Star Formation in the Central Regions of Galaxies

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/68129

Title:	Star Formation in the Central Regions of Galaxies
Authors:	蔡孟均;Tsai,Mengchun
Contributors:	天文研究所
Keywords:	星系;星系核;演化;西佛;星際物質;恒星形成;Galaxy;AGN;Evolution;Seyfert;ISM;Star Formation
Date:	2015-08-27
Issue Date:	2015-09-23 10:47:07 (UTC+8)
Publisher:	國立中央大學
Abstract:	星系中心區域連結了星系核與宿主星系。如果星系中間的黑洞與其宿主星系有共同的演化過程，則在星系中心區域應該會留下演化證據。我們利用星系中心區域的環境特性，例如恒星形成強度，恒星族群及分子雲分佈，來建構一個可能的星系演化模型。在這篇論文中，我們首先研究活躍及普通星系中心區域的特性。我們使用星系核周邊的無線電發射強度來表現其星系核的活躍程度，及使用紅外線波段觀測來表現宿主星系中的恒星形成強度及恒星族群的分佈。我們發現到在一千秒差距半徑的區域中，恒星質量及恒星形成強度在活躍星系中較普通星系中高，但兩者有類似的特定恒星形成速度(SSFRs)。我們也發現到在紅外波段影像中有許多活躍星系擁有明顯內部結構的，這些有內部結構的目標多半為西佛星系(Seyfert)，而含有低游離核發散線區的星系(Low-Ionisation Nuclear Emission-line Region，LINER)只有極少數有內部結構。我們注意到活躍星系中，擁有內部結構者其星系核的無線電強度與其宿主星系的恒星形成率有正相關，而無內部結構者則為負相關。這些結果可能可以用來解釋恒星形成與活躍星系的演化模型。在這個可能的模型中，活躍星系核可能由星系中心的劇烈恒星形成引起，隨著星系演化，星系核強度伴隨著星系中心區域的恒星形成成長，到了演化末期，活躍星系轉變為低游離核發散線區，且星系中心區域伴隨著較弱的恒星形成。為了更深入研究內部結構，我們挑選一個最靠近及明亮且有內部結構的西佛 II 型星系來當例子。在這篇論文中，我們呈現西佛 II 型星系NGC1068 中心區域由次毫米波陣列望遠鏡(SMA)所觀測的CO(3-2)及由歐文斯毫米無線電陣列 (OVRO) 所觀測的CO(1-0)資料。這兩條譜線主要於距星系核約5角秒的區域及沿中心旋臂分佈，但其強度分佈有其不同; CO(3-2)分佈圖的峰值靠近星系核而CO(1-0)則主要分佈於旋臂區域。CO(3-2)/CO(1-0)的強度比值在星系核區域約為 3.1 ，相當於四倍旋臂區域的平均比值，表示這些分子雲受到活躍星系核相當程度的影響。另一方面，譜線強度比值在旋臂區域上可能小到0.24，大到2.34，而平均值約為0.75，這樣的數值與恒星形成區可能的譜線強度比值類似，顯示這些分子雲是受到恒星形成所影響。此外，由 Spitzer 8micron 影像所推算出的恒星形成率，我們看到在旋臂區域 CO(3-2)/(1-0) 比值與恒星形成率有相當緊密的關連性。我們也比較了旋臂上不同區域的CO(3-2)/CO(1-0)比值和恒星形成率，隨著距離星系核距離越遠，這兩個數值也逐漸變小。;The galactic central region connects the galactic nucleaeus to the host galaxy. If the central black hole co-evolved with the host galaxies, there should be some evidence left in the central region. We use the environmental properties in the central regions such as star-forming activity, stellar population and molecular abundance to figure out a possible scenario of the evolution of galaxies. In this thesis at first we investigated the properties of the central regions in the host galaxies of active and normal galaxies. We used radio emission around the nuclei of the host galaxies to represent activity of active galactic nuclei (AGNs), and used infrared ray (IR) emission to represent the star-forming activity and stellar population of the host galaxies. We determined that active galaxies have higher stellar masses (SMs) within the central kiloparsec radius than normal galaxies do independent of the Hubble types of the host galaxies; but both active and normal galaxies exhibit similar specific star formation rates (SSFRs). We also discovered that certain AGNs exhibit substantial inner stellar structures in the IR images; most of the AGNs with inner structures are Seyferts, whereas only a few LINERs exhibit inner structures. We note that the AGNs with inner structures show a positive correlation between the radio activity of the AGNs and the SFRs of the host galaxies, but the sources without inner structures show a negative correlation between the radio power and the SFRs. These results might be explained with a scenario of starburst-AGN evolution. In this scenario, AGN activities are triggered following a nuclear starburst; during the evolution, AGN activities are accompanied by SF activity in the inner regions of the host galaxies; at the final stage of the evolution, the AGNs might transform into LINERs, exhibiting weak SF activity in the central regions of the host galaxies. For further investigation about the inner structure, we choose the most nearby and luminous Seyfert galaxy with inner structure as an example. In this thesis, we present CO(3-2) interferometric observations of the central region of the Seyfert 2 galaxy NGC1068 using the Submillimeter Array, together with CO(1-0) data taken with the Owens Valley Radio Observatory Millimeter Array. Both the CO(3-2) and CO(1-0) emission lines are mainly distributed within ~5 arcsec of the nucleus and along the spiral arms, but the intensity distributions show differences; the CO(3-2) map peaks in the nucleus, while the CO(1-0) emission is mainly located along the spiral arms. The CO(3-2)/CO(1-0) ratio is about 3.1 in the nucleus, which is four times as large as the average line ratio in the spiral arms, suggesting that the molecular gas there must be affected by the radiation arising from the AGN. On the other hand, the line ratios in the spiral arms vary over a wide range from 0.24 to 2.34 with a average value around 0.75, which is similar to the line ratios of star-formation regions, indicating that the molecular gas is affected by star formation. Besides, we see a tight correlation between CO(3-2)/(1-0) ratios in the spiral arms and star formation rate surface densities derived from Spitzer 8 micron dust flux densities. We also compare the CO(3-2)/(1-0) ratio and the star formation rate at different positions within the spiral arms; both are found to decrease as the radius from the nucleus increases.
Appears in Collections:	[Graduate Institute of Astronomy] Electronic Thesis & Dissertation

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