極亮 X-射線源光度函數以及其宿主星系性質之研究

在本篇論文中，我們將研究極亮X-射線源（ultraluminous X-ray sources, Lx > 1039 erg/s）光度函數（luminosity function）以及其宿主星系（host galaxy）的關係。我們將使用錢卓X-射線望遠鏡（Chandra X-ray Observatory）截至目前為止最大量的觀測資料以及新的方法，在不同的宿主星系基本性質間作比較，並證實之前其他人的研究結果。我們的極亮X-射線源數據主要來自5個不同的目錄，而全部的目錄都是使用錢卓望遠鏡的觀測。統計過後，我們有339個都在1億秒差距（100Mpc）內的極亮X-射線源，我們稱這樣的樣本為 ‘全部的極亮X-射線源樣本’（All ULXs sample）；相對於此，我們更進一步篩選並建立了一個其全部可以被錢卓望遠鏡的電荷耦合元件（Charge Couple Device, CCD）完整觀測的樣本，其範圍在D25等光度橢圓（D25 isophotal ellipse）內我們稱之為 ‘完整的極亮X-射線源樣本’（Complete ULXs sample）。這兩個樣本的篩選條件都高於4σ偵測值（detection significance）。其X-射線通量是藉由皮姆斯（Portable, Interactive Multi-Mission Simulator, PIMMS）的估計得出，而當同一極亮X-射線源具有多次觀測資料時，我們將使用其X-射線通量（X-ray flux）最亮的一次觀測製作成X-射線光度函數。除此之外，我們同時也將探討來自不同模型的光譜擬合（spectral fitting）的結果，例如幂次（power law）以及盤狀黑體輻射（disk blackbody）。我們使用四種不同的指標來代表星系的性質，星系形態（galaxy morphology）、恆星誕生率（star formation rate, SFR）由愛惹斯（Infrared Astronomical Satellite, IRAS）的數據估計而來；星系恆星質量（galaxy stellar-mass）由已修正 B 波段星等（BT0）來估計，其已修正銀河系內外消光（galactic and internal extinction）、紅移（redshift）以及傾角（inclination angle），星等數據我們將直接使用第三代亮星系目錄（Third Reference Catalog of Bright Galaxies, RC3）；至星系中心距離（galactocentric distance），我們藉由考慮星系傾角計算得出，我們並發現至中心距離分佈和銀河系中的高質量X-射線雙星（High Mass X-ray Binaries, HMXBs）的相似性。我們分別為了全部的以及完整的極亮X-射線源樣本，建立了兩種的X-射線光度函數，並且使用幂次以及指數截止幂次（exponential cut-off power law）模型進行擬合分析，在這兩種樣本中，再進一步以上述四種星系基本性質建立子樣本（sub-sample）。這是人類歷史上首次使用X-射線光度函數針對極亮X-射線源來進行星系基本性質的研究，並發現截止（cut-off）這樣的特性出現在所有的X-射線光度函數中，因此這可以看成是一個無關於星系基本性質的，而是屬於極亮X-射線源本身的獨立特性。這同時也是第一次，我們提供了根據不同星系基本性質而區分出的數量密度（number density）。擁有最高數量密度的環境是在不規則或奇特星系（irregular/peculiar galaxy）以及高恆星誕生率的子樣本中，更特別的是，以恆星誕生率做為區分的子樣本中顯示：恆星誕生率為影響極亮X-射線源亮度的重要指標。因此我們得到第一個結論是，恆星誕生率是一個對於極亮X-射線源數量密度以及亮度極為重要的因素。同時，我們根據 Kolmogorov-Smirnov 統計測試法（KS-test），來測試兩個子樣本間的差異性，並發現不論是在全部的以及完整的極亮X-射線源樣本裡，最大的差異依舊是發生在根據以恆星誕生率做為區分的子樣本之間。以X-射線光譜特性指標（soft and hard X-ray colors or HR1 & HR2）的統計結果顯示，光譜的特性在任何子樣本間並無顯著的差異。我們同時也根據擁有更多細節（high quality）的X-射線光譜形狀（spectral shape）去比較宿主星系間的基本性質，同樣地，並沒有發現任何顯著的差異。我們同時也簡單比較真．超激亮X-射線源（hyperluminous X-ray sources, HLXs）的宿主星系以及極亮X-射線源的宿主星系，基於我們只有少量的真．超激亮X-射線源的觀測資料下，目前我們發現只有兩個真．超激亮X-射線源的宿主星系有愛惹斯的觀測結果，並且根據遠紅外線（far-infrared, FIR）的數據顯示，此兩個真．超激亮X-射線源的宿主星系都是屬於高恆星誕生率或是活躍星系（active galaxy）。基本上，此一結果和恆星誕生率對於極亮X-射線源的亮度有重要的影響力一致。

