觀測宇宙學暑期研究組會

快速電波爆發的成因及相關應用

時間:109年08月11日(星期二)15:30至17:30

地點:綜合二館天文所501

導讀人:橋本哲也

成員:後藤友嗣、金成鎮、橋本哲也、Alvina、何建彰、王亭文、呂亭誼、桑戴瑞、蕭予揚、林于瑋、凌志騰、吳耕緯、王柏雅、古翊樺

紀錄人:蕭予揚、橋本哲也、王亭文

導讀人介紹:

博士後研究員,專長:快速電波爆發及各種爆發的相關物理。

讀書會進度內容:

快速電波爆發的成因及相關應用、

討論紀錄:

橋本哲也 is recently working on revealing the origins of fast radio bursts (FRBs), which are new astronomical transients. FRBs are mysterious radio signals coming from distant galaxies. Understanding FRBs is one of the central foci in astronomy. Based on FRBs detected with Parkes and CHIME radio telescopes, 橋本哲也 applied so-called 'luminosity function', which is often used in galaxy astronomy, to FRBs. This new approach, for the first time, enabled us to find that occurrence rates of two different populations of FRBs (i.e., non-repeating and repeating ones) evolve in significantly different ways towards higher redshifts. 橋本哲也's finding strongly constrains the origins of FRBs, i.e., non-repeating FRBs are likely associated with old stellar populations e.g. while dwarfs, neutron stars, and black holes, while repeating ones are
related to young stellar population or their remnants with short time scales e.g. supernova remnants, young pulsars, and magnetars.
Furthermore, FRBs are useful to answer two biggest questions in astronomy, i.e., dark energy and cosmic reionization history. In 橋本哲也's previous work, he found that FRBs can be used as a new 'standard candle' over traditional type Ia supernovae. FRBs are the first 'candle' which allows us to investigate the time variability of the dark energy. 橋本哲也 is recently working on how FRBs can constrain the cosmic reionization history. FRBs have a unique observable, dispersion measure, which tells us how much ionized material are there in between an observer and the FRBs as a function of redshift. This observable is much more sensitive to the cosmic reionization history than the cosmic microwave background. 橋本哲也 hope the next-generation radio telescopes such as the Square Kilometre Array will allow us to reveal these subjects in near future.

蕭予揚's research is about using fast radio bursts (FRB) to constrain dark energy. FRBs are the new emerging mysterious signal from the universe which were just discovered from few decades ago. However, the redshift measurement of FRBs needs to be identified counterpart. Therefore, 蕭予揚 purpose the cross-correlation between the galaxies with known redshift and mock FRBs without redshift. Luminosity-duration relation (Hashimoto relation) allowed us to measure luminosity distance once we measure only observed fluence and the intrinsic duration. Based on Hashimoto+2020 and Hashimoto relation, 蕭予揚 adopted the detection ability of Square Kilometre Array to generated the mock data with two different populations of FRBs, repeating and non-repeating FRBs. With these, we are able to constrain the dark energy and other cosmological parameters.

(王亭文:) In order to understand the interaction between the central black hole and the whole galaxy or their co-evolution history along with cosmic time, a complete census of active galactic nuclei (AGN) is crucial. However, AGNs are often missed in optical, UV and soft X-ray observations since they could be obscured by gas and dust. A mid-infrared (mid-IR) survey supported by multiwavelength data is one of the best ways to find obscured AGN activities because it suffers less from extinction. Previous large IR photometric surveys, e.g., WISE and Spitzer, have gaps between the mid-IR filters. Therefore, star forming galaxy (SFG)-AGN diagnostics in the mid-IR were limited. The AKARI satellite has a unique continuous 9-band filter coverage in the near to mid-IR wavelengths. This study group motivates me to search for AGNs with the AKARI satellite. 

