Daily Archives: 2017-05-04

中國天文學家利用新工具在哈勃常數的精確測量領域取得新進展

NGC 4258的多波段數據合成圖
自1929年哈勃提出星系的視向速度與到觀測者的距離之間呈現線性關係(即後來經典的哈勃定律)以來,對哈勃常數的測量始終是天文學研究中最重要的內容之一。哈勃常數,即哈勃關係中的斜率,從它的大小可以直接估算出宇宙的年齡。此外,哈勃常數的獨立精確測量,有助於更好地限定宇宙學常數。然而,源於不同測量者或測量方法之間的系統誤差,對哈勃常數的精確測量並非易事。近日,上海天文台科研人員參與的國際性宇宙學項目組,基於對四個水超盤激微波系統(extragalactic water megaMASER )的長期監測數據,精確測量出哈勃常數,與其它測量方法得到的結果在誤差範圍內一致。

傳統的測量哈勃常數的方法主要依賴於造父變星以及Ia型超新星。儘管相比數十年前而言,不同研究者之間所測得的哈勃常數結果差距已明顯縮小,但為了能更精確地測定哈勃常數,同時也為了檢驗不同測量方法之間的系統誤差,全新的幾何距離測量方法也在不斷地被提出,其中22 GHz水超盤激微波系統的發現為哈勃常數的測量提供了一種全新的可能性。

在該類天體中,在距離河外星系中心超大質量黑洞約一秒差距(parsec,縮寫 pc;一秒差距大約等於三十一兆公里)的吸積盤區域上分佈著大量可以產生22 GHz水超激微波輻射的水分子,並且這些水分子隨著吸積盤圍繞中心超大質量黑洞進行旋轉運動。由於激微波輻射的准直性和輻射放大性,通過甚長基線干涉測量(Very long Baseline Interferometry,簡稱 VLBI)技術,每個激微波斑的空間位置以及視向速度都可以精確的測量出來。

基於對視向速度的測量以及長期監測,可以得到激微波斑到黑洞的物理尺寸。將其與由VLBI測量得到的激微波盤的角尺寸結合,就能得到激微波盤到觀測者的幾何距離;再綜合退行速度,就可以推算出哈勃常數的值。

這種方法並不依賴任何宇宙學模型假設,因此由激微波盤系統所得到的幾何學距離可用於對不同宇宙學模型的檢驗。利用這種方法,NGC 4258成為了首個完成幾何距離測量的激微波系統。 基於對NGC 4258幾何距離測量的成功,由前美國國家射電天文台台長魯國鏞院士等發起倡議、美國國家射電天文台的國際研究團隊於2009年開展了利用河外水超激微波源對哈勃常數進行系統性測量的研究項目。該項目計劃在北天區距離我們50-200Mpc的範圍內搜尋到十個類似NGC 4258的河外水超盤激微波源。若對於每個盤激微波源的幾何距離可以確定到10%的精度,則十個源的結果統計疊加之後可達到3%的精度。

【圖、文:節錄自中國科學院上海天文台網頁;新聞訊息由林景明提供】研究全文刊登在已經出版的《天體物理學報》

激微波 ( MASER,中國大陸音譯作:脈澤)是受激放大微波輻射(Microwave Amplification by Stimulated Emission of Radiation)的頭字母。它指通過受激輻射放大和必要的反饋,產生同一波寬、准直、相干的微波的過程。

研究人員發佈西伯利亞射電日像儀首個太陽觀測結果

5.2GHz太陽射電圖像
俄羅斯科學家發佈使用西伯利亞太陽射電望遠鏡(Siberian Solar Radio Telescope)的新型射電日像儀進行的太陽觀測的首個觀測結果。 西伯利亞射電日像儀(Siberian Radioheliograph)最近開始定期觀察太陽大氣層中的活動過程,從而更好監測太陽活動情況。

西伯利亞射電日像儀是一款具有4至8 GHz工作頻率範圍和10 MHz瞬時接收頻段的48枝天線陣列。射電日像儀安裝在位於距伊爾庫茨克(Irkutsk)約220公里的東薩彥嶺(Eastern Sayan Mountains)射電天體物理天文台(Radioastrophysical Observatory)的西伯利亞太陽射電望遠鏡上。

西伯利亞射電日像儀系統最終將擴展到96枝天線,完成後可以提高分辨率至15角秒,屆時儀器能夠研究日冕物質拋射 (coronal mass ejection,簡稱 CME)的開始及傳播到一至二個太陽半徑高度的過程,從而填補了紫外光和光學範圍內的觀察值之間的差距。

【圖、文:節譯自物理學機構網頁】最初的西伯利亞射電日像儀觀察結果刊登在4月25日科學論文預印本網站(arXiv.org)

光從哪裡去了?哈勃望遠鏡前緣場中透出受抑高紅移星系形成的證據


Public Seminar of RPg Student:- Where’s the Light? – Evidence for Suppressed High-redshift Galaxy Formation from the Hubble Frontier Fields

The standard Cold Dark Matter (CDM) model predicts that galaxies form hierarchically, resulting in an increasing number of galaxies towards lower masses and hence fainter luminosities at all redshifts since the onset of galaxy formation. In the Local Universe, such a dependence had been found for luminosities as low as dwarf spheroidal galaxies, the least massive galaxies known. A similar trend had also been confirmed in the early universe through blank field surveys, which provide a limiting rest-frame UV absolute magnitude of MUV ~ -17 at redshifts z > 4. Does this trend of increasing galaxy space density continue to even lower luminosities as predicted by the CDM model, or eventually turn over as predicted by competing models such the Wave Dark Matter (ψDM) model?