關鍵字

極亮X-射線源；光度函數；錢卓X-射線望遠鏡；恆星誕生率；星系恆星質量；至星系中心距離； KS統計測試法

並列摘要

We present a study of X-ray luminosity functions (XLFs) of ultraluminous X-ray source (ULX) candidates (Lx >10^39 erg/s) and their host galaxy properties. In this work, we present the largest Chandra X-ray observations to date and new methods to compare different ULX host galaxy properties and reconfirm the results from previous studies. Our ULX samples are a collection of 5 different catalogs based on Chandra observations with a total of 339 ULXs within 100 Mpc, and we called it ‘All ULXs’ sample. For comparison, we also build a ‘Complete ULXs’ sample consisting candidates above 4 sigma detection significance, within D25 isophotal ellipse and the whole galaxy is fully covered by Chandra observations. We select the maximum flux of a source estimated by the Portable, Interactive Multi-Mission Simulator (PIMMS) to build XLFs as the parent sample or ‘All ULXs’ sample. In addition to the XLFs of ULXs, we also report X-ray spectral fitting results with power law and disk blackbody models. To characterize the host galaxy properties, we estimate galaxy-wide star formation rate (SFR) based on IRAS observations, while stellar-mass is estimated from the B magnitude (B_T^0) corrected for galactic and internal extinction, and for redshift in the Third Reference Catalog of Bright Galaxies (RC3). We calculate the galactocentric distance of ULXs to the center of the host galaxy by considering inclination angle based on the major-to-minor ratio in the RC3, and find that it is similar to high-mass X-ray binaries (HMXBs) in our Galaxy. We build XLFs in both ‘All ULXs’ and ‘Complete ULXs’ samples and fit them with both power law and exponential cut-off power law models according to morphology, SFR, stellar-mass and galactocentric distance. This is the first time to be done by using XLFs for studying ULX’s host galaxy properties. We find that an obvious cut-off feature in XLFs and suggest that it could be a global feature in terms of host galaxy properties. We select ULXs to construct the corresponding XLFs. For the first time, we also give a number density of ULXs according to different host galaxy properties. The most denser environment are ULXs in irregular/peculiar and high SFR group and we find that the XLFs are significantly difference for high and low SFR. It suggests that SFR is the most important factor than others to affect both number density and luminosity of ULXs. We also apply KS-test to test whether two groups/sub-samples are from the same parent distribution. The results suggest that there is a significant difference between high and low SFR groups in both ‘All ULXs’ and ‘Complete ULXs’ samples. The average HR1 and HR2 colors show no significant difference between groups. We also compare host galaxy properties to each other according to different X-ray spectral shape. These results suggest that the spectral shape may not be affected by different host galaxy properties. However, the luminosity can be affected by host galaxy properties. We also briefly compared the host galaxies of hyperluminous X-ray sources (HLXs) and ULXs. Two of the HLX candidates have IRAS observations. We find that they are located in high SFR galaxy or active galaxy. It would be generally consistent with our results that SFR is crucial factor to determine the luminosity of ULX.

並列關鍵字

ULX ； luminosity function ； Chandra X-ray Observatory ； SFR ； stellar-mass ； galactocentric distance ； KS-test

參考文獻

Begelman, M. C. 2002, The Astrophysical Journal, 568, L97

Buat, V., & Xu, C. 1996, Astronomy and Astrophysics, 306, 61

Bukhmastova, Y. L. 2001, Astronomy Reports, 45, 581

Cash, W. 1979, Astrophysical Journal, 228, 939

Chiang, Y.-K., & Kong, A. K. H. 2011, Monthly Notices of the Royal Astronomical Society, 414, 1329

國際替代計量

極亮 X-射線源光度函數以及其宿主星系性質之研究

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