 

心得感想:

橋本哲也: FRBs are new astronomical transients and have a huge potential. Many astronomers (including me) are now excited in FRBs. More than 10 nature papers about FRBs were published in these two years. I believe that FRBs will bring a breakthrough in astronomy which will totally renew our understandings of the Universe. I would like to contribute to it through my research activities.

蕭予揚:This work is quite interesting because the value of dark energy and Hubble constant is still in debate. Different cosmology such as Type Ia SN, BAO, GW derived the inconsistent cosmological parameters. Nevertheless, FRB is one of the signals which can reach high redshift. Therefore, FRB would play an increasingly important role in constraining cosmology.

王亭文: I not only learned about the traditional methods but the advanced technique. I applied the useful methods I learned from the study group and did research. I benefited a lot from the study group and I hope the activity can continue. 

下一次會更好:

Everything went well in this meeting :) Good Food!!

其他:

Nan

 圖片

^王柏雅認真聽其他成員的報告


Ting-Yi Lu / 2020-09-02

光譜能量分佈的擬合技術及如何將之運用在 AKARI 紅外望遠鏡及其他多波段觀測資料上

 時間:109年08月04日(星期二)15:30至17:30

地點:綜合二館天文所501

導讀人:金成鎮

成員:後藤友嗣、金成鎮、橋本哲也、Alvina、何建彰、王亭文、呂亭誼、桑戴瑞、蕭予揚、林于瑋、凌志騰

紀錄人:何建彰、金成鎮

導讀人介紹:

博士後研究員,專長:多波段觀測資料的疊合、光譜能量分佈擬合。

讀書會進度內容:

1. 光譜能量分佈的擬合技術及如何將之運用在 AKARI 紅外望遠鏡及其他多波段觀測資料上

2. 其他

討論紀錄:

何建彰 talked about his research, which is about the computation of photometric redshift of the AKARI infrared selected sources in the NEP field. In the NEP field, there are ~90,000 interesting infrared luminous sources that lack redshift information. Without redshift, we can not carry out further analysis of the source properties, e.g. luminosity function, or cluster finding. Therefore, I used the public available SED fitting code Le Phare to estimate the photometric redshift of the sources. At z<1.5, we achieve a weighted photo-z dispersion of σ∆z/(1+z)= 0.053 with η= 11.3% catastrophic errors.

 

金成鎮 gave a introduction on how the infrared galaxies in AKARI field help understand galaxy evolution: To understand the evolution of the universe, we've been studying how (various types of) galaxies evolve through cosmic time. Even though it has been already known that baryonic matters occupy only less than ~5% of the whole universe, the evolution of observable baryonic matter in galaxies is closely connected to the dark side (i.e., invisible or undetectable constituents) of the universe.  One of our major targets is the infrared luminous population of galaxies detected by the space telescope AKARI, in particular, which enables us to sample galaxies from the present epoch to cosmic noon (peak of the SFR density) with a unique wavelength coverage in the near- to mid-infrared. Focusing on this specific wavelength, we've discussed how to search/reveal obscured activities of star-formation or in the region of active nuclei.  This activity motivated to finalize establishing catalogs as well as quantifying physical parameters of galaxies, e.g., infrared luminosity and contribution from each component of a galaxy. etc.   

心得感想:

何建彰:Photometric redshift is important. I computed photometric redshift for people that help them to do further research. 

金成鎮: As we obtain more data from the recent observations or ongoing campaigns,  our activities will continue to answer the follow-up questions.

下一次會更好:

希望可以有更多人加入研究並分享AKARI紅外太空望遠鏡的觀測資料。何建彰跟王亭文都要畢業了,未來可能只剩下金成鎮跟桑戴瑞在鑽研這個項目。

其他:

林于瑋分享了近期讀到利用類星體吸收譜研究再電離的期刊論文。王亭文分享了活躍星系核的活躍度如何隨紅移或星系的紅外波段亮度變化。呂亭誼分享了如何利用類星體的電離泡泡研究高紅移時背景光對氫的電離化速率。桑戴瑞分享了其他國家及研究所如何舉辦期刊論文研讀俱樂部。

