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研究生:張敏悌
研究生(外文):Mean-Ti Chang
論文名稱(外文):On the Origin of the Radio Emission of Ultraluminous Infrared Galaxies
指導教授:黃崇源
指導教授(外文):Chorng-Yuan Hwang
學位類別:碩士
校院名稱:國立中央大學
系所名稱:天文研究所
學門:自然科學學門
學類:天文及太空科學學類
論文出版年:2005
畢業學年度:93
語文別:英文
論文頁數:52
外文關鍵詞:secondary electrons
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我們的研究重點主要是—探究次級電子(secondary electrons)在極明亮紅外星系裡所扮演的角色。一般相信,極明亮紅外星系所觀測到的1.4GHz無線電輻射,主要是來自於相對論性電子所產生的同步輻射。然而,我們對於到底是何種機制會產生如此高速的相對性電子,則一無所知。由觀測顯示,大多數的極明亮紅外星系,不但擁有很高的恆星誕生率,同時也有相較於一般正常星系更多的分子雲含量。在另一方面,由於次級電子的產生率,和宇宙射線流以及物質的密度有著正比的關係,因此我們認為在極明亮紅外星系中,由這些次級電子所產生的無線電輻射量必定是不可忽略的。我們所考慮的是一個由宇宙射線質子和分子雲質子碰撞後,進而產生次級電子的穩態方程式。我們藉由理論模型和觀測資料的比對後,發現在某些極明亮紅外星系裡,這些次級電子對於在1.4GHz所觀測到的無線電輻射量,具有顯著性的貢獻。
我們的研究同時也發現,對於那些由大量恆星誕生所觸發的極明亮紅外星系,其中心存在一個緻密的15GHz無線電源,可能是來自於自由-自由的熱輻射(thermal bremsstrahlung)。對於某些極明亮紅外星系,不管觸發機制是活躍星系核、或是大量恆星誕生,其中心所發現的15GHz無線電源,一般均認為和活躍星系核有密切相關。然而,對於這兩種由不同機制所產生的極明亮紅外星系,在比較完它們的無線電譜指數(radio spectral index)後,我們發現兩者的分佈,在統計上有顯著性的不同。這意謂著它們的來源可能是不一樣的。我們的計算結果顯示,對於由大量恆星誕生所觸發的極明亮紅外星系而言,其中心的15GHz無線電源,可以是來自於大量恆星誕生所產生的自由—自由熱輻射(thermal bremsstrahlung),而不是一般認為由活躍星系核所產生的非熱輻射。
We investigate the role of secondary electrons in ultraluminous infrared galaxies. The radio emission in ULIGs is believed to be produced by synchrotron radiation of relativistic electrons. Nonetheless, the source of these relativistic electrons are still unclear. Most of the ULIGs usually have enhanced star formation rates and also posses a huge amount of molecular clouds, which are usually concentrated in a small central region. Since the production rates of secondary electrons are proportional to the cosmic ray fluxes and the density of the ambient gas, the radio emission of secondary electrons in ULIGs might be important. We assume a staedy-state model for the secondary electrons generated from the interaction of the cosmic ray hadrons with dense molecular clouds in ULIGs. We estimate the contribution of secondary electrons in ULIGs by comparing observational results with theoretical calculations for the radio emission in these sources. We find that, for some ULIGs, radio contribtuion from secondary is significant.
We also investigate the radio cores at 15GHz in star-forming ULIGs and find that thermal bremsstrahlung could be the possible mechanism. Compact radio cores at 15GHz are observed both in seyfert-like and star-forming ULIGs. Our calculation shows that radio cores at 15GHz in star-forming ULIGs cloud be from thermal bremsstrahlung other than non-thermal radiation related to AGN.
1 Introduction 1
1.1 Ultraluminous Infrared Galaxies . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Observational Properties of the Molecular Clouds . . . . . . . . . . 2
1.2 Non-thermal Radio Emission . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 The Importance of the Secondary Electrons . . . . . . . . . . . . . . . . . 3
2 Model 6
2.1 Steady-State Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Production Rate of Secondary Electrons . . . . . . . . . . . . . . . . . . . 7
2.3 Cooling Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Synchrotron Radiation of the Secondary Electrons . . . . . . . . . . . . . . 11
3 Comparison with Observation 12
3.1 Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Constraint on Source of Radio Emission . . . . . . . . . . . . . . . . . . . 16
3.3 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Discussion 19
4.1 The FIR/Radio Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Magnetic Field Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.3 Proton Spectrum Assumption . . . . . . . . . . . . . . . . . . . . . . . . . 20
5 The E®ects of AGN on the Radio Emission of ULIGs 24
5.1 The Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.2 The Detection of Radio Cores . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3 The FIR/Radio Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.4 The Origin of Radio Emission at 1.4GHz . . . . . . . . . . . . . . . . . . . 28
5.5 Are Radio Cores AGN Related . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.5.1 Radio Spectral Index . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.5.2 Thermal Bremsstrahlung of Compact Starburst . . . . . . . . . . . 29
5.5.3 Super Giant HII Region . . . . . . . . . . . . . . . . . . . . . . . . 30
6 Conclusion and Summary 41
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