以GOODS-North的星系檢驗塵埃吸收跟紫外線光譜斜率的關係

Abstract

在我們研究星系的恆星形成率的時候，我們很難測量到無線電波的輻射，加上我們清楚知道年青大質量恆星是如何輻射紫外線，所以我們可以透過觀測星系的紫外線，再推算出紫外線的恆星形成率。此過程必須知道星系中的塵埃吸收了多少紫外線，再把被吸收的量修正回去，從而得到正確的紫外線亮度並推算出恆星形成率。由於在星爆星系中，星系的紫外線光譜斜率(β)和其塵埃吸收有相關性，因此現時天文學家主要相信塵埃吸收和星系的紫外線光譜斜率(β)有關，由fλ=λβ定義，其中λ 在1300-3500 Å之間。本論文希望再次檢驗上述關係，先由Morrison et al. (2010)得到星系的無線電波亮度，由星系的無線電波亮度得到無線電波恆星形成率。星系無線電波的產生主要因為大質量年老恆星在超新星爆炸的時候，相對論性的電子被磁場加速，進而產生無線電波的同步輻射，加上無線電波不受塵埃吸收的影響，因此可以由無線電波恆星形成率和紫外線恆星形成率的比值推斷塵埃吸收的本質，再檢驗這和β是否相關。 在星系紫外線的亮度和紅移方面，本次工作採用了Barger et al. (2008)的GOODS-N星系目錄，其紅移範圍比以往的分析廣，該目錄中同時有1.4 GHz的天體最高紅移到逹z~2.6。在星系無線電波的亮度方面用了Morrison et al. (2010)的VLA 1.4 GHz GOODS-N 星系目錄， 再運用Wang et al. (2012)的方法把到無線電波的亮度推導出無線電波的恆星形成率，此為不受塵埃吸收影響的恆星形成率。在比對無線電波恆星形成率和紫外線恆星形成率的比值和β後，我們並沒有發現它們之間有很強的相關性。我們再引入星系的z - Ks顏色，並以z – Ks vs. β的關係定義出兩個新參數，嘗試找出此參數與β的關係，但我們仍然沒有看到參數與β之間有很強的相關性，因此判斷β和塵埃吸收之間並沒有很強的相關性。

When we study star formation rate (SFR) of high redshift galaxies, because we know well how young high mass stars emit UV radiation, we observe the rest-frame UV emission of galaxies, and convert it to UV star formation rate (UV SFR). We also need to know the dust absorption in the UV, and then correct for the absorbed part to obtain the intrinsic UV luminosity and SFR. It is widely believed now that dust absorption is correlated with the rest-frame UV spectral slope (β) of a galaxy, defined as fλ = λβ, where λ is between 1300-3500 Å in this work. This work aims to verify the above correlation; we will derive the radio SFR from the radio flux of galaxies from the radio catalog of Morrison et al. (2010). The radio emission is generated by supernova explosion from high mass stars, in which relativistic electrons are accelerated by magnetic field and produce synchrotron radiation. Therefore the radio emission can also infer high mass star formation rate. Since the radio emission is not affected by extinction, the radio-to-UV SFR ratio can indicate dust absorption. Thus we can test if there are any correlations between the UV and radio SFR ratios and β. For the rest-frame UV fluxes and redshifts, this work uses the GOODS-N catalog from Barger et al. (2008). This catalog covers a broader range of redshift, compared to other works, and the highest redshift object in this catalog with 1.4 GHz radio flux is about z~2.6. We adopt the VLA 1.4 GHz GOODS-N catalog from Morrison et al. (2010) and use the method in Wang et al. (2012) to convert the radio flux to radio SFR. After comparing β with radio-to-UV SFR ratios, we find that there is no strong correlation between them. Then we include z – Ks colors of galaxies and use a z – Ks vs. β diagram to define two new parameters to test if they are correlated with extinction. We do not find significant correlations. Therefore, we conclude that there is not a strong correlation between dust absorption and β.