 

圖片

^桑戴瑞跟金成鎮討論AKARI衛星觀測到的星體性質


Ting-Yi Lu / 2020-08-25

AKARI 紅外望遠鏡觀測到的活躍星系核在不同環境中的狀態

 時間:109年07月28日(星期二)15:30至17:30

地點:綜合二館天文所501

導讀人:桑戴瑞

成員:後藤友嗣、金成鎮、橋本哲也、Alvina、何建彰、呂亭誼、桑戴瑞、蕭予揚、林于瑋、凌志騰、吳耕緯、王柏雅、古翊樺

紀錄人:桑戴瑞、呂亭誼

導讀人介紹:

升碩二,專長:星系演化及星系、類星體在不同環境下的演化 

讀書會進度內容:

     1. AKARI 紅外望遠鏡觀測到的活躍星系核在不同環境中的狀態(詳見討論紀錄) 

     2. 其他

 

討論紀錄:

Previous studies have shown that galaxy properties are affected by the environment of their host galaxies, which refers to the presence of other galaxies in their immediate vicinity. Denser environments mean that galaxies are closer to each other, and this may induce environmental effects that can affect the galaxies’ properties. However, there is no uniform census as to how activities of active galactic nuclei (AGNs) are affected by environment, most especially for infrared (IR) detected galaxies.

This research aims to uncover the relationship between active galactic nucleus (AGN) activities and environments of galaxies using data from the AKARI IR telescope. The AKARI telescope has surveyed the North Ecliptic Pole Wide (NEPW) field, which was also observed by many telescopes, providing us a multi-wavelength data for our galaxy sample (Kim et al. in prep). This also allowed us to calculate the photometric redshifts of our sources which will be needed to do spectral energy distribution (SED) fitting. This is used to constrain the properties of our sources such as stellar mass, star formation rate (SFR), and many others. The galaxy density, on the other hand, is related to the distance to the nth nearest neighbor (where we choose n = 10) and is derived from the Hyper Suprime-Cam (HSC) catalog of optically-detected sources in the NEPW field. The calculated densities are also used to find cluster candidates in the AKARI NEPW field (Huang et al. in prep).

Currently, our results show that a trend is present only for the most luminous galaxies in our sample, the ultra-luminous infrared galaxies (ULIRGs): their AGN contribution fraction (ratio of AGN luminosity to total IR luminosity) decreases with increasing density. In addition, their specific star formation rate (sSFR) and AGN number fraction (ratio of AGNs to total number of galaxies) decreases with increasing density.

 

心得感想:

With the results that I currently have, it seems that AGN and star forming (SF) activities are correlated with each other, which is actually presented by previous studies (von den Linden et al. 2010; Sabater et al. 2012) but with galaxies mainly detected in wavelengths other than IR. However, the trend of decreasing AGN/SF activity with denser environments seems to be present only for ULIRGs, whose luminosity and mass are higher than the rest of the galaxies in our sample. One possible reason for these results is the “over-consumption” scenario. In this scenario, a galaxy’s cold gas, which is the fuel for AGN and SF activities, is stripped out in a very dense environment, and then the galaxy starts to consume its remaining gas reservoir in a time dependent on the stellar mass of the galaxy. More analyses is required to see if this scenario can fit our findings our not.

 

下一次會更好:

目前針對活躍星系核的演化與其環境間的關係的研究沒有統一的結論。希望未來的讀書會我們能整理出一個針對這個主題的整合性的結論。

 

其他:

橋本哲也分享了利用Python疊合影像的方法;金成鎮分享了AKARI望遠鏡的窄波段巡天影像;何建彰分享了昴星團望遠鏡超廣角相機量到的紅移的精確度;吳耕緯分享了太陽系未知的第九行星在阿塔咖瑪宇宙學望遠鏡中的觀測值。

 

