| 摘要: | 在本篇論文中我研究了西弗一型星系NGC 1097的星系核以及銀河系中心的物理暨動力學性質。本篇論文總共發表了四篇國際期刊論文，分別是本論文的第三至六章。
NGC 1097的星系核是由中心分子雲團及一圈星劇增環所組成。
為了了解星爆環的動力性質以及高質量恆星劇增是如何誕生，我們使用了超高解析度的次毫米波陣列和哈伯太空望遠鏡來觀測NGC 1097星系核輻射出的一氧化碳氣體(CO(J=2-1))以及帕申譜線(Pa_alpha)。我們的高解析度CO(J=2-1)圖像首次在星劇增環解析出兩種不同性質的分子雲。第一種分子雲來自於星爆環的塵埃帶，第二種分子雲則是來自於高質量恆星誕生區。塵埃帶分子雲內部的氣體運動速度大於90公里每秒，幾乎是恆星分子雲的兩倍大。我們並發現到了塵埃帶分子雲的動力學以及物理性質與大尺度的星系動力學相關，而恆星分子雲的性質卻與星系動力學無關。此結果是首次發現，並為之後的研究以及理論提供了重要的參考。為了研究NGC 1097中心分子雲團的物理性質，我們也使用了次毫米波陣列觀測了NGC 1097的HCN(J=3-2)及HCO+(J=3-2)譜線。這兩種分子譜線所追蹤的分子氣體密度比CO高一千倍以上。這兩種分子譜線比CO更適合研究恆星形成區域。我們同時比較HCN/HCO+譜線與近紅外線(24 micron-meter)的關係。我們發現了HCN(J=3-2)通量與24 micron-meter輻射有線性的關係，而CO通量與24 micron-meter輻射卻互相沒有關係。此研究證明了高質量恆星形成主要是在星劇增環上誕生，而在塵埃帶的附近並沒有明顯的高質量恆星形成。我們也發現到了在星系核中心分子雲團的HCN豐度比星劇增環高，我們討論了此豐度的增加可能是被來自於中心分子雲團內部的活躍星系核的X-ray輻射所改變其化學性質。
我們接下來研究了銀河系中心的氣體分子物理性質。因為銀河系中心距離我們非常的近，相較於NGC 1097, 銀河系中心是一個很好的研究星系核性質的對象。過去對銀河系中心的研究大部分是利用低激發能量狀態的分子。但是低激發狀態分子卻部份被太陽系和銀河系中心之間的冷氣體吸收，造成研究樣本的偏差。在本論文中我使用了加州理工學院的次毫米波望遠鏡，以高激發能量的分子譜線觀測銀河系中心(CS(J=5-4),CS(J=4-3))。以及使用野邊山毫米波望遠鏡觀測銀河系中心的CS(J=2-1)譜線。借由此高激發能量分子譜線，我們新發現到了銀河系中心靠近大質量黑洞SgrA*的地方有分子噴流及氣泡的遺跡。我們也發現了銀河系中心的分子噴流是從銀河星系盤中被推出星系盤的證據。此分子氣泡包圍著中間的游離熱氣體。這些游離熱氣體可能是來自於數量級約十個的超新星爆發，在爆發的過程中把氣體推出去。我們推算出了這個爆發過程大約是100000年前發生的。這顯示了我們的銀河系中心在過去比現在活躍許多。;In this thesis I present studies of the kinematics and the physical properties of molecular gas galactic nuclei. Two representative bar galaxies; (1) NGC 1097, (2) our own Galactic center, are studied.
The nucleus of NGC 1097 consists of a molecular concentration, ~350 pc in scale, and a starburst ring, ~kpc in scale. To better depict the kinematics and the star formation of the nucleus, the SubmilliMeter Array (SMA) and the Hubble Space Telescope (HST) are used to obtain high angular resolution CO(J = 2-1) line and the Pa_alpha line images. The unprecedented high resolution reveals that the turbulent/diffuse molecular gas (V ~90 km/s) is associated with the dust lane. The less turbulent/dense molecular gas (V ~45 km/s) could be determined to be associated with the starburst ring. The variations of the physical properties of the molecular gas are associated with the large scale dynamics. However, the star formation rate is not significantly affected by such dynamics. For similar type of galaxies, this work initiated the quantitative measurements of the evolution of star formation in the kilo-parsec starburst ring.
The high-J dense gas in the nuclear region of NGC 1097 are investigated with the HCN(J = 3-2) and HCO+(J = 3-2) lines observed by SMA. The purpose is to resolve and study the enhancement of the HCN abundance in the vicinity of the Seyfert 1 nucleus. In the nuclear concentration of NGC 1097, the HCN(J = 3-2) line is found to contribute 30% to the total HCN(J = 3-2) line flux. A self-consistent check of the fractional abundance enhanced by X-ray ionization chemistry of the nucleus is possible with our observation, and the results are consistent with the X-ray chemical model. In addition, the HCN(J = 3-2) and HCO+(J = 3-2) emission lines are optically thin and they show tight intensity correlation with the Spitzer 24 micron-meter emission in the starburst ring. The CO(J = 3-2) line is optically thick and shows poor correlation with the 24 micron-meter emission. This suggests that the dense molecular gas and the dust are of the same origins: the star-forming region hundred-pc scale.
Next studies on the Galactic Cener (GC) are presented.
I obtained the first CS(J = 4-3) and CS(J = 5-4) maps of GC with the Caltech Submillimeter Observatory (CSO).
The main purpose is to study the polar arc, which is a molecular ridge near the SgrA region, with apparent non-coplanar motions and acceleration perpendicular to the Galactic disk. With the new high-J CS maps, a new component in the ridge smoothly connecting the Polar Arc and the Galactic disk is found. This new component is the brightest in the CS(J = 4-3) line. The physical conditions of this new component can be determined using the rotational diagrams and the statistical equilibrium calculation. I found the physical conditions (density, temperature) to produce highest opacity of this new component in the CS(J = 4-3) line. This suggests that this new component is intrinsically the brightest in the CS(J = 4–3) line.
I also lead a project to map the entire central molecular zone (CMZ) of the GC with the CS(J = 2-1) line using the Nobeyama 45m telescope. I present the early results of the central 30 pc of the CMZ in the last part of this thesis. With the low velocity molecular gas surrounding the 20 cm radio halo, I identify a possible expanding shell with a size of ~30 pc near the SgrA complex. In addition, a possible outflow expanding/accelerating perpendicular to the Galactic plane cloud be found. The time scales of these features are 100000 years, which can not be driven by the Galactic center supernova SgrA East (~10000 years). This large scale outflow could be produced by >8 supernova explosions 100000 years ago. |
