Rawlings S.MNRAS,2004,349:1419)通過研究一個3CR的射電星系樣本發(fā)現(xiàn),FR I射電源的噴流功率大于理論上ADAF吸積盤可以得到的最大噴流功率,因此他們認(rèn)為這些FR I射電星系的吸積模式是標(biāo)準(zhǔn)薄盤而不是ADAF,其光學(xué)核和寬線區(qū)是被塵埃環(huán)遮擋了.為了檢驗這些源是否存在塵埃環(huán),第3章中我們用CLOUDY程序模擬3C 288射電源的光致電離模型,發(fā)現(xiàn)理論得到的電離光度大于實際觀測到的光學(xué)核光度,[O III]發(fā)射線光度的理論值高于觀測值,這說明在3C 288中確實存在遮擋效應(yīng).為了進(jìn)一步檢驗這個結(jié)論,本章又根據(jù)塵埃消光來驗證高噴流射電功率星系3C288中的遮擋效應(yīng)是塵埃環(huán)造成的.

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 de fined 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=1025W/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.

In chapter 2,we use a sample of 111 radio galaxies with redshift z<0.3 to investigate their nuclear properties.The black hole masses of the sources in this sample are estimated with the velocity dispersion of the galaxies,or the width of the broad-lines.We find that the excitation index,or the relative intensity of low and high excitation lines,is correlated with the Eddington ratio for this sample.Considering of the empirical relation between the size of the narrow-line region(NLR)and the ionizing luminositywe find that the correlation between the excitation index and the Eddington ratio can be reproduced by the photoionization models using CLOUDY.We adopt two sets of spectral energy distributions(SEDs),with or without a big blue bump in ultraviolet as the ionizing continuum,and find that the modelled correlation between the excitation index and the Eddington ratio is insensitive to the used SED.This means that the difference between high and low excitation galaxies is not caused by the different accretion modes in these sources.Instead,it may be caused by the NLR size.

The unified model of active galactic nuclei(AGN)proposes that all AGNs are composed of the same basic ingredients:a super massive black hole and accretion disk,surrounded bya toroidal mass of dust and gas that obscures the nucleus from some lines of sight.Active galactic nucleus can be separated into “Type 1” objects and “Type 2”objects. “Type 1”objects(those with very broad emission lines in their optical spectra)are those in which we have a direct view of the hot,fast-moving material close to the accretion disk.In “Type 2”objects the torus obscures this material,and we see only narrow emission lines in the optical spectra.

Cao et al.(see Cao X W,Rawlings S.MNRAS,2004,349:1419)analyzed the optical and radio properties of a sample of 3CR FR I radio galaxies,and found that the jet power for more than one-third of the sources in the sample is greater than the maximal jet power extracted from a rapid spinning black hole with advection dominated accretion flow(ADAF)accretion disk.They argued that a significant fraction,at least for the highjet-power sources,of 3CR FR I radio galaxies have standard thin discs,but their optical core and broad line regions can be hidden by torus.In order to examine the existence of torus in the high-jet power sources,we carry out the photoionization model calculations for the radio galaxy 3C 288 using CLOUDY in chapter 3.We found that the theoretical ionizing luminosity is larger than the observed optical core luminosity,as well as the[O III]luminosity.Further,we also calculate the column density by dust extinction.We conclude that there is indeed obscuring torus in 3C288.