圖片

^桑戴瑞分享目所知的類星體演化與環境間的關係的相關研究


Ting-Yi Lu / 2020-08-17

類星體及利用類星體中的萊曼 α 吸收譜觀測宇宙再電離時的中性氫演

時間:109年07月21日(星期二)15:30至17:30

地點:綜合二館天文所501

導讀人:呂亭誼

成員:後藤友嗣、金成鎮、橋本哲也、Alvina、何建彰、呂亭誼、桑戴瑞、蕭予揚、林于瑋、凌志騰、吳耕緯、王柏雅、古翊樺

紀錄人:桑戴瑞、呂亭誼

導讀人介紹:升碩二,專長:宇宙再電離

讀書會進度內容:

     1. 類星體及利用類星體中的萊曼 α 吸收譜觀測宇宙再電離時的中性氫演化(詳見討論紀錄) 

          資料來源:fan06, Becker15, Eilers17, Bosman18, Lu20. 

     2. 其他

討論紀錄:

這次讀書會的導讀人呂亭誼分享了3種利用高紅移的類星體光譜中的萊曼 α 吸收譜研究宇宙再電離時的中性氫演化的方法。除了原理之外,討論也包含實務操作的面向。下列為各方法逐項的討論內容:

方法一:量測萊曼 α 光子光學深度隨紅移的變化

中性氫吸收萊曼 α 光子後電離化,不再吸收萊曼 α 光子。因此,藉由量測星系間介質對萊曼 α 光子的通透率(或光學深度)可以推算星系間介質中的中性氫含量。要量測萊曼 α 光子的通透率,首先要知道背景光源原本發射出多少萊曼 α 光子。類星體常被用作為背景光源,因為類星體亮度為所有已知天體(星星、星系...)中最亮的,就算其距離地球非常遠也可以被觀測到;此外,類星體的光譜特徵固定,所以適合用來量測前景中性氫的吸收光譜並界定星系間介質中的中性氫含量

量測萊曼 α 光子的通透率實務上的作法是將觀測到的類星體光譜除理論預測的類星體光譜,除出來的值就是通透率 。由於光譜中不同波長對應到不同的萊曼 α紅移,我們可以將萊曼 α 光子在不同波長的通透率轉換為其在不同紅移的通透率,進而研究中性氫含量在宇宙再電離時期的演化史。

題外話,因為紅移越高萊曼 α 光子的通透率越低,在紅移值大於6的類星體光譜中都以看到很長一段的萊曼 α吸收譜。此一特性可以被用於尋找高紅移巡天計畫中的類星體。古翊樺在她報告的時候就分享了她如何利用此方法從昴星團資料庫中挑選出候選的高紅移類星體。

方法二:量測Lya光子光學深度在不同紅移區間的分佈

Lya光子的光學深度在不同紅移區間的機率密度函數也被用作於揭示再電離史,因為光學深度的機率密度函數對應到時空中的中性氫分佈,而中性氫分佈又取決於不同星體的數量及分佈、中性氫的分佈,還有Lya光子的平均自由徑。光學深度在計算上是由方法一裡的通透率轉換而來的:光學深度=-ln(通透率)。目前已知紅移值大於6的類星體約有200個,量測一紅移區間光學深度的機率密度函數所用的光學深度樣本即取自於從這些類星體量到的通透率。

近期的觀測數據顯示再電離末期的光學深度機率密度函數隨紅移的變化較大,這有可能歸因於時空中的物質密度變化。然而此光學深度機率密度函數的變化量比理論預測的還大,代表還有其他因子影響光學深度機率密度函數的變化。有的研究團隊試著透過Lya光子的平均自由徑在此時快速增加來解釋此一現象,然而這項理論所需的平均自由徑與觀測不符。其他研究還探討了不同電離化的機制,但要證實是什麼機制導致如此大的機率密度函數變化還需要有相關觀測結果。