To probe even fainter luminosities than is possible through blank field surveys, we took advantage of gravitational lensing provided by massive galaxy clusters that boosts otherwise undetectable faint galaxies to above the intrinsic survey flux limit. Our sample comprises ~600 dropout galaxies identified in five galaxy cluster fields and their adjacent blank fields (observed simultaneously) from the Hubble Frontier Fields program. These galaxies are magnified in brightness by as high as ~100×, thus allowing us to probe approximately five magnitudes deeper than the intrinsic survey limit. Because a given solid angle in the sky is similarly magnified, the volume of space probed decreases in proportion to the magnification. This effect, however, is countered by the expected increase in space density of less luminous galaxies as predicted by the CDM model.

Contrary to such expectation, we discovered a significant deficit of highly magnified galaxies in the data relative to the CDM prediction, reflecting that the diluted apparent galaxy density from the magnified sky area heavily outweighs the addition of faint galaxies brought above the flux limit. This result was found to be in >4σ tension with the standard CDM model, indicating a slow rollover in the faint-end UV luminosity function as is predicted by the ψDM model. In this alternative model, the formation of low-mass DM halos, which seed the growth of faint galaxies, is suppressed by the quantum pressure arising from interfering ultralight dark matter waves with parsec-scale de Broglie wavelengths.

Through a comparison with state-of-the-art ψDM simulations, we found that our results are compatible with the model predictions for a DM boson mass of ~10^-22 eV. The inferred boson mass is consistent with that deduced independently for producing the large constant-density DM cores of local dwarf spheroidal galaxies. In addition, our results also show a marked transition (stronger decline towards higher redshifts) in the cosmic star formation rate density at z ~ 8, a similar redshift as the instantaneous epoch of reionization at z = 8.2 reported by Planck. 
 
時間:2017.05.04(四) 11:30
地點:香港大學 莊月明物理樓105室
講者:Yi Lok Enoch LEUNG (香港大學)
語言:英語

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此活動由香港大學物理系主辦。

富能毫秒脈衝星的X射線和γ射線觀測


Public Seminar of RPg Student:- X-ray and Gamma-ray Observations of Energetic Millisecond Pulsars

Pulsars are fast spinning neutron stars produced by supernova explosions of massive stars and they possess extremely high magnetic fields from 108G to 1012G. Millisecond pulsars (MSPs) are fastest-rotating pulsars with periods from about one to several tens of milliseconds. They are believed to be spun up in binary systems. Their surface magnetic fields are small compared with normal pulsars, but they usually possess large magnetic fields near the outer gaps due to their small light cylinder radii. This makes some MSPs have broadband emission properties similar to young pulsars.

This project aims to understand the high-energy emission properties of energetic MSPs. the speaker focus on three energetic rotation-powered MSPs─B1937+21, B1821-24 and J0218+4232. Firstly, he will talk about the X-ray spectra of these MSPs. Then, the speaker will report the results of timing analysis using NuSTAR data. he will also talk about my research plan, to focus on analyzing gamma-ray data and build a theoretical model to explain the emission. 
 
時間:2017.05.04(四) 10:30
地點:香港大學 莊月明物理樓105室
講者:Wenchao WANG (香港大學)
語言:英語

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星系團MACSJ0329-0211中央巨大橢圓星系的近期恆星形成史


Public Seminar of RPg Student:- Recent Star Formation History of the Central Giant Elliptical Galaxy of the Cluster MACSJ0329-0211

A number of cluster central giant elliptical galaxies(a.k.a. brightest cluster galaxies, BCGs) exhibit recent or ongoing star formation that is most clearly revealed in the UV continuum. The nature of this star formation activity (i.e., origin of the star-forming gas, star formation rate over time) is poorly understood. The best data currently available for elucidating the star formation history of BCGs come from the Cluster Lensing And Supernova survey with Hubble(CLASH), which spans wavelengths from the near-UV to the near-IR in 16 filters. Previous studies using this data have used only the UV filters to estimate star formation rates(SFR) in this BCG assuming a certain star formation history(i.e., nearly constant SFR over 100 Myr). Here, we derive the spatially-resolved spectral energy distribution (SED) of the relatively young stellar population so as to infer its star-formation history through population synthesis modeling. We anticipate our work to set a benchmark for what can be learnt from this unique and high quality data set having unprecedented wavelength coverage.

To obtain the SED of the young stellar population, we first have to subtract the light contributed by the old stellar population in every filter band. To model the 2-dimensional light distribution of the old stellar population, we used images in the longest-wavelength bands where the old stellar population dominates the overall light. In doing so, we found it necessary to mask out nearby cluster members, as well as both the young stellar population and dust based on color images of the BCG. The model parameters for the 2-D light distribution of the old stellar population thus derived was consistent among the reddest filter bands, allowing us to construct a wavelength-averaged model applicable to all filter bands. The images obtained after subtracting the model for the old stellar population have good consistency with the spatial distribution of the young stellar population that dominates the UV images, as well as color images of the BCG.

To make a preliminary look and to obtain global view of the data, we compared a color-color diagram (constructed from three line-free filters) of the young stellar population with population synthesis model. The results, which are only weakly sensitive to metallicity, implies ages ranging from ~10 Myr to nearly 1Gyr for the young stellar population.

We are now in the process of modeling the entire SED to determine: (i) whether the ages thus inferred are compatible with that those inferred from the color-color diagram; (ii) the degree to which we can break the age-metallicity degeneracy; (iii) whether it is necessary to include dust extinction, and the extra degree of degeneracy in doing so. 
 
時間:2017.05.04(四) 10:00
地點:香港大學 莊月明物理樓105室
講者:Junhao LI (香港大學)
語言:英語

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#119

此活動由香港大學物理系主辦。