*橋本哲也在討論光學深度機率密度函數的量法時,分享了如何利用蒙地卡羅法求機率密度函數的誤差。 基本上是藉由隨機生成散佈於觀測到的光學深度的誤差範圍內的"假的"光學深度資料點,量測這些"假的"光學深度的機率密度函數的分佈而求得。 

方法三:量測類星體的電離泡泡大小隨紅移的變化

類星體的電離泡泡是指類星體周圍被高度電離化的氫。電離泡泡的大小取決於類星體的亮度及周圍星系間介質的電離化程度。基本上類星體越亮或周圍星系間介質的電離化程度越高,電離泡泡越大。在已知類星體的亮度並假設其亮度恆定的條件下,透過量測電離泡泡的大小推算出星系間介質電離化的程度。當有若干個不同紅移的類星體時,就可以研究星系間介質電離化程度隨時間的變化了。具體量測電離泡泡大小的做法是測量類星體光譜中Lya通透率初降至0.1的點的紅移,再計算這個紅移到類星體紅移的距離。然而這個測量值可能會因光譜的信噪比不同而有所增減。後藤友嗣因此更進一步討論了利用疊合多個類星體的光譜來降低信噪比並量測一般電離泡泡的大小的可能性。因為要把電離泡泡大小轉換為星系間介質電離化程度需要假設固定的類星體亮度,疊合多個不同亮度的類星體就破壞了這個假設,所以疊合多個類星體的光譜來降低信噪比這個方法不太可行,除非引入更多的模型及假設來修正疊合所造成的誤差。

心得感想:

 這次讀書會我們學到了很多關於宇宙再電離的近期研究進展。這五十至二十年間人們對再電離的理解可以說是爆炸性地增長,令人驚嘆,因為五十年前再電離還只是一個理論預測,現今卻以被證實,並且人們開始深入研究其細節及不同的再電離機制。未來十幾二十幾年人們對再電離的認知想必會有十足的長進,因為觀測技術越來越成熟,不同波段的望遠鏡的建設日漸完備,此外超級電腦的發展也幫助人們在理論預測中引入更細微的物理機制。現在加入研究再電離真是個再適合不過的時機了!

下一次會更好:

再電離牽扯到的物理機制非常繁雜,在觀測上也有五花八門的方法去驗證不同的理論模型。要在僅僅兩個小時內讓所有讀書會成員都理解並體會其中玄妙實在困難!下次可以開一個為期一兩個月密集討論再電離的讀書會,相信成員們在讀書會結束後都能成為再電離的專家!

其他:

在剩餘的時間裡,我們還討論了其他天文相關知識。橋本哲也分享了與伽瑪射線爆伴生的物質有哪些,及這些物質在月球上的含量;蕭予揚分享了如何計算快速電波爆發跟星系的auto-correlation  function,及如何將之轉換為cross-correlation  function;王柏雅分享了阿塔卡瑪大型毫米及次毫米波陣列觀測到的紅移約為7左右(~宇宙大爆炸後1Gyr)的星系;古翊樺分享了如何利用昴星團望遠鏡跟WISE紅外太空望遠鏡的觀測資料尋找z>7的超大質量黑洞;後藤友嗣分享了如何計算星系密度。

 圖片

^呂亭誼介紹萊曼 α 光子的通透率

圖片

 ^與會成員們

 


Ting-Yi Lu / 2020-08-07
快速電波爆發的成因及相關應用 2020-09-02 光譜能量分佈的擬合技術及如何將之運用在 AKARI 紅外望遠鏡及其他多波段觀測資料上 2020-08-25 AKARI 紅外望遠鏡觀測到的活躍星系核在不同環境中的狀態 2020-08-17 類星體及利用類星體中的萊曼 α 吸收譜觀測宇宙再電離時的中性氫演 2020-08-07


Copyright©2007-2009 National Tsing Hua University ALL RIGHTS RESERVED
最佳解析度為1024*768或1280*1024
聯絡我們 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, R.O.C. 30013 新竹市光復路二段101號 代表號:03-5716200 統一編號:46804804